Forum 78 Proceedings

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Acoustics I

Technical Session A: Tues. May 10, 2022 - 8:00 AM to 12:00 PM

Aeroacoustic Prediction and Validation of Variable RPM Rotors and Rotor-Airframe Interactions for Advanced Air Mobility Applications (Paper 1274)
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Mrunali Botre, Daniel Wachspress, Kenneth Brentner, Ze Feng Gan

Aeroacoustic prediction of the tonal noise sources for rotors operating with time-varying rotor rotation rate (RPM) is explored. Enhancements to the CHARM analysis and CHARM/PSU-WOPWOP interface software are described that are required to support acoustic predictions of time-varying RPM multiple rotor and rotor + strut configurations. Predictions are compared with acoustic test data from (Ref. 1) and (Ref. 2) for both constant and time-varying RPM. The configuration with time-varying RPM was considered to understand the tonal noise sources with a focus on loading noise. The effect of change in RPM on aerodynamic interaction between rotor and strut and its effect on loading noise was studied. A comparison between NASA wind tunnel test data and predictions for an isolated ideally twisted rotor is carried out with time-varying rotor RPM data. Sound pressure spectrum plots are generated for raw test data, filtered test data (frequency range 250-1500 Hz), and acoustic pressure time history to perform validation at in-plane and out-of-plane microphones.

Analytic Prediction of Rotor Broadband Noise with Serrated Trailing Edges (Paper 54)
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Sicheng (Kevin) Li, Seongkyu Lee

Trailing-edge serrations are known to be an effective way to reduce broadband noise on wing sections, but their noise reduction capabilities on rotorcraft have not been fully understood. This paper develops a new approach to analytically predicting and investigating broadband noise of rotors with serrated trailing edges. To achieve rotorcraft broadband noise predictions, we extend Lyu and Ayton's semi-infinite serrated wing model to include the finite blade span, the modified scattering coefficient, and the spanwise observer distance. The validations show good agreements with experimental data for serrated trailing-edge noise of a wing section, a hovering rotor, and a forward-flight rotor. Next, the effects of serration parameters on rotor broadband noise are studied. For optimal and realistic rotor broadband noise reduction, the desirable design of a serratedblade rotor has the serration height to wavelength ratio of 2, the radial range of serrations from 50% blade span to the tip. It is also found that the sine-wave, chopped peak, or saw-tooth serration shapes reduce noise most among various shapes that are considered. Finally, noise reduction with serrations is applied to urban air mobility (UAM) aircraft. A 6-passenger eVTOL quadrotor is found to have more than 9 dB noise reduction potential with serrated trailing edges, where higher noise reductions are observed with higher tip speeds and fewer blades. Serrated-blade quadrotors are found to have the dipole broadband noise directivity, and noise reductions are observed at all observer angles.

Comparison of Multi-Fidelity Approaches for the HART-II Rotor Noise Prediction Using CREATETM-AV Helios (Paper 1195)
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Zhongqi Jia, Jeffrey D. Sinsay

This paper presents a comparison of multi-fidelity approaches for the HART-II rotor noise prediction using the high-fidelity and multi-disciplinary rotorcraft analysis software suite CREATETM-AV Helios. The acoustic simulations are performed using the noise prediction tools AARON/ANOPP2 and PSU-WOPWOP. Both permeable and impermeable formulations of the Ffowcs Williams and Hawkings equation are considered for noise predictions. The HART-II baseline and 3/rev high harmonic control (HHC) minimum vibration cases are simulated and validated against test measurements. Compared with the free-wake simulation, the highfidelity CFD/CSD loose coupling simulations agree better with the measured data but still under-predict the advancing side airloads. Both impermeable and permeable surface approaches show better noise predictions than the free-wake approach in terms of noise directivity and amplitude. As the finest wake-grid resolution reduces from 7.5% to 5% chord, the impermeable surface strategy shows an increment of 3 dB in maximum mid-frequency sound pressure level (SPL) on the advancing side of the mid-frequency SPL contour. The maximum mid-frequency SPL on the retreating side of the vehicle is only increased by 1 dB. At a finer wake-grid resolution, the permeable surface approach does not show improvement in blade-vortex interaction (BVI) noise prediction on the advancing side of the rotor disk.

Numerical Analysis of Rotor Aeroacousitc Scattering Characteristics Considering Fuselage Aerodynamic Configuration Parameters (Paper 1302)
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Weicheng Bao, Xi Chen, Qijun Zhao, Dazhi Sun

Based on the FW-H equations, the CFD method and the time domain equivalent source method (TDSEM), the rotor aeroacousitc scattering characteristics considering the fuselage aerodynamic configuration parameters are calculated and analyzed. First, a set of CFD/FW-H/TDESM hybrid analysis method for rotor/fuselage aeroacoustic characteristics is developed. The aeroacoustic characteristics of the UH-1H rotor in hover and a point acoustic source nearby a rigid sphere are calculated, and the employed numerical analysis method is validated through comparisons with reference data. Then, the aeroacoustic characteristics of the Bo-105 in hover (main rotor/fuselage) are analyzed, and the scattered noise is discussed in detail. Finally, the aeroacoustic characteristics of the AH-64 helicopter in forward flight is calculated. In addition, parameters, such as the distance between fuselage and rotor, are quantified, and some conclusions are obtained. The fuselage will influence the directivity of rotor noise, and an appropriate fuselage configuration will reduce the rotor noise at the azimuth of 250-degree(1dB in the present case).

Performance and Acoustics of a Stacked Rotor with Differential Collective Pitch (Paper 37)
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Chloe Johnson, Jayant Sirohi, George Jacobellis, Rajneesh Singh

Performance and acoustics of a 1.108 m radius stacked rotor in hover with an axial spacing between the upper and lower rotor of 0.73 chords were measured. The differential collective pitch of the rotor system, defined as half the difference between the lower rotor pitch and upper rotor pitch was varied along with the azimuthal spacing between the upper and lower blade sets. Coupled comprehensive analysis and acoustics simulations were conducted using the Rotorcraft Comprehensive Analysis System (RCAS), the acoustic solver PSU-WOPWOP, and the broadband noise module from The University of Maryland Acoustic Code (ACUM). Increased sensitivity to azimuthal spacing was observed for large, negative differential collectives, resulting in 76.5% change in thrust and 46% change in power over 11.25o change in azimuthal spacing. The trends in individual and total system thrust were predicted with good accuracy, but the total power tended to be overpredicted. Overall sound pressure level decreased by 3.5 dB at -2o differential collective and 90o azimuthal spacing due to reduced broadband levels, indicating a possible quiet mode of operation for this type of rotor. Acoustic predictions captured thickness and loading noise trends, but significantly underpredicted broadband levels.

The Effect of Boundary Layer Character on Stochastic Rotor Blade Vortex Shedding Noise (Paper 61)
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Christopher Thurman, Nikolas Zawodny, Nicole Pettingill

This work illustrates the effect of a rotor blade's boundary layer on the broadband laminar boundary layer vortex shedding (LBL-VS) self-noise emitted from an optimum hovering rotor through experimental and multifidelity computational studies. Blade surface roughness effects associated with different manufacturing techniques and the effect of adding a boundary layer trip were shown to decrease LBL-VS noise by upwards of 30 dB at the frequency of maximum emission with a slight penalty in aerodynamic performance when compared with smooth rotor blades. Low-fidelity 2-D viscous flow analysis verified the presence of laminar separation bubbles on the rotor blades, which are responsible for LBL-VS noise. Three high-fidelity lattice-Boltzmann simulations were conducted with different wall-functions to predict the boundary layer character correspondent to their experimental counterpart and the resultant presence or absence of LBL-VS noise. Excellent aerodynamic and aeroacoustic agreement was seen between the lattice-Boltzmann simulations and the experimental data for the cases with surface roughness and the boundary layer trip. The broadband noise results from the simulation with fully turbulent wall-functions diverged from the experimental results above 5 kHz. The transitional wall-function simulation, which emulated the smooth experimental blades, underpredicted thrust by 14% and broadband noise by a minimum of 10 dB with an accurately predicted broadband noise trend.

Trailing-Edge Noise Predictions of Rotorcraft Airfoils (Paper 95)
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Hyunjune Gill, Seongkyu Lee

This paper presents the effect of rotorcraft airfoils on turbulent boundary layer trailing-edge noise. Among the numerous airfoils available for rotocraft application, a total of eleven commonly used airfoils are selected and divided in three groups, and their corresponding trailing-edge noise is investigated. The compressible Reynolds Averaged Navier-Stokes (RANS) CFD simulation coupled with a semi-empirical wall pressure spectrum model and a physics-based far-field noise prediction theory is used to predict noise levels at varying angles of attack. In this paper, the CFD tools and the noise prediction model are first validated, and then the trailing-edge noise analyses for the eleven airfoils under the same compressible flow condition are presented. It is found that NACA 0012, VR-12, and SC-1094 R8 airfoils show low trailing-edge noise levels compared to other similar airfoils in each group. The noise characteristics from airfoils like NACA 0012 shows clear separation in the frequency range where the noise from the suction or pressure side is dominant at low and high frequencies, respectively. However, airfoils like VR-12 shows that the suction-side trailing-edge noise can still be important at high frequency at small angles of attack, thus producing a combined effect from both suction and pressure sides. Furthermore, a natural laminar flow airfoil is considered to investigate how the trailing-edge noise responds to the delayed transition locations. It is observed that the delayed transition locations reduce the low- and highfrequency noise at a higher angle of attack.

Acoustics II

Technical Session C: Wed. May 11, 2022 - 1:45 PM to 6:00 PM

A Realistic Rotorcraft Noise Footprint Computation for Low-Noise Trajectory Optimization (Paper 74)
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Pierre Dieumegard, Frédéric Guntzer, Julien Caillet, Sonia Cafieri

This paper introduces recent developments in the computation of rotorcraft noise footprint, implemented in an Airbus Helicopters' internal software. The paper presents the main ingredients that have led to enhance the efficiency and accuracy of such noise footprint computation. This includes taking into account both the particularities of turns in noise emission and the influence of the wind on noise propagation. Furthermore, the software is able to assess a real traffic environmental impact, since computations are done within a realistic 3D simulation environment, taking into account both the curvature of the Earth and the topography of the ground. A variety of noise annoyance indicators can be computed thanks to the coupling with demographic and background noise data. Such realistic noise footprint computation is embedded in a tailored algorithmic scheme aiming at optimizing rotorcraft trajectories in such a way that their associated noise footprint is minimized. The proposed optimization approach has been tested on multiple real-world case studies, showing significant prospective noise reduction compared to reference trajectories.

Analysis of Rotor Noise during Maneuvering Flight based on CLORNS Solver (Paper 1291)
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Xi Chen, Weiqi Wang, Qijun Zhao, Muyang Lin

Based on FW-H equations and the CFD method, aeroacoustic characteristics of rotor during collective pitch aperiodic variation are calculated and analyzed. At first, a set of analysis method for aperiodic rotor aeroacoustic characteristics is developed. The aerodynamic and aeroacoustic characteristics of BO-105 rotor in hover and NACA rotor in a ramp increase of collective pitch are calculated, and the employed numerical analysis method is validated through comparisons with experimental data. Then, the aeroacoustic characteristic of BO-105 rotor in hover and in forward flight during collective pitch aperiodic variation is calculated. Finally, parameters, such as the collective pitch change rate, are quantified, and some conclusions are obtained.

Flyover Noise Comparison Between Joby Aircraft and Similar Aircraft (Paper 1180)
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Jeremy Bain, Greg Goetchius, David Josephson

Joby Aviation is developing a six propeller, all electric vertical takeoff and landing piloted air taxi aircraft. The aircraft is designed for high density operations near residences and workplaces, so it is imperative that the acoustic emissions of the aircraft are minimized for community acceptance. It is important to compare not just the absolute sound levels but also the sound quality with conventional aircraft already known to the public. To showcase the difference between the Joby aircraft and similarly sized aircraft for a level flyover condition, Joby arranged a flight demonstration with two conventional fixed-wing aircraft and three commercial helicopters. All the aircraft were flown at approximately 100 knots (51 m/s) and 1500 feet (457 m) above ground level within minutes of each other in the same location to minimize variability. They were measured with the same equipment and processing methods. The results show that the peak Joby aircraft sound pressure level was 10 to 19 dB(A) below the conventional aircraft. The conventional aircraft noise levels were above the peak Joby level for 43 to 54 seconds during the flyover. The Joby aircraft was measured at 13 to 22 EPNdB lower than the conventional aircraft. Close examination of the spectra showed that Joby aircraft has greatly reduced tonal and low frequency content. When substituting the low ambient noise during the flight test with a realistic urban outdoor cafe environment, the Joby aircraft is below the ambient at all frequencies during the direct flyover which allows it to blend in with the ambient soundscape. The other conventional airplanes and helicopters exhibited large tones that rise above the cafe ambient for approximately one minute, and would impact a much larger area on the ground that is not directly under the flight path.

Ground-based Acoustic Measurements of Small Multirotor Aircraft (Paper 1211)
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Nicholas Blaise Konzel, Eric Greenwood

Experimental procedures are developed for the measurement and characterization of noise from small multirotor aircraft. These procedures are applied to measure the noise of the Tarot X8. The acoustic characteristics of the Tarot X8 are evaluated for a number of level flight flyover conditions. Both tonal and broadband noise is found to be significant, but the relative importance depends on the angle of observation. Variability in noise is assessed both across repeated runs of the same condition and within a run using an array of microphones parallel to the flight track. Variability is found to increase as the distance between the microphone and aircraft increases. Variations within a run are significant (on the order of 2 dBA), but do not explain the greater variation (on the order of 5 dBA) in levels between runs.

Helicopter Noise Source Separation Using an Order Tracking Filter (Paper 30)
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Joel Sundar Rachaprolu, Eric Greenwood

Due to the importance of understanding the aeroacoustics of rotorcraft with continually changing noise sources, this paper presents a new technique for source separation from ground-based acoustic measurements. The source separation process is based on combining a time-domain de-Dopplerization method with the Vold-Kalman (VK) order tracking filter approach. This process can extract rotor harmonic noise even when the sources are continuously changing with time, including impulsive events such as Blade Vortex Interaction (BVI) noise. The advantage of this approach over traditional methods such as harmonic averaging is that the phase and amplitude relationship of acoustic signals is preserved throughout the extraction process. The approach is applied to the measured acoustic data from a Bell 430 helicopter. The measured data were separated into main rotor harmonic, tail rotor harmonic, and broadband residual components. For steady-state conditions, the extracted components could be de-propagated to form acoustic spheres showing the directivity of the separated main and tail rotor components. The source separation process was also applied to a maneuvering flight condition. Each component has different pulse shapes and directivity trends, consistent with aeroacoustic theory.

Measured Acoustic Characteristics of Low Tip Speed eVTOL Rotors in Hover (Paper 1253)
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Jonathan Fleming, Matthew Langford, Jacob Gold, Kyle Schwartz, David Wisda, Nathan Alexander, Jeremiah Whelchel

To document noise characteristics and provide validation data for acoustic modeling of rotor systems appropriate for eVTOL/UAM aircraft, the team of Techsburg, AVEC, and Virginia Tech peformed an outdoor static test of a subscale 5-blade rotor. The testing was carried out as part of a program to demonstrate feasibility and performance of a quiet rotor system in support of the eVTOL industry. Techsburg designed a low-tip speed rotor to approximate performance required by a 4-5 passenger UAM vehicle. A driving design feature was low-tip speed operation (Mtip ~0.27) at system disk loadings of 7 to 8 psf (~3.7 N/m2). The test article was designed as a ground adjustable pitch 5-blade rotor, and 2-blade and 3-blade versions of this rotor were also tested during this project. The test article size of 3 feet diameter (0.91 m) represented a scale factor of approximately 30% compared to a full size vehicles currently in operation or development today. The aerodynamic performance in hover was consistent with other rotor systems tested in the past at Techsburg (Figure of Merit ~0.70), and the effects of naturally occurring turbulence on rotor acoustics was measured using a 180-deg arc array of ground plane microphones at the Virginia Tech Drone Park test facility. The paper closes with a discussion of simulating the experiment with the PowerFLOW LBM solver with and without flow turbulence.

Variation in Helicopter Noise During Approach Maneuvers (Paper 1277)
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Damaris Zachos, Lauren Weist, Kenneth Brentner, Eric Greenwood

The Helicopter Association International (HAI) "Fly Neighborly" program aims to provide helicopter pilots with recommendations to effectively reduce the noise of their operations. One aspect of this program encourages the development of approach trajectories that avoid airspeed and flight path angle combinations associated with high levels of noise generation. These trajectories necessarily involve changes in speed and flight path angle; i.e., there is an acceleration. To understand the effect of acceleration on noise, a parametric sweep of both longitudinal acceleration and time rate of change of flight path angle (vertical acceleration) were completed for a model of the Sikorsky S-76D helicopter. This was done utilizing a PSU noise prediction system that is a coupled flight simulation (PSUHeloSim), rotor comprehensive analysis (CHARM), and noise prediction code (PSU-WOPWOP). These sweeps are compared with noise predictions for trajectories processed through the PSU noise prediction system that follow flight test data collected in the 2019 joint NASA/FAA/Army flight test.

Acoustics III

Technical Session D: Thurs. May 12, 2022 - 10:15 AM to 12:15 PM

An Investigation of Flight Control Strategy on Generic eVTOL Noise (Paper 1290)
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Bhaskar Mukherjee, Kenneth Brentner, Eric Greenwood, Jean-Pierre Theron, Joseph Horn

A preliminary investigation of impact of piloting and flight control strategies on maneuver noise is conducted on a generic eVTOL configuration undergoing a 50 knot level-turn maneuver. The piloting strategy involved control of aircraft pitch to change split between rotor lift and wing lift, while the control strategy involved comparing a rotor thrust control with fixed pitch rotors operating with variable rotation rate and a rotor thrust control strategy with variable pitch rotors operating at constant angular velocity. With the rotors operating in the low tip-Mach number flow regime, it was revealed that broadband noise due to airfoil self-noise dominates the noise levels overwhelmingly. The turbulent boundary layer trailing edge noise contributed the most, with blade stall found to result in significant addition to noise levels (nearly 10 dBA). Deterministic noise was found to be sensitive to rotor thrust control strategies, with control biases offering an additional layer of influence over individual rotor tip-Mach number and thrust levels. Individual rotor thrust and trim were found to be important parameters controlling deterministic noise, while combined rotor thrust levels was found to be the important influence over time-averaged broadband noise levels.

Control Method of the Rotor Noise based on Sound Pressure Cancellation (Paper 1309)
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Yan Ding, Rui Hu, Xi Chen, Qijun Zhao, Guoqing Zhao

The volume of noise has become a major issue in limiting the range of helicopter applications. This paper employs a method of the actively controlled drag piece to reduce the volume of rotating noise produced by rotor blades. The flow field characteristics of the rotor blades are numerically simulated using a high-precision RANS solver-CLORNS to obtain high-resolution sound source information. The rotor noise is then predicted using the Farassat 1A formulation and the rotor aerodynamic noise prediction method. To validate the method, a common UH-1H arithmetic example is used. The standard approach was used to calculate the rotor noise characteristics, and particular results were obtained by focusing on the influence of each parameter of the drag piece.

High Solidity, Low Tip-Speed Rotors for Reduced eVTOL Tonal Noise (Paper 1256)
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Farhan Gandhi, Justin Pepe, Brendan Smith

This paper reports on a computational study conducted on an 8 ft diameter, fixed-pitch eVTOL rotor to examine the potential of using increased solidity and reduced tip speed to reduce the radiated acoustic signature. The study is conducted for the rotor operating in hover and in vertical climb, and at disk loadings between 6-12 lb/ft2. Relative to a "nominal" rotor of solidity ?=0.0646 (with N=2 blades and a root chord, c=15.82 cm), two 3? rotors (the 3?3 rotor with N=3 and root chord of 2c, and the 3?5 rotor with N=5 and root chord of 1.2c) operating at reduced tip speed are considered, as is a single 5? rotor (with N=5 and root chord of 2c) operating at a further reduced tip speed. The high solidity, low tip-speed rotors showed significant reductions in in-plane noise, both in hover as well as vertical climb, and over the range of disk loadings considered. The noise reductions observed with the 3?5 rotor were significantly greater than those obtained by the 3?3 rotor (operating at the same tip speed), and very similar to those of the 5? rotor (operating at a lower tip speed). But the rotor torque and power penalty for the 3?5 rotor was considerably lower than that for the 5? rotor. Overall, a high solidity in the range of 0.2 for eVTOL rotors is quite advantageous, but further increase to around 0.3 appears acoustically unnecessary while being aerodynamically detrimental. At a solidity of 3?, going from 3 wider chord blades to 5 narrower chord blades was hugely influential for in-plane noise reduction. Of the configurations studied, the best (the 3?5 rotor) showed 16-24 dB reductions in in-plane noise in hover, reducing to 14.5-20 dB at 5/ms climb rate, and 12.5-16 dB at 10 m/s climb rate, with larger reductions seen at lower disk loadings. Relative to the solidity-? rotor, the 3? rotors had a torque penalty of 41-44%, and power penalties ranging from 1.5-5% in hover, increasing to 7.5-10% at 10 m/s climb rate.


Advanced Vertical Flight

Advanced Vertical Flight I

Technical Session C: Wed. May 11, 2022 - 1:45 PM to 6:00 PM

Blade Shape Optimization of Mars Helicopter Exploring Pit Craters (Paper 93)
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Masahiko Sugiura, Yasutada Tanabe, Hideaki Sugawara, Keita Kimura, Akira Oyama, Makoto Sato, Kota Yoshikawa, Yuta Buto, Masahiro Kanazaki, Yuki Kishi, Daisuke Kikuchi, Takuma Minajima

Pit craters on Mars are assumed to be used as manned exploration bases and it is highly possible that life signature would be discovered there since its temperature is appropriate and it is less affected by radiation. Regarding pit crater exploration, helicopter which can climb and descend quickly is expected to be utilized. This paper introduces blade shape optimization of Mars helicopter exploring pit craters. Definition of a mission, selection of aircraft types, conceptual design, optimization of blade twist angle and airfoil, and rotor test are conducted. As a result, hexa-rotor which has robustness, resistance to gust, and fault tolerance is proposed. Moreover, it is confirmed that the mission can be accomplished, carrying instruments on the helicopter within payload weight. Hovering performance of the helicopter is improved by optimizing blade twisting angle and airfoil. And it is found that there is a good correlation between experiment and numerical simulation with respect to the helicopter's hovering performance.

Hoverfoil&[trade] Design and Testing (Paper 1282)
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Kelly Echols, Randall Petersen

The Hoverfoil(TM) is an engineered wing-in-ground effect airfoil that can maintain lift while at zero forward velocity. This is done by use of internally routed duct work that channels airflow through the airfoil to the underside, to create an area of high pressure on the bottom of the Hoverfoil(TM). The airfoil shape (excluding ductwork) utilized a NACA 8308 airfoil. A combination of computational fluid dynamic (CFD) models and 3D scale prototyping have been used to study the feasibility of the project. The CFD model has proven the initial idea as feasible, however the implementation of the scale prototype needs to overcome some barriers to complete testing.

Optimization of Coaxial Rotor System for a Gun-launched Micro Air Vehicle (Paper 24)
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Hunter Denton, Moble Benedict, Hao Kang

The paper discusses an experimental parametric study to maximize the hover efficiency of a coaxial rotor system for a micro air vehicle (MAV) that could potentially be launched from a 40 mm grenade launcher to achieve improved mission range and endurance. Prior to testing the rotors in the coaxial configuration, isolated rotor tests were performed on a custom-built hover test stand. The rotor diameter was kept constant at 6 inches (0.015 m), and used two untwisted carbon fiber blades with rectangular planform shape. The blades utilized thin circular cambered plate airfoil sections for improved performance at low Reynolds numbers (Re ? 30,000). The parameters that were varied include blade thickness-to-chord ratio (t/c), chord length, pitch angle, and Reynolds number. Thickness-to-chord ratio did not have a significant impact on figure of merit below 4%. Increasing the blade chord at a constant Reynolds number and t/c significantly improved hover efficiency until a chord length of 16.6 mm (solidity, ?=0.14). Across all the cases tested, the optimal pitch angle was around 16 - 17 degrees. The optimal rotor from the isolated rotor experiments was used as the baseline rotor for coaxial rotor testing where the effect of vertical rotor separation was investigated with the same blade pitch angle for upper and lower rotors. Overall, the rotor separation had negligible effect on the performance of the upper and lower rotors for a separation distance range from 0.5R to 2R. The highest figure of merit obtained for the coaxial rotor was around 0.55 at Re = 30,000. Across all the vertical separations and disk loading the torque-balanced coaxial rotor system produced almost 1.5 times the thrust of an isolated rotor, which was set at the same pitch angle.

Robotic Hummingbird versus Quadrotor: a Flight Dynamics and Gust Response Comparison (Paper 1272)
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David Coleman, Moble Benedict

The paper discusses an experimental effort aimed at quantifying and comparing the relative flight dynamics and gust response of a two-winged, biomimetic, hover-capable robotic hummingbird (RH) with wing kinematic modulation methods for flight control, against those of a quadrotor (QR) with the same mass, inertias, and motor and battery mass fractions as the robotic hummingbird. Comparing the linear flight dynamics models, the RH model had 2-3x greater translational aerodynamic coefficients along the longitudinal and lateral axes, and rotational aerodynamic coefficients of similar magnitude but opposite sign about these axes. The pitching moment response to longitudinal velocity was similar in the two, while the rolling moment response to lateral velocity was 3.5 x greater in the RH model. Control authority in the longitudinal dynamics was found to be greater in the QR, though this was attributed to mechanism limitations in the RH. The lateral and direction control authority was 1.3 and 4.3 x larger respectively in the RH model. Both vehicles were subjected to a gust in the laboratory. Although the average total movement of the RH was greater than the QR, when the gust speed was non-dimentionalized by the tip speed, the RH had a lesser response. Further, the pitching response was similar between the two, but the rolling response was ? 2 x greater in the RH. Finally, the error between the measured and modeled vehicle responses as a function of wind gust velocity was quantified, which generally increased, especially so for speeds > 5 ft/s. The rotational acceleration equations contained the greatest errors. These results suggest that the linear flight dynamics models are not valid for relatively moderate excursions from hovering flight.

Steady and Transient Hover Performance Investigation of Electric Medium-sized Variable-RPM Rotor (Paper 1259)
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Peter Ryseck, Emily Glover, Rajneesh Singh, Mark Lopez, Inderjit Chopra

Electric variable RPM rotors are increasingly being used for propulsion and control of unmanned air vehicles. As these vehicles scale to carry heavier payloads of 50 to 400 lbs (20 to 180 kgs) in the group 2 and 3 UAS category, there are concerns about their aerodynamic performance and handling quality degradation. First, a universal electric powered experimental testing rig designed for use on a hover tower and in a wind tunnel is described. Second, steady hover data is estimated using blade element momentum theory. These predictions incorporate an empirical correction approach and an interpolation approach to capture Reynolds number variation along the span of the blade and variation with RPM. Results show good agreement with the interpolation method for the low Reynolds number rotor tested. Finally, transient step and chirp inputs are presented. The chirp inputs were used in CIFER to validate the results of the experiment and fit transfer functions of thrust and torque due to RPM and RPM2.

Transition Performance Of Tilt Propeller Aircraft (Paper 1189)
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Alex Stoll, Gregor Veble Mikić

A simple model of the transition aerodynamics of Joby Aviation's electric tilt-propeller VTOL aircraft was developed by combining a surrogate model of CFD propeller solutions with semi-empirical models of airframe aerodynamics and interactional aerodynamics. The model was calibrated to match CFD results of the entire aircraft, as well as flight test results of the actual aircraft. Using this model, loads and power requirements across the range of shaft tilt angle and airspeed combinations for trimmed flight - -the "conversion corridor" often described in tiltrotor performance reports - -are analyzed, and resulting performance limitations are discussed. Additionally, a novel approach to transition flight is introduced in which the angle of attack and shaft angle are automatically determined by the flight controller, rather than manually controlled. Using the simple transition aerodynamics model, transition loads and power requirements with this control approach are discussed, and transition flight using this control approach is shown to allow for high performance given reasonable limitations.

Advanced Vertical Flight II

Technical Session E: Thurs. May 12, 2022 - 1:30 PM to 5:30 PM

6-DOF Flight Dynamics Model Identification of a Hybrid-Lift Multicopter in Hover (Paper 1209)
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Mitchell Graham, Subodh Bhandari

This paper presents the identification and verification of a six degrees-of-freedom (6-DOF) flight dynamics model of a hybrid-lift (buoyancy and propulsive lift) multicopter unmanned aerial vehicle using the frequency-domain system identification technique. The hybrid-lift flight vehicle of interest was a dynamically representative (by z-axis CB vs CG location and Buoyancy Ratio) 29% hub-to-hub scale prototype of a full-scale vehicle designed for multi-use operations with a maximum payload of 250 lbs. From the system identification process, it was concluded that stable roll and pitch dynamics can be expected from a hybrid-lift multicopter configuration designed with high Buoyancy Ratio (BR) and a stabilizing buoyant restoring moment. This dynamic behavior is uniquely different from standard multicopters, which exhibit extremely unstable dynamics in those axes. Additionally, from the heavily attenuated yawaxis dynamic control response, it was concluded that a vertical tail or other yaw effector is needed for the control of similarly configured hybrid-lift vehicles.

Benefits of Dual Propellers for a Coaxial Helicopter (Paper 1251)
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Alexander Stillman, Michael McKay, Farhan Gandhi

A model of a coaxial helicopter with a rigid rotor system is modified to have dual side-mounted propellers. The aircraft is simulated in trim at different flight speeds to investigate the potential benefits of the dual propellers with regard to fault tolerance and yaw control authority. At low speed, the dual propellers impact on the main rotor system actuator failure ranges is analyzed, demonstrating an increase in the allowable trim range from 25-30% in the nominal platform to 50-60% with the dual propeller configuration. Relaxation of the stall constraint for the dual propellers expands the upper rotor maximum actuator limits to the maximum geometric limit, minimum actuator limits for the lower rotor are constrained by the upper rotor stall limits. Upper rotor minimum and lower rotor maximum actuator limits are determined by tip clearance restrictions. At mid speed, the dual propellers improve the yaw control power in the 50-100 kt range and allow for a Level 1 aggressive yaw response as defined in ADS-33E. At high speed, the dual propellers provide yaw control redundancy and allow for trim when the rudder is fully deflected for nearly the entire flight envelope.

Design, Development, and Flight Testing of a 70-gram Micro Quad-Cyclocopter (Paper 1292)
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Carl Runco, Moble Benedict

This paper discusses the development of a micro-scale quad-cyclocopter for the purpose of investigating cyclorotor application to low Reynolds number flight. The 70-gram vehicle was the lightest quad-cyclocopter developed to date by an order of magnitude and only the second to achieve forward flight. It utilized two counter-rotating pairs of cyclorotors operating at ?18,600 Re to generate thrust and balance the reaction torque. Each cyclorotor had two control parameters, thrust direction and magnitude, giving the quad-cyclocopter eight independent control parameters. Flight tests were conducted to demonstrate several unique maneuvers the over-actuated system was capable of: changing pitch attitude in a point hover and forward translation via thrust vectoring. Data was collected on longitudinal maneuvers to compare forward flight performance when using strictly thrust vectoring for propulsion versus forward flight achieved by pitching without thrust vectoring. Similar performance was observed between the two modes indicating that they could be used independently or combined based on the desired vehicle state.

Development of a Multi-Rotor eVTOL Using RPM, Collective, and Cyclic Control (Paper 1184)
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Riccardo Roiati, Richard Anderson, Kyle Collins, Patric Hruswicki, Xinyu Yang, Vivek Saini, Nishant Sharma

The Eagle Flight Research Center (EFRC) at Embry Riddle Aeronautical University (ERAU) is investigating the risks and failure modes of distributed electric propulsion (DEP) employed in novel Advanced Air Mobility (AAM) aircraft designs. To certify these aircraft for private and commercial operations, a greater understanding of how the vehicle is controlled in both nominal and off nominal or degraded modes is required. The purpose of the research performed at the EFRC is to assess how the various methods of DEP thrust control scale up to the sizes required for the electric vertical takeoff and landing (eVTOL) mission, in addition to how well the methods perform in both normal and degraded modes of operation. The EFRC team has designed and built a test stand to characterize the capabilities of a single DEP unit as well as a full-scale quadcopter AAM vehicle with RPM, collecting and cyclic control to be used to explore operation in degraded modes. A MATLAB/Simulink model has also been created and validated to simulate the single DEP unit performance.

Flow-Field and Force Measurements on a Cycloidal Rotor Blade in Forward Flight (Paper 1281)
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Joseph Heimerl, Moble Benedict

The paper investigates the unsteady forces and flowfield of a cycloidal rotor blade undergoing forward flight motion through water tunnel experiments. A particle image velocimetry (PIV) system is used in conjunction with an instrumented blade to measure both the two-dimensional flow velocity around the blade and the fluid dynamic forces. The flow-field studies reveal the formation and shedding of strong leading-edge vortices in both the frontal and rear halves of the circular blade trajectory, which plays a key role in generating lift as observed from the blade force measurements. Increasing forward speed diminishes the size and strength of these leading-edge vortices due to the reduction in angle of attack, which reflects in the blade forces. With pitch kinematics symmetric between frontal and rear halves of the cycle the blade produced significantly higher forces in the rear half compared to the frontal half, which was attributed to the dynamic virtual camber and the differences in relative velocity. The thrust vector was observed to be highly sensitive to both pitch phase offset and spin direction at high advance ratios and required a phase angle around 40-degree for positive propulsive force and lift. At very high advance ratios the blade extracts power from the flow over a large region in the frontal half.

M-Star Medical Transport with a Multi-Ducted Angled Rotor Design (Paper 1289)
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Michael Johnson, Richard Catania, Liza Pierce, Stuart Dana, Thomas Johnson

This paper discusses the benefit of the multi-ducted angled rotor (M-DAR) distributed electric propulsion system in a V/STOL aircraft called the M-Star. The M-DAR propulsion system consists of an array of fixed pitched ducted fans. Traditional fan-in-wing designs can suffer from pitch up effects, stall, instability, and momentum drag when transitioning to forward flight as the oncoming air vector increases. In such planar orientations the air must accelerate rapidly into the ducted fan to prevent blade instability. The M-DAR solves these challenges by lowering the angle at which air enters the fans. A 45-degree M-DAR system enables a higher transition speed than 90 degree ducted fans since the air does not need to turn the additional 45-degrees into the ducts. The M-DAR system is comprised of an array of fans which increase the total disk area while the front plate area remains fixed. The fixed fans also eliminate the weight and complexity of actuators-essentially reducing the mechanical challenges of tiltrotor aircraft into programming tasks. In addition, the M-DAR can be operated as a replacement or in conjunction with control surfaces for increased maneuverability and safety. The M-Star incorporates these design benefits which makes the 2-passenger aircraft affordable, safe, and high endurance. SpyDar has successfully developed subscale Group 1 & 2 cargo UAVs using the M-DAR system and intends to scale the concept into the 2-passenger M-Star eVTOL. The mission of this aircraft will be as a medical commuter vehicle for health professionals to rural and underserved areas.



Aerodynamics I

Technical Session A: Tues. May 10, 2022 - 8:00 AM to 12:00 PM

Effects of Active Flow Control on Fan-In-Wing Aerodynamic Performance In Hover and Forward Flight (Paper 1143)
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Chunhua Sheng, Qiuying Zhao

Aerodynamic performance of a fan-in-wing configuration in hover and forward flight is further investigated using an active flow control technique. The previous investigation has demonstrated the efficacy of active flow control to improve the aerodynamic performance of fan-in-wing device operating at a fixed rotational frequency. In the present study, the fan-in-wing device is numerically investigated at a fixed fan pitch angle operating at a series of rotational frequencies under two free stream conditions, one representing the device in hover (zero free stream velocity) and another representing the device in forward flight condition (non-zero free stream velocity). Effects of different active flow control strategies are assessed and compared with the baseline model in both hover and forward flight conditions.

Experimental Evaluation of the Aerodynamic Rotor/Propeller Interactions on High Speed Helicopters, Efforts and Velocity Fields Measurements (Paper 45)
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Lauriane Lefevre, Vianney Nowinski, Jérôme Delva, Antoine Dazin

This paper focuses on the experimental evaluation of the rotor/propeller interactions for hybrid compound configurations. Experiments are conducted in the ONERA L2 large size-low speed wind tunnel with a 1/7.7 Dauphin 365N model and a four-bladed small-scale propeller. Measurements are realized using two six-component balances, accelerometers, and blade pitch, flap and lead-lag angle sensors. PIV measurements are performed to visualize the velocity fields around the helicopter. Different flight conditions (wind speeds and propeller rotational speeds) have been tested. The comparison of the results obtained with an isolated main rotor and an isolated propeller with the complete assembly highlighted the influence of their interactions on their performances. In hover, the propeller is completely immersed in the rotor wake, and the thrust is therefore maximal. At low speed, the propeller is partially immersed and the flow on the propeller disk is highly asymmetrical. No direct interactions are measured at high speed, where the interactions increase the performances of the propeller due to the increased angle of attack of the flow on the propeller disk.

High Speed and Highly Efficient Rotor Blade Design (Paper 1144)
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Byung-Young Min, Vera Klimchenko, Annie Gao, Alex Dunn, Claude Matalanis, Brian Wake

Aerodynamic design of the high speed, highly efficient rotor (HSHER) blade is presented. The main objective was to design a medium-lift utility scale blade with improved high-speed performance and no penalty in hover, suitable for a single-main rotor operating at speeds between 180 and 200 knots. To accomplish this, a design concept was chosen consisting of an advanced passive blade shape along with an adjustable trailing-edge device. Initial studies using a blade-element solver combined with a genetic algorithm were used to quickly survey large portions of the design space. Next, a CFD-based process was established whereby CREATETM-AV HELIOS was used for high-speed forward-flight simulations while hover performance was addressed using a combination of Star-CCM+ for rapid iterations and HELIOS for more detailed evaluation. While fully-automated optimization was not performed, automation was introduced wherever possible to minimize manual effort and keep the engineering effort focused on high-level strategic decisions that are still somewhat difficult to automate. The final result was a new blade design which showed significant improvements in both high-speed performance (?L/D ~1) and hover efficiency (?FM ~0.03) with increased thrust capability.

High-fidelity Numerical Investigation of Complex Propeller Flows (Paper 1139)
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Tao Zhang, Ross Higgins, George Barakos

This paper presents high-fidelity numerical investigations of complex propeller flows using the HMB3 solver of Glasgow. The present study focused on theGATEUR AG25 rotor/propeller integration examined by ONERA. This study compares and evaluates various rotor modelling and turbulence modelling approaches for the simulation of isolated and isolated propellers. Simulations of the baseline isolated propulsors were first carried out and validated against experimental measurements. Simulations of the rotor/propeller integration were then carried out at low speed forward and also at hover. The results were compared with the isolated cases to highlight the performance changes due to the aerodynamic interactions. Effects of the forward speed were compared and discussed in detail. In hover, the aerodynamic interference was found to be considerably exacerbated. Elaborate comparisons were also performed between methods to bring out the gains and losses of different modelling options.

Medium-Fidelity CFD Modeling of Multicopter Wakes for Airborne Sensor Measurements (Paper 50)
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Jonathan Chiew, Michael Aftosmis, Kristen Manies

A steady-state multicopter simulation capability leveraging automated Cartesian grid generation and a blade element source term rotor model was used to investigate multi-rotor aerodynamics in and out of ground effect. Simulations demonstrated that this model is able to predict reasonably accurate thrust in ground effect for both single and multiple propeller cases. The method yielded accurate lift and drag predictions for a commercial quadcopter when compared to recent wind tunnel test data. This aircraft was then simulated in a variety of flight conditions, including both hover and edgewise forward flight, to determine if generalized guidelines for airborne sensor placement could be developed. Velocity perturbation contours showed regions of affected air upstream of the vehicle at low speeds which contracted as the aircraft's speed was increased. Placing the sensors more than one characteristic length ahead of or above the aircraft reduces errors from rotor-induced flow at higher speeds. For low-speed forward flight, the entrainment of flow into the propeller wakes introduces significant flow turning above the aircraft, suggesting that a forward location, where the perturbations are more moderate, could be advantageous for these conditions.

Propeller Ground and Ceiling Effect in Forward Flight
(Paper 72)
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Jielong Cai, Sidaard Gunasekaran

The study of the fixed-pitch small-scale propeller in ground/ceiling effect is extended to forward flight conditions. Force-based experiment and smoke flow visualization is conducted at the University of Dayton Low Speed Wind Tunnel. The propeller is mounted at different incidence angles to simulate the transition to forward flight from hover and forward flight conditions. The effect of propeller incidence angle seems to be negligible at J=0. The power required at constant thrust shows an increasing trend after a threshold advance ratio for each h/D and then decreases steeply upon further increase in advance ratio. Smoke flow visualization revealed the presence of three distinct zones separated by the onset of the fountain vortex and the onset of the ground vortex at each ground distance. At advance ratios below the onset of the fountain vortex lies the optimal ground effect zone. An increase in advance ratio beyond this threshold will result in power increment due to the fountain vortex. Further increase in advance ratio initiates the onset of the ground vortex where the ground effect is significantly reduced. The onset of the fountain vortex and the ground vortex occurs at lower advance ratios for a propeller at an incident angle to the ground.

Aerodynamics II

Technical Session B: Wed. May 11, 2022 - 8:00 AM to 12:15 PM

Computational Analysis of a Quiet Single-Main Rotor Helicopter for Air Taxi Operations (Paper 59)
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Patricia Ventura Diaz, David Garcia, Seokkwan Yoon

A computational study has been done on NASA's quiet single-main rotor helicopter concept for urban air mobility using high-fidelity computational fluid dynamics, rotorcraft comprehensive analysis tools, and computational aeroacoustics. High-order accurate schemes, dual-time stepping, and the delayed detached-eddy simulation model have been employed. A loose-coupling approach between the flow solver and the rotorcraft comprehensive code is implemented to include vehicle trim and blade motions. Acoustic simulations based on the Ffowcs-Williams and Hawkings equations have been performed to compute the rotor noise. Different blade geometries and tip speeds are analyzed, and performance and acoustics results are compared. The vehicle has been simulated in hover and cruise, with flight conditions representative of an air taxi mission. The quiet single-main rotor helicopter is one of the conceptual designs intended to focus and guide NASA's research activities in support of aircraft development for vertical take-off and landing air taxi operations.

Experimental Evaluation of Multi-Rotor Aerodynamic Interactions (Paper 1182)
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Abraham Atte, Daley Wylie, Juergen Rauleder

An investigation was performed into the aerodynamic interactions between the rotors of multi-rotor vehicles in different configurations. The effects of these interactions on the thrust and torque of all individual rotors were quantified in wind tunnel tests. Flow visualization and a limited number of particle image velocimetry measurements were conducted to get further insight into the aerodynamic interactions. The effects of the changes in hub spacings, rotor rotational speeds, and wind speeds were investigated for isolated, tandem, quad-rotor plus and X configurations. The maximum and minimum tip chord Reynolds numbers were 118,000 and 73,000, respectively. Results showed that the aft rotors experienced detrimental aerodynamic interactions in all configurations. In all examined multi-rotor configurations, an increase in the hub spacing caused a decrease in the thrust deficit between the aft rotor and the isolated rotor. However, the differences in the configurations also affected the measured loads. In the tandem configuration, the aft rotor experienced up to 24% reduction in the thrust coefficient at a hub spacing of 2.1R when compared to the isolated rotor at the same rotational frequency and wind speed. The aft-most rotor in the plus configuration experienced as large as a 28% decrease in the thrust coefficient when compared to one of the aft rotors in the X configuration for the same hub spacing and flight conditions. Good correlation was found between these wind tunnel tests and flight tests for the front and side rotors in X and plus configurations (7.9-14.2% difference), but a larger difference of 30-41.9% was found for the aft rotors, which is likely due to the different rotor trim conditions.

High-fidelity Simulations of Rotors in Compact Configuration (Paper 1235)
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Sebastian Miesner, Manuel Keßler, Ewald Krämer

High fidelity CFD simulations of seven rotors in compact configuration at three different flight scenarios are performed. The cases are hover, as well as 50 km/h and 100 km/h forward flight. For comparison each rotor is simulated isolated with the same RPM and pitch angle as in the configuration. Additionally, a bigger isolated rotor with the same area as the complete configuration is simulated. For the configuration as well as the bigger rotor a flight mechanic trim is performed using the flight mechanic tool VFAST. The CFD simulation is performed with FLOWer. In hover, only the center rotor showed a significant Figure of Merit (FM) drop of 16% compared to the isolated rotor. The thrust of the outer rotors is increased at the tip areas facing outwards, while the tip areas towards the center and the tip areas of the center rotor showed reduced thrust compared to the isolated rotor. The wake contraction at the outer rotors is increased compared to the bigger rotor. For the 50 km/h forward flight the efficiency of the front rotors is increased (10%-17%) and the rear ones decreased (11%-16%). In this case the wake is directly convected from the front rotors into the rear rotor planes and strong vortex interactions occur. For the 100 km/h case the efficiency gain of the front rotors is reduced to 3%-11% and the decrease to 5%-9%. Due to the higher pitch the wake of the rotors flows away from the rotor plane and the rotor-rotor interactions are reduced.

Quasi-Steady Aeromechanic Helicopter Simulations Using Mid-Fidelity Aerodynamics (Paper 65)
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Philipp Kunze, Tobias Ries

A numerical simulation process based on loosely coupling a helicopter flight mechanics code and a 3D panel and freewake aerodynamics method is extended for arbitrary steady flight states and quasi-steady maneuver analysis. First, the basic coupling procedure and their building blocks are introduced. Then, the extensions for arbitrary steady flight states are illustrated and a newly developed procedure for approximate maneuver simulations using a sequence of steady aeromechanically coupled simulations is presented. The aim of this procedure is to provide a rapid method for evaluation of loads, performance and stability in the helicopter design and development process. It includes an improved physics-based modeling of aerodynamic interactions as compared to low-fidelity aerodynamics models used in comprehensive codes. At the same time the computational effort is dramatically reduced as compared to highfidelity aerodynamics methods. Finally, validation results for the coupling procedure and first applications of the maneuver simulation process are shown. The computed results are compared with flight tests and blade element momentum theory based results. In many cases the solution accuracy can be significantly improved by using the coupled simulation procedure. But for maneuvers involving fast pilot inputs or considerable translational or rotationalaccelerations of the helicopter, the quasi-steady coupling approach is not suitable and an extension to fully unsteadyloose coupling or the implementation of a tight coupling approach is proposed.

Time Efficient Methodology for the Evaluation of Aerodynamics and Flight Mechanics of the RACER Compound Helicopter in Hover under Cross Wind Conditions (Paper 1121)
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Ronan Boisard

This paper describes a new mid-fidelity, steady RANS based, method to predict the aerodynamic interactions between rotating and fixed parts of the compound rotorcraft RACER of Airbus Helicopters. The method models main rotor and side propellers with source terms, hence avoiding near-body grids and unsteady overset grid techniques. The method is then applied to the analysis of the flight mechanics of the RACER in hover flight under cross-wind conditions, as part of the de-risking of the coming flight testing. The method is validated by comparison with higher-fidelity, URANS based, results taken from the literature. Trends on aircraft controls and pitch and roll attitudes are found to be consistent. The new mid-fidelity methodology provides physical insight on how the aerodynamic interactions between main rotor, fuselage, wings, rear parts, side propellers and cross wind impact aircraft controls and attitude to maintain a stabilized hover under cross-wind.

Transient and Quasi-steady Numerical Simulations of Tiltrotor Conversion Maneuvers (Paper 1173)
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Steven Tran, Hyeonsoo Yeo

The goal of this work is to characterize the ability of current numerical tools such as Comprehensive Analysis (CA) and Computational Fluid Dynamics (CFD) to model the aeromechanics of a tiltrotor undergoing a transient conversion maneuver. This is accomplished in two parts. First, a simplified model is used to quantify the differences in predicted loads between a simulation of a transient maneuver and those of several quasi-steady simulations. Ultimately this analysis showed that quasi-steady simulations were an efficient and accurate method for modeling transient maneuvers. Following the results from this analysis, the full XV-15 tiltrotor was studied using coupled CFD/CA quasi-steady and standalone CA transient maneuver simulations. It was found that nonlinear effects such as blade/wake interactions and aerodynamic interferences between the rotor and the wings were significant at the beginning and end of the conversion maneuver, respectively. As such, CA showed good agreement with CFD/CA at higher airspeeds during conversion, but struggled at low speeds and during cruise. Overall this work highlights the need for coupled CFD/CA analysis for capturing the complexities of tiltrotor conversion maneuvers. Coupled together, the simulations leverage the strengths offered by each tool and have the capability to accurately model the aerodynamic and structural dynamics of proprotors and tiltrotors at relevant operating conditions.

Uncertainty Quantification Approach for Rotorcraft Simulations (Paper 1239)
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Buvana Jayaraman, Andrew Wissink, Rohit Jain, Manas Khurana

The goal of this work is to quantify the uncertainty and sensitivity of freestream velocity and wind direction on wing download and rotor thrust predictions for the Joint Vertical Experiment (JVX) tiltrotor configuration in hover. Even light winds can have a significant impact on hover performance. Accordingly, an accurate representation of hover performance with uncertainties due to variability in atmospheric wind conditions needs to be understood. To support this effort, mid-fidelity simulations with a Reduced Order Aerodynamic Model in CREATETM-AV Helios is used to generate training and testing data for constructing the surrogate models. Uncertainty propagation is facilitated using a surrogate-based approach which integrates stochastic expansions based non-intrusive polynomial chaos method in the Dakotaenvironment. The first test case considers wind velocity and directions treated as epistemic uncertain variables. Post uncertainty analysis, parameter sensitivities are established using Sobol indices to rank the relative contribution of input parameters to the total uncertainty in download and thrust. Sensitivity analysis showed that the interaction of wind velocity and direction has the largest influence on download predictions. The second case includes collective as an uncertain input and additionally carries out a sensitivity analysis. Computed Sobol indices identified collective as the major contributor of uncertainty. Ultimately, uncertainty quantification procedure laid out in this work can facilitate informed design decisions based on quantifiable data that is formed using validated computational approaches integrated with established data science principles with statistical metrics.

Aerodynamics III

Technical Session C: Wed. May 11, 2022 - 1:45 PM to 6:00 PM

Analytical Model Development for Rotors Hovering Above Heaving Surfaces (Paper 1299)
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Joseph Milluzzo, John Tritschler, Scott Davids

A potential flow model was developed for rotors hovering in-ground-effect above a heaving surface using a similar methodology to classical, analytical, static ground effect models. Experimental performance measurements for rotors hovering above a surface undergoing single degree-of-freedom heaving motion were used for model validation, and speculative mission trends were generated for a representative naval helicopter. Unlike prior empirical models, the current model was able to capture the effect of rotor hub height, as well as ground motion parameters. A new thrust ratio was proposed that compared the thrust produced in-ground-effect above a heaving surface to that produced above a static surface. Better agreement was found to occur at the higher collective pitch settings and hub heights, with the model predicting the classical thrust ratio within 5% of the measured value for approximately 75% of the test points. The thrust ratio relative to a static surface was found to produce better agreement and approximately 80% of the experimental tests points were predicted within 5% of the measured value.

Automated Inference of Vortex Core Physics of Hovering Rotor Wakes Using Machine Learning Techniques (Paper 1280)
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Jennifer Abras, Nathan Hariharan

Machine learning can significantly enhance the insights gained from large aerodynamic computational data sets. Specifically, 3D rotor wake computations produce detailed vortex structures, but typically, only a small fraction of the data is leveraged. A critical limiting factor is the time needed to process these large 3D data sets and infer the correct physics. The development of machine learning methods to process these data sets reduces the data processing burden on the analyst and also increases the volume of data that can be processed within a reasonable amount of time. Recent efforts in this area have resulted in the development of a novel machine learning-based vortex-core data extraction methodology. The present study demonstrates the application of this method to hovering rotor wakes to compare different blade tip geometries, ground effect, wake grid spacing, and collective changes and discusses the extracted vortex physics for each case.

Helicopter Shipboard Operation: Effect of Atmospheric Boundary Layer on Turbulent Ship Airwake and Rotor Aerodynamic Loads (Paper 1220)
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Neda Taymourtash, Myles Morelli, Alberto Guardone, Giuseppe Quaranta

This paper studies the effect of two different boundary layer models on the airwake of the Simple Frigate Shape 1 and further on the unsteady aerodynamic loads of a scaled helicopter model operating inside the ship airwake. The unsteady airwake of the isolated SFS1 is computed in a time-accurate approach using the open-source SU2 solver, implementing two types of boundary conditions: a Uniform Flow (UF) and a steady Atmospheric Boundary Layer (ABL), where the reduction of the velocity due to the surface roughness is also considered. The simulations are performed in two wind conditions, including headwind and 30? from the port-side (R30). The airwake data are implemented into a multibody simulation of a scaled helicopter model, developed using the open-source multibody software MBDyn, based on the one-way coupling approach. Hover tests are performed at different positions with respect to the deck and the unsteady aerodynamic loads are compared in frequency domain. In addition to the increase of unsteadiness in the red wind compared to the headwind simulation, the results indicate that in both wind conditions, the unsteadiness is reduced with the presence of the steady ABL. Since the unsteady loads are expected to directly affect the pilot workload, the results highlight the importance of modelling a realistic boundary layer considering both steady and turbulent profiles.

Investigation of Dynamic Stall Leading-Edge Flow Features at a Low Transitional Reynolds Number (Paper 103)
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Jagdeep Batther, Seongkyu Lee

The unsteady laminar separation and subsequent dynamic stall vortex (DSV) formation is investigated on a NACA 0012 airfoil section subject to a constant pitch rate motion using Delayed Detached Eddy Simulations (DDES) in NASA's OVERFLOW 2.3 solver. This study focuses on the complex flow features during the initial DSV formation and analyzes the distinct mechanisms from which the vortex is formed. It is shown that DDES accurately predicts the bursting of a laminar separation bubble (LSB), which triggers the onset of a DSV. In parallel to studying the feasibilty of DDES in terms of capturing distinct flow features compared to Large Eddy Simulation (LES) results, a turbulence model study is also carried out, analyzing the influence of stateof-the-art turbulent and transition models on the DSV formation and subsequent stall onset. These include the fully turbulent Spalart Allmaras (SA) turbulence model and three different transition models: SA Coder Amplification Factor Transport (AFT), SA Medida-Baeder ? ?R~e?t , and the Shear Stress Transport (SST) Langtry-Menter ? ?R~e?t . It is found that the Coder SA AFT model provides the closest results with LES and the SST Langtry-Menter model predicts the earlier onset of the DSV. The fully turbulent model shows an abrupt development of a turbulent separation bubble and the under-prediction of the lift coefficient at lower angles of attack. At higher angles of attack, after the collapse of the separation bubble, all the models provide similar trends with each other and LES results.

Large Eddy Simulation of the Wakes of Three Urban Air Mobility Vehicles (Paper 63)
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Denis-Gabriel Caprace, Andrew Ning

Recent advances in urban air mobility have driven the development of many new VTOL concepts. These vehicles often feature original designs and futuristic shapes. Due to their novelty, the wake characteristics of such aircraft are unknown. However, large wake-induced velocities, should they exist, may be dangerous for any other vehicle evolving in their close proximity. Therefore, improved knowledge about the wakes of VTOL vehicles is needed to guarantee the safety of urban air mobility operations. In this work, we study the wake of three VTOL aircraft in cruise by means of large eddy simulation. We present a two-stage numerical procedure that enables the simulation of long wake ages at a limited computational cost. The analysis of our simulation results reveals that the wakes of rotary vehicles feature larger wake vortex cores than a typical airplane wing. The vortex circulation decay is also faster due to the self-induced turbulence generated during the wake roll-up. Finally, we introduce a model of the vortex circulation distribution that fits the numerical measurements with satisfactory agreement across space and time, and can be used to evaluate induced velocities.

Aerodynamics IV

Technical Session E: Thurs. May 12, 2022 - 1:30 PM to 5:30 PM

A Combined Experimental and Computational Analysis of the Flow Field of a Coaxial Counter-Rotating Rotor in Hover (Paper 1157)
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Patrick Mortimer, Jayant Sirohi, Jimmy Ho, Mark Lopez, Ashwani Padthe

The flow fields of a 2m-diameter two-bladed single rotor and a 22-bladed coaxial counter-rotating rotor were measured using Particle Image Velocimetry, and computed using a coupled RCAS-VVPM analysis. Time-resolved measurements were performed on the single rotor at 44/rev with at least 552 flow realizations per azimuth. Similar measurements were performed for the coaxial, counter-rotating rotor at 64/rev with at least 260 flow realizations per azimuth. The goal of this study was to compare the flow features of these rotor configurations and validate current numerical analyses. Overall, there was good correlation between the measurements and calculations for the single rotor configuration. Numerical computations captured the axial velocity distribution across the blade radius and rotor azimuth. The radial distributions of axial velocity revealed less agreement in the tip region due to differences in the tip vortex trajectories. The tip vortex positions extracted from the measurements revealed a radial displacement of Dr=R = 0:112 and an axial displacement of Dz=R = 0:230 over one rotor revolution. The numerical computations captured the radial position for the first blade passage, but over predicted the radial position after blade passage. The axial positions were under-predicted over the entire azimuthal range. Comparison for the coaxial, counter-rotating rotor configuration revealed a consistent over-prediction in both radial and azimuthal velocity profiles for all spatial locations investigated. The axial position of the tip vortex were captured well with slight under-prediction for the upper rotor. The radial tip vortex positions were over-predicted over the entire azimuthal range investigated. Further validation of the numerical model is required before it can be used to extract state-space inflow models for single and coaxial, counter-rotating rotors

CFD Simulation of Flow Around ROBIN-Mod7 Fuselage with PSP Rotor Using an Immersed Boundary Method (Paper 120)
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Hee Sung Park, Daniel Linton, Ben Thornber

Computational fluid dynamics simulations of the flow around the ROBIN-mod7 fuselage with PSP rotor are conducted using an immersed boundary method and an actuator surface model in OpenFOAM. The ROBIN-mod7 fuselage is represented by the immersed boundary method, while the unsteady rotor is modeled using the actuator surface model. The integration of the immersed boundary method and actuator surface model is straightforward; there is no fundamental reason to be conflicted with each other in both theory and practice. A comprehensive analysis of the generic helicopter configuration is carried out for the hovering flight condition; the isolated fuselage is simulated to provide its baseline aerodynamics, and the isolated rotor and rotor-fuselage cases are studied to measure the rotor performance in hover and the fuselage effect on the performance. The validation of each test case is conducted against both experimental measurements and computational data from the literature. The surface pressure data from the isolated fuselage case shows good agreement with the experimental measurements. Also, the rotor performance predicted on the isolated and installed rotors (rotor-fuselage) has excellent agreement with the reference data; in particular, the performance data on the installed rotor agrees with the experimental data better than the previous numerical study does. The fuselage effect has been analyzed by comparing the isolated rotor and rotor-fuselage data sets. The computational effort for different grid levels of each test case is provided. Overall, the results have demonstrated an equivalent level of accuracy compared to the previous high-fidelity simulation results at their fraction of setup and computational expenses.

Comparing Strategies for DNS Based Optimization of Airfoils for Martian Rotorcraft (Paper 33)
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Lidia Caros, Julian Blank, Oliver Buxton, Peter Vincent

Martian atmospheric conditions present various challenges when designing rotorcraft. Specifically, the thin atmosphere and lower speed of sound compared to Earth requires Martian rotor blades to operate in a low-Reynoldsnumber (? 103 to 104) compressible regime, for which conventional airfoils have not been designed. Airfoils with sharp leading edges and flat surfaces have been shown to perform better than conventional airfoils under these conditions. In order to find the optimal airfoils, several studies have explored optimizing non-conventional airfoils with evolutionary techniques. These algorithms usually require many cost function evaluations, and hence, they typically employ Reynolds-Averaged Navier Stokes (RANS) solvers because of their low computational cost. However, RANS solvers have limited predictive capability when the flow becomes unsteady and separated at moderate angles of attack. Enabled by recent advances in solver technology and GPU hardware, we are able to overcome this limitation by undertaking optimization using high-fidelity Direct Numerical Simulations (DNS), able to capture the flow physics, via the compressible flow solver in PyFR ( In order to reduce the cost of the optimization, given that it involves expensive cost function evaluations, the current study compares two multi-objective optimization strategies using pymoo ( as the optimizer. Specifically, the study compares the cost of Genetic Algorithm (GA) optimization with two-dimensional DNS used to evaluate the cost function, with the cost of surrogate-assisted GA optimization where the model is generated and updated with two-dimensional DNS. Results help elucidate efficient strategies for high-fidelity two- and three-dimensional DNS optimization for aerospace applications, specifically rotorcraft airfoils in Martian atmospheric conditions.

Improved Initial and Boundary Conditions for Hovering Rotor CFD Simulations (Paper 1196)
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Feilin Jia, Qiqi Wang, Philippe Spalart

The new flow fields originate in a 2012 paper by Spalart (Ref. 1), which addresses the far-field behavior induced by a momentum source, such as a hovering rotor or a static jet. This model incorporates entrainment into the turbulent far wake, and supersedes the time-honored model based on a sink field. In the present study, a new initial condition as well as boundary condition based on a modified version of the above far-field model are investigated by unsteady simulations of the XV-15 rotor in hovering flight condition at collective pitch angle ? = 10?. The idea is to inject an approximation to the rotor's wake under it. From our results, compared to the freestream initial condition, the far-field model based new initial condition can quickly establish the low-speed induced flow and achieve faster convergence of thrust and torque to statistically steady state. The chaotic starting vortex can be effectively flushed out by the new initial condition to prevent the oscillations of thrust due to blade-vortex interaction shown in the simulation from the freestream initial condition. Results of the modified far-field model as a far-field boundary condition to hovering flight in multiple reduced computational domains are also presented. Although the new boundary condition doesn't show evident speedup in convergence of thrust and torque, its wake can be developed much faster than the freestream boundary condition due to its prescribed velocity field at the boundary. This new initial and boundary condition could be beneficial to simulations of multi-rotor aircraft.

Numerical Simulations of the Adverse Effects of Rain on Airfoil and Rotor Aerodynamic Characteristics (Paper 1203)
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Aishwerya Gahlot, Ritu Eshcol, Lakshmi Sankar, Richard Kreeger

There is a significant interest in improving the performance of rotors under adverse operating conditions. However, there is a very limited understanding of the performance implications on 2D airfoils and rotor blades under adverse effects of rainfall. Furthermore, the fundamental physical phenomena causing the loss in performance are not clearly understood. In this study, low fidelity models are first developed to rapidly estimate the water layer formation on 2D airfoils and assess the resulting impact on lift and drag characteristics. The low fidelity simulations are also useful to obtain quick estimates of water layer thickness as a function of liquid water content and droplet diameter. Subsequently, computational fluid dynamics studies for 2D airfoils and a small-scale rotor in hover are done to obtain more accurate estimates of the effects of rain on airfoil performance and match test data where available. Higherfidelity parametric studies for various airfoils were conducted by varying angles of attack, the liquid water content in the rain droplets, and the droplet diameters to capture trends in performance degradation. The resulting trends match the trends from the test data reasonably well. The higher fidelity airfoil loads are subsequently used within a classical combined blade element-momentum model (BEM) to assess the loss of performance attributable to rain for a smallscale rotor.


Aircraft Design

Aircraft Design I

Technical Session A: Tues. May 10, 2022 - 8:00 AM to 12:00 PM

Acoustic Analysis for Conceptual Design via Comprehensive Analysis (Paper 1234)
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Matthew Floros, Phuriwat Anusonti-Inthra, George Jacobellis, Michael Avera

A process for estimating acoustic performance of rotorcraft from conceptual design is presented. A comprehensive analysis model is automatically generated from the conceptual design, and the higher-fidelity aerodynamic loads and blade motions are taken as inputs to an acoustic propagation tool. Five example configurations were tested to demonstrate the ability to process arbitrary vertical lift configurations including single or multiple rotors and wings. Selected comparisons of thrust, torque, flapping, and control angles are presented to illustrate the consistency between the conceptual and comprehensive models. Acoustic predictions were made for the configurations, and each configuration exhibited unique noise characteristics. The configurations exhibited varying directivity patterns and different relative contributions from thickness, loading, and broadband noise, as well as different contributions from the different rotors in each model.

CFD Based Design Optimization of Tail Boom Strake for Hover and Sideward Flight Performance (Paper 1273)
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Furkan Kurban, Osman Gungor, Murat Senipek, Alper Ezertas

CFD based multi-point multi objective optimization method is performed to search optimum tail boom strake geometry in order to improve helicopter hover and sideward flight(10kts, 20kts, 30kts and 40kts) performance at hot and high atmospheric conditions. Upper and lower strake geometries are parametrized by using NX CAD tool and effect of the designed strake geometries are analyzed at specified flight conditions by using computation fluid dynamic (CFD) methods. Star CCM+ is used as mesher and solver. Downwash of the main and tail rotor is modelled by using built-in virtual disk model in Star CCM+. Optimization is performed by using multi-point multi objective optimization tool HEEDS and multi-objective SHERPA optimization algorithm is preferred for optimization. Accumulated pedal improvement and total power reduction are specified as objectives. At the end of the optimization workflow, improvements of the best strakes on pedal input and power required at hover are investigated. Furthermore, sensitivity of the design parameters, population of the design space and flow field around tail boom are examined in detail.

Effect of Lifting Surface and Tail Configuration on the Aerodynamics and Flight Mechanics of VTOL Aircraft (Paper 1237)
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Abhijnan Dikshit, Tom C.A. Stokkermans, Shawn Lim Kin Yip, James Wang

To increase the cruising range for VTOL aircraft it has become necessary to add a wing or wings so the aircraft can vertically take off like a helicopter but cruise like an airplane. This paper compares the aerodynamic efficiency, stability, and handling qualities of four different cruise configurations: conventional wing-tail airplane, canard airplane, flying wing and tandem wing. An additional aim is to perform a parametric study of the tandem wing aircraft configuration because that has become a popular choice among eVTOL aircraft designers. This paper does not examine the hovering flight portion of the mission. The study is carried out using a vortex lattice model and a RANS CFD model. The paper reveals the pros and cons of each configuration in terms of aerodynamics and flight mechanics. The parametric study of the tandem wing illustrates the effect of parameters such as the relative wing sizes and wingspan on the performance of a tandem wing aircraft. The paper also shows examples of how the four baseline configurations can be applied to different eVTOL aircraft.

Helicopter Rotor Blade Planform Optimization Using Parametric Design and Multi-Objective Genetic Algorithm (Paper 1160)
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Yonghu Wenren, Joon W. Lim, Luke D. Allen, Robert B. Haehnel, Ian D. Dettwiller

In this paper, an automated framework is presented to perform helicopter rotor blade planform optimization. This framework contains three elements, Dakota, ParBlade, and RCAS. These elements are integrated into an environment control tool, Galaxy Simulation Builder, which is used to carry out the optimization. The main objective of this work is to conduct rotor performance design optimizations for forward flight and hover. The blade design variables manipulated by ParBlade are twist, sweep, and anhedral. The multi-objective genetic algorithm method is used in this study to search for the optimum blade design; the optimization objective is to minimize the rotor power required. Following design parameter substitution, ParBlade generates the modified blade shape and updates the rotor blade properties in the RCAS script before running RCAS. After the RCAS simulations are complete, the desired performance metrics (objectives and constraints) are extracted and returned to the Dakota optimizer. Demonstrative optimization case studies were conducted using a UH-60A main rotor as the base case. Rotor power in hover and forward flight, at advance ratio ???? = 0.3, are used as objective functions. The results of this study show improvement in rotor power of 6.13% and 8.52% in hover and an advance ratio of 0.3, respectively. This configuration also yields greater reductions in rotor power for high advance ratios, e.g., 12.42% reduction at ???? = 0.4.

The Effect of Blade Tip Sweep Angle on Forward Flight Performance of a High-speed Helicopter (Paper 1236)
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Keita Kimura, Yasutada Tanabe, Masahiko Sugiura

This paper discusses the change in forward flight performance and its mechanism when a sweep angle is added to the rotor blades of a high-speed helicopter, based on the results of CFD (computational fluid dynamics) analysis. The results of the study of the concept of maintaining the rotor disk horizontal in flight confirmed that a blade with a forward sweep angle can be expected to provide several percent additional performance improvement. This is due to the fact that the dynamic pressure changes with azimuth angle at the point where the sweep angle is added to compensate for the effect of rotor wake, which reduces the thrust at the aft half of the rotor, making it easier to balance the rotor moment. A detailed discussion of this mechanism is included based on the wake speed distribution and the blade pressure distribution at the relevant azimuth angles.

Wing Lift Enhancement from Aft Rotor Induced Suction (Paper 1268)
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Richard Healy, Farhan Gandhi

This study examines the aerodynamic interactions of rotor-wing units in which lifting rotors are mounted below and behind a wing. The rotor-wing units are simulated using CFD, and their performance is compared to isolated rotors and wings in order to understand the interference effects. Simulations are performed using the commercial Navier Stokes solver, AcuSolve, with a delayed detached eddy simulation (DDES) model. Rotor-wing units with three wing incidence angles (7-degree, 10-degree and 13-degree) as well as three rotor disk loadings (6, 9 and 12 lb/ft2) are considered. By simulating the flow and comparing the pressure distribution around an isolated wing to one with the rotor installed, the rotor is seen to introduce a low pressure region that extends over the wing's top surface. The additional rotor-induced suction on the top surface of the wing augments wing lift by up to 134%, and provides some stall mitigation at 13-degree incidence angle. Suction near the leading edge of rotor-installed wings also counters the nominal wing drag, introducing a net propulsive force on the wing at all incidence angles considered. On the rotor, downwash induced by the wing's bound circulation introduces a rotor thrust deficit up to 10% nominal thrust and torque penalty up to 4% nominal torque. Despite the rotor performance penalties, interactions between the rotor and wing lead to equivalent lift to drag ratio improvements ranging from 47% - 52% over a range of wing angles. As disk loading is increased, the rotor-induced suction strengthens, extending the 66% wing lift increment at 6lb/ft2 up to 115% at 12 lb/ft2. These results suggest that the interactional aerodynamics associated with mounting a rotor below and behind a wing can introduce enhanced system performance over a range of wing angles and rotor loadings.

Aircraft Design II

Technical Session B: Wed. May 11, 2022 - 8:00 AM to 12:15 PM

Design and Optimization of Innovative Tiltrotor Wing Control Surfaces through Coupled Multibody–mid-fidelity Aerodynamics Simulations (Paper 1174)
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Alberto Savino, Alessandro Cocco, Andrea Zanoni, Alessandro De Gaspari, Alex Zanottti, João Cardoso, Daniel Carvalhais, Vincenzo Muscarello

The present work proposes a novel approach, based on a series of co-simulations combined with an optimization procedure, used to perform the preliminary sizing of the control surfaces of the NextGen Civil TilRotor (NGCTR) and the relative actuation systems. The activity is collocated in the framework of the FORMOSA Clean Sky 2 project which has the aim of designing an innovative solution for the wing movable surfaces able to incorporate multiple functions (download alleviation, flap, aileron) thus reducing the complexity of the actuation system. The optimization procedure, based on a Design of Experiment approach, is then exploited to define the best aileron configuration to improve the roll performance, trying to reduce the time to bank.

Design and Test of an Active Pneumatic Trailing Edge Flap for High-Speed Rotorcraft (Paper 1130)
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Matt DiPalma, Tim Conti, Claude Matalanis, Preston Bates, Joe Szefi

As part of the High-Speed, Highly Efficient Rotor (HSHER) program, a novel trailing-edge flap concept is evaluated. A finely tuned internal laminate topology, coupled with a lightweight pneumatic actuation system, enable a performant trailing-edge flap technology that does not require electronic or mechanical actuators within the rotor blade. The trailing-edge flap is experimentally shown to provide a 12-degree range of motion between the downward and upward deflected configurations under pressures which can be generated passively by the rotation of the rotor blade. The structure is shown to be sufficiently stiff against aerodynamic pressures and moments, is resilient to strains resulting from large blade deflections, and can hold its shape in the event of pneumatic actuator failure. Additionally, the test data confirmed the strong predictive capability of the finite element analysis for highly-compliant laminate designs such as this. The design is highly customizable and can accommodate a wide variety of airfoils, flap parameters, and loading scenarios. Details of the design, fabrication, and testing of the trailing-edge flap are presented.

Design, Development, and Flight Testing of a 25-Kilogram Quad-Cyclocopter (Paper 1260)
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Ramsay Ramsey, Saul Sanchez, David Coleman, Atanu Halder, Moble Benedict

This paper describes the design, development, and tethered flight testing of a quad-cyclocopter weighing 25 kilograms and 1.8 m x 1.4 m x 0.8 m in dimension. This cyclocopter features four cantilevered cyclorotors with a unique five-bar blade pitching mechanism. The cyclorotor design is chosen through systematic parametric studies using an in-house 2-D computational fluid dynamics (CFD) solver. Based on the parametric studies, the final design selected is a 6-bladed cyclorotor with 0.67 chord-to-radius ratio, symmetric NACA 0015 airfoil, and pitch amplitude of 45-degree because it provided high thrust and power loading (thrust/power) at a low operating rotational speed of 700 RPM. The cyclorotor blades are manufactured with a foam core and carbon fiber skin resulting in lightweight blades with large bending and torsional stiffness. The rotor supporting structure and transmission is designed to be lightweight and resilient to large centrifugal loads and dynamic torques, respectively. A minimum mass airframe is designed and constructed from carbon fiber tubes to handle the vibratory loads from cyclorotors, mount electronics and batteries, and provide attachment points for the carbon fiber cowling. The attitude control strategy utilizes a combination of RPM control and thrust vectoring of the four cyclorotors (eight independent control parameters), resulting in an overactuated system. A custom autopilot uses a closed-loop cascaded proportional-integral-derivative (PID) controller to stabilize the aircraft in hovering flight. The vehicle has been flight tested while tethered and achieved lift off.

Examining a Tandem-Rotor Configuration for the Electric Urban Air Mobility Mission (Paper 90)
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Erik Scott

Electric vertical takeoff and landing (eVTOL) vehicles for the urban air mobility mission have seen increased interest in the past five years spurred by rapid improvements in battery and brush-less motor technology. Because of ease of implementation and perceived increase in safety, most designs utilize distributed propulsion through numerous directly-powered small lifting rotors or propellers. This investigation instead considers the largely-overlooked tandemrotor configuration, presents three notional designs, and considers the potential benefits and limitations of such a configuration for the urban air mobility role.

Modeling and Optimization of Propulsion Systems for eVTOL Aircraft (Paper 1181)
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Willem Anemaat, Drew Darrah, Bruno Moorthamers, Wanbo Liu

UAV platforms often use commercial off the shelf electric propulsion units for thrust generation. The manufacturers of these electric motors often only include the maximum efficiency or a list of efficiencies coupled with specific rotors. Taking into account the effect of the electronic speed control, many combinations are possible, making it hard to predict the final propulsion and efficiency of the complete propulsion unit. In this paper, a novel method of analysis is discussed to predict the flight endurance of any motor, battery, propeller combination. Using flight testing, the model is verified for four specific configurations. The model is then used to present some trends on flight endurance and cost as a function of vehicle weight using a total of 13 configurations. In future, the model could be used by UAV designers to better predict the performance of their aircraft using COTS components.

Rotorcraft Conceptual Design Methodology by Using Fully Parametric CAD Model with Embedded Empirical Formulations (Paper 108)
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Hasan Ibacoglu, Abdullah Enes Coskun, Tolga Kayabasi

Rotorcraft design needs a synchronous design method that connects different design decisions. This is due to the reciprocal nature of design drivers, and their corresponding consequences. Therefore, there is a need for a comprehensive design methodology that includes and connects different design considerations including performance, configuration, ergonomics etc. This paper introduces a rotorcraft design methodology which utilizes a fully parametric computer aided design (CAD) model. In the CAD model, performance calculations, weight estimations, and volume allocations are empirically formulated, and embedded into the model. This study enlarges the boundaries of conceptual design scope as it includes ergonomics factors such as pilot view angle, seating types and seating configurations. Moreover, alternative landing gear configurations, fuel tank configurations, different cabin types and design choices with their corresponding effects on weight and performance are discussed.

Structural Optimization of a Co-Axial Compound Rotorcraft by using Three-Dimensional Finite Element Representation (Paper 69)
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SunHoo Park, ByeongUK Im, TaeYong Chun, SangJoon Shin

A three-dimensional detailed design procedure is proposed for the internal layout optimization of a co-axial compound rotorcraft. The present design procedure consists of the following two phases. In its first phase, the detailed three-dimensional configuration is obtained by a robust procedure. This procedure constructs the detailed internal layout based on the structural integrity and aeromechanical stability analysis. Along with such, structural optimization is attempted, and the structural weight will become more precise than that estimated by the trend formula. It is found that the first phase has the capability to predict the component weight accurately. And then, the second phase is performed for estimating the more realistic internal layout. The present procedure accounts the volume of the components and the internal layout will be reconstructed by including the realistic component volume. By applying those phases, the internal layout of the compound rotorcraft will become more realistic.

Aircraft Design III

Technical Session E: Thurs. May 12, 2022 - 1:30 PM to 5:30 PM

38th Student Design Competition Graduate Winner: ALICORN – 2025 Unmanned Vertical Lift for Medical Equipment Distribution (Paper 1323)
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Dilhara Jayasundara, Spencer Fishman

(No abstract available.)

38th Student Design Competition Undergraduate Winner: GARRA – 2025 Unmanned Vertical Lift for Medical Equipment Distribution (Paper 1324)
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Derek Safieh, Andrew Lent

In response to the Vertical Flight Society's 38th Annual Student Design Competition's Request for Proposal (RFP), the University of Maryland's undergraduate design team presents Garra. The name is inspired by the Garra Rufa fish also known as the doctor fish. In nature, this fish brings wellness and joy to those around it. Garra is an innovative thrust compounded single main rotor featuring a novel open bottom structural design. The vehicle meets and exceeds all the RFP requirements in terms of safety, compactness, and block time. The SMR design has been approved by the FAA in the past, ensuring a smooth path to certification by 2025.

Biomimetic Adaptive Airframe Technologies (BAAT) for Rotorcraft Design and Optimization (Paper 1230)
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Allen Davis, Bochan Lee, Moble Benedict, Darren Hartl

Mission-adaptive aerostructural design considers the alteration of structural geometries to improve multi-objective performance across multiple aerodynamic environments associated with flight conditions derived from specific mission profiles. To accomplish this, adaptive structures design requires evaluating the aerostructural responses for each possible geometry to determine the optimal configuration for each mission stage. This work develops a mission-driven design framework combining aerodynamic, structural, mission, and optimization computational tools to design and optimize adaptive vertical lift aerostructures. Aerodynamic conditions and vehicle properties are driven or defined by mission requirements, where a mission is defined as a sequence of specified flight phases (inputs) with a unique set of performance objectives (outputs). Aerodynamic and structural analysis tools iteratively trim the vehicle for each mission phase while considering desired geometric changes, viable actuation methods, and actuator sizing required. Together these comprise a complete structural description. Preferred geometries for each mission phase are then determined via optimization to improve performance metrics defined by mission objectives and requirements. The computational framework searches algorithmically for structural configurations that improve mission-driven objectives based on trim flight for each mission phase using a novel algorithm to consider both adaptive and fixed design variable selection to effectively solve this complex design problem. It will be shown that both the optimal placement of adaptive structures and levels of morphing can be determined concurrently via a novel optimization and postprocessing procedure, leading to mission-wide performance improvements (in this case, reduced required power) not possible with static structures.

Revisiting Legacy Weight Relationships Using Machine Learning Techniques (Paper 114)
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J. Michael Vegh, Andrew Milligan

This paper investigates the application of K-Means Clustering algorithms to traditional aircraft conceptual-level weight estimation techniques. As a proof of concept demonstration, application was narrowed to fuselage basic weight estimation with expansion to additional component weights as a planned follow on activity. A variety of weight sources were parsed and curated to produce a large, diverse dataset consisting of 82 separate aircraft with a corresponding new universal baseline regression to compare against. A K-Means Clustering algorithm was then employed that sorted aircraft into groupings based on configuration as well as topology and created an associated regression for each grouping. Configuration-based groupings utilized information such as a high-level abstraction of the structural layout as well as whether the aircraft is a fixed-wing or rotary-wing vehicle. Topology-cased groupings utilized information such as landing gear location and possession of a cargo ramp or wing. The configuration-based groupings showed modest improvement compared to the baseline regression while the topology-based groupings consistently outperformed both the baseline regression as well as the configuration-based regressions. Under all conditions, a subset of the data associated with fixed-wing aircraft was shown to be an outlier in regards to error as a result of a large range of weight and speed scales, as well as possible secondary pressurization impacts. Special treatment of the winged dataset led to further reduction in error based on unique design features, presenting an overall fuselage weight estimation methodology that leverages machine learning algorithms that can improve and inform existing best practices.


Avionics and Systems

Avionics and Systems

Technical Session B: Wed. May 11, 2022 - 8:00 AM to 12:15 PM

Accuracy Improvement Methods to a Deep Neural Network Model in Computer Vision (Paper 40)
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Grace Chrysilla

This paper covers machine learning approaches to improve the accuracy of a Deep Neural Network (DNN) inference in computer vision, particularly in image segmentation. A few benefits of object recognition aided by computer vision is to help aircraft crew avoid adversaries in various weather conditions, to offload the crew's attention from finding desired objects as well as estimating the position of those objects throughout missions. Since DNN inference is trained without explicit programming, it is challenging to precisely locate its sub-optimality when it performs less accurate prediction. This paper explores three machine learning approaches that improve a DNN model prediction accuracy: 1) Prediction result analysis using the author's original work called Directional Pixel Delta as well as Jaccard's Intersection over Union (IoU) loss function 2) Bayesian optimization hyper-parameter tuning 3) Training data optimization via Bootstrap Aggregating.

An Evaluation of Human Performance with a Large Area Touchscreen in a Simulated Rotary Wing Environment (Paper 113)
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Margaret Lampazzi, Catherine Daly, Jason Browning, Kathryn Guy

This paper presents results of a human machine interface (HMI) evaluation that examined representative flight deck tasks with a large area touchscreen installed in a ride quality simulator that replicated rotary wing vibration profiles. Touchscreens have made their way onto the flight deck of many fixed-wing aircraft and recently into rotary-wing cockpits as well. As such, there is a need to better understand how task performance is impacted by the unique vibratory environment encountered in helicopters. A large area touchscreen (LAD) was evaluated by 14 pilots conducting various touch tasks (target selection, data entry, swiping, long press and zoom), under three varying levels of vibration with and without flight gloves. Performance was assessed objectively (time to completion, touch accuracy, error rates) and subjectively (usability, musculoskeletal discomfort, and video footage). Design recommendations are made for display interface design, including target size, data entry and use of gestures. This study was reviewed and approved by an independent Institutional Review Board for the protection of human subjects.

Evaluation of Flight Control System Architectures for the AH-64 (Paper 109)
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Bryan Chu, Gary Klein, Russell Enns

The AH-64 would significantly benefit from an improved flight control system, particularly as the aircraft's requirements have evolved and continue to do so. To address this, multiple Vehicle Management System architectures are developed and presented for the AH-64 attack helicopter, each addressing several demanding and possibly conflicting future requirements, including Level 1 handling qualities, operating in degraded visual environments, autonomy, high-speed flight operation, and multiple vehicle coordinated operations. Architecture tradeoffs are performed with the understanding that the current AH-64 flight control system is mechanical with electrical partial authority augmentation, but also possesses a non-redundant full authority fly-by-wire emergency backup system. The various architectures are assessed as to how they satisfy the requirements. They are also assessed with respect to their relative costs, both for typical costs such as design, recurring, operating and sustainment, and training as well as other indirect costs such as size and weight. A methodology is developed to compare and contrast the architectures using informal qualitative-based scores assessed by subject matter experts from both industry and customer organizations, as well as scoring from Boeing program management with various operational experiences. The methodology assigns weightings to each requirement and cost criteria through the use of the Analytical Hierarchy Process. The results suggest that there are several likely cost-effective options, possibly with a higher potential return on investment than a conventional fly-by-wire (FBW) architecture. Some of these solutions are also incremental in nature, thereby providing more flexibility than the conventional FBW system.

Mission System Needs for Small Unmanned Systems (Paper 1170)
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Max Taylor, Matthew Cunnien, James Kleveland

The United States (US) Department of Defense (DoD) is looking to reverse the trend of new programs costing significantly more than their predecessors while providing advanced capabilities to the warfighter by supplementing existing manned platforms with small Unmanned Air Systems (sUAS). Traditionally sUAS were leveraged for limited tactical objectives with two-way communication to a single entity such as a ground station or as part of a single manned-unmanned team (MUM-T). However, advancements in collaborative networks, mature autonomy and continued miniaturization of key technologies have expanded the potential for a broader operational use of sUAS. The multi-domain connected battlespace of the future envisions significant strategic roles for sUAS to provide actionable information more broadly to the joint forces.1 The expanded use of sUAS platforms is evident in the future US Army strategy to augment the existing and future capabilities of its own vertical lift platforms. In order to dis-integrate and exploit enemy threat systems the US Army intends to leverage sUAS systems such as Air Launched Effects (ALE) and Future Tactical Unmanned Aircraft Systems (FTUAS) (Ref. 1). These will be part of the Future Attack Reconnaissance Aircraft (FARA) ecosystem allowing extended reconnaissance, security, and attack operations. The US Air Force is also identifying new Concepts of Operation (CONOPS) which can leverage sUAS as a force multiplier to help against emerging threats (Ref. 2). This includes existing continued enhancement of intelligence, surveillance, and reconnaissance (ISR) capabilities as well as new MUM-T and swarming CONOPS. In this new role, there are several challenges that emerge for sUAS mission systems. ? Existing sUAS security boundaries are focused on vulnerabilities between the aircraft and the ground station or controlling vehicle. Introduction of sUAS in the connected multi-domain battlespace opens the security boundary to include all participants consuming data from these vehicles. This results in additional attack vectors for adversaries requiring new security considerations for a sUAS. ? Secure and available communications are key to supporting multi-domain battlespace doctrine at the timing and tempo required to gain advantage on the adversary. Introduction of sUAS to this assumes the ability to interconnect securely with existing and future communication protocols at a significantly reduced size, weight and power. Availability of secure communication from sUAS is challenging when considering using these unmanned systems to support operations in contested environments. ? Autonomous operations and processing on the edge are key to reaping the benefits of the sUAS operating in a MUM-T environment. Moving the processing of key capabilities to the edge allows for the quicker response times and the ability for the sUAS to continue operations in contested environments and report back when secure communications become available. It can be difficult to combine the processing resources and power required to perform the needed advanced autonomous behaviors in an extremely small form factor. ? With advancements in technology, the emerging threats to warfighter are outpacing upgrades of existing mission systems (Ref. 3). The ability to rapidly update mission systems will be required to counter these threats. The mission system architectures for sUAS will need to be designed with Modular Open System Approach (MOSA) solutions that can allow rapid updates to hardware and software. This ability for fast third-party system update and integration will be required to keep sUAS relevant and maintain an operational advantage. Presented at the Vertical Flight Society's 78th Annual Forum & Technology Display, Ft. Worth, TX, USA, May 10-12, 2022, Copyright 2022 by the Vertical Flight Society. All rights reserved. 2 The paper will offer analysis of the implications of the emerging role for sUAS with an emphasis on potential impacts to the vertical lift community. This resultant paper will examine how sUAS performing a more interconnected role will impact overall battlespace security. In addition, the paper will analyze and assess the impacts to the attributes of sUAS including size, weight, power, and cost (SWAP-C), life-cycle cost, mission system and payload integration and upgradability. Finally, the paper will identify technology considerations to address sUAS interoperability, safety, security, qualification, and accommodations for new, as well as legacy technology.

Observers for Robust Rotor State Estimation (Paper 1128)
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Omkar Halbe, Manfred Hajek, Florian Holzapfel

The paper discusses the synthesis of linear and nonlinear observers to estimate rotor states from fuselage state measurements alone. First, the paper reviews two forms of the classical Luenberger linear observer applied to the rotor state estimation problem and identifies some limitations thereof. Thereafter, the paper proposes a new robust nonlinear discontinuous observer based on the sliding mode theory to simultaneously estimate rotor flapping and lead-lagging states from fuselage state measurements. For this new nonlinear observer, the paper presents stability analyses to determine conditions that guarantee rotor state estimation accuracy despite unknown but bounded turbulence input. The nonlinear observer also lends itself to the online estimation of the unknown turbulence input. Simulation results in calm and turbulent air conditions highlight the efficacy and performance of the nonlinear discontinuous observer. Such rotor state observers could provide an independent source of online rotor states estimates to complement or supplement in situ rotor state measurement apparatus for various flight control and health monitoring applications.


Crash Safety

Crash Safety

Technical Session A: Tues. May 10, 2022 - 8:00 AM to 12:00 PM

Advanced Seat Belt System for Occupant Restraint (Paper 1241)
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Marv Richards

Current passive occupant restraints do not adequately restrict upper torso/head motion during a crash, especially with a combined forward and vertical acceleration component. The belt geometry of conventional webbing restraints allows the upper torso to rotate forward, increasing the probability of secondary impacts. In addition to the suboptimal belt geometry issues, some occupants are not familiar with the current 4-point and 5-point restraints. Safe, Inc. has developed a passive restraint system with improved belt routing geometry to reduce the occupant's motion and resulting injury risk. The restraint supplements the familiar automotive 3-point restraint system with a second, mirror image diagonal shoulder belt to provide upper torso restraint in all directions. It is intuitive to don with minimal installation instruction to encourage more frequent and proper use. The restraint would be beneficial to all vehicle types and is well suited for the emerging Urban Air Mobility (UAM) platforms.

Development and Analysis of Energy Absorbing Subfloor Concepts to Improve eVTOL Crashworthiness (Paper 1179)
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Jacob Putnam, Justin Littell, Mercedes Reaves, Nathaniel Gardner

To help ensure safe transportation within electric Vertical Take-off and Landing (eVTOL) vehicles the National Aeronautics and Space Administration (NASA) has been developing novel energy absorbing (EA) design technologies to improve crashworthiness within the unique design constraints of these vehicles. As part of this effort, a series of lightweight energy absorbing subfloor concepts were developed for potential use within eVTOL vehicle design. The capability of the subfloor designs was first evaluated through finite element (FE) model simulation in both component and vehicle level impact conditions. Knowledge gained from these analyses were used to iterate upon the design prior to fabrication. Fabrication and testing of the subfloor designs has begun and will be used to verify predicted capability. Results from FE model analysis was used to down select to a final subfloor geometry for additional component level optimization and full-scale test validation.

Fatal Rotorcraft Accidents for a Ten-Year Period (Paper 1267)
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Amanda Taylor, Joseph Pellettiere

The Federal Aviation Administration implemented regulations and guidance material to protect occupants in the event of a rotorcraft crash. These rules apply to all newly developed rotorcraft. There has been a recent push to develop similar standards for the existing fleet and for the newly constructed rotorcraft. Two major areas of concern are injuries and fatalities resulting from either blunt force trauma or post-crash fire. To understand the effects of these two factors on survivability outcomes, the National Transportation Safety Board accident database was queried for rotorcraft accidents from January 1, 2009 to December 31, 2018. During this period, a total of 1489 accidents occurred, and after refinement, 187 listed fatal. Of the fatalities, blunt force trauma was by far the greatest cause. Though thermal injuries resulted in fewer fatalities, post-crash fires were still a frequent occurrence and protective measures should be put in place for both.

Test and Analysis Methodology for Validating Crashworthiness of AW609 Tiltrotor (Paper 1310)
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Ryan Miller, Jim Waterman, Fabrizio Turconi, Andrea Di Renzo, Marco Anghileri

The AW609 tiltrotor is the first new class of air vehicle pursuing civil certification in over fifty years under FAA order 21.17B. The primary safety objective for crashworthiness for any aviation vehicle is to maximize the probability of survival and to minimize the probability of post-crash fires. A unique feature of the AW609 design is a high mounted wing above the fuselage, with nacelles at the end of the wing that contain heavy items of mass such as engines, rotor systems and transmissions. In order to create a viable aviation product with suitable empty weight fraction a controlled wing failure mechanism at the wing root is included to shed wing and nacelle mass in high g landing events. This allows for protection of the fuselage and the occupants up to the full regulatory vertical landing requirement of 12 g's. An analysis supported by test methodology is presented that includes explicit dynamic finite element modeling, coupon, element and full-scale drop testing of a full span AW609 wing. The objective of the testing is to characterize the equivalent quasi-static g level for damage initiation and progressive damage progression for the tested configuration, and to validate the associated analysis. The dynamic analysis, once validated by test, shall be used to analyze other weight and center of gravity combinations relevant to the anticipated AW609 mission profiles.


Crew Stations and Human Factors

Crew Stations I

Technical Session C: Wed. May 11, 2022 - 1:45 PM to 6:00 PM

3D Conformal Pilot Cueing for Rotorcraft Shipboard Landings: On-Demand Trajectory Generation used with a Flight Path Marker Cue (Paper 1175)
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Robert Walters, Mahmoud Hayajnh, Karen Feigh, J.V.R. Prasad

Shipboard landings are one of the most challenging piloted maneuvers. Trajectory guidance can aid the pilot and reduce workload. Bezier curves provide a mathematical framework that is stable, can be easily constrained, and executes quickly on a computer. These properties allow for dynamic trajectory generation for use in pilot cueing. A pilot-in-the-loop study was used to test the impact on pilot workload and performance of these trajectories as part of an advanced cueing system. The results show that perceived pilot workload decreased while maintaining or improving performance across the measured metrics.

Development and Evaluation of Coupling Modes and Haptic Functions enabled by Active Inceptor integration in Cockpits (Paper 1145)
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Laurent Binet, Raphaël Perret

The potential uses of Active Side-Stick Units (ASSU) technology far exceed solely the case of fully programmable haptic feedbacks to one single pilot or the capability to generate haptic cueing for Flight Envelope Protection functions. Indeed, in dual pilot configurations, the capability to electronically link ASSU enables the coupling of the motions of the Pilot Flying (PF) and Pilot Monitoring (PM) controllers and opens potential new modes of communication between pilots, as well as between crew and Automatic Flight Control System (AFCS) when upper modes are engaged. The EFAICTS (Ergonomic impact and new Functions induced by Active Inceptor integration in CockpiTS) project, started in December 2018, received funding from the Clean Sky 2 Joint Undertaking under the European Union's Horizon 2020 research and innovation program under grant agreement No 820884. ONERA was the project coordinator and Safran Electronics & Defense the Topic Leader. The EFAICTS project proposed to develop and integrate coupling and haptic functions for both Pilot/Co-pilot and Crew/Autopilot interactions along a Human-Centered Design approach, in which the end-users (i.e. pilots) were at the heart of the development, from the beginning of the project to the final evaluation phase. The paper focuses on the flight scenarios definition; the PF/PM/AFCS interactions definition; the development of specific haptic feedbacks and ASSU coupling functions. All the concepts developed and assessed through modelling and intermediate tests were finally evaluated by experienced pilots on the ONERA simulation bench. The main results are presented in the paper.

Differential Role of Gravitoinertial Cues for Active and Passive Control in Degraded Visual Environments (Paper 1314)
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Martine Godfroy-Cooper, Jean-Christophe Sarrazin, Edward Bachelder, Joel Miller, Benoit Bardy

Future vertical lift (FVL) missions will be characterized by increased agility, degraded visual environments (DVE) and optionally piloted vehicles (OPVs). Increased agility will induce more frequent variations of linear and angular accelerations, while DVE will reduce the structure and quality of the out-the-window (OTW) scene. As rotorcrafts become faster and more agile, pilots are expected to navigate at low altitudes while traveling at high speeds. In contour terrain flight, the perception of self-position and orientation provided by visual, vestibular, and proprioceptive cues can vary from moment to moment due to visibility conditions and body alignment as a response to gravitoinertial forces and internally/externally induced perturbations. As a result, erroneous perceptions of the self and the environment can arise, leading ultimately to spatial disorientation (SD). In OPV conditions, the use of different autopilot modes transforms the pilot's role from active pilot to systems supervisor. This shift in paradigm, where pilotage is not the primary task, and where feedback from the controls is not available, has important consequences. Indeed, space perception can be strongly modulated by the nature of the displacement in space. Considering the relationships between the level of automation (LOA) and sense of agency (SoA), it is of particular interest to examine whether motor control mechanisms can modulate the level of visual-vestibular integration in tasks of movement perception vs. movement control. An experiment was conducted using the NASA AMES vertical motion simulator (VMS) to evaluate the effects of optical and gravitoinertial cues in the assessment of altitude in contour terrain flight. Seven U.S. Army pilots participated in the experiment. The aim of the proposed research was a) to establish the relative contribution of visual and gravitoinertial cues as a function of the quality of the visual cues (good vs. degraded) and the presence or absence of gravitoinertial cues; b) to determine the role of manual control vs. supervisory monitoring control on the estimation of altitude, and c) study the interactions between the nature and the quality of the sensory cues and the type of control. For the supervisory control condition, the results showed that the gravitoinertial component played a significant role in the estimation of ground height, but only in the case where the optical structure did not efficiently specify the actor-environment interaction. Meanwhile, the results for the manual control task provided evidence, at multiple levels, that the acceleration information, specified by the variations of the gravitoinertial field, has a relative character. Altogether, these results are in line with the Sensory Weighted Approach of perception, which proposes that each sensory cue is weighted depending on this reliability: gravitoinertial information is attenuated when the visual information is relevant while it enhances performance when the visual information is poor.

Further Simulated Flight Trial Assessment of Novel Autorotation Cueing Methods (Paper 1201)
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Mushfiqul Alam, Michael Jump, Jonathan Rogers

Autorotation maneuvers in helicopters are generally performed in an emergency following some form of catastrophic mechanical or system failure. It is a complex maneuver to perform because the pilot is required to perform several tasks simultaneously and the timing of each of them needs to be precise. Workload can be high and the consequences of getting things wrong can be fatal. Following on from a series of studies that investigated the use of symbology presented on a Head-Up Display to try to assist a helicopter pilot to fly the autorotation maneuver more safely and accurately, this paper presents a pilot-in-the-loop flight simulation study to explore the use of haptic cueing to help the pilot maintain indicated air- and main rotor speeds. Various entry conditions to autorotation maneuver are assess via simulated flight trial at Liverpool's HELIFLIGHT-R full motion flight simulator. Subjective evaluation of the results show that the haptic cues are useful to pilots in terms of reducing the workload to perform a successful autorotation landing.

Crew Stations II

Technical Session D: Thurs. May 12, 2022 - 10:15 AM to 12:15 PM

Individual Differences in Cueing Utilization in Rotary-Wing Aviators (Paper 66)
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Kathryn Feltman, Jason Boggs, Michael Wilson

A preliminary qualitative review of participant comments from a series of three studies was completed. The three studies evaluated different aspects of multisensory cueing developed by the Army's Degraded Visual Environment-Mitigation program. A Template Analysis was used to identify themes across the studies in which participants identified aspects of the cueing that they preferred or did not prefer. Outcomes suggest that individual differences in perceived usefulness of cueing may exist. Further research, including qualitative analysis of additional studies and experimental research with a larger sample, is recommended.

Methodology Proposal to Better Integrate Human Factors in Aviation Maintenance. (Paper 112)
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Fabien Bernard, Raphael Paquin, Georges Devilliers, Mohsen Zare

In industry, and more particularly in the aviation maintenance industry, Human Factors/Ergonomics (HFE) is increasingly considered by maintainability stakeholders in the aircraft development process. However, most of the stakeholders are not specialized in HFE, therefore the compromise between HFE and design criteria is not optimized. This paper introduces a methodology proposal to enhance integration of HFE in aviation maintenance by maintainability stakeholders without HFE skills and knowledge. This methodology, called PEAM (Preliminary Ergonomics Analysis in Maintainability) will not replace the HFE specialist but will help all maintainability stakeholders to anticipate the maintenance operator's activity in the preliminary phases of aircraft design. This paper will also introduce the first results regarding PEAM deployment efficiency.

Task Analysis and Predictive Workload Modeling for Autonomous Aircraft (Paper 116)
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Margaret Lampazzi, Carl Pankok Jr.

The desire to transition to single-pilot operations (SPO) has led to research and development of autonomous technologies that can take over tasks normally handled by two pilots and create a new paradigm that supports SPO. To safely achieve single-pilot operations (SPO) of an existing dual crew aircraft, the workload split between the two pilots needs to be analyzed and candidate tasks for offloading identified. Simulation is a valuable tool to model different task allocation strategies for such systems. This paper presents the methodology that was used to analyze shared tasks between a two-pilot crew and identify candidate tasks that could be handled by the autonomous system. A simulation tool called Improved Performance Research Integration Tool (IMPRINT), developed by the U.S. Army was used as part of the design process for an autonomous flight control system. IMPRINT was used to guide cognitive walk-throughs and model pilot workload to inform task allocation between autonomy and the human operator. Advantages and disadvantages of this method will be discussed as well as recommendations for future work.

The Portable Helicopter Oxygen Delivery System (PHODS) in the Altitude Chamber: Cerebral and Peripheral Blood Oxygen Saturation and Perceptual Vigilance. (Paper 81)
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Leonard Temme, Bobby Bowers, Amanda Hayes, Paul St Onge, Aaron McAtee, Dennis Ard, Frank Petrassi

The Portable Helicopter Oxygen Delivery System (PHODS) provides supplemental oxygen (O2) to Army aircrew at altitudes up to 18,000 feet (ft) above mean sea level (MSL). Previous PHODS tests and evaluations (T&E) used conventional pulse oximetry to monitor peripheral blood O2 saturation (SpO2). The present T&E incorporates near infrared spectroscopic (NIRS) measures of regional cerebral blood O2 saturation (rSO2). Army aircrew (N = 22) assessed PHODS functionality and effectiveness in an altitude chamber during three challenges: 10 minutes of a visual reaction time test, 5 minutes of text reading, and 2 minutes of a physical workload challenge (self-paced squats). Throughout the T&E, SpO2 and rSO2 were measured continuously at ground level and at pressure altitudes of 14,000 and 17,800 ft MSL. Results indicate that the PHODS maintained constant SpO2 throughout all testing but not rSO2 in the presence of workload. The possible operational significance of this finding is discussed.



Dynamics I

Technical Session B: Wed. May 11, 2022 - 8:00 AM to 12:15 PM

An Integrated Three-Dimensional Aeromechanical Analysis of Lift Offset Coaxial Rotors (Paper 83)
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Mrinalgouda Patil, Paulo Arias, Anubhav Datta, James Baeder

This paper presents the first use of integrated three-dimensional (3D) aeromechanics modeling, defined as the coupling of 3D solid finite elements based comprehensive analysis (CA) with 3D Reynolds-Averaged Navier-Stokes (RANS) computational fluid dynamics (CFD), to study modern lift offset coaxial rotors. The goal is to demonstrate the development of this methodology and assess its capabilities. The X3D structural dynamics solver is coupled with the UMD Mercury CFD framework for this analysis. Metaltail - a notional hingeless coaxial rotorcraft, developed as an open-access model for the U.S. Army / DoD's High Performance Computing Framework Helios is used as the test case here. The analysis is performed at a low speed transition flight and preliminary predictions of airloads and three-dimensional stresses are discussed. Modern coaxial rotors are compromised by heavy rotor hubs reducing their performance. This paper aims to demonstrate a high-fidelity capability that can help overcome this barrier through accurate predictions of 3D dynamic stresses of blade and hub.

Evolution in Buffet Loads Determination for Tilt Rotor (Paper 1126)
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Carlo Cassinelli, Maria Ludovica Dall'Aglio, Nicola Donini, Alberto Angelo Trezzini

This work presents the state-of-the-art of the validated buffet loads determination methodology developed by Leonardo Helicopters, with particular attention to the aerodynamic modelling and the related structural loads calculation. The modern CFD, validated by WT data, is the first major topic addressed in the paper: simulations are performed with ANSYS Fluent to characterize the oscillatory pressure distributions generated by the wing stall. The structural loads calculation is the next part of this process, with a focus on the interface with the aerodynamic input and on the necessary statistical analyses to predict safe loads. Buffet is considered a random phenomenon, which is robust and repeatable only in terms of statistic quantities, and, to estimate the maximum likely values, a probabilistic approach is used to calculate a reasonable safety factor. The last topic is the analysis of actual flight data in compatible high angle of attack conditions in order to substantiate both the load analysis deterministic results and the statistical approach to predict peak values for aircraft design.

H145 New Comfort Experience: Upgrading From Four-Bladed HMR To Five-Bladed BMR (Paper 1146)
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Raphael Rammer, Martijn Priems, Stefan Dreher, Oliver Dieterich

In 2020, Airbus Helicopters achieved EASA type certification of the BK117 D-3 (branding name H145), which is the latest variant of the BK117 family. It entails a significant upgrade from the BK117 D-2. The major change, a new five bladed main rotor, required extensive investigations with respect to dynamics. This paper gives a short overview of the history of the BK117 family development from its beginnings up to now and on the investigations performed in the fields of structural dynamics, aeromechanics stability and equipment integration for its latest variant. A development and certification methodology, relying on the use of numerical tools validated by using the results of extensive laboratory, ground and flight testing was applied. It is shown that the upgrade of the H145 with the five bladed rotor allowed to reduce the helicopter's empty weight by omitting landing gear dampers and anti-vibration measures as well as thanks to the reduced weight of the rotor system itself.

Higher Harmonic Control Simulation with Actuators Designed by the Physics-Based Approach (Paper 1212)
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Kunhyuk Kong, Byeonguk Im, SangJoon Shin

A higher harmonic control simulation along with actuators designed by the physics-based approach is attempted in this paper. The object rotorcraft used in the simulation is UH-60A Black Hawk and a multibody dynamics analysis program DYMORE is used for the simulation. The three actuators are located upon the non-rotating swashplate, and represented by the prismatic joints. Pitch angles of the rotor blades are adjusted by the combination of linear motion of the actuators. The rotor system is verified by the comparison against the references via the modal and trim analysis. The response of the fuselage is reflected regarding its entire hardware by the order reduction according to Herting's method. The fuselage is finally modeled as the beam element. A higher harmonic control with the transformation from the harmonic coefficients to the displacements of the servo actuators is to be simulated. By LQG based algorithm that is proposed by the authors, the vibration reduction capability of the present control algorithm will be verified.

Lichten Runner-up Paper: High-Fidelity Modelling of Actuation Systems for Nonlinear Aeroservoelastic Analysis of Tiltrotor (Paper 1163)
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Federico Pellegrino, Alberto Angelo Trezzini, Giuseppe Quaranta

Recent evolutions in regulatory practices place an increasing emphasis on nonlinear aeroservoelastic analysis to prevent adverse phenomena like Limit Cycle Oscillations (LCO). Thanks to enhanced computational capabilities, this field is set to take an even more important role in the certification process of new aircraft, and in particular of tiltrotor, together exploiting the increasing ability to build multidisciplinary and high-fidelity modelling of complex aeronautical systems. In this paper, a comprehensive model of a tiltrotor control surface actuation system is introduced, capable of accurately representing the main nonlinear features and helping to better comprehend their origin. The model includes the entire control chain, starting from the Flight Control System (FCS) logic and getting to the control surface structure, passing through the hydraulic servo-actuator and the related mechanical linkages. The behavior of the servo-actuator model as a subsystem and the one of the entire control chain are compared to experimental data. Finally, a simplified nonlinear aeroservoelastic simulation shows the potential effects of the nonlinearities present in the system.

Model-Based Optimization of a Tethering Device for Ultralight Helicopters (Paper 97)
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Benjamin Rothaupt, Walter Fichter, Benedikt Grebing

This paper presents the process of flight mechanical analysis and optimization of a helicopter tethering frame that is used in pilot training and research projects. An existing linear state space model of the CoAX 2D ultralight helicopter is fusioned with a parametrized analytical model of the tethering device. Using the eigenvalues of the free flying and tethered helicopter as a similarity measure, optimal values for the geometric parameters are calculated through bounded parameter optimization. In a separate section, the castor wheels that enable horizontal motion of the tethered helicopter are analyzed, yielding the parameters that are relevant for the dynamic behavior of the tethered helicopter and possible changes that should mitigate the flight mechanical effect of the wheels.

Relative Rotor Phasing for Vibratory Load Minimization for a Coaxial Multicopter (Paper 60)
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Gaurav Makkar, Robert Niemiec, Farhan Gandhi

This study focuses on vibration reduction for a coaxial multicopter with 2-bladed, synchronized RPM, variable-pitch rotors through the use of rotor phasing. The study also examines the effect of aerodynamic interference between the rotors of a coaxial pair on the vibration predictions. A set of seven multi-rotor phase parameters are defined - a crossover azimuth for each the four coaxial pairs along with three aircraft level modes - pitch phasing, roll phasing, and differential phasing. The phase modes are examined in both a cross- and plus-configuration multicopter. Irrespective of whether interference was included or not, crossover azimuth of 0? tends to minimize the 2/rev lateral loads, while moderate values of crossover azimuth reduces the 2/rev longitudinal loads, for a coaxial rotor pair. For minimizing overall vibratory moments at the C.G., crossover azimuth corresponding to minimum 2/rev thrust is chosen for all the coaxial rotor pairs. It was observed that when interference is not included in the model, vibrations at the aircraft level are almost zero for pitch phasing and very low for the roll phase mode. However, when interference is included pitch phase is no longer a free parameter. With interference modeled, the 2/rev forces for the cross-configuration are ? 65% lower than the plus-configuration, while the 2/rev moments are ? 70% lower.

Dynamics II

Technical Session C: Wed. May 11, 2022 - 1:45 PM to 6:00 PM

Blade Load Reconstruction from Embedded Strain Measurements (Paper 1198)
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Pierangelo Masarati, Carmen Talamo, Roberta Cumbo, Daniela Rigamonti, Paolo Bettini, Daniel De Gregoriis

Rotorcraft blades are subject to significant dynamic loads both in standard and critical operating conditions. The knowledge and the prediction of the produced aerodynamic loads could represent an advantage in preventing failures on the rotorcraft, but also to avoid unnecessary inspections and reduce the downtime of the aircraft. This work applies the Kalman filtering technique to estimate the aerodynamic loads on a helicopter rotor blade at wind-tunnel model scale, representative of that of a medium-weight helicopter (900 mm span and 72.5 mm chord, corresponding to a 1:5-1:8 model scale). The loads estimation is based on strain measurements provided by Fibre Bragg Grating sensors embedded in the blade at several spanwise sections. Two different test campaigns have been done: a static one to characterize the experimental set-up followed by a wind-tunnel test campaign. The results show that the Fiber Bragg Grating sensors could represent an alternative choice with respect to strain gauges for strain measurements in in-flight health monitoring.

Cheeseman Award Paper: Damper Model Identification Using Hybrid Physical and Machine Learning Based Approach (Paper 1322)
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Michele Zilletti, Ermanno Fosco

In this paper the identification of a time domain model of a helicopter main rotor lead-lag damper is discussed. Previous studies have shown that lead-lag dampers have a significant contribution to the overall aircraft dynamics, therefore an accurate damper model is essential to predict complex phenomena such as, instabilities, limit cycles, etc. Due to the inherently nonlinear dynamics and the complex internal architecture of these components, the model identification can be a challenging task. In this paper, a hybrid physical/machine learning based approach, has been used to identify a damper model based on experimental test data. The model, called grey box, consists of a combination of a white box, i.e. a physical model described by differential equations and a black box, i.e. regression numerical model. The white box approximates the core physical behaviour of the damper while the black box improves the overall accuracy by capturing the complex dynamic not included in the white box. The paper shows that, at room temperature, the grey box is able to predict the damper force when either a multi-frequency harmonic or a random input displacement is imposed. The model is validated up to 20 Hz and for the entire damper dynamic stroke.

Effect of Lag Damper Failure on Helicopter Ground Resonance (Paper 1245)
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Farhan Gandhi, Jonah Whitt

This paper examines ground resonance of a helicopter with a 4-bladed rotor with degradation in one of the lag dampers. The analysis is conducted with lag equations in individual blade coordinates solved using Floquet theory, lag equations in multi-blade coordinates solved using Floquet theory, and lag equations in multi-blade coordinates simplified using a constant coefficient approximation and then solved as an eigenvalue problem. From the study it was observed that regardless of whether the blade lag motions are in individual or multi-blade coordinates, the predicted stability levels are identical if the analysis is conducted using Floquet theory. In multi-blade coordinates, collective and differential lag needs to be retained in the analysis, unlike the case of a classical ground resonance analysis where only the cyclic lag modes and body motions are required. Using the constant coefficient approximation in multi-blade coordinates it is equivalent to smearing the damping loss of a single damper equally over all the damper. With the constant coefficient approximation predicts a smaller reduction in damping with damper degradation than the Floquet method, with the differences increasing as the level of degradation increases. For a completely failed damper, the loss in system damping predicted using the constant coefficient approximation was 46% of that from the Floquet analysis for an articulated rotor, and 55% for a hingeless rotor.

On the Impact of Flight Control Systems on Kinetosis of Helicopter Passengers (Paper 44)
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Süleyman Özkurt, Walter Fichter, Christian Fischer, Heinrich Bülthoff

Disorientation, nausea, and vomiting of passengers that result from vehicle vibrations are characterized as kinetosis or motion sickness. It is mainly the low-frequency movements of these vehicles that contribute to the kinetosis of passengers. Frequencies that provoke kinetosis lie within the rigid-body flight characteristics of helicopters, which depend, among other things, on the design parameters of the flight control system. Moreover, the flight control system's reaction to atmospheric turbulence leads to low-frequency vibration that can result in kinetosis of the passengers and decrease the ride comfort of the helicopter. It is necessary to understand which movements of the helicopter cause kinetosis to develop countermeasures. Therefore, an extensive experimental campaign was conducted as part of a national research project. The campaign's results show that kinetosis in helicopters depends on the number of motion degrees of freedom and that the existing assessment method in ISO2631-1 is not able to reflect this correctly. Furthermore, based on the results, it can be assumed that the extent of kinetosis depends on the flight control system since the investigated flight control systems lead to different vibration spectra that expose the passengers. Therefore, consideration of kinetosis and thus ride comfort is recommended for the design procedure of future flight control systems.

Pilot Biomechanics for the Definition of a Rotorcraft-Pilot Interaction Experiment (Paper 118)
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Andrea Zanoni, Alessandro Cocco, Davide Marchesoli, Carmen Talamo, Pierangelo Masarati, Francesca Colombo, Sarah Kemp, Ermanno Fosco

Understanding Rotorcraft-Pilot Couplings phenomena is important for improving Human-Machine Interface design in rotorcraft. Although substantial progress has been made in the past decades, further effort is needed in enabling RPCfree or RPC-insensitive pilot-vehicle interface design. The design of an experimental testbed dedicated to investigating RPC interactions is presented, with a special focus on numerical simulation activities. The first results obtained in test campaigns involving non-skilled individuals are encouraging, showing that the testbed can enable more in-depth experimental analysis in the near future.

Pre-test Analysis on Slowed Mach-scaled Thrust Compounding Rotorcraft (Paper 1279)
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Shashank Maurya, Inderjit Chopra, Anubhav Datta

This paper details the propeller analysis in forward flight to finalize the wind tunnel test matrix of the thrust compounding rotorcraft model. First, the University of Maryland Advanced Rotorcraft Code (UMARC) is validated with main rotor hover test data at the tip Mach number of 0.2 and 0.5. It is further validated with slowed rotor (tip Mach number 0.2) wind tunnel test data up to a high advance ratio of 0.7. Next, the lifting-line-based propeller analysis is integrated into UMARC and validated with the propeller hover test data at various RPMs and propeller pitch settings. This analysis examines the isolated propeller performance in forward flight followed by a combined main rotor and propeller analysis using Maryland Freewake in the time domain. The wind tunnel test will be carried out at the propeller pitch of 30, and 35 and RPM will be varied between 1400 to 3500 up to a wind speed of 100 knots. A rotorcraft system-level comparison study is conducted using previous Mach-scaled wind tunnel test data at different shaft tilts and the propeller analysis at the corresponding speeds. Based on this coupled main rotor-propeller analysis, it is found that the thrust compound does not have much impact on rotorcraft efficiency, L/D. However, it helps extend the speed by using the propeller for propulsive force and the main rotor shaft tilted backward for efficient lift.

The Inter-2-Blade Lead-Lag Damper Concept (Paper 117)
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Pierangelo Masarati, Leonardo Frison, Andrea Zanoni

A novel concept of inter-blade damper rotor hub arrangement is proposed. Inter-blade lead-lag dampers currently used in some helicopters connect adjacent blades; their damping effect is associated with the relative lead-lag motion of the blades. In the proposed arrangement, called 'inter-2-blade,' dampers connect non-adjacent blades. The proposed arrangement improves ground-resonance stability and reduces harmonic components of the loads in the dampers. The Deutsch criterion for the sizing of lead-lag dampers is generalized to the inter-blade and inter-2-blade arrangements.

Time-Frequency Analysis of Experimental and Analytical Hub Loads of a Rotor Undergoing a Rotor Speed Change (Paper 39)
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Martin Sekula, Carl Russell

A two-part analytical study was conducted examining a small-scale, two-bladed rotor undergoing a change in rotor speed. First, a parametric study was conducted to understand how rotor transient response, determined using a time-marching solution, is affected by the choice of rotor wake model, blade elasticity, and inclusion of an elastic support structure model. The second part of the study compared results from an analytical model of the NASA Multirotor Test Bed (MTB) undergoing a rotor speed change to wind tunnel data. Since the analytical and experimental time histories were nonstationary signals, a Stockwell transform was employed to analyze the data in lieu of traditional Fourier transform-based methods. The analysis included extraction of time-varying frequency content of the rotor thrust and hub motion, damping ratios, instantaneous rotor speed, and instantaneous phase difference between thrust and hub motion. Use of a free wake model was found to be necessary to predict higher-harmonic thrust, however, it underpredicted the amplitude of the unsteady loads. Modeling the elasticity of the stiff rotor blades installed on the MTB resulted in minimal impact on the computed vibratory loads compared to a rigid blade model. The current analytical model of the MTB, including an elastic support structure, properly predicted the measured trends in the frequency content of the thrust.


Electric Vertical Takeoff and Landing (eVTOL)


Technical Session A: Tues. May 10, 2022 - 8:00 AM to 12:00 PM

Design of a Rotary Engine based High-Power Density Hybrid-Electric Power Generation Testbed (Paper 1278)
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Roydon Fernandes, Jayaprakash Shivakumar, Mariana Gehrmann, Nicholas Miller, Kyle Collins, Patrick Currier, Richard Anderson

This paper details the design and components of a high-power density Hybrid-Electric Power generation testbed that is being built by the Eagle Flight Research Center at Embry-Riddle Aeronautical University, Florida. The system consists of a twin-rotor rotary Wankel engine, a radial flux Permanent Magnet Synchronous Machine used as the generator along with its inverter/controller, a 400 V Lead-acid battery pack, a vehicle control unit, and the associated thermal systems. The system weighs 324 lbs. (147 kg) before fuel and is estimated to achieve peak power of 134 hp (100 kW) with the High-Voltage battery and sustained power of 70 hp (52 kW) with just the hybrid-electric system. With 8 gallons of fuel, the system is estimated to realize a specific energy of 0.37 hp-h/lb. (0.61 kWh/kg), and a specific power of 0.46 hp/lb. (0.76 kW/kg). The system control was implemented on the Vehicle Control Unit using a feedforward-feedback control loop with user-defined speed and output power values.

Development of eVTOL Aircraft For Urban Air Mobility At Joby Aviation (Paper 1185)
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Alex Stoll, JoeBen Bevirt

Typical existing VTOL aircraft - notably, helicopters - are limited in their suitability to civil transportation due to high operating costs, high noise levels, and safety levels below other forms of commercial aviation. Modern electric propulsion technology offers potential solutions to these drawbacks and potentially allows for practical VTOL aircraft configurations less compromised than traditional solutions. Through a multiyear study of various configurations, Joby selected the tilt-propeller as the optimal approach to safely achieve relatively high speed, long range, and low noise in urban air mobility operations. Full-scale flight testing began with a demonstrator phase, demonstrating successful transition between thrustborne and wingborne flight in 2017, and proceeded to a pre-production prototype phase, demonstrating high performance, including a 249 km (134 nm) VTOL flight and a true airspeed of over 322 km/h (174 kt), as well as low noise, measuring 45 dB(A) at 500 m (1,640 ft) equivalent in flyover and under 65 dB(A) at 100 m (328 ft) equivalent in hover.

Electromechanical Modeling and Testing of a Phase-Controlled Stacked Rotor (Paper 1240)
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Matthew Asper, Jayant Sirohi, Michael Ricci, Jack Brewer

Electric power has enabled unique propulsion architectures to improve eVTOL performance. This paper discusses independently-controlled electric motors to enable active phase angle maneuvers for thrust variation of stacked, corotating rotors. This can potentially increase the bandwidth of thrust control compared to varying rotor speed. A two-meter diameter phase-controlled stacked rotor system with independent electric drives was tested. Both isolated rotor and stacked rotor tests were performed actively varying the position, or phase angle of the rotors. Blade collective pitch was set to 11-degree and rotor speed was set to 950 RPM. Dynamic and steady state performance was predicted with an electromechanical model incorporating blade element momentum theory, field oriented control logic, and equations of motion. Tests evaluated the performance of varying both the magnitude of the phase change as well as the rate of the phase change. Isolated rotor tests showed good agreement with the model, with slower phase changes modelled better. For an isolated single rotor at a blade loading CT/? = 0:1175, changing the rotor phase by -5-degree at a rate of 50 -degree/s could be completed in 0.11 seconds, or less than 2 rotor revolutions, with an additional 49.7% of nominal rotor torque. Stacked rotor testing showed that there is significant coupling during phase angle changes, which limited the total phase angle achieved. At an axial spacing of 0.7 chord, thrust changes of up to 16.5% in 0.4 seconds were observed with an increase in power required by 44.6% for a phase change rate of 25-degree/s. Faster rates had comparable thrust changes, but increased the power required. Future work will include designing a control architecture to account for the aerodynamic coupling between rotors during angle changes and mitigate the drift each rotor experiences.

Fabrication, Testing, and Comparative Analysis of Lithium Sulfur and Lithium-Ion Electrochemistries (Paper 1257)
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Emily Fisler, Anubhav Datta

This paper addresses the fundamental barriers of eVTOL aircraft - energy and power. Lithium sulfur and lithium-ion coin cells were fabricated with identical overhead for a clear and consistent comparison of specific energy and power. The characteristics measured were discharge cycles, cycle life, impedance under conditions unique to electric vertical takeoff and landing aircraft namely high C-Rates, half cycles, and high transients. Equivalent circuit models were developed and validated to predict the steady-state and transient behavior of these cells. The key conclusions are lithium sulfur provides more than twice the specific energy of lithium-ion up to currents of almost C/2. At 1C, it is comparable. Above 1C it drops drastically and by 4C the energy vanishes almost entirely. This is traced to an order of magnitude higher impedance of these cells. The price to pay for high energy is low cycle life. However, it appear this problem can be eliminated by half cycles. The dynamic behavior of lithium sulfur is richer in comparison to lithium-ion. The response is still capacitative, hence first order, but the complex Warburg and constant phase elements have far greater influence. The behavior is harder to model as it does not fit neatly into linear equivalent circuits. The key conclusion is that lithium sulfur appears to be an attractive alternative to lithium-ion with characteristics that have significant ramifications on future eVTOL design and infrastructure.

Power System Characterization and Flight Testing of a >55lb Hybrid-Electric Multirotor (Paper 1214)
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Michael Ricci, Vishaal Varahamurthy, Robert Hicks

Hybrid electric propulsion is a promising technology to enable new aircraft configurations with excellent overall vehicle performance. Electric propulsion enables new types and configurations of rotorcraft, but the poor energy density of batteries has limited the overall endurance of electrically propelled aircraft. A hybrid architecture can take advantage of the high energy density of chemical/liquid fuels; but also utilize the advantages of electric propulsion technology. This advantage of a hybrid system can only be realized if the hybrid system is light enough and efficient enough. If not done in a highly optimized way the hybrid system can end up being too heavy and too inefficient - the worst of both worlds instead of the best of both worlds. This paper presents details of the LaunchPoint EPS hybrid electric power system and results of flight tests showing aircraft endurance improvements of at least 4 times longer over a pure electric version of the same aircraft.


Technical Session B: Wed. May 11, 2022 - 8:00 AM to 12:15 PM

Advanced Thermal Analysis Methodologies to Support eVTOL Propulsion System Optimization (Paper 1288)
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Lee Rogers, Thomas Holdstock

eVTOL vehicles have seen rapid advancements in electric propulsion technologies supported by complex high-fidelity simulations that allow subsystem performance to be maximized. Optimizing subsystem performance can improve propulsion system power density, especially through improved cooling. However, current work on holistic system level approach to eVTOL propulsion system designs [1,2] can benefit from earlier consideration of the interactions between inverter, motor, transmission, and thermal management systems. This can give a better understanding of the trade-offs between increasing power density and more complex, active cooling systems. These trade-offs must be understood as active cooling elements can lead to additional redundancy and certification requirements, which can increase the time-to-market in a highly competitive environment. This paper proposes an approach that quantifies these trade-offs early in the design process, so that the right propulsion system can be brought to market in the right timeframe.

Comprehensive Simulation for eVTOL Aircraft-Diagnosing Coupled Airframe-Propulsion Dynamic Instabilities (Paper 53)
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Felix Brenner, Llorenc Foraste Gomez, Jan Goericke, Matt Hasbun, Patrick O'Heron

The design, development, and testing of an eVTOL aircraft requires multiple physics-based simulation applications to address technical challenges and optimize the configuration for stringent mission requirements. While existing rotorcraft simulation and analysis tools are utilized for the analysis of eVTOL configurations, the adoption of an electrified propulsion system introduces new challenges. These tools are tailored for analysis of coupled system dynamics including elastic structures, airloads, and wake interactions, but are lacking capability for detailed electrified drive system dynamics. As a result, the dynamics of the electric drive system are often analyzed separate from the flight and structure dynamics. This can cause important coupled interactions to be overlooked and require costly late-stage design changes. This paper describes and demonstrates the coupling of GT-SUITE, a detailed electric propulsion modeling and analysis tool, with the comprehensive rotorcraft analysis tool, FLIGHTLAB. The objective of coupling these applications was to provide eVTOL analysis capabilities that enable improved designs in terms of mission range, operation capabilities, and flight safety. The successful coupling of these applications was demonstrated with lift and cruise propellers based on the Uber Elevate configuration.

Drag Performance Degradation Due to Icing of eVTOL (Paper 1202)
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Geoffrey Karli, Sihong Yan, Robert McKillip, Jose Palacios

An empirical model was developed to estimate the liquid water content (LWC) and mean volume diameter (MVD) of super-cooled water droplets of an icing cloud based on the torque variation of a single rotor. The model was developed for eVTOL configurations that can actively monitor on-board torque via an electric speed controller (ESC). Experimental data was collected in the Penn State Adverse Environment Rotor Test Stand chamber under various icing conditions. From experimental data, a relationship between torque degradation and LWC was developed. This LWC is coupled to the FAR Appendix C Icing Envelopes to determine an MVD range of possible values. The range discrepancy is resolved by running Blade Element Momentum Theory (BEMT) and the Han-Palacios Correlation (HPC) for a given set of condition and finding the results that best match with the recorded torque. This process is repeated over the flying duration, using the actively incoming torque information from the ESC. Through the coupling of BEMT and HPC, torque degradation due to blade icing can be modeled and predicted. Being able to predict torque degradation means that the time until a critical torque is reached can also be estimated. This estimated time is then relayed to the vehicle as the available flying time in the current cloud environment. The proposed model was verified by comparing experimental data to the predicted values. The BEMT prediction deviated from the constant measured torque value by 6.0%. For HPC verification, 11 experimental cases were compared to estimated results. On average HPC deviated from the experimental cases by 14.8% at 20 MVD and 16.6% at 40 MVD. With the model verified, a blind study was performed using this method. A case that emulates possible flying conditions in a realistic environment was chosen. Torque data was acquired during the icing event. Within 90 seconds of data available during testing, the MVD and LWC were predicted within 1.39% and 20% respectively. Every 10 seconds the prediction for 10 more seconds into the future was compared to the 'blind' experimental data. On average, the prediction overestimated the torque by 6.5%. Based on the final calculations it was determined that 5.25 minutes of flying time was available to this particular rotor operating within this cloud environment. The presented effort introduces and verifies the capability to determine icing cloud conditions and available flight time available based on the proposed approach.

Experimental Evaluation of Panel-Method-Based Path Planning for eVTOL in A Scaled Urban Environment (Paper 1215)
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Zeynep Bilgin, Murat Bronz, Ilkay Yavrucuk

In this study, previously proposed panel method based path planning for electric vertical take off and landing vehicles in urban environments is tested in a high fidelity simulation environment and with real-life drones in an indoor flight arena. Panel method is a numerical tool, borrowed form fluid dynamics domain, that can generate collision free paths for multiple vehicles in environments with arbitrarily shaped obstacles while guaranteeing obstacle avoidance and convergence to global minima with little computational load. In this study, panel method based path planning is further improved with introduction of novel safety source element that enables a safety perimeter around obstacles without losing convergence guarantee. Furthermore, path planning capability of panel method for electric vertical take off and landing vehicles in urban environments is demonstrated with hardware experiments in a scaled urban environment. Experiment results indicate that panel method is a promising tool for path planning in urban environments.

Flight Characteristics of AAM/UAM-Scale Quadcopters Under Atmospheric Turbulence (Paper 1129)
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Matthew Bahr, Etana Ferede, Farhan Gandhi

The flight responses of quadcopters of various scales (50-1200 lb) are evaluated in the presence of a 20 kt turbulent headwind. Nominal flight controllers are tuned for each aircraft to meet HQ specifications, where the controllers will reject disturbances to the aircraft groundspeed, attempting to maintain zero groundspeed. Froude-scaling is performed to scale the handling qualities specifications of the smaller aircraft, utilizing the additional agility. The nominal and scaled controllers are then compared in rejecting disturbance due to the turbulent headwind. Non-linear dynamic simulations are performed to evaluate the rigid body response of each aircraft, finding that scaling the flight controllers (on aircraft less than 1200 lb) reduces the pitch and roll attitude response so that all aicraft have similar peak-to-peak values. Following scaling, the peak-to-peak roll and pitch attitudes for all four aircraft are within 0.5? and 0.4? of each other, respectively. The improvement in disturbance rejection capabilities from scaling comes at the cost of 24% increase in the RMS current, and 2% additional current margin for the 50 lb aircraft (which is the largest increase). Overall, the increase in actuator activity due to scaling the controllers is less than the current margin required for maneuvering, resulting in the motors not needing to increase in size.

High-Fidelity Analysis of Six-Passenger Quadrotor Air Taxi Concept (Paper 34)
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Cameron Druyor, Li Wang

A code-to-code comparison has been performed for high-fidelity simulations of NASA's six-passenger quadrotor air taxi concept vehicle. The multidisciplinary simulations combine comprehensive rotorcraft dynamics with high-fidelity fluid dynamics obtained from an unsteady Navier-Stokes computational fluid dynamics code. An internal overset-grid assembler, Yoga, developed at the NASA Langley Research Center, is employed to efficiently handle the communications between component grids particularly for the present large-scale, unstructured-grid systems. The simulation results are then compared with those in the literature. A quantitative comparison of converged trim angles has been performed and normal force, chord force, and pitchingmoment coefficients are presented for qualitative comparison. Workflow changes to meet the unique demands of multirotor vehicle analysis are also discussed.

Parametric Investigation of Flow over a Rotor-Blown Wing using High-fidelity Simulations (Paper 58)
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Ullhas Udaya Hebbar, Farhan Gandhi, Onkar Sahni

This study models an infinite rotor-wing unit based on the CRC-20 quad-rotor bi-plane at an angle of attack of 8? and rotor modeled using the actuator line method (ALM). Parametric variations to the rotor-wing geometry are considered. These include rotor-wing chordwise separation, rotor-wing vertical offset and rotor-rotor spanwise separation. Large eddy simulation (LES) and delayed detached eddy simulation (DDES) approaches with and without the transition model are used to analyze the baseline configuration. DDES with the transition model is found to compare well with LES and is selected for the parametric study to balance the computational cost. Compared to isolated wing and rotor cases, baseline rotor-wing case shows 5.46% lower power loading, 14.42% higher lift and 4.45% higher L/D ratio. From the parametric study, varying the rotor-wing chordwise spacing did not significantly influence rotor power loading but placing rotor further from the wing improved L/D ratio by 7.64% compared to baseline due to reduction in sectional drag. The rotor-wing vertical offset cases show that placing the rotor below the wing significantly reduces the L/D ratio while placing it above yields similar L/D ratio to baseline but lowers the power loading by 6.69%. Finally, the spanwise rotor-rotor separation cases show that higher separation yields a 5.66% improvement to L/D ratio with no effect on the rotor power loading, again due to reduction in sectional drag.


Handling Qualities

Handling Qualities I

Technical Session A: Tues. May 10, 2022 - 8:00 AM to 12:00 PM

Analysis of Handling Qualities for an Urban Air Mobility (UAM) eVTOL Quadrotor with Degraded Disturbance Rejection and Control Response (Paper 1285)
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Jeremy Aires, Shannah Withrow-Maser, Allen Ruan, Carlos Malpica, Stefan Schuet

A piloted handling qualities study of urban air mobility (UAM) electric vertical take-off and landing (eVTOL) quadrotors was performed utilizing the Vertical Motion Simulator (VMS) facility at NASA Ames Research Center. Rotor speed and variable pitch-controlled variants of a six-passenger conceptual design vehicle were assessed with different levels of degradation to control response and disturbance rejection bandwidth (DRB) in the heave axis. In previous work, preliminary trends across several handling quality rating categories reflected the effects of these degradations. Additionally, the impact of using different test standards and turbulence on the ratings were discussed. This paper elaborates on those results, but also provides insight into unexpected trends observed during the study including: a disharmony in attitude response, subpar ratings for the baseline Level 1 performance vehicle, and excessive drift and yaw couplings observed in a lateral reposition maneuver. Moreover, shortcomings of the handling quality scales and comparisons of power consumption among the vehicles in the various test conditions are presented.

Evaluation of Helicopter Ship Deck Landing Control Laws in Piloted Simulations (Paper 1148)
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Arti Kalra, Alexander Štrbac, Malte-Jörn Maibach

This paper describes the development and implementation of newly designed helicopter ship deck landing control modes and their evaluation in a piloted simulation study. The ship deck landing modes are embedded in a modelfollowing controller architecture. The employed control design is a complete model-following control system which imposes the desired command model dynamics on the controlled helicopter. Different command types combined with various hold functions are implemented to make the task easier for the pilots. Three basic command types and three advanced command types, one without ship communication and two with ship communication, are implemented. A piloted simulation study was performed in a simulator to evaluate and compare the implemented control modes within a complete maritime scenario design. The evaluation of control modes is based on the success of helicopter ship deck landings which is assessed by a quantitative as well as a qualitative assessment. Simulation results demonstrate that the advanced command types improved the task performance as well as reduced the pilot workload extensively in comparison to the basic command types.

Evaluation of Pilot Assistance Systems for Helicopter Ship Deck Landing (Paper 1147)
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Alexander Štrbac, Malte-Jörn Maibach, Daniel Greiwe, Arti Kalra, Anthony Gardner

The operation of helicopters on ships is one to most challenging tasks due to adverse weather conditions, the lack of visible cues, turbulent airwakes behind the ship and a moving confined landing spot on the ship. Currently, only a very limited number of pilot assistance systems are available to ease helicopter ship deck landings. The focus of this paper is the evaluation of a Head-Down Display (HDD), a Head-Mounted Display (HMD) and two different Attitude Command Attitude Hold (ACAH) flight control architectures for ship deck landings based on piloted simulation. A ship deck landing scenario at the research flight simulation facility Air Vehicle Simulator (AVES) has been extended to include turbulent ship airwakes from high-fidelity Computational Fluid Dynamics (CFD). The pilot assistance systems have been implemented at the simulator and evaluated by four helicopter pilots. In particular, the results show a favorable potential of the Head-Mounted Display and the flight control architectures.

Flight Test of Scaled MTEs for VTOL Certification using an Ultralight Coaxial Helicopter (Paper 16)
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Tim Jusko, Michael Jones, Benedikt Grebing

New methods are currently being investigated to perform certification of future electric Vertical Take-Off and Landing (VTOL) aircraft. Due to the expected mission requirements, classical certification methods do not lend themselves to this type of vehicle. One aspect being investigated is a new approach to evaluate Handling Qualities (HQs) as means of compliance demonstration. The EUROCAE Working Group 112 has proposed new Flight Test Manoeuvres (FTM) that could be used for this purpose. These FTMs are scaled based on vehicle geometry. This paper presents first results obtained from a flight campaign to test a set of the proposed FTMs. The flight tests were performed with a coaxial ultralight helicopter and conducted at airport Obermehler (EDCO) in Germany. One pilot flew seven proposed FTMs. Pilot feedback and analysis of results are used to propose modifications to the FTMs.

Handling Qualities Considerations in Control Allocation for Multicopters (Paper 1186)
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Christina Ivler, Will Hunter, Emily Vo, Kate Russell, Carlos Malpica, Shannah Withrow-Maser

Due to multicopters' role in advanced air mobility, there has been an increasing interest in their handling qualities. This paper investigates the effect of rotor speed control allocation of multicopters on handling qualities. Typically, multicopter aircraft are trimmed with equal speed on each rotor. The force generated by each rotor can be modeled, where thrust and torque of the rotor are related to the square of the rotor speed. The dynamic equations indicate that trimming pairs of motors at higher speeds results in an increase of control effectiveness in the associated axis, improving bandwidth and disturbance rejection. A tradeoff occurs where power usage is no longer minimal at trim, but handling qualities are improved in that control axis. A blade element model for multicopter, RMAC, was used to simulate how this off-nominal control allocation would affect the dynamics of a multicopter. System identification flight tests were used to validate the trends seen in RMAC. Control systems were optimized to provide improved disturbance rejection bandwidth in the axis associated with increased rotor speed. Unmanned mission task elements were performed in flight test, showing improved tracking in the lateral axis by using off-nominal mixing to increase trim speed on the rolling motors. The method was also investigated on a full-sale UAM hexacopter, which showed similar trends, but only small changes in the dynamics were achievable because of significantly smaller power margin as compared to the small scale hexacopter.

Hover Dynamics & Flight Control of a UAM-Scale Quadcopter with Hybrid RPM & Collective Pitch Control (Paper 1177)
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Ariel Walter, Michael McKay, Robert Niemiec, Farhan Gandhi, Tom Berger

Hover trim and dynamic analyses were performed on a UAM-scale quadcopter with both variable rotor speed and variable collective blade pitch. The bare-airframe dynamics were first considered at three different hover trim points, where power consumption is increased to improve authority. The control and stability derivatives were examined at each trim point and an increase in base RPM caused increased authority for pitch inputs (and decreased authority for RPM inputs) in thrust-dominated axes. Explicit model following control laws wre then optimized using CONDUIT to meet ADS-33E-PRF handling qualities specifications. Design margin optimization was then performed on each axis. Heave and yaw responses of the linearized system were examined for the three trim points with either RPM or pitch control. It was found that pitch-control outperformed RPM-control in heave, while the opposite was true for yaw. Hybrid control mixing was considered using a complementary filter, so that it uses pitch actuators for short-term responses, and RPM for trim. Effects of changes in motor time constant and complementary filter cutoff frequency were examined. The benefits of hybrid control were demonstrated through simulations that involved transition between trim points. Hybrid control required lower maximum power during thrust-driven maneuvers by allowing the aircraft to accelerate using pitch actuators, and recovers the original stall margin by using the rotor speed to re-trim. For a drop-off of 176 lbs of payload, hybrid control provided 5.0% lower trim power than pitch control with the reduced weight. Hybrid control also allowed a 3.9% reduction in power compared to pitch control at a flight speed of 30 kts.

Toward a UAS Handling Qualities Specification: Development of UAS-Specific MTEs (Paper 1187)
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Christina Ivler, Kate Russell, Anthony Gong, Tom Berger, Mark Lopez

Unmanned Aerial Systems (UAS) are becoming more prominent in the airspace and offer solutions to the limitations of manned rotorcraft. The ability to perform autonomous and/or remotely piloted tasks make them popular for both private and public use. As UAS become commonplace, the need to define handling qualities requirements is a critical task. This paper builds upon previous work towards a VTOL-UAS handling qualities framework to propose two UASspecific maneuvers along with mission-appropriate performance specifications. Flight test results on the University of Portland hexacopter (Group 1 UAS) were collected to validate performance specifications for both the UAS-specific maneuvers and Froude-scaled ADS-33E-PRF mission task elements. A new performance metric based on Froude dynamic scaling of the ADS-33E-PRF attitude bandwidth metric was also developed. This Froude-scaled Level 1 attitude bandwidth criteria was then evaluated in flight test on the UP Hexacopter as a predictive criterion for Level 1 MTE performance. The Synergy 626, a single main rotor helicopter Group 1 UAS, was used to further validate the MTE performance specifications and the scaled attitude bandwidth results, showing this work is applicable beyond multirotor configurations. The key outcomes of the work are the proposed UAS-specific maneuvers, the validation of performance specifications, and validation of the Froude-scaled ADS-33E-PRF Level 1 attitude bandwidth metric as a predictive metric.

Handling Qualities II

Technical Session B: Wed. May 11, 2022 - 8:00 AM to 12:15 PM

An Explanation of Unanticipated Yaw Phenomenom on Helicopters (Paper 1125)
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David Ferullo

The helicopter community has been plagued during the latest forty years by accidents due to unanticipated yaw, also called Loss of Tail rotor Effectiveness (LTE). Thanks to official reports, the first part of this paper will be devoted to the genesis of this problem and recommendations provided at that time. An analysis of accident databases, available in different countries, is astonishing with 18 accidents occurring on average each year. Surprisingly, three out of four accidents took place close to the ground where the recovery actions recommended in AC 90-95 are not applicable. A part of the mystery remains and especially no clear explanation of the phenomenon was provided at that time. The benefit of this paper is to provide an explanation of the phenomenon using the pedal curve approach. During hovering flight with wind, the pedal curve is useful to understand how and under what conditions unanticipated yaw can develop. The lack of effectiveness of the tail rotor perceived sometimes by the pilot is a consequence of a too limited action on the pedal during the recovery maneuver. Finally, flight tests carried out on AH130 confirm the explanation given thanks to the pedal curve and provide a clear recommendation to recover from such situation. A quick and large action on the pedal (up to the stop if necessary) in the opposite direction of the yaw is the only possible response to unanticipated yaw, regardless of the yaw rate reached during the phenomena. Authorities, Manufacturers and Schools must convey the same message towards the pilot community in order to reduce the number of accidents and to restore confidence in the effectiveness of the tail rotor.

Load Alleviation Control vs. Load Limiting Control: Pros and Cons. (Paper 1255)
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Chams Eddine Mballo, J.V.R Prasad

The development of life extending control schemes is important to ensure the longevity of critical helicopter components. Perhaps, equally important is the ability to choose the appropriate component life extension scheme for a specific application. In this paper, the benefits and drawbacks associated with two control schemes for component life extension are discussed: namely Load Alleviation Control (LAC) and Load Limiting Control (LLC). Understanding such benefits and drawbacks can help to not only motivate future research that will improve these schemes but also in the selection of a suitable scheme for a specific application.

Neuromuscular Response Comparison for Center and Side Stick Positions (Paper 1216)
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Edward Bachelder, Jeff Lusardi, Bimal Aponso

Four Army pilots participated in a simulation experiment to examine the influence of stick location and sensitivity (gain) on pilot neuromuscular (NM) response, performance, and workload. The experiment employed an active inceptor that was positioned between the pilot's legs or, adjacent to the pilot's right side with an armrest. Two stick sensitivities that varied by a factor of four were evaluated using a single-axis compensatory tracking task in the longitudinal and the lateral axes. The experiment results identified two prominent NM modes at roughly 10 and 25 rad/s; stabilizing the elbow implicated the 10 rad/s mode with forearm motion, and wrist/finger motion with the mode at 25 rad/s. With the longitudinal task using the low stick gain, workload ratings were significantly higher for the side stick than the center stick. A preliminary analysis indicated that the greater resisting force between the forearm and non-compliant armrest (side stick configuration) relative to the resisting force between the forearm and leg (center stick configuration) may be a key factor in the higher workload. This suggests that a side stick's gain in the longitudinal axis should be a function of task such that control displacements are generally small. Overall workload during manual tasks would benefit if this approach were applied to all control axes. A second study was conducted to investigate the significant effect of stick gain on crossover frequency that was observed in the first experiment. These results showed that the ratio of stick rate to stick amplitude is directly proportional to crossover frequency, and that a tradeoff between rate and amplitude reflected by changing crossover is similar to the phenomenon described by Fitts Law, where manual movement time is related to the distance travelled. The implications for design are that stick travel can affect performance much more than stick force provided the stick dynamics do not adversely interact with the NM system. It is recommended that the feel system mode should lie between the forearm and wrist/finger NM modes, and that stick sensitivity selection should be based on mission and operating environment.

NGCTR-TD Tiltrotor Autorotation Numerical Investigation (Paper 1120)
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Fabio Riccardi, Matteo Pecoraro, Andrea Ragazzi, Valentina Giuliani, Giuliano Prando, Federico Del Grande

Within the Clean Sky 2 Fast Rotorcraft platform, a Next Generation Civil Tilt-Rotor Technology Demonstrator (NGCTR-TD) is under development. The design phase has been successfully completed and first flight phase is planned within 2023. As part of design verification, the capability to perform a safe landing procedure after losing all engines power is investigated, with dedicated analysis planned in order to improve the transport category performance requirements, as verified for Legacy tilt-rotor platform. Such procedure consists in 4 different steps when starting from AP mode, and the analysis are focused on VTOL steady autorotation phase, being the one where tilt-rotor configuration shows higher peculiarity with respect to helicopter configuration, due to the presence of lifting surfaces. As first, preliminary considerations from helicopter theory are cited, and tilt-rotor peculiarities listed. Then, adopting the developed FlightLab numerical model, different solutions are investigated, starting from characterization in airspeed for a baseline configuration. Different sensitivity analyses are conducted to map the major effect for the specific topic, and best trade-off autorotation configuration/control strategy is obtained for NGCTR-TD. Such solution is investigated in the complete Weight/CG envelope to ensure control strategy robustness.

Seeking Lift Share: Design Tradeoffs for a Winged Single Main Rotor Helicopter (Paper 62)
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Mark Lopez, Ashwani Padthe, Emily Glover, Tom Berger, Eric Tobias

A single main rotor helicopter with a wing is one proposed design for the U.S. Army Future Vertical Lift (FVL) Capability Set 1 (CS-1). While there are many single main rotor aircraft, there are a few with stub wing designs and even fewer with a large wing appropriate for high speed operating conditions expected for FVL. In this work, a flight dynamics model of generic single main rotor helicopter with a wing is designed to meet the FVL CS-1 class of aircraft. The generic design is derived from open literature and well studied UH-60A, Bo-105, and XV-15 aircraft. The design is implemented in a blade element flight dynamics model which is subsequently used to develop a preliminary flight control design. The complete aircraft design approach is detailed and design tradeoffs with respect to trim, flight dynamics, and maneuverability are presented.

Towards Handling Qualities and Automation Assessment for Certification of eVTOL Aircraft (Paper 1261)
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Michael Jones, David Klyde, Martin Schubert, David Mitchell, David Sizoo, Ross Schaller, Johannes van Houdt, Michael Feary, David Webber, Rick Simmons

Led by the Federal Aviation Administration (FAA), research is being conducted to develop certification means of compliance methods for small aircraft including the emerging class of advanced air mobility vehicles that will transition from thrust-borne flight to wing-borne flight and vice versa. These vehicles typically feature highly augmented fly-by-wire flight control systems that offer advanced flight control modes designed for simplified vehicle operations and in many cases completely autonomous flight operations. The emerging means of compliance methods will feature special classes of flight test maneuvers that address the assessments of the vehicle system, flying qualities, handling qualities, and increasing automation. Inspiration for this approach comes from the Army's Aeronautical Design Standard ADS-33-E-PRF that introduced a mission-oriented approach to address handling qualities via predictive requirements and specified flight test maneuvers - Mission Task Elements. Whereas ADS-33E-PRF prescribes methodologies through a procurement process, the FAA seeks a means to determine that a minimum safety standard has been met by an aircraft presented for certification. Thus, the flight test maneuvers as conceived here form a holistic approach to determine acceptable handling qualities via a process that encompasses the period between Type Certification (TC) application to the point where the TC is granted. To facilitate the process further, a team led by Systems Technology, Inc. (STI) is developing a flight test guide that will support the safe and repeatable execution of these flight test maneuvers. This paper provides an overview of not only the holistic approach to handling qualities assessments, but also the key elements of the flight test guide.


Health and Usage Monitoring Systems (HUMS) - Condition Based Maintenance (CBM)


Technical Session D: Thurs. May 12, 2022 - 10:15 AM to 12:15 PM

Ensemble Integration Methods for Load Estimation (Paper 1124)
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Catherine Cheung, Zouhair Hamaimou

Helicopter component load estimation can be achieved through a variety of machine learning techniques and algorithms. To increase confidence in the load estimation process, ensemble methods are employed combining multiple individual load estimators that increase predictive stability across flights and add robustness to noisy data. In this work, several load estimation methods are applied to a variety of machine learning algorithms to build a large library of individual load estimation models for main rotor yoke loads from 28 flight state and control system parameters. This paper explores several ensemble integration methods including simple averaging, weighted averaging using rank sum, and forward selection. From the 426 individual models, 25 top models were selected based on four ranking metrics, root mean squared error (RMSE), correlation coefficient, and interquartile ranges of these two metrics. All ensembles achieved improved performance for these four metrics compared to the best individual model, with the forward selection ensemble obtaining the lowest RMSE, highest correlation, and closest load signal prediction visually of all models.

Helicopter Gearbox Mechanical Classification based on Vibration Pattern Recognition (Paper 1137)
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Valerio Camerini, Valentin Aubin, Ammar Mechouche

In this paper, unsupervised data analysis methods are used in order to characterize helicopter gearbox mechanical components based on their vibration pattern. Differently from classical vibration health monitoring systems applications, where the objective is to detect early symptoms of impending mechanical degradations, the proposed methodology aims at characterizing the components vibration in their nominal state. The purpose is to identify systematic manufacturing or installation deviations affecting the vibration signature. This allows on one hand for a proactive analysis of the consequences of such deviations, and on the other, it helps explaining vibration indicators variability within traditional health monitoring applications. By combining features extraction, data reduction and clustering techniques, it is shown how it is possible to detect patterns in the vibration signatures from a set of helicopter pinions, ultimately leading to characterizing their main manufacturing differences. These first results give encouraging perspectives for further developments.

Rotor Fault Detection and Identification on Multicopters based on Statistical Time-series based Data-Driven Methods: Experimental Assessment via Flight Tests (Paper 1265)
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Airin Dutta, Jianxi Wang, Fotis Kopsaftopoulos, Farhan Gandhi

A robust framework for fault detection and identification of rotor faults in multicopters is validated with data from experiments with a quadcopter and a hexacopter. The rotor fault detection and identification methods employed in this study are based on excitation-response signals of the aircraft under atmospheric disturbances. A concise overview of the development of the statistical time series model for healthy aircraft using the aircraft attitudes as the output and controller commands as the input is presented. This model is utilized to extract quality features for training a simple neural network to perform effective online rotor fault detection and identification. A proper justification of choosing the method of time-series assisted neural network has been given. It is shown a statistical time-series assisted neural network employed for online monitoring in the quadcopter and hexacopter achieves accuracy over 96% and 95%, respectively. It is effective under gusts and experimental variability encountered during outdoor flight and is sensitive to even partial loss of rotor thrust.

Validation of Probabilistic Regime Recognition and Damage Estimation with Large Fleet Datasets (Paper 1136)
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Dakota Musso, Jonathan Rogers, Subhasis Sarkar

Regime recognition (RR) and damage estimation is a critical step in the creation of usage and damage spectra and in development of component replacement timelines. While a variety of methodologies exist for regime recognition based on deterministic classification, recent work has demonstrated the benefit of approaching the problem of regime classification and damage estimation from a probabilistic standpoint. However, while studies have shown the successful deployment of probabilistic algorithms on simulated data, little work has been done to validate their use on large fleet datasets that contain actual flight data. This paper seeks to address two main points. First, the damage estimation scheme proposed in prior work is extended to incorporate uncertainty in the component damage rates, allowing for a complete treatment of the damage estimation problem that fully captures all major sources of uncertainty. Second, this paper exercises the probabilistic regime recognition and damage estimation algorithms published in previous work on 500 hours of actual fleet data, validating the efficacy of the approach. Detailed analysis of the results show that the probabilistic algorithms are effective in estimating flown regimes and component damage fractions and offer unique insights that are not available from comparable deterministic algorithms.


Technical Session E: Thurs. May 12, 2022 - 1:30 PM to 5:30 PM

Active-Sensing Acousto-Ultrasound-based Rotorcraft Structural Health Monitoring via Adaptive Functional Series Models (Paper 1286)
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Fotis Kopsaftopoulos, Peiyuan Zhou, Shabbir Ahmed

In this work, the experimental assessment of the damage diagnosis performance of a full-scale rotorcraft blade is performed via stochastic time-varying time series models in the context of active sensing acousto-ultrasound guided wave-based damage detection and identification scheme. Ultrasonic guided waves, that are dispersive in nature, are represented via functional series time-varying autoregressive (FS-TAR) models. Next, the estimated time-varying model parameters are employed within a statistical decision making framework to tackle damage detection and identification under predetermined type I error probability levels. Damage detection and identification based on coefficients of projection (COP) as well as time-varying model parameters are shown. Both damage intersecting and non-intersecting paths are considered in a full-scale rotorcraft blade as well as in an aluminum plate in pitch-catch configuration for the complete experimental assessment. The detailed damage diagnosis results are presented and the method's robustness, effectiveness, and limitations are discussed.

Development and Integration of a Fiber-Optical Sensor System for Rotor Blade State Observation (Paper 1266)
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Florian Berghammer, Verena Heuschneider, Manfred Hajek

Health and Usage Monitoring Systems (HUMS) for helicopter rotor blades arise the demand for non invasive surveillance methods for slender composite structures. Integrating optical sensor systems within the manufacturing process of rotor blades holds challenges in fiber bonding, handling, the layup process and curing. Examining the steps of fiber integration using a co-bonding approach, the present work enlightens the challenges faced while manufacturing instrumented rotor blades. Apart from sensor integration it is crucial to show feasibility of integrated optical sensing networks. Measuring strain from Fiber Bragg Gratings (FBG), a deformation algorithm is applied to determine blade deflection along the airfoil section of a rotor blade using a NACA 0012 profile. Results from static deformation tests are used to validate the algorithm. In conclusion an experimental modal analysis is conducted and results obtained by accelerometers are compared to the ones from finite element simulation and to the eigenfrequencies measured by the FBG system.

Exploring Bearing, Shaft and Gear Monitoring using an Optical Fiber Fabry–Perot Bragg Grating Cavity (Paper 1164)
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James Washak, Paul Dunning, Christopher Holmes, Ling Wang

This paper presents a Fabry-Perot cavity Bragg grating (FP-FBG) technique for vibration monitoring. The FP-FBG is created by placing two identical and uniform Fibre Bragg Grating (FBG) with the same Bragg wavelength to produce a Fabry-Perot (FP) cavity between them. This has the effect of enhancing the vibration monitoring through tailorable enhancement of the spectral response, which scales proportionately with relative grating spacing. In this work, a single fibre containing four FPFBG of varying cavity sizes (4mm-72mm) was fabricated and tested on both a square meatal plate mounted on a mechanical shaker and bearing test rig to test the overall sensitivity of FP-FBG. The cavity size of the FP-FBG and physical positioning was investigated. The findings so far indicate that overall, FP-FBGs are effective at vibration monitoring, and initial tests are shown to detect the shaft rotation speed from a ball bearing with no faults. Future work shall look at FP-FBG capability to detect faulty bearing conditions and design parameter further.

Improving the Performance of Bearing Analysis (Paper 25)
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Eric Bechhoefer, Josh Kethan

Health and usage monitoring systems (Aka “HUMS”) have typically not been associated with CFR Title 14 type 27 normal category rotorcraft (weighing less than 7000 pounds, with a seating capacity of 9 or less), due in part to the cost of such systems relative to the asset value. This paper describes performance improvements to HUMS bearing diagnostics methodologies to enhance functionality and improve the business case for HUMS.




Technical Session E: Thurs. May 12, 2022 - 1:30 PM to 5:30 PM

A Brief History of Kaman Innovative VTOL Aircraft and System Designs (Paper 1207)
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Jacques Virasak

Nowadays, helicopter manufacturers have been acquired or merged within conglomerated aerospace giants worldwide. However, only a few companies remained independent from their original owners, such as Kaman Aerospace Corporation. Thus, on December 12, 2021, Kaman celebrated its 75th anniversary. This paper revisits some VTOL aircraft and system designs that Kaman designers have achieved since 1945.

Ernst Otto Schmidt 1872-1938: Passionate Helicopter Inventor (Paper 4)
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Berend G. van der Wall, Gert Dieter Schmidt, Klausdieter Pahlke

Ernst Otto Schmidt is practically unknown in the row of aeronautical and especially helicopter pioneers. This is be-cause his profession was a decoration painter and he earned good reputation at the time. However, he also was fasci-nated by the idea to vertically take off and land with a machine, experimented more than half of his life with models and held two patents for his designs (issued 1924 and 1939). Oral reports state that one demonstration flight was performed in 1929 and a U.S. businessman offered funding that Schmidt denied. Schmidt was afraid of being laughed at and kept his experiments secret for a long time. Without a wealthy sponsor his designs never came to fruition, partly because he also lacked any aeronautical engineering education. However, some ideas he had were realized by others at much later times. A book about Ernst Otto Schmidt was published by his grandson, the co-author of this paper. Additional personal correspondence from his family archive and further material found by the author are provided here for the first time.

God's Machine: The Miracle at Gander (Paper 23)
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Paul Fardink

After crossing the North Atlantic Ocean in the early morning hours of September 18, 1946, a Sabena Airlines DC-4 OO-CBG, carrying 37 passengers and seven crew members, crashed in dense fog and drizzle 22 miles short of the Gander, Newfoundland Airport. Considered by many to be the first major civilian airliner crash, it was also the first major U.S. Coast Guard rescue mission with helicopters. Working together, the Coast Guard, the U.S. Army, and Canadian personnel used Sikorsky helicopters and fixed-wing aircraft to successfully complete this heroic rescue of 18 survivors in what the media considered “the Miracle at Gander.” This inter-service and civilian team exhibited unrivaled courage, innovation, and compassion, resulting in a nearly flawless operation. What happened at Gander would also serve as the turning point for future Coast Guard rescue mission use of the helicopter, which many, then and since, have considered to be “God’s machine.”


Manufacturing Technology and Processing

Manufacturing Technology I

Technical Session D: Thurs. May 12, 2022 - 10:15 AM to 12:15 PM

Air Entrapment Prediction in Composite Manufacturing (Paper 1154)
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Michael West, Yih-Farn Chen, Benjamin Cournoyer, Felix Nguyen

Rotorcraft are highly engineered systems designed to perform missions in extreme locations around the world. To accommodate harsh environments and non-trivial cyclical loading conditions, components are commonly fabricated using advanced materials including composites. Dynamic load bearing thick laminate components are particularly challenging to produce due to the potential for entrapped air to be present within a fully cured component. A proof-of-concept model was developed to predict entrapped air and out-gassing. Thermo-chemical and flow-compaction finite element analyses were completed using a combination of commercial-off-the-shelf software and a custom proof-of-concept model developed by Convergent Manufacturing Technologies, Inc. Predicted results were compared against defects observed within laminated composites fabricated in a laboratory environment. Initial finite element predictions agreed moderately well with entrapped air observations.

Digital Transformation of a 6000-Year-Old Process (Paper 1149)
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Heather Woodworth

Integration of advanced technologies in the casting commodity through digital transformation efforts is essential to future success of rotorcraft transmission castings. This paper reviews the advanced technologies that have been integrated into the casting process for transmission housings. The transformation in casting designs, solidification modeling, mold fabrication, casting techniques, thermal and mechanical post processing, inspection, and quality management are all impacted by new technologies and driven by the continued digital thread in the manufacturing process. The casting manufacturing process is 6000 years old, but the complexity of aircraft requirements and part designs has outpaced capability of standardized casting processes. Digital transformation driven technologies create repeatable processes that reduce production scrap and development lead times.

Manufacturing Innovation for Bell's Future Factory (Paper 68)
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Amber Pike

Bell Textron's Manufacturing Technology Center (MTC) is an innovative proving ground, commissioned on March 25, 2021. At the MTC, Bell tests and refines technologies to support the manufacturing readiness and build processes for future vertical flight aircraft. The MTC is designed to address the biggest rotorcraft manufacturing challenges, including cost, schedule, and quality. The facility is connected and controlled by a network of IT, Industrial Internet of Things (IIoT) and cybersecurity systems that manage its processes and operational data. The MTC provides a breadth of capability and houses both manufacturing and quality inspection processes in one site that produces composite blades, gears, cases, tooling, and more. The MTC focus areas include manufacturing of rotorcraft dynamic components and advanced composites, but also include integration for aircraft assembly, digital infrastructure, and digital manufacturing. This paper shows how Bell's advances in manufacturing support high-rate rotorcraft production and optimal manufacturing.

Paint Scheme & Marking Digital Transformation (Paper 48)
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Mary DaSilva

(No abstract available.)

Manufacturing Technology II

Technical Session E: Thurs. May 12, 2022 - 1:30 PM to 5:30 PM

Computational Analysis of a Model Coaxial Rotor Hub Wake (Paper 1233)
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Forrest Mobley, James Coder

A model counter-rotating coaxial rotor hub in free-air was simulated using computational fluid dynamics as a comparison against experimental results. The simulation is performed using NASA's OVERFLOW 2.3d Reynolds averaged Navier-Stokes solver, and flow conditions are based on experiments performed in the 12-inch diameter water tunnel at ARL Penn State. Surface forces were examined, and harmonics in this data were computed and analyzed. The turbulent wake of the rotor hub was analyzed using frequency content as well as turbulent quantities related to the production and transport of turbulent kinetic energy. The lift spectrum showed different dominant frequencies for each counter-rotating hub, and the drag spectrum showed the expected dominant frequency for both parts of the hub. Frequency content of the velocity components in the wake showed positional biases towards the advancing side of each hub, opposing the results from past analyses of similar single rotor hubs. Reynolds stresses showed similar positional biases, and were also consistently concentrated in relatively small areas within the wake. The individual wakes of the hubs did not show signs of interaction until the far wake.

Sikorsky Leakage Criteria and Prevention Methods (Paper 1319)
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Altayeb Hashim, Joe Buzzeo, Cody Donecker, Anthony Chory

Helicopter gearbox seals typically exhibit a zero-leakage condition, especially on new helicopters. Improved lip seal materials and molding techniques, special attention to grinding seal liners, application of break-in grease, and smooth lead-in chamfers to prevent assembly damage have all contributed to excellent seal performance. Operational leakage criteria, especially for military aircraft is established for completion of missions and exceeds what is expected by operators of new helicopters or by crew chiefs and pilots who are accustomed to zero leakage or minor seal wetness. There are, on occasion, conditions where wetness or drips occur that are questioned by operators because a norm has been established. There are also occurrences where miniscule scratches in seal liners barely visible by the naked eye or microscopic nicks in seal rubber can allow minor seepage. Seal replacement can result in removal of rotors or shafting, requiring reservicing with all new lubricants and serviceability inspections, which can cause mission or delivery delays and put an extra burden on maintenance crews. This paper describes methods to eliminate leakage and provides recommended leakage criteria for safe and reliable operation.

Tolerance Relief on Transmission Castings for Producibility (Paper 1318)
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Brad Gimbutis, Griffin Palmer, Justin Beardsley, Holly Ann Quinn

Helicopter gearbox transmission castings must be designed for adequate strength, adequate stiffness for gear meshes and flight loads, clean-up of all machined surfaces, adequate edge distance for all clamped connections and minimum weight. Castings manufacturers generally request relatively large tolerances due to the nature of the casting processes in the order of +/- 0.100 inch while designers will require casting tolerances in the order of +/- 0.030 inch for cast surfaces. Excess casting material can lead to assembly interferences and excess weight while insufficient material can lead to thin wall conditions, lack of damage repair capability and potential performance issues. This paper describes how skewed (unilateral or unequally disposed) casting tolerances can eliminate casting rejections, eliminate Material Review Board (MRB) delays, eliminate reworks, ensure proper performance, and have negligible impact on weight.


Modeling and Simulation

Modeling and Simulation I

Technical Session A: Tues. May 10, 2022 - 8:00 AM to 12:00 PM

Comprehensive Analysis of a Control Loading System for a Rotary Wing Flight Simulator (Paper 1166)
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Giorgio Guglieri, Marco Rinaldi, Pierluigi Capone

This paper presents the design methodologies of a Control Loading System (CLS) drive model for the Research and Didactics Simulator (ReDSim) of the Zurich University of Applied Sciences (ZHAW). The analysis of the CLS hardware and interface software is presented. The CLS drive model is intended as an irreversible flight control system of a rotary wing aircraft's collective control, with trim system and force feedback capabilities, mainly as a function of the lever's position. The CLS motor's control parameters are set to fit the application's performance requirements. The drive model is developed in MATLAB/Simulink and is integrated with the XV-15 tiltrotor model of the ReDSim. Two different system identification techniques, the Least Squares and a convex relaxation-based Set-Membership approach, are used to estimate the parameters of the CLS hardware and consequently assess its fidelity to a model of reference. The implementation in the ReDSim has been rated by the pilot according to the Cooper-Harper handling qualities rating scale.

Dual-Engine Failure Emergency Procedure Development Using an Engineering Simulator (Paper 1190)
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Ilgaz Doga Okcu, Kaan Sansal, İlker Uysal, Gokhan Virlan

Assessment of rotorcraft performance following an engine failure is necessary to ensure its safe operation. During the rotorcraft certification process, demonstration of compliance to power-off landing requirements is one of the most challenging flight test phases. For these types of flight activities, using a simulation approach not only provides safety, reduction of cost and effectiveness but also allows training of the flight test team beforehand. Since loss of engine power is regarded as one of the most critical failure scenarios, extensive training of the flight test team is required to improve autorotation performance, i.e., managing available energy at the rotors. This paper is focused on evaluation of autorotation characteristics of the T-625 light utility helicopter in a simulator environment. Experience gained from this work guided both design engineers and flight test team towards initial preparation of dual-engine failure emergency procedure in the rotorcraft flight manual.

Evaluation of Simulator Cueing Fidelity for Rotorcraft Certification by Simulation (Paper 1123)
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Philipp Podzus, Jur Crijnen, Michael Jones, Stefan van't Hoff, Paul Breed

Before their entry into service, newly developed rotorcraft must go through an extensive certification process in order to receive a type certificate from the certification authority. A vital and long-lasting phase of this process is the compliance demonstration. This phase involves a large amount of flight testing, which leads to high expenses for the rotorcraft manufacturer and can be considered as high-risk when it comes to rotorcraft safety, especially for scenarios including control system or engine failures, as in the case of a category-A rejected take-off (CAT-A RTO). The Rotorcraft Certification by Simulation (RoCS) CleanSky2 research project aims to reduce the amount of flight testing required for compliance demonstration by using flight simulation, to achieve an increase in safety (less hazardous situations) and effectiveness, and a reduction in certification duration and costs. Within the project, pilot-in-the-loop simulator test campaigns were conducted at DLR and NLR, investigating the visual cueing fidelity required for performing a CAT-A RTO scenario. Emphasis was put on varying the available field of view (FoV) for the pilot and investigating the suitability of virtual reality (VR) devices. Subjective and objective results from these simulator campaigns, as well as pilot comments are presented in this paper.

Global Model Identification for a Coaxial Helicopter (Paper 1199)
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Barzin Hosseini, Franz Sax, Julian Rhein, Florian Holzapfel, Lukas Maier, Aaron Barth, Manfred Hajek, Benedikt Grebing

In this paper, the Blade Element Momentum Theory (BEMT) hast been applied to setup a nonlinear model structure for a coaxial helicopter with a maximum take-off weight (MTOW) of 600kg. The model includes unknown aerodynamic parameters, which are estimated using flight test data. An extensive flight test campaign, with a fully instrumented CoAx 600 helicopter has been carried out to generate the required flight test data for parameter estimation. In this study, we use the maneuvers performed in the vicinity of the hover flight condition for system identification. The nonlinear model can be trimmed and linearized for the analysis of the system dynamics and control design. This has been done for one trim point in the study and the corresponding pole plan has been provided.

Rotorcraft Pitch-Surge Motion Cueing Requirements for a Simulated Offshore Approach Task (Paper 94)
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Josephine Roscoe, Mark White, Steven Hodge, Gareth Padfield

The paper describes findings from a rotorcraft pitch-surge motion fidelity study conducted in the University of Liverpool's HELIFLIGHT-R flight simulator. Two test pilots flew a decelerating offshore approach maneuver during which a disturbance was encountered. The approach was flown without simulator motion as a 'baseline' case and with simulator motion using different combinations of pitch and surge motion filter coefficients. The pilot-awarded handling qualities and motion fidelity ratings for each motion configuration, and task performance and control activity data were gathered. The study concluded that for the approach flown without motion, the dominant driver of workload was vertical rate control, and this was most significant at airspeeds less than 50kts. When the approach was flown with a disturbance, good motion cues were judged to be achieved through careful selection of the pitch and surge motion filter gains, the most favored being a surge gain equal to that of the pitch gain. Additionally, pilots preferred pitch-surge cues that were in the correct sense, rather than being particularly 'strong'. Increasing the pitch gain was only beneficial with a moderate level of pitch break frequency of 0.4 rad/s.

System Identification and Stitched Modeling of the ADAPT&[trade] Winged Compound Helicopter Scaled Demonstrator (Paper 8)
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Samuel Nadell, Tom Berger, Christopher Dimarco, Mark Lopez

Frequency-domain system identification was performed for the ADAPT™ Winged Compound Helicopter Scaled Demonstrator, a 10% scale version of the Piasecki X-49A, at four flight conditions spanning its flight envelope. Since the aircraft has eight redundant control effectors – lateral cyclic, longitudinal cyclic, collective, Vectored Thrust Ducted Propeller (VTDP) RPM, rudder, differential flaperon, symmetric flaperon, and elevator – and exhibits a large amount of inter-axis coupling, the Joint Input-Output (JIO) Method was used for system identification in addition to the Direct Method. Based on the identified frequency responses, a hybrid model structure, which explicitly includes the coupled fuselage-rotor flapping dynamics and a first-order model for VTDP RPM lag, was used. State-space models were identified at each flight condition, and combined with trim data to form a full flight envelope stitched simulation model. A detailed analysis of the trends of the stitched model trim, stability and control derivatives, eigenvalues, and frequency responses was performed.

Modeling and Simulation II

Technical Session B: Wed. May 11, 2022 - 8:00 AM to 12:15 PM

A Comparison of High-Fidelity Simulation Approaches for Interactional Aerodynamics of Multirotor Systems in Forward Flight (Paper 1132)
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Abhishek Chopra, Richard Healey, Farhan Gandhi, Onkar Sahni

Recent advancements in distributed electric propulsion for urban air mobility applications have made interactional aerodynamics more common on modern rotorcraft designs. Rotor-to-rotor interactions are associated with complex flow features, and require high-fidelity numerical tools for adequate analysis and prediction. The computational cost associated with resolving these aerodynamic interactions can become prohibitively expensive, particularly when simulations over a multitude of operating conditions/configurations are desired. As an alternative to the typical bladeresolved DDES (BR-DDES) approach, an actuator line model with LES (ALM-LES) is considered for its high fidelity aerodynamic prediction capabilities at a reduced computational cost. In this study, flow field and rotor performance predictions using ALM-LES are compared to BR-DDES in order to evaluate the merits of ALM-LES for interactional aerodynamic analysis. Overall, the wake structure of the two-rotor system shows good agreement between the two methods. Integrated thrust is also predicted similarly, with a difference of 2.4% for the front rotor and 4.3% for the aft rotor. While integrated thrust is predicted well by ALM-LES, some discrepancies in sectional thrust are observed in areas with blade-vortex interaction (BVI). The vortex position compares well between the two methods, so the sectional thrust difference is tied to differences in vortex strength and how well ALM is able to represent a BVI. Sectional thrust differences are also observed on the aft rotor and are associated with secondary vortices convecting into the rotor plane. Despite differences in parts of the rotor disk with BVI, ALM-LES is shown to be capable of predicting the interactional aerodynamics of a two-rotor system in forward flight at about 1% the computational cost of BR-DDES.

A Multirotor Inflow Model Based on Combined Momentum Theory and Simple Vortex Theory (CMTSVT) for Flight Simulations (Paper 42)
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Feyyaz Guner

This paper extends the recently developed combined momentum theory and simple vortex theory (CMTSVT) multirotor inflow model for dynamic simulations. Previous comparison studies of the CMTSVT inflow model have shown that the model captures fundamental rotor-on-rotor inflow interference effects of various multirotor configurations in steady flow conditions. In this study, an output-coupled model structure widely used in the literature is followed to transform the formulation of the CMTSVT inflow model to a form that can be used in time marching simulations. Inflow components of the new formulation converge to the original values in steady flight conditions, which indicates a successful reformulation. The proposed model can be used for rapid estimation of vehicle performance as well as flight dynamics simulations and handling qualities analyses.

Design, Modeling, and Flight Dynamics Analysis of Generic Lift-Offset Coaxial Rotor Configurations (Paper 1131)
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Ashwani Padthe, Mark Lopez, Tom Berger, Ondrej Juhasz, Eric Tobias, Emily Glover

Lift-offset coaxial compound rotorcraft designs have been proposed for the U. S. Army Future Vertical Lift (FVL) programs. Without a need for the tail rotor and equipped with a pusher-propeller, the coaxial compound configuration is capable of achieving high airspeeds and perform maneuvers not feasible for the conventional rotorcraft. To facilitate an independent assessment of the lift-offset coaxial configurations by the U. S. Army, two designs are presented in this paper. Physics-based flight dynamic models are developed using open source data and implemented into the HeliUM-A comprehensive analysis code. On-axis frequency responses from the models are validated against available flightdata showing good agreement. The models are trimmed for a range of vehicle airspeeds, pitch angles, and rotor speeds. Trends in trim controls, hub loads, and power consumption are studied. Linearized time-invariant (LTI) models are extracted from the nonlinear HeliUM-A models. The LTI models are used to study trends in system eigenvalues, stability and control derivatives. Vehicle flight dynamics are examined through frequency response plots at various flight conditions.

Implementation and Linearization of a Rotor Simulation with a State-Space Free-Vortex Wake Model (Paper 6)
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Umberto Saetti, Joseph Horn

This paper describes the implementation and linearization of a free-vortex wake model in state-variable form as applied to a helicopter rotor. Following a detailed mathematical description, the wake model is implemented for a UH-60 rotor and tested in forward flight and for simple control inputs. A periodic solution to the wake model is found by time marching the coupled rotor and vortex wake dynamics. Next, linearized harmonic decomposition models are obtained and validated against nonlinear simulations. Order reduction methods are explored to guide the development of linearized wake models that provide increased runtime performance compared to the nonlinear and linearized harmonic decomposition wake models while guaranteeing satisfactory prediction of the periodic response of the wake.

Implementation and Linearization of a State-Space Free-Vortex Wake Model for
Flapping-Wing Flight
(Paper 5)
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Umberto Saetti, Joseph Horn

This paper describes the extension of a free-vortex wake method in state-variable form originally developed for rotarywing applications to flapping-wing flight. Following a detailed mathematical description, the wake model is implemented for a hovering insect representative of a hawk moth. A periodic solution to the wake model is found at hover using a modified harmonic balance algorithm. Next, linearized harmonic decomposition models are obtained and validated against the nonlinear dynamics using simulations. Order reduction methods are explored to guide the development of linearized wake models that provide increased runtime performance compared to the nonlinear and linearized harmonic decomposition wake models while guaranteeing satisfactory prediction of the periodic response of the wake.

Physics and Improved Simulations for Computational Modeling of Synthetic Jets (Paper 1172)
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Aaron Crawford, Marilyn J. Smith

With the rise of active flow control techniques several approaches of flow reattachment have been studied, including the use of direct momentum injection through a zero-net mass flow synthetic jet. The focus of this paper is to further understand the role of turbulence in the physics of synthetic jets and the sensitivity of external crossflow on the prediction of the jet and its downstream behavior while in cross flow. Computational results are correlated with recent experimental data obtained by the U.S. Army. The full actuator geometry is computationally modeled and to develop unsteady and phase-averaged boundary conditions at the jet/outer mold line interface at the relevant spatial and temporal levels. This study also considers the effect of turbulence model, as well as the associated turbulent quantities, and their influence in the prediction of the local synthetic jet flowfield in cross flow. Results indicate that turbulent fluctuations in three-dimensional flows with large eddy simulation wakes are required to predict the jet interactional effects. The external crossflow has a significant impact on the magnitude of the turbulent characteristics, but the trends observed in experiments are captured with the additional of turbulence in the jet.

Modeling and Simulation III

Technical Session C: Wed. May 11, 2022 - 1:45 PM to 6:00 PM

A Comparative Assessment of the NATO-GD and SFS2 Ship Airwakes and their Influence upon Helicopter Aerodynamic Loading (Paper 1222)
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Nicholas Fernandez, Neale Watson, Ieuan Owen, Mark White

The purpose of this study was to assess and compare the airwakes of two generic ship models, the NATO-GD and the SFS2, and their impact upon helicopter aerodynamic loading in operations over a ship's deck. Airwakes were computed using Delayed Detached Eddy Simulation, a time accurate CFD approach, for each ship model in a headwind and a Green 30 wind. The CFD airwakes were then integrated with a helicopter flight dynamics model, representative of an SH-60B Seahawk. The helicopter model was held stationary at a number of points along a lateral traverse profile over the flight deck of each ship, and the unsteady aerodynamic loads on the fixed-position helicopter were recorded and compared for the two ships. It was seen that the NATO-GD airwake created higher levels of turbulence and greater unsteady loads on the helicopter model than did the SFS2 airwake, implying greater workload for a pilot.

Design and Integration of a Tilt-Rotor Flight Simulation Platform (Paper 1167)
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Stefano Primatesta, Federico Barra, Pierluigi Capone, Giorgio Guglieri

This paper introduces a tilt-rotor flight simulation platform for research and teaching purposes implementing a real-time simulation of the Bell XV-15 aircraft. The mathematical model of the XV-15 aircraft has been implemented including simplified models for the aerodynamics of the whole aircraft, rotors, and engine dynamics. Hence, the simulation is performed in a graphic environment to reproduce the simulated flight and to interact with it using commands given by the pilot. The simulation platform is implemented using MATLAB/Simulink, while the input commands are set using USB peripherals, i.e., a flight stick and a pedal board. Instead, the visualization environment is performed using FlightGear, an open-source and cross-platform software that is widely used in research. The result is a portable tilt-rotor simulator to be executed on a commercial pc, while ensuring real-time performance. The tilt-rotor flight simulator is also validated by a licensed helicopter pilot returning positive feedback regarding the flight experience.

The Effect of Lift Position on Helicopter Recovery to a Twin-Island Aircraft Carrier (Paper 1151)
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Neale Watson, Ieuan Owen, Mark White, Richard Lynn

This paper describes an investigation of the air flow over the flight deck of a twin-island aircraft carrier with the ship's lifts in a raised and lowered position, and the subsequent change in the aerodynamic loads on a helicopter in hover over the deck. The unsteady flow over the flight deck was simulated using Computational Fluid Dynamics for a 40 kt wind from 60 off the starboard. The turbulence intensity and velocity flow field produced over the flight deck for each lift configuration was analyzed and compared. An experimental study was conducted in which the mean and unsteady flow over a 1:200 scale model the aircraft carrier submerged in a large recirculating water channel was measured with the lifts in the raised and lowered position. The CFD prediction of the flow over the full-scale aircraft carrier was compared to the experimental velocity data. To analyze the effect of the two airwakes on a helicopter in hover, a technique known as the Virtual AirDyn was used to quantify the unsteady forces and moments acting on the aircraft. Thirty seconds of time-varying velocity data simulated for each lift configuration was integrated with a flight dynamics model of a helicopter representative of a Seahawk SH-60B. The helicopter was fixed in hover positions over the flight deck and subjected to each unsteady airwake. The resultant unsteady aerodynamic loads acting on the aircraft were analyzed to assess the effect of the lift positions on the helicopter in hover. The results show that for the conditions considered, while the lifts' positions do affect the air flow and the aerodynamic loading, there will only be a limited impact on a recovering helicopter.

Towards Certification by Simulation with model-based continuous Engineering Processes showcased on eVTOL Application (Paper 1242)
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Marcel Gottschall, Bastian Binder, Alexis Castel

Aerospace industry OEMs and suppliers are progressing their engineering processes and performance to the next maturity level gearing to digital thread solutions. Current challenges like continuous engineering, virtual certification, distributed development, consolidated virtual proving grounds, homologation, digital twin and operational applications, require well informed decision making in a comprehensive, reliable, traceable and customizable environment. In particular, in aerospace domain, with widespread tight collaborative ecosystems between integrators and suppliers, the capability of tracing each decision and its underlying artifacts becomes a key value of an engineering platform. This paper will outline a middleware approach to reuse generated artifacts and their relationships in a federated engineering environment supporting a "best tool for the job" approach by introducing a layer providing unification and consistency throughout all managed artifacts. Based on an exemplary eVTOL setup, the benefits of integrated data and workflows from specification to virtual design verification are highlighted to motivate their value towards realisation of MBSE methodologies.

Verification, Validation and Calibration Under Uncertainty for a Scaled Experimental Rotor Model (Paper 64)
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Neda Taymourtash, Giuseppe Abate, Matteo Daniele, Giuseppe Quaranta

Virtual Engineering (VE) in rotorcraft design is a well-established approach to support decision-making throughout the rotorcraft's life-cycle (Ref. 1). The effectiveness of structural design, analysis and CAD software, CFD solvers and rotor aeromechanics codes is crucial throughout rotorcraft life-cycle, from preliminary design phase development to certification process (Ref. 2). The required fidelity level and reducing the number of physical tests during development can be achieved by building a reliable and repeatable protocol for Verification, Validation and Uncertainty Quantification (VVUQ) of numerical models (Refs. 3-5). UQ can be exploited to both determine how much the variability of numerical and physical parameters affect the simulation outcome and perform a calibration of numerical models. This paper aims to perform the three steps of VVUQ for a digital twin of a scaled helicopter designed for wind tunnel tests. In this study, the experimental setup and numerical model will be introduced. Then, the statistical methods implemented for each step will be explained in more detail.

XV-15 Rotor Simulation in Flow360 using the Blade Element Theory (Paper 1192)
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John Moore, Feilin Jia, Qiqi Wang

This paper presents results of the numerical study of the Bell XV-15 rotor with the flow solver Flow360 using a coupled Navier-Stokes/Blade Element Theory model. Results are presented for both steady-state and transient simulations. The resulting thrust, torque, and blade loadings show good agreement with high-fidelity DDES results computed with Flow360 and experiments.




Technical Session E: Thurs. May 12, 2022 - 1:30 PM to 5:30 PM

Expectations, Test Results, and Lessons Learned, Flight Testing an RF DILR ALE (Paper 80)
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Bob Burzynski, Sid McManus, Mike Lengyel, Dan Ferrante

VFS 78 Summary: In this paper we frame the events and report the outcomes of a series of Radio Frequency (RF) Detect, Identify, Locate and Report (DILR) Air Launched Effect (ALE) laboratory, ground, flight tests and demonstrations. The flight testing and demonstrations were accomplished at the U.S. Army Yuma Proving Ground (YPG) in AZ. Elbit America, in cooperation with Area-I and the U.S. Army Combat Capabilities Development Command Aviation & Missile Center (AvMC) presented a paper in the VFS 77 Operations Forum entitled Air Launched Effects Payload and Air Vehicle Integration. In the VFS 77 paper we described the intricacies of development, integration, laboratory testing and CONOPS of an RF payload to Detect, Identify, Locate, and Report (DILR) the whereabouts of a field emitter. The objective was to demonstrate an integrated UAS/payload capability suitable for battlespace operations to help defeat enemy Integrated Air Defense Systems (IADS). Scheduling conflicts mandated the VFS 77 paper be presented prior to conducting field trials so no results were included in the VFS 77 paper.
VFS 77 Abstract: ALE is a new operational concept for vertical lift warfighting and is an important element of the future warfare paradigm. All U.S. service branches have ALE capability development programs depicted in their evolving operational views and Army Aviation is the leader for vertical lift concepts. Operations with ALE will provide on-demand, stand-in effects deployed from vertical lift vehicles at very low altitudes and from unmanned aircraft systems. The U.S. Army is developing and evaluating ALE concepts of operation, air vehicle systems and payloads via capability demonstrations and field trials. A coordinated effort between multiple Industry and DoD organizations is needed to achieve success. This paper discusses an ALE capability to rapidly DILR an embedded field RADAR. The cooperative integration and evaluation effort included Elbit Systems of America with their EW DILR payload, Area-I and their ALTIUS 600 UAV and the U.S. Army AvMC ALE team.

Impact of Transformative Air Vehicle Operations on Logistic Supply Chains (Paper 1316)
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Kaydon Stanzione, Daniel Schrage, Richard Ruff, Suresh Kannan

Global supply chains have reached critical capacity, resulting in escalating prices, long delivery times, and has had a crippling effect on manufacturing, health care, agriculture, and commerce in general. In addition, the cascading effects of the COVID-19 pandemic as well as other intra-supply chain problems have resulted in a 40-year high inflation and degradations in quality of life, employment, GNP, and personal and national security. During the pandemic, shortages in the workforce and all transportation carrier modes forced the acceleration of digitally powered solutions for capacity improvements. At the same time, emerging technologies in autonomy, electrification, fuel cells, and low carbon powerplants are pushing the envelope of OEM development for over-the-road trucks, aircraft, and maritime vessels. These new transportation modes offer clean, low noise, and high safety operations, which promise to move people and goods faster, cleaner, and ultimately more affordably between origin and destination. This paper provides an overview of general logistics issues that can assist VTOL designers and operators in determining critical configuration parameters, location demographics, flight operations, and emerging Aircraft On Ground (AOG) technologies to maximize near-term ROI for transformative VTOL aircraft.

Operations Study on a Multimodal Transport using Cargo eVTOL Aircrafts and High-Speed Rail (Paper 1306)
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Yu Ito, Haruki Tsuge

This study discusses how cargo eVTOL operations can collaborate with high-speed rail (HSR) in the context of rapid freight delivery. Operational aspects of eVTOL and HSR is discussed to reveal that their combination will enable a flexible and economic rapid delivery. Although HSR logistics have not been largely introduced in Japan so far, collaboration between a long-undergoing development project regarding HSR - the free gauge train (FGT) - could have synergy with eVTOL logistics. The discussion here also shows how the operational design will be affected by local constraints and social features.

Scenario-based study of using Civilian Cargo eVTOL Aerial System for Counter Catastrophic-Disaster Mission (Paper 1246)
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Haruki Tsuge, Yu Ito

In recent years, the use of Unmanned Aerial Vehicles (UAVs) in natural disaster situations such as earthquakes or heavy rain has attracted a great deal of attention. The flight and safety performance of UAVs continues to evolve over the years. Because UAVs are more maneuverable than helicopters and other manned aircraft, they are being used to quickly assess damage at the scene after an actual natural disaster has happened. UAVs are also expected to be used for transporting relief supplies in the event of a disaster. Therefore, test flights and demonstration tests are being conducted across Japan to transport relief supplies in the event of a disaster. However, there have not been many studies on UAV logistics during large-scale disasters. This paper analyzes the effectiveness of eVTOL aircraft that is expected to perform a role as a next generation parcel delivery service in normal operations for relief supplies transportation in a large-scale natural disaster, which is likely to occur within a few decades. Based on the results of the analysis, this paper proposes an operational model for eVTOL aircraft for logistics in large-scale natural disasters.

SmartHangars and SAE International Aircraft Charging Standard (Paper 1210)
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Joshua Portlock, Richard Watson

Electro.Aero has been supporting the emerging electric aviation industry for nearly a decade, including the development of chargers, electrical management systems, battery management systems and electric propulsion systems to numerous electric aircraft development projects. During that time, it was identified that the industry needed standardization, especially regarding the charging coupler, so the SAE AE-7D committee was founded by Joshua Portlock in 2018 to help standardize electric aircraft charging and energy management. The AS6968 standard for light conductive charging of electric aircraft was the first standard the committee worked on and is now approaching completion through this year. Furthermore, while the charging coupler standard was proving to be of high priority to the industry, there has also been a noticeable trend that typical airport hangars don't have sufficient electrical power to charge the growing power demands of future electric vertical take-off and landing (eVTOL) aircraft. Therefore, SmartHangar technology was developed, whereby the limited grid connectivity is supplemented by solar and stationary battery energy storage systems to help deliver the peak power demands of electric aviation charging, whilst also helping airports and vertiports transition to more self-sufficient sustainable energy.

The Bambi Bucket: Evolution of the Most Versatile Aerial Firefighting Tool (Paper 1178)
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Robert Roedts

From humble beginnings in the 1940s, helicopters have become an integral part of managing and fighting wildfires throughout the world. Starting in the 1950s and over the next several decades, various government agencies increased the utilization of helicopters in their management of wildfires across the US and deployed new technologies. One of the most impactful technologies developed was using a slung bucket to dump water. Initially using repurposed construction and agricultural equipment, these early generation buckets were dipped in sources of water and released using a simple mechanical gate system. In parallel with these early generation buckets, the technique of long-line operations were being perfected that allowed flight crews to increase separation from terrain and increased control of the external load. Over the next several decades, these buckets were steadily improved and incorporated into firefighting operations across the work. In addition, the use of long lines for external loads allowed the use of remote water sources and increased precision water drops. In 1982, Don Arney developed the ubiquitous "Bambi Bucket." This bucket was innovative in many ways: easily scalable, lightweight, collapsible, stowable, and reduced drag. Today, helicopters have become the main aerial resource to fight wildfires across the world. Furthermore, helicopters outfitted with Bambi Buckets account for 90% of all firefighting operations.


Product Support Systems Technology

Product Support

Technical Session A: Tues. May 10, 2022 - 8:00 AM to 12:00 PM

Application of MSG-3 Maintenance to the Bell 525 (Paper 1269)
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Corey Mooney, Jim Ciazinski

In the early days of aviation, maintenance requirements were determined by a few experienced mechanics with assistance from the Original Equipment Manufacturer (OEM). As aircraft became more complex, a more sophisticated method of developing an aircraft maintenance program was needed. The approach aimed for a data driven maintenance philosophy. Just as sequential aircraft designs introduced new enhancements, each revision to the maintenance logic improved the maintenance approach in terms of effectiveness. The latest approach to this maintenance philosophy is known by the acronym MSG-3, for Maintenance Steering Group 3. As scheduled maintenance requirements for aircraft continue to change, the procedures need to be effective, reliable, and economically reasonable. The approach and benefits of an MSG-3 program are discussed in reference to the Bell 525, a new fly-by-wire, 16-passenger commercial helicopter under development at Bell Textron Inc. The development of the 525's MSG-3 maintenance program and its benefits to operators are discussed. The MSG-3 process schedules aircraft maintenance tasks necessary to maintain the stated levels of reliability and safety, reducing direct maintenance costs by 30% while maximizing aircraft availability (Ref 1).

Automated Tracking and Forms Management of Serialized Parts (Paper 1168)
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Steve Schoonveld, Corey Pearce, Mike Augustin, Dy Le, John Moffatt

Ongoing aircraft status, maintenance actions, and operational history are mostly entered manually using a variety of data forms. This manual process inherently introduces errors, inaccurate data, and outdated configuration into aircraft maintenance records. These data errors can either endanger aircraft by delaying required maintenance actions or trigger unnecessary maintenance activities resulting in increased down time of aircraft that are fully mission capable. The US Army has identified a fundamental need for autonomous parts tracking and management as an underlying capability to alleviate maintenance burdens and the associated increased cost. Further, autonomously tracking remaining useful life (RUL) based on actual mission profiles can safely extend component Time-on-Wing (TOW), significantly reducing maintenance costs. Using mature components, it was demonstrated how manual data input for parts tracking, maintenance records management, and report generation can be automated, greatly increasing the health state awareness of each aircraft. The development of this technology is mission-critical and allows the Army to realize unprecedented levels of readiness, cost effectiveness, and safety.

Bolt Hole Corrosion and Fatigue Damage Repair in Hybrid Vertical Lift Structure (Paper 1244)
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Jude Restis, Mike Dubberly

Cold expansion has been successfully used in aerospace structures, including vertical lift airframes to repair and prevent fatigue damage for more than 50 years. PartWorks is innovating the cold expansion process, parts and tooling for use in repairing corroded fastener holes (U.S. Patent 11,255,371). PartWorks is participating in a two phase development and demonstration program, first for the US Navy (Office of Naval Research/ONR) starting in 2017, and more recently for the United States Air Force (Air Force Research Lab/AFRL) for repairs to bolt holes on aerospace structures with metal/carbon-fiber composite skins. These locations in aerospace structures for vertical lift or fixed wing have demonstrated greater levels of corrosion when compared to all-metal structural skin due to galvanic corrosion between metal and carbon fiber composites. Existing repair methods for these metal/carbon fiber composite skin bolt hole/fastener sites often involve extensive removal of corrosion, non-standard or oversized holes, and extensive modeling/validation to prove repair effectiveness. Existing repairs can also lead to premature structural component replacement. This project is evaluating a new repair method that uses cold expansion with thin wall bushings and/or a rivetless nut plate (RNP) to restore fatigue life to the metal bolt hole if damage is missed or potentially without having to remove all the damage/corrosion.

Optimizing Battery Life in Conventional Helicopters: Selection, Charging, Storage, and Maintenance (Paper 22)
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Mike Neus, Jerome Powell, Collin Zreet

Conventional helicopters are equipped with a main battery to provide power for many situations including pre- and post-flight operational checks, engine starting, and emergency in-flight backup power. Historical field data indicates the average replacement interval for main batteries on commercial helicopters are on the order of 1,000 Flight Hours. This makes main batteries one of the most frequently replaced non-consumable items on fielded helicopters. Other industries, such as renewable energy and space flight, show us much longer lifetimes are possible. This paper discusses the factors that influence helicopter main battery service lives for Absorbed Glass Mat (AGM) and Lithium Iron Phosphate (LFP) battery types. The most common main battery failure modes and their causes are discussed. Considerations for battery selection during system design, associated charging algorithms, storage, and periodic maintenance requirements are identified. These factors and knowledge of battery failure modes are used to provide guidance to optimize AGM and LFP main battery life.

Rotorcraft Digital Twin: Exploiting On-board Data for Enhancing Sustainment and Operational Availability (Paper 1206)
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Avinash Sarlashkar, Matt Harrigan, Raymond Beale, Jared Kloda, Mark Kruse, Dennis Vanill

Rotorcraft Digital Twin (RDT) is a Digital Transformation initiative to leverage on-board flight data, design data, maintenance records, and advanced analytical models to enhance the current sustainment paradigm, increase availability, reduce life-cycle cost, and to enable optimization of future designs. Many core capabilities have been integrated into RDT, such as load estimation, advanced regime recognition, gross weight estimation, fatigue damage accrual calculations, among others, which leverage a rich body of prior work. Multiple algorithms enable component life extensions, predictive maintenance, selection of the optimal set of assets for a particular mission or deployment to minimize the likelihood of unscheduled maintenance, supporting the Rotorcraft Structural Integrity Program (RSIP) requirements, and feeding field data back into the design process for continuous product improvements. A robust, generic, and reusable suite of algorithms and associated framework has been developed utilizing a common data model where possible. RDT has been developed with a modern cloud-native microservice software architecture which glues together carefully selected Free and Open-Source Software (FOSS) components.

Scheduled Maintenance Program Development for the Leonardo AW609 Tiltrotor via the Maintenance Review Board Process Utilizing MSG-3 (Paper 1243)
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Titos Gosalvez, Nicholas Flynn

Scheduled maintenance remains an important aspect within the aviation industry to uphold the inherent safety and reliability of aircraft so that ownership of such aircraft should remain justifiable to the operators. The Maintenance Review Board Process utilizing MSG-3 logic describes a collaboration between the national aviation authorities, aircraft manufacturers, aircraft operators, and equipment manufacturers to derive a scheduled maintenance plan with the purpose of ensuring safety and reliability of an aircraft, while remaining cost-effective. As the industry pursues means to improve this aspect, a review of the Maintenance Review Board process utilizing MSG-3 logic applied to the Leonardo AW609 Tiltrotor demonstrates the effectiveness of these current standards for a new technology. The history of the certification process for the AW609 sets a foundation that will serve as a point of reference for future programs with coming advancements such as Advanced Air Mobility (AAM), and electric vertical takeoff and landing (eVTOL). This case study reports details of the process for the AW609 to provide insight from a practical perspective. A brief review of MSG-3 and unique features of the AW609 are provided to support the case study. The subsequent review of selected challenges encountered during the process provide valuable lessons learned.



Propulsion I

Technical Session B: Wed. May 11, 2022 - 8:00 AM to 12:15 PM

Advanced Data analytics for Dynamic Systems Monitoring (Paper 1162)
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Océane Martin, Ammar Mechouche, Emmanuel Mermoz

On the last generation of Airbus Helicopters rotorcrafts such as H175 or H160, dynamic systems data are collected in a systematic manner in order to perform advanced analytics. Main gearbox (MGB) oil temperature and oil pressure are key parameters to assess the overhaul status of the lubrication and cooling systems. This paper describes new ways of monitoring lubrication and cooling systems behavior, taking advantage of big data capabilities and advanced analytics such as machine learning and physical modeling based approaches.

CFD Modeling Framework Development for Robust Rotorcraft Design (Paper 1140)
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Shyam Neerarambam, Donald Lamb, Jon Frydman, Dustin Coleman, Charles Berezin, Rebecca Cotton, Michael Alexander, Nolan Birtwell, Dana Halline, Daniel Bernier

Reducing development cycles, developing advanced capabilities at reduced technical risk, and containing overall program development costs are key goals of next generation rotorcraft development programs. Achieving these goals requires a significant investment in digital transformation in all phases of the aircraft development; design, test, manufacturing, and certification. CFD tools have traditionally been used to shape and influence various aspects of rotorcraft design. However, workflow inefficiencies typically limit use of CFD to key flight conditions of the rotorcraft flight envelope. Robust design requires analytical design assessments across a very broad range of environmental conditions - ambient temperature, wind, gross weight etc., across a very broad range of rotorcraft mission spectra. Development of a CFD modeling framework and streamlining/automating all building blocks for rotorcraft CFD analysis is essential to enabling robust analysis driven design. This paper provides an overview of the digital transformation efforts on CFD modeling framework development for rotorcraft design and productivity improvement realized on practical use cases with ongoing work.

Dynamic Load Analysis of Motion Converter Ball Bearings in a Pericyclic Transmission (Paper 89)
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Nick Weinzapfel, Nathan Bolander, Tanmay Mathur, Hans DeSmidt, Edward Smith

Performance of ball bearings in the motion converter subassembly of an internally driven, single-speed, torque-split, twin configuration pericyclic transmission prototype is evaluated to extend the analytical knowledge base on this innovative transmission concept. A dynamic model of the transmission is developed with high-fidelity models of the installed rolling element bearings to determine their reactions. Attention is focused on the pair of ball bearings supporting the motion converter subassembly which are subjected to a complex combination of loads, including radial and axial forces, moments, carrier motion, and possibly internal preload. Then the influence of internal axial clearance and preload on the behavior of the rolling elements is analyzed with a fully dynamic ball bearing model. Provisions to consider the carrier motion and a robust integration algorithm for component orientations are presented. Finally, a microstructure-based fatigue life simulation of the critical bearing component is performed to demonstrate the effect of clearance/preload on bearing reliability.

Dynamic Simulation of a Rotor System with Variable Speed (Paper 1217)
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Michael Weigand, Mario Bischläger, Christopher Gross, Jonas Koch, Agnes Poks

Current concepts for fast rotorcraft and tiltrotor-/tiltwing aircraft require a speed variation in a wide range; this wide range cannot be covered by varying only the turbine speed. Such new aircraft under development require a rotor system with a large rotor speed variation but constant turbine speed which leads to the consequence that a transmission with a variable ratio during flight is necessary. TU Munich, TU Wien and Zoerkler gears work in the funded research projects VARI-SPEED and VARI-SPEED II on a solution "constant turbine speed / variable rotor speed" that consists of a transmission with variable ratio and a rotor system with rotor head and rotor blades for variable rotor speed. The helicopter UH-60A from Sikorsky Aircraft is used as reference. The project VARI-SPEED showed the feasibility of a rotor system with variable speed. In the current project VARISPEED II the dynamic simulation of the system is built up. The paper describes the approach to the simulation and shows first results for each component. After completion of the simulation the elaborated rotor control will be implemented in a flight simulator for evaluation by pilots.

Integrating a Torque Measurement System for the GE T408 Engine into a CH-47D Chinook (Paper 99)
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Daniel Kakaley, Gary Martin, Caryn Kopay, Scott Kennedy, Robert Kyff, Kenneth Durbin, Gregory Lane, Kevin Ignatuk

This paper presents an overview of the efforts associated with integrating a new torque measurement system for T408 engines into an NCH-47D airframe and performance of the torque measurement system during operation on the aircraft. The effort described was performed collaboratively between GE Aviation, The Boeing Co, Parker LORD, and DEVCOM AvMC. The Torque Measurement System was utilized in the T408 Engine Control System during its demonstration on an NCH-47D airframe at Ft. Eustis, VA. In addition, based on DEVCOM AvMC program requirements, health monitoring capability was successfully implemented and demonstrated during the NCH-47D T408 Engine Demonstration. Lessons learned from the torque measurement system's health monitoring features are included.

Relative Adhesive Wear Performance of Rolling Element Bearing Material Pairs (Paper 96)
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Robert Sadinski, Carl Hager, Dave Prock

The relative adhesive wear performance of rolling element bearing material pairs was evaluated using a constant entrainment stepped slide-to-roll (SRR) test protocol. Raceway materials included M50, M50NiL, 440C, and powder metallurgy M62 (M62 PM). Rolling element materials included M50, 440C, and Si3N4. The material interfaces of both all-steel and hybrid configurations were ranked based on the survived SRR. The Si3N4 and steel hybrid pairing increased the adhesive wear resistance, reduced the tractive effort of the contact, and enabled operation with reduced component temperatures throughout each test. Post-test wear track morphology and chemistry were investigated using light microscopy, scanning electron microscopy, and energy dispersive spectroscopy. Steel alloying elements and elevated levels of oxygen were identified within the Si3N4 wear track from each hybrid material pair.

Propulsion II

Technical Session D: Thurs. May 12, 2022 - 10:15 AM to 12:15 PM

A Semi-empirical Method to Predict Motor Heat Transfer Coefficient for SUAS Conceptual Design (Paper 1284)
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Farid Saemi, Moble Benedict

The brushless DC motors of small unmanned aerial systems typically rely on axial air-cooling. Modeling the motor's thermal dynamics requires knowledge of the convective heat transfer coefficient. However, the coefficient is an extrinsic property which depends on surrounding conditions. As such, motor manufacturers provide no relevant data which stymies conceptual design and modeling. This paper presents an analytical method to predict the motor's heat transfer coefficient in various operating conditions using readily-available parameters. The paper also shares experimental measurements of heat transfer coefficient collected on a custom-instrumented hover stand. The analytical predictions matched experimental trends for a range of operating shaft speeds. The experimental measurements indicate that the heat transfer coefficient increases with shaft speed before reaching a maximum. The combined results provide guidance for eVTOL conceptual designers in (1) predicting a motor system's steady-state temperature, (2) maximizing convective cooling, and (3) understanding tradeoffs for compact & thermally-conductive motors.

Design Concepts to Meet EASA SC-VTOL-01 Single Failure Criteria (Paper 1294)
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Patrick Darmstadt, Sheevangi Pathak, Tim Krantz, Mark Valco

The objective of the current work is to discuss European Union Aviation Safety Agency (EASA) SC-VTOL-01 single failure criteria, VTOL.2250(c). VTOL.2250(c) increases safety metrics compared to existing Vertical Takeoff and Landing (VTOL) regulations, creating new engineering challenges that must be addressed. Additionally, research and development targeting compliance against VTOL.2250(c) will more broadly benefit the VTOL industry, providing guidance for safer system designs. Prior studies have developed concept distributed propulsion and flight control (DPFC) system architectures and found they comply with EASA SC-VTOL-01 probabilistic failure criteria, VTOL.251(a). Prior work developed two all-electric DPFC systems utilized in a quadrotor concept aircraft developed by the National Aeronautics and Space Administration (NASA); one uses interconnecting shafts and gearboxes to interconnect redundant motors with each rotor system and the other uses gearboxes to connect redundant motors locally, near each rotor. Common between the two electric DPFC systems were rotor shafts, epicyclic systems, and motors. The current work explores Category I failures in drive systems, relevant research to support fail-safe design practices for gear systems, research and adjacent industry trends in motor fail-safety and reliability, and proposed design concepts to comply with VTOL.2250(c). Continued research in fail-safe design concepts and design guidance will benefit eVTOL and conventional rotorcraft, alike. Continued research in these areas will benefit eVTOL certification against SC-VTOL-01, and could optimistically translate to more widespread adoption of similar fail-safe design concepts into new rotorcraft designs certified against CS-29.

Rotorcraft Propulsion System Hybridization for Enhanced Safety and Performance (Paper 1304)
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Peter Giannola, Gerry McCann, John C. Ho, Mars Chan

Herein, the case for hybridization will be made, primarily as a means for enhancing safety. Utilizing a case study of a popular light turbine helicopter, it will be shown that augmentation with a parallel hybrid electric system, under two critical operational failures, safety can be significantly enhanced. Conceptual design of two powertrain architectures for parallel turbine-electric hybrid propulsion systems will be examined in sufficient detail to quantify their relative impact in comparison with the existing conventional turbine-only powertrain. Initial trade studies were performed to minimize system weight, utilizing current state-of-the-art components & technology. Commercially available motors, inverters and battery systems were selected for the study. Two architectures were chosen and exercised for three operational scenarios: requirements for 21/2 minutes of emergency power, 5 minutes of emergency power and 10 minutes of emergency power at takeoff/climb power level. Relative cost & benefit are quantified by performance impacts including payload capacity impact, range, rate of climb, and high-altitude performance for the different sized battery packs required for each of the 3 emergency power durations. The key benefits are safe recovery from two serious failure modes: inoperative engine and main gearbox failure. The rough "performance cost" for this safety enhancing modification is a loss of about 1/3 of the nominal useful (passenger + baggage) payload, assuming full fuel capacity operating at maximum gross takeoff weight.




Technical Session C: Wed. May 11, 2022 - 1:45 PM to 6:00 PM

A Method to Reduce Rotorcraft Development Risk by Integrating Historical Quantitative Risk Assessment into Fault Tree Models (Paper 1276)
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John Hewitt, Loan (Joan) Pham

Fault Tree Analysis (FTA) is performed during vertical lift product development, but only an estimation of the probability of component failures can be made at that point of product design and development. Estimation of component failure probability during FTA typically does not account for component aging, installation effects, maintenance actions, and other factors encountered in operation, which can lead to under prediction, resulting in identification of hazards during test, evaluation, and deployment. Quantitative Risk Assessment (QRA) is typically performed during fleet operation. Efforts to eliminate hazards or mitigate risks are less effective and much more costly in this phase of the product lifecycle compared to proactively addressing hazards early in development. If the risk of failures could be accurately predicted earlier, hazards could be addressed early in the process. Such a method is presented here, where historical QRA for similar hazards can be integrated into the FTA. This would reduce cost, schedule, and safety risks by reducing the risk of failure during ground and flight test and in fleet operation.

A Structured and Comprehensive Air Vehicle Risk Assessment (Paper 76)
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Laurence Mutuel

In recent years, U.S. Army programs have leaned on the Department of Defense Standard Practice for System Safety (MIL-STD-882E) and commercial Aerospace Recommended Practices (SAE ARP4754A/ARP4761) to guide the implementation of System Safety processes in air vehicle design and sustainment. System Theoretic Process Analysis (STPA) adds to safety requirement capture and provides a unique pathway to integrating Human Factors Engineering and Cybersecurity assurance with System Safety. As part of the U.S. Army's Future Attack Reconnaissance Aircraft (FARA) program, Bell is implementing a novel blended approach that combines techniques from MIL-STD-882E, SAE ARP4754A/4761, and STPA to improve the overall performance of the System Safety process. The result is a more complete hazard identification process, a more thorough risk assessment (including identification of causal factors), a richer set of hazard control requirements, and a more contextual traceability to verification test cases. The Blended System Safety Framework (BSSF) is shown to be straightforward and results in the streamlined development of airworthiness artifacts while maintaining full traceability to content and framework source requirements.

Physics-Based Detection of the Proximity to Loss of Tail Rotor Effectiveness Within the Helicopter Flight Data Monitoring Program (Paper 41)
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Paola Zanella, J.V.R. Prasad, Charles Johnson, Dimitri Mavris

Loss of tail rotor effectiveness (LTE) is a critical helicopter flight characteristic and a major factor in numerous fatal accidents. To support LTE risk mitigation, the Helicopter Flight Data Monitoring (HFDM) program provides pilots with constant flight evaluation reports. Nevertheless, the existing LTE safety metric used within the HFDM environment presents several pitfalls that hinder the reliability of the results, contributing to a misleading pilot's training and low LTE awareness during flights. This paper is part of a larger ongoing effort that aims to develop an expanded safety metric to enhance the detection of proximity to LTE within the HFDM program. While LTE has been defined to include different complex flight conditions, this work focuses on the phenomenon of running out of pedal (tail rotor collective) for trim, a critical event that may lead to LTE behavior. A parametric surrogate model for the detection of proximity to running out of pedal events is formulated by combining results from physics-based simulations with machine learning techniques. The new safety metric allows for operator fine-tuning of proximity to critical boundaries, and hence, it provides flexibility in risk management from post-flight analyses. The results confirm the accurate prediction of running out of pedal for trim, providing an improved detection of the proximity to LTE within flight data. This aims to promote better awareness of LTE within the pilot community and support the proactive mitigation of LTE accidents.

Safety Considerations on the Operation of Electric Vertical and Takeoff Landing (VTOL) Aircraft at Airports and Vertiports (Paper 104)
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Gaël Le Bris, Loup-Giang Nguyen

Design innovation for electric vertical takeoff and landing (eVTOL) aircraft is accelerating rapidly. By 2025, eVTOL aircraft might be operating commercial flights. Aviation facilities may have to distribute new energy vectors used by eVTOLs. Accommodating these aircraft will require careful planning, as well as design features, standards, and operational procedures to ensure aviation safety. Indeed, these new technologies cannot be turned into a viable air transportation system if they cannot be integrated into existing and future aviation facilities. With the first eVTOL certification on the horizon, a study of potential airside integration issues is timely. This paper identifies and analyzes potential aviation safety hazards and risks specific to the introduction of eVTOL aircraft and hydrogen technologies at airports and vertiports. A review of existing industry standards and ongoing standardization tasks suggests ways to further reduce the most prominent risks through industry standards and practices promoting operational flexibility and interoperability.

The Safety of Advanced Air Mobility and the Effects of Wind in the Urban Canyon (Paper 1152)
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Sharon Schajnoha, Guy Larose, Maryam Al Labbad, Hali Barber, Alanna Wall

The safe integration of vertical takeoff and landing aircraft (VTOLs) into an urban setting requires careful consideration of the effects of the wind flows through the urban canyon, which are complex and cannot be predicted with the reliability level necessary to enable safe operations. The assessment of wind in the urban canyon is critical to the siting and design of vertiports and safe operations of VTOLs. Vertiport design guidance related to the effects of wind flows in the urban canyon is limited and draws from helipad design guidance. The guidance recommends that wind conditions be assessed across a vertiport, but the guidance does not suggest how the study should be conducted nor does it provide design thresholds for wind. RWDI, the National Research Council Canada and Transport Canada have joined forces to identify and study the wind characteristics that would be important for the aerodynamic performance of VTOL and how these characteristics would be influenced by buildings and structures in the urban environment. In the framework of this research effort, an experimental approach was developed, centered on several sites in Canadian cities, representing a range of urban densities and surrounding topographies, as wind conditions are highly dependent on the built environment, including building footprint, building features, combinations of buildings and the local climatology. The work involved a quantitative approach in which physical models of several urban sites were built, instrumented and tested in boundary layer wind tunnels. In the wind tunnel experiments, "red flag" wind conditions were identified and measured. Detailed data capturing these flow features were collected around roof tops of lower and taller buildings, representing prospective vertiport locations and along prospective flight paths. This paper provides the details of the wind tunnel testing conducted for one of the urban sites; a site with an existing helipad at a hospital and a low-rise parking garage. The wind tunnel measurements were compared to published helipad design thresholds and combined with long term meteorological conditions representative of the site to determine how often vertiport operations will be limited by unfavourable urban wind conditions on an annual basis and by season. The wind tunnel study provides a case study and demonstrates how to evaluate the safety of a vertiport now, based on limited availability of VTOL aerodynamic performance capabilities and into the future as more information becomes available on VTOL performance in turbulent winds.


Structures and Materials

Structures and Materials I

Technical Session C: Wed. May 11, 2022 - 1:45 PM to 6:00 PM

Comparison of Helicopter Component Fatigue Test Results to Analysis (Paper 75)
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James Michael, Leigh Altman, Nathan Green

The safe-life fatigue methodology approach, combined with fatigue testing, is used throughout the industry to qualify helicopter components. In the early stages of helicopter development, limited component fatigue test results are available. As a result, component designs risk failure in down-stream testing. Alternatively, an over-designed component risks adding unnecessary weight to the aircraft. This paper discusses an analytical approach for fatigue evaluation with the purpose of minimizing these risks. The approach, applied to a flight-critical metallic component, utilizes finite element modeling and coupon fatigue test data to assess the component's design early-on. Analytically predicted component fatigue strength is assessed and compared to measured component fatigue test results to draw conclusions regarding the validity of the analytical approach. In summary, the analytical method compares favorably to the component test data.

Development of a 3D Braided Preform for a Rotorcraft Flex-beam (Paper 1127)
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Christopher Pastore, Dean Nguyen, Tommy Chen, Stanley Ng, Benny Ong, Hoa Lam, Maury Grudza, Dave Jann, Robert Ciccarelli, Tiffany Liao

A novel 3-dimensional braiding concept, 4StepPlus, suitable for rotorcraft components where high damage tolerance and fatigue cycles are critical is explored in this paper. 3D textiles such as woven and braided fabrics provide through the thickness reinforcement and thus improved interlaminar strength properties over laminated composites. However, there are challenges in creating shapes with varying cross-sectional shapes and areas. A 4StepPlus 3D braiding approach is used to fabricate a subscale flexbeam with substantial changes in cross-section throughout the length of the part. The mechanical properties of 3D braided composites employing IM7 carbon and S2-glass with RTM epoxy are investigated. The effects of fiber bundle sizes is also assessed as it plays an important role in the performance as well as manufacturing of the component. The results indicate no significant effects of yarn size on the performance of the composites.

Housing and Bearing Deformation Interaction of Large Size Bearings (Paper 43)
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Biqiang (Johnny) Xu

The traditional studies [1, 3] of bearing deflection and load cannot consider the interaction of bearing deflections at radial, axial and tilting directions. Swashplate bearings, rotor bearings, and etc. are usually of large size and mounted on flexible bearing housing, it makes the housing and bearing deform in all directions. The interaction between housing and bearing races is no longer negligible. On the other hand, modeling contact status between rolling elements and bearing race way in the housing-bearing assembly finite element model is too complicated and not practical for engineering analyses. In this study, multi-scale finite element model is developed for the system with rolling element bearings to investigate the interaction of housing and bearing race deformations. Rolling elements are simplified as spring element to make the system level finite element model executable. Hertzian contact theory is employed to determine roller compression non-linear stiffness. Clearance and preload can be considered in the stiffness curve. The result provides correct stiffness and load distribution for housing design. FEA submodel is used for local detailed roller contact analysis. It provides detailed bearing contact analysis of interested individual roller for detailed bearing design.

Improved Strain Gage Instrumentation Strategies for Rotorcraft Blade Measurements (Paper 88)
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Timothy Davis

In this presentation, a new strategy is presented to wire, calibrate, and measure strain gages for rotor blade testing that will provide more information and is robust to individual gage loss. The additional information can be used in several ways. including reducing redundancy, offering rapid identification of damage locations, and in some cases reducing risk allowing tests to continue to collect calibrated data after one or more sensors have failed. This strategy replaces the classical four-gage full Wheatstone bridge with four separately wired quarter bridges that are combined into a full bridge in the data acquisition system using a calculated channel. This strain gaging concept also provides the engineer with data from the four individual stresses, as well as the calibrated full bridge output that has been converted into engineering units.

Nonlinear Finite Element Analysis of a Sharp Radius on the Shank of a Ring Locked Stud (Paper 1150)
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Lin Liu, David Binney

This paper documents an investigation of the effect of a sharp radius on the shank of a ring locked stud. An industry specification for studs does not define a minimum size for the radius and very sharp radii have been found during inspection of some studs. Finite element analysis (FEA) is used to evaluate the stud as part of an assembled joint subject to fatigue loading. A parametric study is conducted, and high-fidelity modeling is adopted to correlate the FEA results with stud testing results. After a validated FEA model is confirmed, the elastic-plastic stress strain curve is applied in the FEA model. Three sizes of the radius are introduced in the model to evaluate the radius size effect on the vibratory stress under operational loads. The evaluation demonstrates that the radius size has a significant impact on its vibratory stress which in turn affects the calculated fatigue life.

The Application of Sandwich Technology to the Airframe Structure of Helicopters (Paper 73)
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Rainer Arelt, Stefan Goerlich, Jan-Christoph Arent, Benjamin Hailer, Mathias Dambaur

The H160-B is the latest helicopter design from AIRBUS HELICOPTERS with the extensive use of sandwich technology in the airframe. A sandwich with face sheets from CFRP and honeycomb cores is a robust outer skin of a helicopter. Furthermore it shows a very good tolerance to impact damages and a very good reparability. At Airbus Helicopters great experience is available which is required to understand and to control all manufacturing parameters, that are driving the quality of such parts. Powerful inspection technologies are in place to maintain the high level of manufacturing quality. In this paper an overview of the parts on this Helicopter made with sandwich technology will be given. These are cowlings and structural parts as well as principal structural elements (PSE) on main load paths. The respective certification requirements and related means of compliance demonstrations will be explained in detail. Special attention is paid to the applied methods for the damage tolerance demonstration of sandwich. The design and strength analysis was done with a combination of FEM analysis and analytical method, using basic allowable derived and validated by tests.

Vitrimer Carbon Fiber Composites for Rotorcrafts Components with Fatigue Reverse Ability (Paper 1141)
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Mithil Kamble, Sikharin Pranompont, Catalin R Picu, Nikhil Koratkar

As rotorcrafts enter new generation of their design, they are expected to be subjected to more stringent performance requirement, Increased loads and operational frequency necessitates use of structural components with higher fatigue life. Carbon fiber reinforced polymer composites (CFRP) are popular as structural material due to their superior performance while being lightweight. However, fatigue originating in weaker polymer limits their fatigue life, moreover the fatigue damage introduced accumulated irreversibly resulting in catastrophic failure. The damage is irreversible due to permanent crosslinked nature of thermoset polymers used in CFRP. If the crosslinks are made dynamic i.e. reversibly crosslinked, the fatigue damage may be reversed imparting ultra-high fatigue life to the components. Vitrimers are such epoxy based networks which may be ideal candidate for this application as they possess ability to dynamic crosslinking at elevated characteristic temperature. Here we report a vitrimer based CFRP i..e., vCFRP which has properties comparable to conventional CFRP which has ability to retain its original properties in fatigue tests when they are subjected to periodic heating. The fractographic analysis suggests that periodic heating serves dual purpose of enabling dynamic crosslinking as well as repairing small scale fiber-matrix interface failure. Thus, rotorcraft components made with vCFRP may have very high fatigue life compared to conventional CFRP components.

Structures and Materials II

Technical Session D: Thurs. May 12, 2022 - 10:15 AM to 12:15 PM

Airframe Structural Sizing Automation (Paper 1229)
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Anthony Joseph, Robert Daley, Anne Koeppel

Determining the optimum cross section for each primary structural member of an airframe structure has always been an iterative process since changing the stiffness of one member redistributes loads. Each iteration of internal loads calculations with a global aircraft finite element model (GFEM) followed by strength and stability checks results in further cross section changes (sizings) to reduce weight or regain positive margins of safety. The handoffs between tools and the update process for the next iteration is time consuming and has many opportunities for errors. This paper will describe a tool developed at Sikorsky to automatically iterate sizings saving development time and executing more sizing iterations than historically possible which saves weight. The tool can operate on metallic and composite structures. The development time and weight savings is critical to support ever shrinking time to fielding/market for commercial and military models.

Verification and Validation (V&V) of Numerical Helicopter Airframe Model for Dynamic and Static Finite Element Analysis (Paper 7)
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Junhyeon Seo, Srivatsa Bhat Kaudur, Mayank Agarwal, Rakesh K. Kapania, Courtney Fisher, Larry Pilkington

In this research, Columbia Helicopters, Inc. (CHI) and Virginia Polytechnic Institute and State University collaborated to conduct the numerical model Verification and Validation (V&V) of a Global Finite Element Model (GFEM) of a tandem rotor helicopter developed by CHI. The V&V process is followed based on the ASME V&V guide for computational solid mechanics. The target mathematical model is verified with a convergence study by improving the mesh density and quality. For the model validation, the authors compare the dynamic and static finite element analyses (FEA) with the experimental results. During 1980s, NASA along with some industry participants pursued a Design Analysis Methods for Vibration (DAMVIBS) project to develop and validate accurate FEM based framework for dynamic analysis of helicopters. This work utilizes NASA’s DAMVIBS project results to validate the dynamic responses for vertical, lateral, and pitch loading cases. In addition, for static validation, the authors have used CHI’s static pull experimental data. This data includes measured strain values at various locations on the fuselage structure. Both dynamic and static FEA results match within 10 % of the DAMVIBS and static pull experimental results, respectively. Thus, this study successfully validates the reliability of the numerical model.


System Engineering Tools/Processes

System Engineering

Technical Session B: Wed. May 11, 2022 - 8:00 AM to 12:15 PM

Fifty Years of Innovation in Performance Limit Indicator (A success story of Airbus Helicopters contribution, to helicopter fleet safety , by simplifying piloting and reducing workload) (Paper 38)
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Serge Germanetti

Since their invention, helicopters were typical flying object that requested very specific competencies to control their flight. The control of the rotors power, delivered by the engine, was subject to multiple indicators that were installed in the cockpit. Depending on the various technologies, engine temperature, turbine rotation speed, and torque are measured; main gearbox torque, or free turbine speed are monitored. The pilot has the responsibility to ensure that none of them are at their limit, depending on the environmental conditions mainly outside temperature and atmospheric pressure.

Preview of an Enterprise Product Architecture (EPA) to Support a Modular Open Systems Approach (MOSA) (Paper 1226)
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Thomas DuBois

The potential for significant MOSA benefits appears unachieved to date. Recently, the requirement to use a MOSA has been made statutory and regulatory (Ref. 1-5). Previous work (Ref. 6) suggested that a customer-controlled component architecture is a necessary activity to achieve MOSA technical and business benefits across multiple programs. This paper extends that premise while previewing the concept of an Enterprise Product Architecture (EPA) for use in an acquisition environment where there is a requirements organization is contractually separated from the development organization. The requirements organization is composed of an enterprise that is responsible for multiple subordinate programs, and each subordinate program is responsible for developing major systems with their suppliers. Industry product line approaches developed the concept of an EPA (Ref. 7). This paper explores how an EPA can be used to set an enterprise-level MOSA to increase opportunities to achieve expected MOSA benefits in the context of a multi-tiered acquirer with multiple programs using different contracts with different suppliers. This paper is a preview for a more detailed version of this concept planned for a future publication. The technical approach previewed in this paper leads to a product line strategy for the enterprise focused on software and data. This paper defines key terms used for the development and application of an Enterprise Product Architecture (EPA). Products to be acquired would be accomplished through the interaction of the subordinate programs and their supplier(s). The development of the EPA begins with the establishment of functional boundaries. Interfaces between functional boundaries will be defined as a result of operational analysis that recognizes how different functional groupings interact to accomplish operational behaviors. Interfaces between functional groupings will be defined at a logical level to enable the incorporation of diverse products to accomplish requirements in the functional grouping. This paper previews the use of three independent categories of functional product groupings: Mission, Utility, and Infrastructure. EPA development includes the establishment of functional groupings that enable capabilities to be agnostic of the underlying infrastructure, hence the reason for the separately categorized Infrastructure functional groupings. Utility functional groupings represent a class of applications that provide operational functionality used by more than one Mission functional grouping. The Utility functional groupings facilitate integration and orchestration of Mission functions. Mission functional groupings represent the rest of the non-infrastructure capabilities needed for the programs to accomplish their operations.

Sikorsky Airframe Full Spectrum Customer/Supplier Collaboration (Paper 1122)
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Darryl Toni

Sikorsky airframe collaboration with suppliers and military customers has been evolving over the past 20 years to continually decrease the cycle time for design development including customer concurrent oversight and airworthiness certification ("Information Week - Communication Aids Design", Reference 1). Future Vertical Lift (FVL) aircraft rapid development schedules including the Raider-X Future Attack Reconnaissance Aircraft (FARA) Competitive Prototype (CP) fly-off and subsequent production design have mandated the need for concurrency in contractor / customer awareness of design details. To address this need, Sikorsky has developed a uniquely collaborative system to share structural analysis applications, structural models and substantiating data as it emerges real-time in development from multiple sources with customer oversight to assure consistency and accuracy to satisfy requirements for rapid certification.


Test and Evaluation

Test and Evaluation I

Technical Session C: Wed. May 11, 2022 - 1:45 PM to 6:00 PM

Automatic Category A Takeoff for H145 – Development and Flight Testing (Paper 111)
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Carl Ockier, Daniel Reber, Marc Salesse-Lavergne, Paul Prevost

Most multi-engine helicopters are certified to operate in Category A. This means that in the event of an engine failure during takeoff or landing, the helicopter can either land safely on the helipad or continue the takeoff and establish level flight. As part of an Airbus effort to improve flight safety, the Category A takeoff from helipads was automated and first certified on the H160 helicopter. Automatization warrants correct maintenance of the flight path during takeoff and ensures a timely initial response to engine failure. On the H145 helicopter, development of the automatic Category A takeoff included elevated helipads and was adapted to the already certified H145 Category A takeoff procedures. Several hundred automatic takeoffs were performed on the H145 to develop, fine-tune, and certify the automatic takeoff mode including failure and abuse cases. Flight tests confirmed excellent mode performance, reduction in pilot workload and gain in flight safety.

Energy-Maneuverability as a Framework for Performance and Handling Qualities Testing (Paper 1300)
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John Tritschler, Brandon Dotson

Energy-maneuverability representations are an important tool that operational pilots use to understand helicopter maneuver performance across a wide range of conditions, though the potential for these representations to provide a unifying framework for performance and handling qualities testing has not been previously explored. The present work reports the results of an investigation in which handling qualities test results are combined with predictions of specific excess power in order to present a more holistic view of aircraft capability. The flight testing involved the break turn mission task element, which was flown across a range of airspeeds and bank angles to assess handling qualities over a broad flight envelope. The results of the flight test show that energy-maneuverability diagrams are uniquely suited to present an aircraft's performance and handling qualities in a single visualization. This study also proposes the concept of usable aircraft performance - i.e., the combination of both the performance to maneuver as well as favorable handling qualities - and suggests ways in which this concept can provide important insights into flight test results as well as a basis for developing future, integrated requirements for aircraft performance and handling qualities.

eVTOL Component Testing for Supporting Algorithmic Icing Detection (Paper 1248)
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Robert McKillip

Component testing in support of the development of a model-based algorithmic Icing Detection Filter (IDF) for use on eVTOL aircraft is described. The IDF represents an extension of prior work on holistic aircraft icing detection where data from individual electric motor controllers is monitored to track performance degradation effects due to accreted icing on rotating components. Icing accretion tests were conducted at a variety of conditions on representative components from a Lift+Cruise eVTOL configuration in both an icing hover chamber and an icing wind tunnel. Several different simulation tools were used to assess their capability to predict observed trends from the test data, and while icing accretion on rotating components was modeled sufficiently, non-rotating components appeared to accumulate ice more uniformly than suggested from the droplet trajectory analysis used.

Flight Test Validation of Thrust Axis Tactile Cueing System on AW609 Tiltrotor (Paper 1262)
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Lucia DeNicola, Luca Belluomini, Sid Xiao, Ahmad Haidar

Rotorcraft generations are defined by concurrent paradigm advances, and a key theme of next generation rotorcraft is the reduction of pilot workload through state-of-the-art fly-by-wire systems. A means of reducing pilot workload is envelope protection - allowing the crew to spend less time managing physical aspects of the aircraft and more time managing the mission. Tiltrotors operate in a wide envelope, hovering near the ground and cruising at 25,000 feet. The power management system needs to be versatile to protect against operating above transmission limits in helicopter mode and engine limits at high altitude in airplane mode. In this implementation, tactile cueing is used to provide the pilot immediate feedback to and management of the transmission torque. Through flight test, the methodology of power management using a first-limit based tactile cueing system is validated. This paper describes the feedback scheme as realized in the flight control system, then the dedicated engine handling flight test campaign and subsequent experimental results are presented.

Full-Scale Investigation of Rotor/Obstacle Interactions using an Elevated Fixed Platform (Paper 1153)
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Mark Silva, Eric Hayden, Leighton Myers, John Tritschler, John Holder

A unique shore-based facility and flight tests designed to investigate rotor/obstacle aerodynamic interactions under low wind speed conditions were planned and carried out in 2020 at Naval Air Station (NAS) Patuxent River. A temporary elevated fixed platform (EFP) sized to represent the aft half of an LPD-17 flight deck was built out of stacked CONEX shipping containers on a closed taxiway. The EFP walls were instrumented with ultrasonic anemometers to gather velocity flow field measurements as various rotorcraft executed simulated recovery profiles to and hover ladders near the EFP. The EFP was subsequently reconfigured to conduct a confined landing area investigation. The simultaneous acquisition of aircraft performance data and flow field data will be invaluable for the validation of the Navy's shipboard operations modeling and simulation tools, maximizing the Navy's return on investment in building the temporary EFP facility.

Lichten Award Paper: Automated Optical Rotor Blade Tip Clearance Tracking Using Artificial Intelligence Algorithms (Paper 1325)
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Timothy Schmidt

Analytical methods used to determine weapon projectile trajectory relative to helicopter rotor blade tips during most conceivable maneuvers tend to provide insufficient safety margins to accurately define a safe firing envelope. The flight test team at Sikorsky, a Lockheed Martin Company, was given the challenging task of measuring the weapon-to-rotor-blade-tip clearance during these maneuvers. The first-generation methodology was entirely manual using a flat vertical projection screen tangentially aligned with the rotor tip path plane to visualize the intersection between the weapon's projectile and blade tip path. While the first flight test program utilizing this technique was successful in its own right, by the time a second opportune flight test program was initiated, software tools had been developed to help automate the process, drastically increasing the amount of data that could be used to correlate with other aircraft state parameters. During the data processing, a training set was created that was used to build an Artificial Intelligence (AI) capable of performing the same task while bringing the processing time down from 4-5 frames per second (fps) to 130 fps. This enables real-time AI inference to be taken from digital video cameras on the helicopter and clearance measurements real-time processed and recorded to the instrumentation system or displayed for the pilot's awareness.

Thermodynamic Modeling for the Analysis of Non-Stabilized Flight Test Temperature Data (Paper 15)
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Denis Hamel, Carson Allgood, Thomas Cooper, Alexander Kolarich

Temperature is the result of underlying thermodynamically independent heat transfer modes of conduction, convection and radiation. Flight test methods have evolved to collect targeted data in support of most engineering disciplines – flight mechanics, aerodynamics, performance, dynamics – but little has changed in the field of thermal analysis, which relies mainly on stabilized temperature data requiring extended ground and flight test conditions. Traditional methods are time consuming and may result in imprecise extrapolation of steady-state temperatures. Temperature response is an exponential time decay function with time constant and asymptotic values defined by material properties, compartment volume, and heat transfer mode. A generalized heat transfer block diagram and matrix state space model presents each heat transfer mode in a multi-zonal system of helicopter compartments. The model is defined via system identification tests that isolate the heat transfer effects – electrical heating, passive cooling, solar radiation, and airspeed effects. The resultant model can be used with time-variant standardized hot day solar radiation and ambient temperature profiles to yield temperature versus time profiles for direct comparison with equipment qualification criteria. This methodology is the basis for more efficient and more accurate results than current evaluation techniques.

Test and Evaluation II

Technical Session D: Thurs. May 12, 2022 - 10:15 AM to 12:15 PM

Dynamic Stall Investigation on a Rotating semi-elastic Double-swept Rotor Blade at the Rotor Test Facility Göttingen (Paper 110)
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Martin Müller, Armin Weiss, Johannes Niklas Braukmann

Experimental investigations of three-dimensional dynamic stall on a four-bladed Mach-scaled semi-elastic rotor with an innovative double-swept rotor blade planform are presented. The study focuses on the coupling between the aeroelastic behavior of the blade and the underlying aerodynamics. Blade bending moment and flap displacement measurements were conducted using strain gauges and optical tracking of blade tip markers. The aerodynamic behavior was characterized by means of unsteady surface pressure measurements using unsteady pressure-sensitive paint (iPSP) across the outer 65 % of the blade span and fast response pressure transducers at discrete locations. Different cyclicpitch settings were investigated at a rotation frequency of frotor = 23.6 Hz, that corresponds to blade tip Mach and Reynolds numbers of Mtip = 0.282 - 0.285 and Retip = 5.84 - 5.95 x 105. The findings reveal a detailed insight into the non-linear behavior in the flap movement during downstroke. iPSP and pressure transducer data indicates that this non-linear flap behavior is caused by a radially phase-shifted dynamic stall process at the forward and backward swept part of the blade.

Model Based Blade Attachment Stiffness Evaluation of the MERIT Rotor in Hover with Photogrammetry and Digital Image Correlation (Paper 1252)
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Verena Heuschneider, Florian Berghammer, Amine Abdelmoula, Manfred Hajek, Jayant Sirohi

The paper focuses on the analyses of photogrammetry measurement results on the two-blade, hingeless MERIT rotor with diameter 1.8 m at the two rotor speeds 900 and 1800 RPM and collective pitch angles 0-12 in hover and their comparison to simulation results calculated in CAMRAD II. Blade tip displacements in flap, lead-lag, axial, and torsional direction are shown as a function of collective pitch and rotor speed. Radial displacements in flap and lag direction depict the influence of the pitch bearing play and blade attachment. The structural blade model is validated by using static DIC deformation measurements and shows very good agreement. Calculated and measured thrust polars match very well with the use of a free wake model in the simulation. The combination of measured flapping angle and calculated flapping moment gives a stiffness estimation for a virtual flap hinge. The influence of hinge offset and stiffness are shown by parameter adjustment. Flap deformations of the rotating blades leave a tip flap offset of less than 1 mm in average, which corresponds to the statically measured effect of the bearing play at the blade tip. The study shows that photogrammetry is a valuable tool to identify and tune parameters of a numerical model.

Practical Aspects of Designing Background-oriented Schlieren (BOS) Experiments for Vortex Measurements (Paper 1224)
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Clemens Schwarz, Johannes N. Braukmann

Setup-related aspects of background-oriented schlieren (BOS) experiments are discussed focusing on a sensitivity parameter S, which represents the relation between light deflection and resulting BOS signal, and the geometric blur. An analytic expression for the geometric blur by means of the circle of confusion (CoC) was derived which shows a proportional relation to the sensitivity factor S. The theoretical findings were validated in a reference experiment using generic distortions in glass plates. It was found that the filtering effect of the blur decreases the maximum background shift and its influence can be expressed with a blur loss factor B, which depends on the size of the CoC in relation to the investigated object. Multiplying the setup sensitivity S with the blur loss B results in the effective sensitivity Seff that determines the maximum achievable BOS signal of a schlieren object. For the investigated reference objects, the maximum effective sensitivity Seff was found to occur at CoC sizes in the object domain from 2.5 to 3.8 times the extent of the investigated objects. A step-by-step method is proposed for designing BOS experiments to obtain a maximum signal strength. The design parameters are further discussed specifically in regard to rotor tip vortex visualization, for which a variety of previously reported experiments are compared. A simple prediction method for the BOS signal of blade tip vortices is proposed and validated with experimental data from a rotor test stand. The application of the method to rotor systems of different size shows the requirement for increasingly higher sensitivity values for visualizing vortices of small-scale rotors.

RACER Compound Helicopter: Operational Wireless FTI Data Transfer from ROTOR's up to Fuselage (Paper 1171)
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Marc Seznec, Jean Grégoire Ivanoff

Acquiring helicopter rotor data is always a very sensitive point that requires at least "effort and special attention". This data acquisition is generally managed by a "physical link" (slip ring for example) whereas wireless products are now present everywhere with a technology more than promising. The objective of this paper is to show how the wireless technology was developed within the framework of RACER COUMPOUND HELICOPTER to monitor the three rotors in accordance with CS29 regulations for the mechanical assembly, DO160 rules for environmental constraints and IRIG 106 standard for Flight Test Instrumentation domain notwithstanding that this wireless acquisition means will be used on a daily basis to monitor the data from the three rotors of the RACER. The paper provides an overview of this project, supported by the CEE (Horizon 2020/CS2), and from the initial requirement up to the operational results obtained during the flight test campaigns carried out on the H175. Finally, this paper, based on this use case ( RACER oriented) , aims also to open the perspective of using such a wireless application on helicopters in service in the frame of health monitoring system


Unmanned VTOL Aircraft and Rotorcraft

Unmanned I

Technical Session B: Wed. May 11, 2022 - 8:00 AM to 12:15 PM

A Novel Co-simulation framework for Verification and Validation of GNC Algorithms for Autonomous UAV (Paper 77)
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Victor Dezobry, Federico Cappuzzo, Domenico Bianchi, Stefano Di Gennaro

The use of autonomous Unmanned Aerial Vehicles (UAVs) in commercial applications has the potential to disrupt several industries. To effectively cover the broad spectrum of possible applications, UAV integrators require the ability to develop drone platforms that meet the requirements specified for the missions to accomplish. This is not limited to the correct sizing of the UAV physical subsystems, but also on the Guidance, Navigation, and Control (GNC) algorithms that enable the UAV to operate autonomously in an efficient way. System simulation can provide a valuable support to both activities. In the predesign phase, it allows exploring the design space, analyzing the performance of multiple UAV variants, and selecting the most promising concepts. Once this phase is completed, the resulting dynamic and multi-physics performance model of the UAV presents a sufficient fidelity to support the continuous development of GNC algorithms. Thanks to a novel co-simulation framework proposed in this paper, the performance model can then be integrated in a co-simulation framework to include perception sensors, mission environment, and GNC algorithms. Two use cases based on the framework are presented and discussed.

Acceleration of Heuristic Motion Planning for Unmanned Aerial Vehicles (Paper 1200)
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Neelanga Thelasingha, Kaushik Nallan, A. Agung Julius, Sandipan Mishra

In this paper, we propose a heuristic-based fast motion planning framework which can be readily incorporated by the on-board path planner of Unmanned Aerial Vehicles (UAVs) to generate safe and efficient trajectories while traversing through challenging environments cluttered with obstacles. The proposed planning technique is effective for the scenarios where the exact obstacle locations need to be detected during flight and the obstacle detection range is limited by degraded environmental conditions like fog. Unlike many kinematic based planning strategies, the generated planned trajectories can be tracked effectively as they preserve the dynamics of the UAV. The planning problem is graphically represented by discretizing input and state spaces to facilitate usage of discrete search algorithms. We also propose a heuristic calculation strategy based on dynamics relaxation to accurately encode the obstacle. The Bellman optimality condition is used to modify the heuristic to facilitate faster search. This faster planning contributes to requiring a reduced minimum obstacle detection range for receding horizon planning. The proposed algorithm has been compared against an off-the-shelf nonlinear program solver and the proposed method produced superior planning times and feasible trajectories avoiding collisions. Further, we analyzed the sub-optimality of the planned trajectories and the minimum obstacle detection range required for the receding horizon planning framework.

Feature-Based Vision For Stochastic Motion Tracking Under Partial Occlusion (Paper 1208)
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Abhishek Shastry, Anubhav Datta, Inderjit Chopra

This work is a continuation of the ship-deck landing research presented at VFS Forum 2021, which established a vision-based method to track ship-deck motion before landing when conditions are favorable. This paper makes advancements on two fronts: 1) Demonstrating the effect of velocity feedback on tracking stochastic deck motion, and 2) Developing a new feature-based vision system to detect and track decks in a wide range of challenging environments. The new vision system can detect decks in occlusion as high as 95%. It can also handle a wide range of illumination conditions varying from 20,000 lux of a bright day to 0.01 lux of a dark room. Moreover unlike fiducial-based vision systems that require a custom well designed marker or tag to work, the new vision system can work with generic ship-decks or landing platforms.

Transition Trajectory Planning and Control for Quadrotor Biplanes in Obstacle Cluttered Environments (Paper 1228)
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Kristoff McIntosh, Jean-Paul Reddinger, Jae Woo Kim, Sandipan Mishra

This paper presents a trajectory planner and a control architecture capable of guiding a quadrotor biplane tailsitter (QRBP) through obstacle cluttered environments. The trajectory planner is formulated as an optimization problem that uses a differentially flat, point-mass model of a QRBP that considers wake effects on the aerodynamic forces generated during transition. Obstacle avoidance is realized as a state constraint in the optimization problem that defines 'no-flight zones' or regions where the QRBP cannot enter based on obstacle size and safety clearance requirements. The 6DOF control architecture is designed as a set of cascaded dynamic inversion controllers that use the aerodynamic feedforward signals produced by the trajectory planner to complete the inversion in the outer loop. To show the effectiveness of the obstacle avoidance path planning methodology, time-optimal trajectories are generated for two flight missions (the hover to forward flight and vice versa) through cluttered environments. The control architecture is validated on these two cases using a high fidelity flight dynamics simulation of a QRBP. The computational efficiency of the trajectory planner and the tracking performance of the control architecture are then empirically validated.

Unmanned II

Technical Session D: Thurs. May 12, 2022 - 10:15 AM to 12:15 PM

A Study of the Influence of Motor-ESC Dynamics on Multi Rotor Vehicle Disturbance Rejection (Paper 91)
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Rhys Lehmann, Stef Ceelen

Unmanned aerial systems (UAS) are being operated in increasingly complex environments, and robust disturbance rejection is critical in enabling these systems to operate effectively in these environments. Previous studies have indicated that the actuator dynamics of multirotor systems can be a major limiting factor in the overall disturbance rejection. In this paper, a detailed study of multirotor actuator dynamics was conducted based on experimental results collected during bench tests and in a small wind tunnel. Time and frequency domain system identification techniques were utilised to characterise the actuator response across a broad range of throttle levels. Comparisons were made between various Electronic Speed Controller (ESC), motor and rotor configurations, highlighting the spread of actuator dynamics with different hardware selections. In addition, a range of factors associated with improved actuator response characteristics were identified. Subsequently, two actuator configurations were selected for evaluation in a flight test using a small quadrotor vehicle, based on results from laboratory tests. A measureable improvement in vehicle disturbance rejection from the baseline configuration to the updated hardware was demonstrated during the flight test, validating the lab results.

Flight Test Measurement of Quadrotor Performance at Varying Sideslip Angles (Paper 1258)
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Matthew McCrink, Dhuree Seth, Sage Herz

This research investigated the effects of the aerodynamic interaction of the rotor-rotor and the rotor-body as the vehicle transitions between the plus (+) and cross configurations (x), through flight testing. A highly-instrumented vehicle was used to measure the change in rotor performance across skew angles ranging from -45-degree to 45-degree for various vehicle speeds. In addition, the work also addresses the uncertainty associated with external sources such as wind, which has a first-order importance to the trim attitude, and hence the thrust and moments generated by the rotors. The flight-test data demonstrates the variation in performance owing to interactions between the rotors and body structure for a range of vehicle flight speeds and skew angles.

Simulation Model Update and Optimized Control Design of a Sub-Scale Flybarless Helicopter (Paper 107)
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Jeffry Walker, Mark Tischler

This paper demonstrates simulation model updates using black-box filters and the application of Froude scaled ADS-33 specifications to design an inner- and outer-loop control system for a sub-scale flybarless helicopter. The black-box filters provide a simple and effective update to the vehicle plant resulting in accurate translation of design to flight. Design specifications for manned rotorcraft specified by ADS-33 are Froude scaled and applied as design minimums with adjustments made to accommodate limiting vehicle dynamics. The control system is optimized using CONDUIT and design margin optimization is used to maximize the vehicle performance. Flight testing is conducted to validate the control laws using system identification and the vehicle performance is demonstrated using scaled Mission Task Elements.

Validation of Scanned Propeller Geometry for Simulation Modeling (Paper 1231)
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Jean-Paul Reddinger, Michael Avera, Rajneesh Singh

3D scanning technology is used to acquire the airfoil geometry, twist, taper, and chord distribution of several propellers spanning 10-15" in diameter. Sectional airload properties are then derived from the geometry using the HAM2D computational fluid dynamics solver and construct an RCAS model of each propeller. A comparison between the scan-derived RCAS model and experimentally obtained thrust and torque measurements in hover shows generally reasonable agreement for integrated airloads, with predictions within 5% of thrust and 8% of torque. Representative scanning accuracy error is injected in the model to identify the mode of error which has the largest impact on the predictions, between airfoil geometry (camber), chord length, and twist distribution. It is determined that imprecise scanning and error in the measured twist distribution provides the greatest source of modeling error(5.9%) through this process, likely due the the reliance of these parameters on as little as two scan points. The importance of Reynolds effect modeling at this scale is also confirmed.