Abstract:
The CH-53K® heavy-lift helicopter, contracted under the Heavy Lift Replacement Program, is nearing the end of development, and was recently approved for Full Rate Production (FRP). The CH-53K features a full authority Fly-By-Wire (FBW) flight control system with Model Following Control Law architecture, chosen to address several overarching requirements including Level 1 Handling Qualities in accordance with ADS-33E-PRF. Early system identification of the air vehicle and tuning of the Flight Control System (FCS) allowed for an incremental expansion in airspeed, altitude, and maneuvering envelope. During the maneuver envelope expansion forward cyclic control margin (B1s) became a limiting factor. The reduced margin was noted during aft CG configurations when expanding load factor. Control margin limits and Do Not Exceed (DNE) criteria were established for safe expansion early in the test planning process. These criteria were based on best practices, specifications, desktop, and simulation models as well as engineering judgment prior to first flight. Some influence of the criteria likely derived from the defunct MIL-H-8501A Military Specification 'General Requirements for Helicopter Flying and Ground Handling Qualities,' with a notable absence of quantitative criteria in ADS-33E-PRF. As the full authority FBW control system was demanded to work within a smaller forward portion of the control authority envelope for mission-representative aft CG configurations, the DNE criteria challenged the long-standing general guidelines regarding control margins. Furthermore, control margin DNE's were experienced primarily during maneuver recovery-not through pilot input but via a closed loop response to the Model Following architecture and had the potential to restrict unnecessarily the boundaries of the Operational and Service Flight Envelope (OFE/SFE). Challenged with the task of envelope expansion, the Integrated Test Team (ITT) comprising contractor and government flight test engineers and pilots, worked to refine flight test technique and continuation criteria to validate the maneuvering requirements of the Air Vehicle Specification (AVS). Ultimately, the ITT developed a criterion for expansion of a modern FBW system to demonstrate the full capability in all corners of the envelope and deliver a superbly flying aircraft to the end customer, the United States Marine Corps (USMC).