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Abstract
This paper presents a methodology to analyze the coupled structural dynamic response of an elastic airframe and engines of a helicopter in response to main rotor and tail rotor hub loads. Transfer functions of individual components (airframe, engine, mount struts and torque tube) are coupled together using a sub-structuring approach to obtain consistent coupled solutions of the entire system. Using this approach, the two-engined and four-bladed S-92 helicopter is analyzed using NASTRAN-based models of the Sikorsky airframe and GE CT7-8 engine. This ultra-efficient substructuring approach is validated against the fully coupled NASTRAN model using forced response studies. Characteristics of the mount properties, i.e., the torque tube stiffness, and aft mount stiffness and damping are systematically varied to study their effect on the engine vibration response. The fore and aft mount element properties for minimizing the 8/rev engine response are identified without increasing 4/rev response. A compromise between 4/rev and 8/rev response is also identified from parametric studies of rear mount properties, using just 3 parameters to represent the design space. Using the sub-structuring approach presented here, future studies can be performed to rapidly match airframe characteristics with available engines at approximately 1000 times the speed of running a detailed finite element model (millions of degrees of freedom), without any reduction in accuracy.

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  Investigation of Engine-Airframe Vibration Due to Main Rotor Hub Loads Using a Substructuring Framework

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