INVESTIGATION OF COUPLED ROTOR-DRIVETRAIN-AIRFRAME-ENGINE VIBRATIONS USING DYNAMIC SUBSTRUCTURING

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Date
2020
Authors
Sidle, Stacy
Advisor
Chopra, Inderjit
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Abstract
Excessive vibrations continue to be a problem that rotorcraft designers are unable to address until flight testing. Accurate prediction of vibrations throughout the aircraft is hindered by the complexity of the models required. This dissertation focuses on utilizing frequency based modeling and substructuring methods to enable full coupling between the comprehensive analysis modeling of the rotor aeromechanics and finite element models of the drivetrain, airframe, and engines. A substructuring methodology is developed 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, a twin-engine, four-bladed helicopter is analyzed using NASTRAN-based models of the airframe and engines. 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 8P engine response are identified without increasing 4P response. A compromise between 4P and 8P 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. Comprehensive vibration analysis of a rotor-airframe-engine-drivetrain system using a time-domain modal coupling approach was conducted. Pair-wise couplings of components were performed to isolate the contribution of each component to the complete coupled system, and the effect of each component (airframe and drivetrain/engine) on rotor loads and hub loads was studied. The drivetrain model was a 6-dof model consisting of inertia and torsional spring elements, while the airframe model used was a NASTRAN superelement of a detailed finite element airframe model. Although drivetrain coupling resulted in elastic twist of the shaft by less than 0.02 degrees, there were noticeable reductions in the 4/rev chordwise blade bending moments as well the 4/rev vertical hub force. The airframe coupling produced very small hub motions, less than $1\times10^{-4}$ inches, and showed almost identical trends in both the blade loads and hub loads. 8/rev hub forces and moments were significantly affected by both the airframe and drivetrain coupling.
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