Coupled Vibration Prediction of Rotor-Airframe-Drivetrain-Engine Dynamics

Comprehensive vibration analysis of a rotor-airframe-engine-drivetrain system using a time-domain modal coupling approach was conducted. Pair-wise couplings of components (airframe and drivetrain/engine) were performed to isolate the contribution of each component to the complete coupled system, and the effect of each component on blade loads and hub loads was studied. The drivetrain model is a 6-dof model consisting of inertia and torsional spring elements, while the airframe model is a NASTRAN superelement of a detailed finite element airframe model for a medium-lift utility helicopter. Drivetrain coupling resulted in elastic twist of the rotor shaft by less than 0.02 degrees, but there were noticeable reductions in the chordwise blade bending moments as well the 8/rev hub torque. The airframe coupling produced very small hub translation amplitudes, less than 5×10⁻⁴ inches, however it had a significant impact on the higher harmonic flap bending, increasing the 9/rev flap moment by up to 60% near the root.