AN IMPROVED ROTOR AIRFRAME COUPLING METHOD FOR NASTRAN AIRFRAME VIBRATION ANALYSIS

An elastic, multiple-degree-of-freedom rotor model is developed for use with finite element models of helicopter airframes. The method incorporates representative hub impedances into the finite element model at all frequencies of interest, rather than simulating the rotor as a lumped mass, which is the conventional method of representing the rotor in airframe dynamic analyses. The method uses rotor blade mode shapes developed in the rotating-coordinate system to derive the rotor equations of motion in the airframe fixed-coordinate system. The rotor model is developed as blade force-displacement transfer functions and is then reformulated into a set of second-order differential equations with frequency-independent coefficients. These equations are appended to the airframe math model. The rotor math model includes structural damping and two-dimensional strip theory aerodynamics. The method is demonstrated using a simple cantilever example and calculations for the Bell Helicopter Model 4BW development aircraft. It is partially validated by comparing calculated response transfer functions with vibration test results of a 0.2-scale semispan wind tunnel model of the Bell-Boeing V-22 Osprey. The coupled rotor-airframe math model presented is particularly suitable for airframe frequency tuning with respect to rotor natural modes and for airframe forced-response calculations.