An Improved Rotor-Airframe Coupling Method For Nastran Airframe Vibration Analysis

An elastic, multi-degree-of-freedom rotor model is developed for use with finite element models (FEM) of helicopter airframes. The method incorporates correct hub impedances into the FEM at all frequencies of interest, rather than simulating the rotor as a lumped mass, which is the current/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 forcedisplacement transfer functions and is then reformulated into a set of second order differential equations with frequency independent coefficients. These equations are then appended to the airframe math model. The rotor math model includes structural damping and two-dimensional (2-D) strip theory aerodynamics. The method is demonstrated using a simple cantilever example and is validated by comparing calculated response transfer functions with vibration test results of a 0. 2-scale semispan wind tunnel model. An alternative method based on component mode synthesis is also presented and discussed. The coupled rotor/airframe math model presented herein is particularly suitable for airframe frequency tuning with respect to rotor natural modes and for airframe forced response calculations.