Nonlinear Dynamic Characteristics of the Rotor Bifilar Absorber

A coupled rotor/bifilar/airframe analysis has been developed under contract to the U.S. Army Structures Lab (AVRADCOM). This analysis has been used to study the dynamic characteristics of the centrifugally tuned, rotor-hub-mounted, bifilar vibration absorber. The performance of the bifilar absorber is shown to be a function of its basic design parameters: dynamic mass, damping and tuning. Since the absorber acts to reduce the motion of its attachment point to the main rotor hub, hub impedance is of equal importance. For the case of a bifilar absorber which reacts cyclic response (e.g., 3/rev rotating response on a four-bladed rotor), the degree of symmetry of hub impedance in the two orthogonal directions also influences absorber effectivity. Because of manufacturing tolerances, every individual bifilar mass can be slightly different from the others in terms of its basic parameters. Results are presented which show the cause of dissimilar bifilar mass motions and the resulting non-N/rev excitation of the airframe. Flight test results for the BLACK HAWK helicopter which demonstrate the key results from the analysis and their correlation are also presented.