Comparison of Tiltrotor Whirl Flutter Analyses with Gimballed and Hingeless Hub Models

This study investigates and compares the whirl flutter characteristics of two different hub models, gimballed and hingeless hubs, for the tiltrotor aircraft in cruise flight. This study uses the rotorcraft comprehensive analysis code, CAMRAD II, to model the proprotor, semi-span wing, and pylon, and to investigate the whirl flutter instability. The two analytical tiltrotor models are based on two proprotors developed by the U.S. Army/NASA, and KARI. The proprotor frequency characteristics depend on the hub type and have influence on the wing-rotor interaction to cause whirl flutter. In addition, it is shown that the rotor regressive lag mode frequency is an important system design parameter for a hingeless tiltrotor, whereas the rotor regressive gimbal mode is important for a gimballed tiltrotor.