Advanced analysis on tiltrotor aircraft flutter stability, including unsteady aerodynamics

The whirl-flutter-instability phenomenon imposes a serious limit on the forward speed in tiltrotor aircraft. In this paper, an advanced analysis is formulated to predict an aeroelastic stability for a gimballed three-bladed rotor with flexible wing based on three different types of aerodynamic model. Among them, the one with the full unsteady aerodynamics is most sophisticated, because it may represent more realistic operating conditions. A nine-degree-of- freedom model is newly developed to predict the complete tiltrotor aircraft. Numerical results are obtained in both time and frequency domains. A generalized eigenvalue is used to estimate whirl-flutter stability in the frequency domain, and the Runge-Kutta method is used in the time domain. Control system flexibility is further included in the present analysis to give the capability for a more accurate stability prediction. Results from such an improved analysis are validated with the other existing predictions and show good agreement. The present model with unsteady aerodynamics will be extended to further consider an aerodynamic interaction between the rotor and wing.