Influence of Geometric Coupling on the Whirl Flutter Stability in Tiltrotor Aircraft with Unsteady Aerodynamics

Abstract

A further improvement is attempted of an existing analytical model for an accurate prediction of the aeroelastic stability of a tiltrotor aircraft. A rigid-bladed rotor structural model with the natural frequencies selected appropriately in both the flapping and lagging motions is used. The geometric coupling between the wing vertical bending and torsion is also included. The pitch-flap and pitch-lag couplings are also added. Three different aerodynamic models are combined with the structural model: two quasi-steady models and one full unsteady aerodynamics model. Frequency domain analysis is conducted to predict the whirl flutter stability boundary. It was found that the geometric coupling must be included at an appropriate level to predict the whirl flutter boundary accurately. The addition of the wing bending/torsion coupling and the control system flexibility improves the prediction accuracy significantly. Unsteady aerodynamics influences the stability prediction. The whirl flutter boundary is predicted to be less when quasi-steady aerodynamic models are used as compared with that for unsteady aerodynamics. For these two cases, different behaviors regarding the intersection among the relevant structural modes, especially between the lower frequency rotor modes and the wing modes, are observed from the frequency and damping prediction.