Stability analysis of whirl flutter in a rotor-nacelle system with freeplay nonlinearity

Tiltrotor aircraft are growing in importance because of their unique flight envelope. However, aeroelastic stability - particularly whirl flutter stability - is a major design influence that requires accurate prediction. Research efforts to make future tiltrotor aircraft larger and faster result in more difficult prediction of whirl flutter onset. Additionally, several nonlinearities that may be present, such as freeplay, are often neglected in analyses for simplicity, or they are investigated using stability analysis methods that do not capture their effects. However these nonlinearities can be significant, sometimes even reversing the stability predictions from linear analysis methods. This paper investigates the effect of a freeplay nonlinearity in the pitch degree of freedom of two rotor-nacelle models of contrasting complexity. The modelling approach and the stability analysis methods employed are explained. Ultimately the freeplay nonlinearity is shown to have a complex effect on the systems' dynamics, including creating the possibility of whirl flutter in parameter ranges that linear analysis methods predict to be stable. This effect is demonstrated via a comparison of stability boundaries for the linear and freeplay versions of the basic model.