Whirl flutter is an aeroelastic instability that affects propellers/rotors and the aircraft on which they are mounted. The complexity of its behaviour and analysis increases significantly with the addition of nonlinear effects. With their long and flexible rotor blades, tiltrotor aircraft are particularly susceptible. This paper investigates the impact of structural nonlinearity on the whirl flutter stability of a basic gimballed rotor-nacelle model, compared to a baseline linear stiffness version. A 9-DoF model with quasi-steady aerodynamics and blades that can move both cyclically and collectively in both flapping and lead-lag motions was adopted from existing literature. The nonlinearities investigated in this paper are cubic and quintic softening and hardening introduced to the gimbal flapping stiffness. The investigation is conducted through a combination of bifurcation and eigenvalue analyses, supplemented by time simulations. In some cases, the nonlinearities are shown to cause whirl flutter behaviour to exist in parameter value regions that are predicted to be stable by linear analysis. This impact is fully captured in the redrawn system stability boundary.
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