Vortex particle method whirl-flutter predictions of a tiltrotor with wing extensions

This work seeks to investigate the impact of wing extensions and aerodynamic modeling on tiltrotor aeroelastic stability predictions. While performance calculations, particularly for improvement from wing extensions, are typically conducted using higher-fidelity aerodynamic tools which include interactional effects, whirl-flutter predictions commonly use no inflow model or uniform inflow. Three semispan configurations, baseline, reduced stiffness, and wing extension reduced stiffness, of the XV-15 tiltrotor are used to compare stability predictions for the wing beam bending, chord bending, and torsion modes using uniform inflow, dynamic inflow, and a vortex particle method. In addition, comparisons are made between linearized and transient stability predictions. Several key conclusions are made from the results. Modal interaction between the rotor lag mode and wing beam bending and torsion modes occurs during certain airspeeds in a nonlinear dynamic interaction in which the linearized assumption in the model breaks down and transient predictions are needed. In general, the wing chord bending mode both does not have a similar nonlinear interaction with the rotor lag mode and is not strongly effected by aerodynamic model. Additionally, frequency predictions and low-speed damping predictions show insensitivity to analysis method. The speed at which the wing beam bending mode becomes unstable increases with increasing levels aerodynamic fidelity, with variations of up to 75 knots. Finally, the effectiveness in whirl-flutter mitigation through the addition of a wing extension varies with different aerodynamic models. Uniform inflow predicts a delay in whirl-flutter of 25 knots, dynamic inflow predicts 43 knots, and the vortex particle method predicts 70 knots.