Optimal Tiltrotor Blade Twist to Extend Whirl-Flutter Stability Boundaries

The paper investigates how rotor blade twist in tiltrotor aircraft influences the whirl flutter stability boundaries. Preliminary optimizations indicate that the whirl flutter speed can be increased if the blade twist slope is reduced. This positive effect results from the shifts in the overall blade thrust toward the blade tip, increasing the flap wise bending moment at the blade root and, consequently, the trim coning angle. This, in turn, generates a positive pitch-lag coupling that improves the whirl flutter stability boundaries. However, the shift of high sectional thrust forces toward the blade tip sections returns a higher induced drag, showing the tendency to increase the power required. The paper shows that by using more sophisticated blade twist laws and considering also other parameters in the optimization problem, like the wing thickness, it is possible to identify configurations that are able to improve the whirl flutter performance, without penalizing the power required both in forward flight and hover. A detailed tiltrotor model representative of the Bell XV-15 is used to display the effectiveness of the proposed approach. The examples show that increases up to 18% of the whirl flutter speed are achievable without penalties in the performances and with the additional benefit of a benign impact on rotor pitch link loads since the section aerodynamic center is shifted, reducing the control moments.