Design Optimization for Soft-Inplane Tiltrotor Whirl Flutter Stability Improvement

This paper examines the influence of parametric variation in individual rotor/wing design variables on whirl flutter stability improvements of a soft-inplane tiltrotor. This is followed by the use of formal gradient-based design optimization. The simulations are based on a full-scale soft-inplane Boeing Model 222 rotor on a semi-span wing. For this baseline configuration the flatwise and chordwise flexibility are both outboard of the pitch bearing and produce destabilizing pitch-flap and pitch-lag couplings. The most influential rotor parameter in improving flutter stability characteristics is the distribution of flatwise flexibility vis-à-vis the pitch bearing, and large improvements are observed when flatwise flexibility is moved inboard. However, this can result in large increase in lag frequency and turn the design into a stiff-inplane configuration. Using frequency constraints during optimization of rotor parameters and carefully selecting the airspeed range over which optimization is conducted results in improvement in sub-critical wing mode damping and increase in critical flutter speed. Other rotor parameters changes that have some benefit are reduction in overall chordwise stiffness, positive increase in pitch-flap and pitch-lag coupling, and increase in control system stiffness. Wing design parameters are unable to increase the critical flutter speed but can improve the sub-critical damping, primarily through an increase in wing torsion frequency and a simultaneous reduction in wing beam mode frequency. Optimization using a combination of rotor/wing design parameters yields a 35 knots increase in whirl flutter speed (from 390 to 425 knots), and an increase in sub-critical damping from less than 1% to in excess of 2% critical.