Design Optimization for Improved Soft In-Plane Tiltrotor Aeroelastic in Airplane Mode

This paper examines the influence of parametric variation in individual rotor/wing design variables on whirl flutter stability improvements of a soft in-plane tiltrotor. This is followed by the use of formal gradient-based design optimization. The simulations are based on a full-scale soft in-plane Boeing model 222 rotor on a semispan wing. For this baseline configuration, the flatwise flexibility 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-a-vis the pitch bearing, and large improvements are observed when flatwise flexibility is moved inboard. However, this can result in a large increase in lag frequency and turn the design into a stiff in-plane configuration. Using frequency constraints during optimization of rotor parameters and carefully selecting the airspeed range over which optimization is conducted results in improvement in subcritical 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 subcritical 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 kt increase in whirl flutter speed (from 390 to 425 kt) and an increase in subcritical damping from less than 1 % to in excess of 2% critical.