Rotor Design for Whirl Flutter: An Examination of Options for Improving Tiltrotor Aeroelastic Stability Margins.

摘要

Rotor design changes intended to improve tiltrotor whirl-flutter stability margins were analyzed. A baseline analytical model similar to the XV- 15 '23% thick wing' was established, and then a 15% thick wing design was developed. A simplified finiteelement model of the airframe was used for the structural design. This thinner wing is representative of a wing for a high- speed tiltrotor with good aerodynamic performance. While it has lower drag, it also has lower stiffness, reducing the flight speed for whirl-flutter instability. Changes to the rotor blade design were investigated with the objective of increasing the stability speed margin for this thinwing design. Small rearward offsets of the aerodynamic center with respect to the blade elastic axis and pitch axis created large increases in the stability boundary. The effect was strongest for offsets at the outboard part of the blade, where an offset of 10% of tip chord improved the stability margin by over 100 knots. Forward offsets of the blade center of gravity had similar but less pronounced effects. A limited investigation of blade loads in helicopter and airplane configuration indicated that proper choice of parametric variations can avoid excessive increases in rotor loads.