Rotor design options for improving tiltrotor whirl-flutter stability margins

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 was designed to be representative of a high-speed tiltrotor. While the thinner wing has lower drag, it also has lower stiffness, reducing whirl-flutter stability. The r otor blade design was modified to increase the stability speed margin for t he thin-wing design. Small rearward offsets of the aerodynamic-center locus with respect to the blade elastic axis created. large increases in the sta bility boundary. The effect was strongest for offsets at the outboard part of the blade, where an offset of the aerodynamic center by 10% of tip chord improved the stability margin by over 100 knots. Forward offsets of the bl ade center of gravity had similar but less pronounced effects. Equivalent r esults were seen for swept-tip blades. Combinations of tip sweep, control-s ystem stiffness, and delta-three were also investigated. A limited investig ation of blade loads in helicopter and airplane configuration indicated tha t proper choice of parametric variations can avoid excessive increases in r otor loads.