Rotor design changes intended toimprove tiltrotor whirl-flutter stability margins Were analyzed. A baselineanalytical model similar to the XV-15 (23% thick wing) Was established, and then a 15% thick wing Was designed to he representative of a high-speed tiltrotor. While the thinner wing has lower drag, it also has lower stiffness, reducing whirl-flutter stability. The rotor blade design was modified to increase the stability speed margin for the thin-wing design. Small rearward offsets of the aerodynamic-center locus with respect to the blade elastic axis created, large increases in the stability boundary. The effect was strongest for offsets a t 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 blade center of gravity had similar but less pronounced effects. Equivalent results were seen for swept-tip blades. Combinations of tip sweep, control-system stiffness,and delta-three werealsoinvestigated. Alimited investigationofbladeloadsin helicopter and airplaneconfiguration indicated that proper choice of parametric variations can avoid excessive increases in rotor loads. Notation blade section aerodynamic center, positive afI of EA blade chordwise center of gravity, positive forward of EA thrust coefficient, divided by solidity elastic axis blade quarter chord, positive aft of EA rotor radius wing thickness-to-chord ratio change in blade chordwise QC or CG position kinematic pitch-flap coupling ratio advance ratio (flight speed divided by tip speed)

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