Rotor design options for improving tiltrotor whirl-flutter stability margins

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Rotor design options for improving tiltrotor whirl-flutter stability margins

Acree, C.W. jun. / Peyran, R.J. / Johnson, W.

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 Iower drag, it also has Iower stiffness, reducing whirl-flutter stability. The rotor blade design was modified to increase the stability speed margin for the thin-wing design. SmaII 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 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 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 were also investigated. A limited investigation of blade Ioads in helicopter and airplane configuration indicated that proper choice of parametric variations can avoid excessive increases in rotor loads.