Effects of Rotor Design Variations on Tiltrotor Whirl Mode Stability

Tiltrotor performance is constrained by coupled wing/rotor whirl-mode aeroelastic instability. Two parallel research efforts are currently underway at NASA Ames Research Center to investigate methods of expanding the whirl-mode stability boundary for proprotors. Results of the first effort, analyses of design changes to the V-22 rotor, indicate that moderate amounts of blade sweep at the tip substantially increase the stability of the symmetric wing beam bending mode. Adding either a chord-balance mass or a tip mass on a boom considerably enhances the beneficial effects of sweep. To validate the predictions at minimum cost, a second re-search effort is to carry out a small-scale wind-tunnel test of a modified rotor. Cost will be minimized by exploiting the technology of radio-controlled models, from which off-the-shelf components can be used in a dynamically-scaled wind-tunnel model with only minor modifications. This report summarizes the results to date for the V-22 and describes the proposed wind-tunnel proprotor model