Optimized Aeroelastic Couplings for Alleviation of Helicopter Ground Resonance

Abstract

Formal optimization methods are used to determine a combination of aeroelastic coupling parameters that can alleviate ground resonance instability of a soft-inplane rotor. Optimization at a prescribed rotational speed is unable to stabilize ground resonance, as the stability objective function is satisfied merely by moving resonance to a lower rotational speed. A moving *point optimization procedure, attempting to stabilize at the rotational speed where damping is a minimum, during each optimizer iteration, is successful in stabilizing the regressing lag mode at moderate collective pitch. However, these optimal aeroelastic couplings are destabilizing near zero thrust conditions. A multi-point optimization procedure attempting to stabilize ground resonance simultaneously at low as well as high collective pitch conditions is able to restrict the instability to small values over a broad range of variation in collective pitch, and eliminate the destabilizing trend at resonance with increasing collective. This configuration could then be completely stabilized by increasing body roll damping. Negative pitch-lag coupling, positive pitch-flap coupling, flap flexibility outboard of pitch, and lag flexibility inboard of pitch, were found to be most beneficial. Decrease in roll inertia was destabilizing for the baseline as well as optimized configurations, but level of instability, if any, was lower for the optimized configuration.