OPTIMIZED AEROELASTIC COUPLINGS FOR ALLEVIATION OF HELICOPTER GROUND RESONANCE

Formal optimization methods are used to determine a combination of aer oelastic coupling parameters that can alleviate ground resonance insta bility of a soft-in-plane rotor. Optimization at a prescribed rotation al speed is unable to stabilize ground resonance, as the stability obj ective function is satisfied merely by moving resonance to a lower rot ational speed. A moving-point optimization procedure attempting to sta bilize at the rotational speed where damping is a minimum during each optimizer iteration is successful in stabilizing the regressing lag mo de at moderate collective pitch, However, these optimal aeroelastic co uplings are destabilizing near zero-thrust conditions. A multipoint op timization procedure attempting to stabilize ground resonance simultan eously at low as well as high collective pitch conditions if able to r estrict the instability to small values over a broad range of variatio n in collective pitch and eliminate the destabilizing trend at roll re sonance with increasing collective. This configuration could then be s tabilized completely by increasing body roll damping. Negative pitch-l ag coupling, positive pitch-flap coupling, nap flexibility outboard of pitch, and lag flexibility inboard of pitch were found to be most ben eficial.