Active control to augment rotor lead-lag damping

Hingeless and bearingless rotor designs are today well accepted for modern helicopters. Continued development, however, revealed some deficiencies in the area of aeromechanical stability and vibration. In general there is a good basic understanding of how to avoid these instabilities. But since it becomes more and more desirable to focus rotor design on aerodynamic features and flight performance, these aeromechanical instabilities gain new importance due to the difficulties to provide the required damping. Since all rotor concepts suffer from the lack of sufficient natural lead-lag or inplane damping most designs in use show artificial lead-lag dampers to overcome aeromechanical instabilities. Additionally, active control offers the possibility for an artificial stabilisation of aeromechanical instabilities. Meanwhile, many research activities focus on active control to augment rotor lead-lag damping and many authors demonstrate the potential inherent in this approach. The paper shortly repeats the problem of aeromechanical instabilities of hingeless rotor-systems. A simple rotor blade model with flap, lag and pitch DOFs is used to derive the coupled set of differential equations. The emphasis of this paper is to demonstrate the potential of active control and to gain physical understanding. Lead-lag damping augmentation of an isolated rotor blade with lead-lag rate and attitude feedback even in forward flight is shown. However, some problems that may limit the success of an active control approach are discussed.