Reduction of Helicopter Vibration through Cyclic Control of Variable Orifice Dampers

In the present study it is demonstrated that cyclically varying the damping coefficient of controllable lag and flap dampers can reduce the 4/rev vibratory hub loads of a 4-bladed hingeless rotor helicopter in high speed forward flight. For ease of introduction into a comprehensive rotorcraft aeroelastic analysis, controllable orifice dampers are modeled as linear viscous dampers, whose damping coefficient can be modulated by applying a voltage to the orifice; and it was verified that the reduced order model and a full fluid dynamics based model produced identical hysteresis cycles for both constant as well as cyclically varying orifice voltages. A gradient-based optimization scheme is used to determine the optimal multi-cyclic damping variation inputs that simultaneously minimize all six components of vibratory hub loads. The results showed that optimal 2/rev and 3/rev lag damping variations could virtually eliminate the vibratory hub drag force and yawing moments, and produce small reductions in the vibratory hub side force. The optimal lag damping variations, interestingly, produced increases in the 3/rev and 5/rev components of the blade root drag shear, that cancelled the contributions of the blade root radial shear to the vibratory in-plane hub forces. Although there were increases in higher harmonics of blade response, damper loads, and blade and flexbeam root loads, the lower harmonics and the peak-to-peak values changed little, so blade and damper fatigue life will not be adversely affected. The study also examined the effect of changes in lag damper configuration and size, in terms of its ability to reduce vibration through cyclic damping variations. When optimal 2/rev and 3/rev variations in flap damping were introduced in conjunction with the optimal lag damping variations, 30% reductions in the hub vertical vibrations were obtained, in addition to the reductions in the vibratory in-plane forces and yawing moment. The cyclic flap damping variations resulted in a reduction in the higher harmonics of the blade root vertical shear. Reductions in hub vibration levels were obtained over a range of forward flight speeds.