Vibration Control in a Helicopter with Semi-Active Hydraulic Lag Dampers

This paper describes the research on the use of semi-active hydraulic lag dampers for vibration control in helicopters. A semi-active vibration control of the hydraulic lag damper is investigated in the context of a midsized production helicopter in steady level flight. The semi-active operation of the damper is achieved via periodic flow restrictor modulation. The influence of the modulations and constraints on the nonrotating hub loads of a five-bladed main rotor is evaluated. The selected harmonic modulation frequencies are determined by the combination of the hydromechanical coupling in the damper and the mechanical filtering effect of the symmetric rotor. The modulation frequencies of 3, 5, and 7 per revolution are used to modify the 5-per-revolution lateral and longitudinal hub loads, whereas the frequencies of 4 and 6 per revolution are efficient at modifying the 5-per-revolution hub yaw moment. The harmonic modulations larger than seven per revolution are not used due to limited damper responses at these frequencies. The out-of-plane hub loads and moments cannot be significantly influenced by the rotor-damper configuration considered. A maximum load reduction of 72% on the lateral hub load component decreases to 58% when considering a limited damper response authority due to the relief valve activation.