AN ACTIVE CONTROL SYSTEM FOR HELICOPTER VIBRATION REDUCTION BY HIGHER HARMONIC PITCH

An analytical study has been conducted to define the basic configuration of an active control system to reduce helicopter vibration. The active control system utilizes the standard swashplate control configuration to input higher harmonics of blade cyclic pitch once every rotor revolution. The controller for the vibration loop is based on adaptive optimal control theory and is designed to adapt to any flight condition. The prime characteristic of the vibration control system is its real-time capability. Advanced control techniques utilizing a real-time identifier are coupled with a transfer matrix approach representing the plant so that the transfer matrix is completely updated once every rotor revolution and commanded higher harmonic blade cyclic pitch input. This enhancement of the transfer matrix approach provides the capability to handle all flight conditions including maneuvers and transients without gain scheduling and also without having to open the loop and perturbate the system to determine the transfer matrix. Theoretical results are presented for a nonlinear four-bladed single rotor helicopter simulation which demonstrate the effectiveness of the active control system. Four per rev fuselage vibration reductions on the order of 80-90 percent are achieved with the active controller for airspeeds up to 150 kn . The amplitude of higher harmonic pitch required to achieve this level of vibration reduction is on the order of one degree at 3, 4, and 5 per rev. The rotor performance penalty associated with this level of vibration reduction is about 1-3 percent and the increase in rotor blade stresses is low to moderate for the particular case studied.