Helicopter vibratory loads and vibrations reduction using higher-harmonic control

This paper studies vibratory hub loads and cabin vibrations reduction using higher harmonic control. The dynamic analysis of the rotor is performed using a comprehensive analysis tool coupled with a gradient based optimization algorithm to evaluate the higher harmonic cyclic input which is expected to minimize vibratory hub loads. The hub force minimization problem is converted into a cabin vibration reduction problem using a comprehensive aeroservoelastic helicopter model which includes a linear time invariant aeroelastic rotor model, rigid and elastic fuselage modes, servo actuators and sensors for vibration measurement. The optimal higher harmonic control inputs are then applied and the accelerations are measured at the selected sensor locations and additionally aeroservoelastic stability analysis is performed. For the same aeroservoelastic model, a cabin vibration reduction procedure is formulated considering direct acceleration reduction at sensor locations. All studies are conducted on a light utility helicopter. Vibratory hub loads and sensor accelerations are presented and the differences in the objectives of minimum force at hub and minimum vibration at sensors are discussed.