Helicopter Vibration Reduction Using Discrete Controllable-Stiffness Devices at the Rotor Hub

Abstract

The present study demonstrates that optimal multi-cyclic stiffness variations of discrete controllable stiffness devices in the rotor blade root region can reduce the 4/rev vibratory hub loads of a four-bladed hingeless rotor helicopter. The controllable stiffness devices (flap, lag, and torsion devices) are modeled as discrete springs whose stiffness coefficients can be varied, and a gradient-based optimization scheme is used to determine optimal multi-cyclic device stiffness variations that minimize a composite index comprising of all six components of vibratory hub loads. Multi-cyclic stiffness variations of the flap and lag devices are most influential, and when optimal 2/rev and 3/rev stiffness variations of these devices were used in combination the vibratory hub drag force was practically eliminated and the vibratory hub side force was reduced by 55%. No significant detrimental effects were observed on the first through the fifth harmonics of the vibratory blade root loads. Multi-cyclic (3/rev and 4/rev) stiffness variations of the torsion device produced only small reductions in the 4/rev hub vertical force. Multi-cyclic stiffness variations of the flap and lag devices were seen to be effective in reducing hub vibration even when there were changes in fundamental rotor properties such as the flap, lag, and torsion stiffness of the root (flexure) element, and the cruise speed.