Airframe Vibration Reduction for an Unmanned Lift-Offset Compound Helicopter Using Active Vibration Control System

A simulation study using an active vibration control system (AVCS) to reduce airframe vibrations of an unmanned compound helicopter is conducted. The present unmanned compound helicopter uses wings, propellers, and a lift-offset coaxial rotor system, with two blades per rotor for high-speed flight. The airframe vibration responses are reduced using the AVCS at 230 knots. The hub vibratory loads of a lift-offset coaxial rotor are calculated using a rotorcraft comprehensive analysis code, CAMRAD II. The obtained rotor vibratory loads excite the airframe structure, represented as a one-dimensional stick (elastic line) model. The finite element analysis software, MSC.NASTRAN, is used to model the structural dynamics and analyze the vibration response of the airframe. The AVCS in this work uses the Fx-LMS (Filtered-x Least Mean Square) algorithm to determine the vibration cancellation signal produced by the force generators, which has the same amplitude but opposite direction to the airframe vibration signal. Herein, the AVCS consists of four force generators and six accelerometers and the simulation framework is constructed using MATLAB Simulink. Ten AVCS simulations are conducted, considering the different locations and various directions of the vibration cancellation forces from the force generators. When AVCS is applied to the airframe at a flight speed of 230 knots, the 2/rev longitudinal and vertical vibration responses at the specified airframe positions, such as the remote cockpit device, wing root, and wing tip, are reduced by 81.39–99.93%, compared to the baseline results without AVCS application.