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. KeywordsLift-offset compound helicopter • Airframe vibration • Active vibration control system • FX-LMS algorithm B Jae-Sang Park
This study using IBC (Individual Blade pitch Control) attempts to reduce the hub vibratory loads of a lift-offset rotor in high-speed forward flight while the rotor performance is maintained or improved. The rigid coaxial rotor of XH59A compound helicopter is considered as a lift-offset rotor for the present work and CAMRAD II is used to calculate the hub vibration and performance of a rotor. The single harmonic input the using 3/rev actuation frequency, amplitude of 1°, and control phase angle of 270° (3P/1°/270°) for the IBC minimizes the vibration index of the XH-59A rotor at 200 knots by about 62%, but it reduces the rotor effective lift-to-drag ratio by 3.43%. When the 2/rev actuation frequency with the amplitude of 2° and control phase angle of 270° (2P/2°/270°) and the 3/rev actuation frequency using the magnitude of 1° and control phase angle of 210° (3P/1°/210°) are used in combination for the IBC with multiple harmonic inputs, the rotor vibration is reduced by about 62%, while the rotor performance increases by 0.37% at 200 knots. This study shows that the hub vibration of the lift-offset rotor in high-speed flight can be reduced significantly (by above 50%) but the rotor performance is maintained or improved moderately, using the IBC with multiple harmonic inputs.
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