Control mechanism of the thickness noise based on sound pressure cancellation

The low harmonic in-plane thickness noise has an important influence on the airworthiness and inaudibility characteristics of helicopter. A new numerical simulation by using the cancellation of opposite sound pressure waves from on-blade active actuator is developed to study the control mechanism of rotor thickness noise. Firstly, the composition of the thickness noise and parametric study to examine the effects of aspect ratio and so on are respectively performed for AH-1/OLS rotor. The characteristics of sound waves formation and the influence of the amplitude and harmonic of active tip force on the cancellation of rotor thickness noise are then deeply assessed. The influence of on-blade active actuator on the thickness noise reduction characteristics is analyzed in different flight conditions. The results show that, the sound waves from the change rate of tip force, opposite to thickness pressure waves, which is the largest in the advance side of rotor, can reduce the peak thickness noise levels in the front of rotor. Compared with low harmonic tip force, high harmonic tip force is found to need small amplitude to cancel the noise at the target observer location, which means that the performance penalty of the additional force is slight. But, the cancellation effectiveness occurred over a narrow region.