Helicopter Noise Generation and Detectability.

A helicopter is easily detected and identified by its acoustic signature. During high speed flight and under certain landing approach flight conditions there occurs the helicopter-characteristic periodic impulsive noise ("blade slap"). In the absence of such dominating impulsive noise phenomena, the periodic low-frequency sound of the main-rotor and the more high pitch "propeller-like" sound of the tail-rotor acoustically typifies a helicopter. There are a number of noise generating mechanisms at work, individually related to the operation of the main-rotor, of the tail-rotor, and/or to the interaction of the two. This presentation discusses the physics of these various mechanisms and shows ways and means of how to reduce the acoustic detactability and the acoustic annoyance caused by a helicopter in flyover