Abstract
The fluctuating lift on a helicopter rotor blade passing close to a tip vortex shed from a preceding blade may generate an intense cyclic banging noise, called blade slap, which is one of the most offensive of all helicopter noises. Such blade/vortex interactions are modeled by a finite aspect ratio wing flying at uniform speed over a carpet of equally spaced, infinitely long, line vortices. By using linearized lifting surface theory the harmonic blade loads are expressed as a Fourier series with coefficients involving the same “sinusoidal gust transfer function” that figures prominently in the analysis of aircraft response to atmospheric turbulence. An established theory ( Lowson and Ollerhead, 1969 ) can be used to calculate the radiated noise harmonics in terms of the Fourier coefficients. Consideration of the noise propagated to the far field of a blade-fixed reference frame indicates that blade slap noise is only weakly influenced by aspect ratio. For a blade passing over the vortices at fixed height, acoustic power generation is proportional to the inverse third power of the height and is efficient only if the vortex spacing is about five times the height.
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