Sources of helicopter noise

The myriad noises that one can hear when near to the helicopter is derived from various parts and processes involved in flying the helicopter. The sources of the noise are as follows:

Firstly, the thickness noise, which basically is dependent on the shape and motion of the blade, and is usually caused as a result of displacement of the air by the rotor blades. It is usually directed in the plane of the rotor.

Secondly, loading noise which is an aerodynamic adverse effect due to the acceleration of the force distribution on the air around the rotor blade due to the blade passing through it, and is directed primarily below the rotor. However in general, loading noise can include numerous types of blade loading. Changes in blade-section motion relative to the observer as the steadily loaded propeller rotates, is generally referred to as "loading" noise. This source usually dominate at low blade speed.

Thirdly, blade vortex interaction (BVI) occurs when a rotor blade passes within a close proximity of the shed tip vortices from a previous blade. This in turn causes a rapid, impulsive change in the loading on the blade resulting in the generation of highly directional impulsive loading noise. BVI noise can occur on either the advancing or retreating side of the rotor disk and its directivity is characterized by the precise orientation of the interaction. In general, advancing side BVI noise is directed down and forward wheraas retreating-side BVIs cause noise that is directed down and rearward. It has been shown that the main parameters governing the strength of a BVI are the distance between the blade and the vortex, the vortex strength at the time of the interaction, and how parallel or oblique the interaction is.

Fourthly, broadband noise is yet another form of loading noise. Broadband noise consists of various stochastic noise sources. The noise sources being the turbulence ingestion through the rotor, the rotor wake itself, and blade self-noise.

Fifthly, high-speed impulsive (HSI) noise is caused by transonic flow shock formation on the advancing rotor blade, and is distinct from loading noise. The source of HSI noise is basically the flow volume around the advancing blade tip, hence it cannot be captured by examining only the acoustic sources on the surface of the blade. HSI noise is typically directed in the rotor plane forward of the helicopter, like thickness noise.

Sixthly, Whereas most noise from a helicopter is generated by the main rotor, the tail rotor is a significant source of noise for observers relatively close to the helicopter, where the higher-frequency noise of the tail rotor has not yet been attenuated by the atmosphere. Tail rotor noise is particularly annoying to the human listener due to its higher frequency (when compared to the main rotor) which places it directly in the band in which the human ear is most sensitive.

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