While 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.
Tail rotor noise is particularly annoying to the human listener due to its higher frequency (as compared to the main rotor) which places it directly in the band in which the human ear is most sensitive.
|Blade slap ...... you've heard it ... here it is|
|Written by US Source|
|Wednesday, 11 April 2007|
In 2000 a New York City study of helicopter noise and its effect on residents reported that helicopters were not being forced to comply with noise and pollution regulations. The council said that there is no provision in the Clean Air Act regulating pollution from aircraft.
"There is an urgent need for noise relief," said Cunningham, author of the report. "Helicopter noise is annoying because of blade slap and low-frequency noise that results in building vibration."
She said residents also should be concerned about potentially harmful toxins released into the air by helicopters. City Councilwoman Christine Quinn (D-Chelsea) said she had received all sorts of complaints from constituents about helicopter noise.
Helicopter noise is inherently more complex to model than jet aircraft noise.
Helicopter noise tends to have a pulsating quality caused by the blade passing frequencies of the rotors. Under certain conditions, this can become pronounced and is termed ‘blade slap’. The forward motion of a helicopter in normal flight results in variations of blade speed through the air during each rotation formed by a combination of the forward speed of the aircraft and the rotation of the blade.
This variation of blade speed through the air results in complex noise propagation. This effect applies to both the main and tail rotor. There is also an interaction between noise produced by main and tail rotors.
A short list of references to research on helicopter noise:
(Source CAA website)
|Last Updated ( Monday, 07 May 2007 )|