Helicopter Noise Control at-aerospaceservice

Vai ai contenuti

Menu principale:

The noise produced by a helicopter introduces different typologies of sources of noise that can be reassumed in: The rotor that generates different types of noise: Thickness noise is caused by the blade periodically displacing air during each revolution. This sound propagates in the plane of the rotor. Moreover, a rotating blade at a non-zero angle of attack imposes rotating forces onto the surrounding air, causing blade loading noise. This sound generally propagates in a direction perpendicular to the plane of the rotor. These two types of noise always occur, even in a hover condition.
another type of noise produced as that just described, is that produced by the noise mechanisms for the anti-torque system are basically identical to the rotor description in the paragraph above. However, due to its position behind the main rotor, the anti-torque device can also be subject to non-uniform inflow caused by the main rotor wake. This leads to additional interaction noise phenomena. Another typology of noise is that produced by the engine of the helicopter in which the noise emitted by turboshaft engines is basically composed of the mainly rotational noise produced by the radial and/or axial compressor(s) and turbine stages and broadband noise generated in the combustion chamber. 
The turboshaft engine compressor fan typically generates a high frequency tone emanating from the engine inlet and attenuates quickly through the atmosphere. Turboshaft engine exhaust noise has a broadband character and can become more prominent once the helicopter has passed overhead of the observer when rotor noise sources become less dominant.Piston engines are typically used on smaller helicopters and can be one of the prominent noise sources for those aircraft. Exhaust noise typically dominates piston engine noise emissions and, for helicopters, most piston engine noise reduction has been focused on use of upturned exhausts, mufflers and resonators. Unsilenced exhaust noise is broadband with the highest levels at low frequencies. The exhaust noise spectrum contains strong tones associated with the rate of cylinder firings. Engine exhaust noise can be controlled successfully by relatively advanced technology.
Piston engines are typically used on smaller helicopters and can be one of the prominent noise sources for those aircraft. Exhaust noise typically dominates piston engine noise emissions and, for helicopters, most piston engine noise reduction has been focused on use of upturned exhausts, mufflers and resonators. Unsilenced exhaust noise is broadband with the highest levels at low frequencies. The exhaust noise spectrum contains strong tones associated with the rate of cylinder firings. Engine exhaust noise can be controlled successfully by relatively advanced technology.
The contributions of the individual noise sources to the global helicopter noise spectrum perceived on the ground differ considerably depending not only on the flight condition but also on the observer position. Even though each helicopter configuration might have particular characteristics some general trends can nevertheless be observed. although the control of the global noise produced by the helicopter represents a very complex problem, numerous they are the searches in the field mainly contemplated to the control of all the individual sources of noise you know in the circle of aerial noise that vibrational that interest the geometry of the blades, the aeroacoustic etc. Our Technical Engineering Office thinks that
 
necessary to relatively conduct a Research and Development program to the silencers type reactive for intake systems and exhaust systems and for the acoustic improvement of passive acoustics performance inside the cabin; such acoustic improvement besides the choice of fit coverings phono-insulation and phono-absorbent, could be gotten in an experimental way analyzing the modal resonances inside the cabin and with the further one the analysis experimental wave-acoustics of the noise transmitted through the structures by to operate with Laser Scanning Vibrometer and analysis FFT of the transfer function with Analyser FFT connected multichannel is to the microphones internally positioned to the same cabin that to the driving signal in entry from the electrodynamic Shaker that fed by a generator of white noise it puts in excitement the whole structure and allows once gotten such function of transfer to operate all the necessary structural changes to reduce to the least one the noise introduced in the cabin and due to vibrational transmission and with the modal analysis also that transmitted by air.
 
 
Torna ai contenuti | Torna al menu