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Quiet helicopters

Matej

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We all know that some devices to make helicopters quiet were developed and some of them tested. I am especially interested in equipment with microphones and loudspeakers, that are creating opposite sound waves to negate rotor and engine noise. Does any information exist about its using on operational helicopters and on which types? I mean outside using for special ops units, not to lower noise inside the cockpit.
 

Jemiba

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A number of such systems were developed and, I think some are already in use..
to counter the noise inside of a helicopter. But I think to us it against the outside
noise, generated by the helicopter is very difficult, because such systems are
working only for quite a small area. The sound generating area of a heli, the rotor
disc is very large and every blade generates different frequencies with constantly
changing phases. The heli transmits its noise in a circular pattern and not two points
on this circle will have the same frequency, amplitude and phase. To counter this
noise, you would have to put the microphone to the place, where the listener is,
I think, but that's not, what you've asked for.
The only way to make helicopter quiet, is to reduce engine noise and vibrations.
Active allround noise supression for all directions cannot be achieved, I think.
 

RP1

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There will be issues with the range of frequencies generated, also. I used to live out in the countryside, and found I could detect helicopters long before they flew over, by hearing a very low frequency wump-wump-wump. IIRC, this is also used by the "windchimes" type helicopter detector, and would be quite difficult to cancel using a small speaker, more suited to higher frequencies.

RP1
 

Jemiba

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I had a look again at the back issues of vertiflite and found an article in the winter'04
issue about the work of several companies in the field of vibration and noise reduction.
Here, generally accelerometers are used for measurement of vibrations and the output
signal is used to counter balance vibrations by influencing the rotor. Here several different
ways are explained, on of them is shown in hte diagram below, using flaps at the rotor
blades. Another system is the active camber rotor blade airfoil concept.
And the Sikorsky S-92 is mentioned, as the first production delivery utilizing an AVC
(active vibration control) system. The main aim is to decrease vibration loads and so allow
for a lighter structure and enhance crew comfort, but decreasing noise level probably
is achieved as well.
 

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Matej

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Once again something new for me. Thanx.

Edit: Another possibility is to fly as low as possible ;)

http://www.aviapedia.com:8080/helicopters/BO-105/tiger_1.wmv
 
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