Quiet helicopters

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.

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.

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

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.

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

Anyone remembering the 1997 movie Conspiracy Theory (Julia Roberts, Mel Gibson)? They had an special method for noise reduction written into the script, a kind of technospiracy in itself. When the men in black in their helo descend, cut into the hightech interior of the helo, where on the overhead dashboard a switch is manually activated: „whisper mode“. Immediately the overall noise the helicopter generates is reduced to a … yes … whisper.

I talked to people who believed it was real tech.

Blue thunder had it back in the 80's

bob225 said:

Blue thunder had it back in the 80's

Click to expand...

Yesss, even better … that was the image I remembered, and I put it in the 1997 movie, where they do what I described above, but without the dashboard actually having written „whisper mode“ upon it. However, in the same scene intercut is Gibson, driving a cab and ranting on about the black men in their silent helicopters, using … and he says it: „whisper mode“. Chances are it is a reference to Blue Thunder.

Tail rotors, oil cooler fans, even the transmission itself also generate and propagate sound in a number of frequencies. Added to the low frequencies mentioned getting the whisper mode is a challenge. A way to reduce the noise is to slow the rotor system (if possible) that reduces the propagation off of the rotor tips, but that is very challenging without having to develop rotors that may not be optimized across the rotor speed range.

Some helicopters are less loud than others. The Eurocopter EC135 / Airbus Helicopters H135 isn't all that loud, and it doesn't have the "flap flap flap" or "wump wump wump" sound that many other helicopters still have. I'm sure the enclosed tail rotor and rigid main rotor have something to do with it.

perttime said:

Some helicopters are less loud than others. The Eurocopter EC135 / Airbus Helicopters H135 isn't all that loud, and it doesn't have the "flap flap flap" or "wump wump wump" sound that many other helicopters still have. I'm sure the enclosed tail rotor and rigid main rotor have something to do with it.

Click to expand...

The Royal Canadian Mounted Police fly both EC120 A Star and EC130 helicopters around Vancouver.
EC 130 makes a unique "buzzing" sound that is distinct from all the other helicopters in this airspace. I can hear when an EC 130 is overhead without looking.

Now…if blades are very wide—-could you have something you can fast-rope out of while auto-rotating down?

No engine noise at all. Winch synchronized to pulls up as craft descends to deposit squad at zero velocity.