Silent Helicopters?

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water wings
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Silent Helicopters?

#1 Post by water wings » Sun Jul 23, 2006 1:41 pm

OK, kids.... i am a fixed winger, and have a really odd question:
I am watching "Conspiracy Theory" right now (Mel and Julia) and the boys in the helicopter hovering over the city just said something about switching to silent... just when i said to myself WTF? one of the pilots said "confirmed silent" . I have no idea what kind of rotorcraft it was, either.
Someone please explain if this is Hollywood hooplah, or the real thing.
(it sounded cool - thrump thrump thrump)
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#2 Post by sky's the limit » Sun Jul 23, 2006 2:00 pm

Hollywood Hoopla as you say.

No such thing.

There are some very quiet (relatively speaking) helicopters out there, such as the Eurocopter Ec 120/130, MD's NOTAR(no tail rotor) technology equiped machines. Machines are being developed that are getting quieter all the time, but I'm sure the limit will still be much louder than Blue Thunder ever was in "whisper mode." LOL They are working hard on blade design and the like to cut down on noise for urban applications like police and air ambulance work. HAWK One in Calgary is a good example, MD 600(520N maybe?) NOTAR.

Hope that helps.

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#3 Post by rotorfloat » Sun Jul 23, 2006 2:57 pm

Also the way the aircraft is handled will help minimize (relative) the amount of unnecessary noise produced. Rapid control inputs, steeper turns, shallow approaches and shallow departures usually will lend to noisier flight.

In the US, noise abatement is a big issue. I think it was the FAA that publised the 'Fly Neighbourly Technique'. A manual listing most types of civil helicopters and which regimes of flight produced the most blade slap; the popping noise we are all familiar with helicopters.

Tail-rotor equipped helicopters will always be the noisiest on the same side the tail rotor is located. Flying neighbourly would mean keeping noise sensitive ares on the other side of the helicopter.

For most helicopters, avoiding the 400-600 fpm ROD will nearly eliminate blade slap. For neighbourly approach, this would mean a steep, low-power, 1000+ ROD approach, keeping IAS above 70mph, and then flaring and bringing in the power at the bottom. I had to fly this technique for weeks, and for sure, not a hint of slapping all the way down.

Have you ever watched wood ducks land in a pond or creek? All they do is fly overhead, fold up their wings and plop right in. I felt like that sometimes, coming in so fast and steep.

One job I was on was working over an urban environment, and flying neighbourly was the order of the day. Even as far as flying over other noise producing areas like freeways to help mask your own noise.

For NOTAR and fenestron equipped helicopters flown gently, all you hear flying overhead is a gentle hiss of the turbine.
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#4 Post by sky's the limit » Sun Jul 23, 2006 3:25 pm

Hey Rotor,

Yeah, in the 212 you have to fly neighbourly for sure, took a bit of getting used to, the technique you're describing. Where you flying?

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#5 Post by Walker » Wed Aug 09, 2006 2:38 pm

Are you guys 100% sure about that; I seem to recall some engineering firm recruiting people here at UVIC last year. They didn’t exactly say what they did but they were involved with rotorcraft and active noise cancellation. They were looking for people in computers, fluid dynamics, acoustics, and electronics…. They also stated that you would need to pass a security clearance as a req of getting hired.
However I can't for the life of me remember what they were called.
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#6 Post by Lommer » Wed Aug 09, 2006 6:30 pm

Well I suppose it depends on your definition of "silent". The EC120 is quiet, but I'd hardly call it silent. The American Comanche attack helicopter was supposed to be super-quiet, but it got canned a couple years ago. If there's firms doing top-secret work on active noise cancellation in helicopters then who knows, maybe there's a successor to the comanche that is darn-near silent.
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#7 Post by sky's the limit » Wed Aug 09, 2006 10:43 pm

I wouldn't worry about one sneaking up on you anytime soon.... :wink:

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#8 Post by rotorfloat » Thu Aug 10, 2006 11:13 am

There was a thread on pprune a while ago about a Hughes 500P that was silenced and used by Air America in Vietnam.

I'll let Lu Zuckerman finish:
A silent helicopter is not as impossible as you might think. During the Vietnam War (or was it a police action?) Hughes Helicopters under contract from the Defense Advanced Research Projects Agency (DARPA) modified an OH-6 so that it was almost silent during a fly over. Hughes using a special test stand was able to run the helicopter with no rotor, with no transmission, with no tail rotor gearbox and without a tail rotor. All of the dynamic loads were absorbed by a waterbrake dynamometer. They also monitored the amount of sonic energy broadcast by the aluminum skin and the primary structure. Once the noise characteristics were established Hughes did the following. The structure and skins were covered on the interior with insulation to minimize noise transmittal. The gearboxes were covered with insulation. The engine was fitted with a muffler. A new five-blade rotor system was fitted and it ran at a lower speed and an X type tail rotor was fitted which also turned at a lower speed.

I saw this helicopter fly and it went overhead at about 500 feet and all you heard was a Whooshing sound. After the first few flights this helicopter disappeared and I assume it was sent to Vietnam
http://leeaviationconsulting.com/AirAmerica500.pdf
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#9 Post by sky's the limit » Thu Aug 10, 2006 12:44 pm

Well you don't say.... Interesting.

You can hear my 500 coming 5 miles away...lol.

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#10 Post by w squared » Thu Aug 10, 2006 5:08 pm

Imagine what they could do with a NOTAR 500/530 today, even without ANR.
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