How are helicopters silenced?

I keep seeing movies where helicopters have the ability to silence themselves. How does that work? Websites that are supposd to tell you are no longer working.(surprise!)

what do you mean by "silence themselves". Do you mean to that you can't hear the engines?

They have a silence mode where they push a button and the engine goes silent.

"They have a silence mode where they push a button and the engine goes silent. "

Thank you Captain Obvious!!!!!!!!!

LOL

Originally posted by xtremist
They have a silence mode where they push a button and the engine goes silent.

Indeed. It functions extremely well in the movies doesn't it?

In reality, helicopter engines make lots of noise, and those giant blades go whop-whop-whop-whop-whop pretty loud.

Now... that being said. Some military systems have a means to reduce engine power, thus reducing noise, while increasing the attack angle of the rotor blades, maintaining some lift (but reduced so that such quiet mode operations can't last every long).

Also a five bladed rotor makes helicopters much more slient.

I believe the system for silencing also uses a form of air compression, similar to silencers on guns, to react against the noise pollution created by the engine. I am not certain of this, however.

airwolf had a silent mode,
Needed Jan michael vincent to operate it tho.

There exist a technology to silence anything you want by playing a "negative" image of the same sound. At least in theory it's posible.

I had this idea a long time ago, but I never had the chance to do research or experiment. But anyway I saw once on the Discovery channel that some other guys have build a device that does just that.

It "records" the sound of an airplane for example, then it produces an "negative" or "opposite" image of the sound wave and then play it back. If it's a "static" sound it'll be easy to "cancel" the wave. But since natural sound is mostly dynamic you'll need to do it in real time and I don't know if they continued research or not, but I am 100% sure that they have at least one prototype of that device.

So who knows, it may sound crazy but it's just one possibility.. Computers can do almost everything these days.

I don't think noise reduction can be done by sending out the negative of a sound. In school we ran a computer simulation where we placed 2 speakers directly in front of each other. The 2nd speaker produces the negative of the first one.

We saw that it depends on your position from where you are listening. In some places you can barely hear the sound, in other places the sound is amplified. Because sound travels in every direction, you can't cancel the sound in every direction.

I would go for air compression to solve this problem.

is adding more blades (as was mentioned)

Here's a link...

www.ainonline.com...

Other methods are likely classified....

Originally posted by Seadud
I don't think noise reduction can be done by sending out the negative of a sound. In school we ran a computer simulation where we placed 2 speakers directly in front of each other. The 2nd speaker produces the negative of the first one.

We saw that it depends on your position from where you are listening. In some places you can barely hear the sound, in other places the sound is amplified. Because sound travels in every direction, you can't cancel the sound in every direction.

I would go for air compression to solve this problem.

Indeed, if you place them directly in front of eachother it won't work for 100% that's what they showed on descovery channel. But there are other ways to do it. But like I've said before in theory it should work, don't know how far they are with the technology.

Very interesting Gazrok,

More blades need less engine power en so less noise. By using accoustic absorbing materials on the gas intake and outlet you can further decrease the noise.

So is there a maximum number of blades you may use to decrease engine power? If not, is Leonardo Da Vinci 's design of a spiral shaped helicopter the most efficient design ever built? Did it actually fly?

I'm really not understanding the engine power thing, as a helicopter's rotor speed is not altered during flight. The blades and rotor head are used to change speed and direction.
For example, on the blackhawk, which has four mains, two are used for lift when the other two are used for direction.
The CH-53 has 5 main blades, but that is one noisy beast, but its a huge one.
The Apache is quiet, but again, its not because of any secret component but the shape and number of blades.

The only switch in a helicopter that I am aware off that will silence it is labled OFF or kill. There is only one proplem in using this device. The motar will stop and the craft will generally 'glide' to the ground as only a helicopter can. Sprial may be a better word fall also come's to mind.

According to "Blackhawk Down," rocket propelled grenades silence them straight away.

Originally posted by Maddas
The only switch in a helicopter that I am aware off that will silence it is labled OFF or kill. There is only one proplem in using this device. The motar will stop and the craft will generally 'glide' to the ground as only a helicopter can. Sprial may be a better word fall also come's to mind.

Might you be looking for the word "autorotation"?

As far as I'm concerned, helicopters are in a constant state of controlled crash, anyway.

I think it has something to do with having 2 rotors (two separate rigs on top of each other, not like the chinook)

It has something to do with balancing the sound and the airflow through each blade, its a very interesting concept, it could also be done by making the blades ultra-aerodynamic so that the sound created by drag would be minimal.

I believe that is exactly what he's looking for. And I believe he was being rather facetious with the "glide" comment...bein's the glide ratio would be 0.

Yes autorotation would be the word that I was looking for. I know and understand that a heleicopter's blades would still rotate as as it falls, giving it some control.
I like Willam's explanation make sence, along with the number of blades.

Had to look up what facetious means. Not!

There was a time that I belived there was a silencer in choppers.