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Another study question for all us helicopter nerds...


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ok let see if I can explain this in laymans terms. first the basics

 

When an airplane is in motion at subsonic speeds, the air is treated as though it was incompressible. As airplane speed increases, however, the air loses its assumed incompressibility and the error in estimating, for example, drag, becomes greater and greater.

 

The question arises as to how fast an airplane must be moving before one must take into account compressibility.

 

A disturbance in the air will send pressure pulses or waves out into the air at the speed of sound. Consider the instance of a cannon fired at sea level. An observer situated some distance from the cannon will see the flash almost instantaneously, but the sound wave is heard (or the pressure wave is felt) some time later. The observer can easily compute the speed of sound by dividing the distance between him and the cannon by the time it takes the sound to reach him. The disturbance propagates out away from the cannon in an expanding hemispherical shell.

 

the sweped forward tail rotor blade allows the outer most portion of the blade to reach near the speed of sound first, the sound wave compression is held to the outet most part of the disc while the inner portion remains under the speed of sound thus able to continue to move the air through the disc.

 

hope this makes sense to y'all :D

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here's another answer from the skunk-works people

 

From: skunk-works-digest-owner@pmihwy.com

To: skunk-works-digest@pmihwy.com

Subject: Skunk Works Digest V6 #13

Reply-To: skunk-works-digest@pmihwy.com

Errors-To: skunk-works-digest-owner@pmihwy.com

Precedence:

 

 

Skunk Works Digest Sunday, 2 February 1997 Volume 06 : Number 013

 

In this issue:

 

Re: Speed of helicopter blades

RE: Speed of helicopter blades

Helicopter blade tip speed

RAM covered helmets

RE: Speed of helicopter blades

Re: Skunk Works Digest V6 #12

"Jetsons Here We Come"

 

See the end of the digest for information on subscribing to the skunk-works

or skunk-works-digest mailing lists and on how to retrieve back issues.

 

----------------------------------------------------------------------

 

From: "A.J. Craddock"

Date: Fri, 31 Jan 1997 10:21:26 -0800

Subject: Re: Speed of helicopter blades

 

However, it is also public knowledge that the Armed Forces possess

helicopters that are silent.

 

The Wall Street Journal even had an article about it a year or so ago.

 

Apparently the trick is to broadcast simultaneously a noise profile that is

equal and opposite to the primary noise generated, which cancels out the

noise.

 

Am sure some of the list members can add more details.

 

Tony Craddock

 

*************

 

At 10:02 AM 1/31/97 -0800, you wrote:

>I looked in my Larousse, but it is entitled "Larousse Gastronomique" (or

some

>such) and had nothing on helicopter blades. I then contacted a friend who

>works for NASA designing helicopters and received the following reply:

>

>"Hey Bernie!

>

>Original helicopters like the Huey UH-1 had blades that traveled

>supersonically. But, they have since toned down rotor tip speeds because of

>noise and vibration problems. Rotor blades now typically travel at tip

speeds

>anywhere from 650 to 790 ft/sec depending of the type of helicopter or

>tiltrotor. At ambient sea level conditions the blades on

>not moving at supersonic speeds. But, as you may

know,

>when you gain altitude, temperature decreases and so does the speed of

sound:

>

>a = sqrt (gamma*R*T)

>

>where a is the speed of sound, gamma is 1.4, R is a gas constant, and T is

>temperature

>

>OK, so maybe you are not interested in equations. But you get the idea.

:-)

>This is how Chuck Yeager broke the sound barrier with the X-1. He gained

>altitude where the speed of sound was lower in order to go supersonic.

>

>So, the answer to your question is: it depends. It depends on what altitude

>the copter is flying at. It depends on the temperature at the altitude the

>copter is flying at. But, it also depends on the aerodynamic flows around

the

>airfoils on the blade. Even if the blade is not traveling at supersonic

>speeds, the air flow as it accelerates over the upper surface of the blade

can

>experience supersonic speeds. Also, you have to consider the relative

forward

>velocity of the blades as they travel, i.e. tip speed + forward speed.

Now,

>when you're talking about noise, that's a whole other ball of wax. Steve's

>the expert in this area. :-) The noise of helicopters are not just due to

>shock waves coming off the blades. A second noise generated by

helicopters is

>called blade vortex interactions (BVI's). The bound circulation of the

blade

>trails off into concentrated tip vortices. When these vortices slap into

the

>following blades as the helicopter rotor system rotates, BVI's occurs.

>

>This is a very simplified explanation of aerodynamics, fluid dynamics, and

>helicopter theory, but I hope it answers your question.

>

>Alex"

>

>

>And there you are!

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been a long time since I've studied this, but it has to do with blade tip speed and the speed of sound barrier I do beleive :blink:
A cranked-forward tip will reduce noise somewhat, but a swept-back blade tip is somewhat more effective (like the new Bell helicopters or the new Agusta 109 T/R blades).
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