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Old 01-24-2008, 07:21 PM
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Why do helicopter go "chop chop"?


They're called choppers for a reason. What is it about a rotary wing aircraft that creates the chopping sound that's so familiar? I assume it's something about the acoustical nature of the air and the speed of the blades.
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Old 01-24-2008, 07:28 PM
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Real simple version: Each time one of the main rotor blades goes by the tail rotor there is an interaction between the flow coming off each. The down flow from the main rotor crashes into the sideways flow from the tailrotor.

About the worst case is a 2-blade main & a two-blade tail, both turning slowly. Which is why the Viet Nam-era Huey had such a distinctive sound.
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Old 01-25-2008, 07:53 AM
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Additionally, helicopters with no tail rotors don't make that sound at all. It's more of a hum or buzz. No chopping at all.

Then there's Blue Thunder in stealth mode!
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Old 01-25-2008, 07:56 AM
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Quote:
Originally Posted by LSLGuy
About the worst case is a 2-blade main & a two-blade tail, both turning slowly. Which is why the Viet Nam-era Huey had such a distinctive sound.
Does the Chinook make such a racket from having two flows of air in opposite directions come together then?
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Old 01-25-2008, 08:39 AM
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Quote:
Originally Posted by Pushkin
Does the Chinook make such a racket from having two flows of air in opposite directions come together then?
It's different, but there is a distinct sound that lets you know there is a big arse helicopter near you. It's a 'heavier' sound -- much deeper.
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Old 01-25-2008, 08:49 AM
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Quote:
Originally Posted by GaryM
Additionally, helicopters with no tail rotors don't make that sound at all. It's more of a hum or buzz. No chopping at all.
Helicopter with no tail rotor? How does it fly without the pilot getting really dizzy?
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Old 01-25-2008, 08:53 AM
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Oh I know that sound so well. I used to listen to them flying overhead at night, thrilling to think of how it looked sweeping through the sky. Big moment when I saw my first one

Looking back my childhood was kind of strange. Until I moved to a much more religiously motivated sound here in Northern Ireland, my friends and I at primary school pored over aviation books arguing over which was the one we'd see the day before. Then we moved and my class mates would brag about whose dad had sworn best at the soldiers at the checkpoints
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Old 01-25-2008, 08:55 AM
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Quote:
Originally Posted by friedo
Helicopter with no tail rotor? How does it fly without the pilot getting really dizzy?
The NOTAR system replaces the traditional tail rotor on some machines.
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Old 01-25-2008, 09:07 AM
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Quote:
Originally Posted by Pushkin
The NOTAR system replaces the traditional tail rotor on some machines.
That's the one I was alluding to. But there are/were some helecopters developed that predated that system. One was the Hiller Hornet Wiki It was powered by ramjets mounted on the end of each rotor blade.

There are also helecopters with coaxial mounted rotor sets that rotate in opposite directions, thereby countering the torque trying to spin the fuselage.
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Old 01-25-2008, 09:11 AM
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Blade Slap - The dominant noise produced by helicopters consists of a broadband spectrum generated by vortex formation and shedding in the flow past the helicopter blade. In addition to the discrete frequency noises at harmonics of the blade passage frequency, superimposed on the broadband spectrum for helicopters is a rotational noise known as blade slap. This high amplitude periodic noise plus highly modulated vortex noise caused by fluctuating forces on the blade due to the cutting of one blade's tip vortices by another blade and transonic shock. Blade slap is a distinctive, low frequency throbbing sound which increases during certain descent, maneuvering and high-speed cruise operations.
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Old 01-25-2008, 12:58 PM
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Helicopters don't fly, they beat the air into submission. Thats where the sound comes from.

Oh, I've slipped the surly bonds of earth
And hovered out of ground effect on semi-rigid blades;
Earthward I've auto'ed and met the rising brush of non-paved terrain
And done a thousand things you would never care to
Skidded and dropped and flared
Low in the heat soaked roar.
Confined there, I've chased the earthbound traffic
And lost the race to insignificant headwinds;
Forward and up a little in ground effect
I've topped the General's hedge with drooping turns
Where never Skyhawk or even Phantom flew.
Shaking and pulling collective,
I've lumbered The low untresspassed halls of victor airways,
Put out my hand and touched a tree.
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Old 01-26-2008, 12:22 AM
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Quote:
Originally Posted by Loach
Helicopters don't fly, they beat the air into submission.
If Chuck Norris hasn't beaten said helicopter into submission first.
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Old 01-26-2008, 05:28 AM
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I have heard those in the know claim the blade tips break the speed of sound, and you hear mini sonic booms. But, I've never been in a setting where I could question them on this. This makes no sense at all for several reasons. First, flying at the speed of sound is very unstable; therefore, it is not done. Second, crossing the sound barrier requires opposing aerodynamic profiles. Third, how could there be a never-ending string of sonic booms? The blade tips would have to go sonic, sub-sonic, sonic, sub-sonic...how stable would that be?

I wonder, though, if any other SDopers had been told the same thing and if this idea has any basis whatsoever. ...Oops! I guess that may be what the above Poster means by "transonic shock", but how is this physically accomplished while keeping the bird in flight? I simply cannot picture how this can be! (FYI: Even the Concorde had to change its profile to cross the sound barrier.)

So, the blades DO continually crosses and uncrosses the sound barrier? Can anyone explain this further?

- Jinx

Last edited by Jinx; 01-26-2008 at 05:33 AM.
  #14  
Old 01-26-2008, 06:24 AM
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I love your poem, Loach. Is it yours, or by the ubiquitous Anon?
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Old 01-26-2008, 07:06 AM
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Quote:
Originally Posted by DesertDog
I love your poem, Loach. Is it yours, or by the ubiquitous Anon?
Original by John Magee.
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Old 01-26-2008, 07:13 AM
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I don't know about helicopter blades but some aircraft prop blades come close to the speed of sound at full power resulting in an unearthly roar.

I think you have a misconception about sonic booms. A sonic boom is a sound continuously produced by something travelling at transonic and supersonic speeds. It is caused by shockwaves. If you imagine standing in the water on a beach, when a wave passes you you experience it just once but the wave itself is continuous. So you don't need to be passing in and out of the speed of sound to get multiple booms, you just need to hear the passing of the object multiple times, this is what happens with blade tips.

Quote:
Originally Posted by Jinx
I have heard those in the know claim the blade tips break the speed of sound, and you hear mini sonic booms. But, I've never been in a setting where I could question them on this. This makes no sense at all for several reasons. First, flying at the speed of sound is very unstable; therefore, it is not done. Second, crossing the sound barrier requires opposing aerodynamic profiles. Third, how could there be a never-ending string of sonic booms? The blade tips would have to go sonic, sub-sonic, sonic, sub-sonic...how stable would that be?

I wonder, though, if any other SDopers had been told the same thing and if this idea has any basis whatsoever. ...Oops! I guess that may be what the above Poster means by "transonic shock", but how is this physically accomplished while keeping the bird in flight? I simply cannot picture how this can be! (FYI: Even the Concorde had to change its profile to cross the sound barrier.)

So, the blades DO continually crosses and uncrosses the sound barrier? Can anyone explain this further?

- Jinx

Last edited by Richard Pearse; 01-26-2008 at 07:14 AM.
  #17  
Old 01-26-2008, 08:38 AM
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Quote:
Originally Posted by Jinx
I have heard those in the know claim the blade tips break the speed of sound, and you hear mini sonic booms.
Rotor blades at the tip are going round at about 300 or 400 kts. Of course the retreating blade is going the same speed backwards, which limits the helicopter's forward speed due to retreating blade stall.
Quote:
Originally Posted by DesertDog
I love your poem, Loach. Is it yours, or by the ubiquitous Anon?
As 1920s Style "Death Ray" points out, the poem is based on McGee's High Flight; but the heli version is by Mr. Anon.
  #18  
Old 01-26-2008, 09:18 AM
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Here is another modern alternative tail rotor design http://en.wikipedia.org/wiki/Fenestron
The net effect is a less pulsy output & helos with this design have a loud whine instead of a slow chop chop sound.
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Old 01-26-2008, 01:53 PM
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This is the one my Army buddy brought back from his second tour as a Huey pilot in Nam.

LOW FLIGHT

Oh, I have slipped the surly bonds of Earth
And hovered out of ground effect on semi-rigid blades;
Earthward I've auto'ed,
and met the rising brush
Of non-paved terrain - and done a thousand things
You would never care to -
skidded and drooped and flared
Low in the heat-soaked roar.
Confined there,
I've chased the earthbound traffic, and lost
The race to insignificant headwinds.
Forward, and a little up, in ground effect
I've topped the General's hedge with drooping turns
Where never Skyhawk, or even Phantom flew.
And, shaking and pulling collective, I've lumbered
The low trespassed halls of Victor Airways,
Put out my hand, and touched a tree.
  #20  
Old 01-26-2008, 04:00 PM
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Quote:
Originally Posted by LSLGuy
Here is another modern alternative tail rotor design http://en.wikipedia.org/wiki/Fenestron
The net effect is a less pulsy output & helos with this design have a loud whine instead of a slow chop chop sound.
I can attest to the sound they make.

The British Army use the Aerospatiale Gazelle for reconnaissance and when someone was on the run in Northern Ireland one of those would sit on top of the town for a while, keeping a look out. When they finally moved off, the silence they left was really quite profound.

I used to be able to ID the helicopters the British Army used from way off just by the sound. I've not had practice in that for many years, so that's one skill that is long lost to me.
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Old 01-26-2008, 04:09 PM
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I've noticed that a helecopter hovering (not moving) above me doesn't make a chop chop sound, just growling and whirring and jet noises, and asked a pilot why.

He said the chop chop chop sound came because of two effects: first, the blade speed through air is different by twice the flight speed for the blades on either side of the craft, and second, you hear the blades coming at you or going away from you.

If it's all the stuff about the tail rotor, why don't choppers hovering overhead chop?
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Old 01-26-2008, 10:19 PM
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I've heard that the "chop chop" sound that helicopters make is so distinctive that there are audio helicopter detectors that can hear them coming from very far away, long before humans can hear them. I think they use some sorta DSP to analyze the audio spectrum...
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