Why do helicopters "chop"?

Kyle_in_Tex · Mar 18, 2004

Re: Why do helicopters \"chop\"?

A whole bunch of skill, parts, and energy,
has to happen just right,
Just to stay 1" above the ground.

A very high percentage of helicopter accidents are declared "pilot error-failure to maintain rotor RPM"

BigDave · Mar 18, 2004

Re: Why do helicopters \"chop\"?

( So that brings us to the variable pitch of a helicopter blade. To produce directional thrust the pitch is varied through the swashplate mechanism. The pitch will vary through a single revolution thus the shock waves will vary through that same revolution. I can see how this would produce the unsteady chop sound. )

Okay, I guess I short changed you a bit by not giving you a more thorough answer. The blades on a helicopter vary pitch to account for load and to propel the helicopter in a direction. This is where the tail rotor plays bigger role that just negating rotational effects of the main blade. In a foward thrust situation, the tail rotor offsets the rotation caused by the non-symmetric blade pitch.

Not all of the "chop" is caused by changing the intensity of the shock waves. The loudest chop is helped by increased pitch of forward flight, because the blade is at it's highest angle of attack during the heaviest load. For hovering, the blades will maintain a constant pitch, except to compensate for wind. A helicopter in hover still has some chop. The reason you as a person standing at a point can hear the chop even though the is a continuous mach wave in fornt of each blade, is because the shock wave is not continuous from your perspective. The tail of the pressure wave reaches you, and you here a chop, then as it rotates around the helicopter back to your position you hear a chop again. Inside the helicopter should be chop free, if there are no shock wave interactions/reflections with the body. There is body interaction, so chop-chop.

BTW, my wife is getting suspicous about all of the questions I am asking her about helicopters. She was an engineer at Bell Helicopter. /forums/images/graemlins/blush.gif /forums/images/graemlins/shocked.gif /forums/images/graemlins/smirk.gif /forums/images/graemlins/wink.gif /forums/images/graemlins/smile.gif /forums/images/graemlins/grin.gif

Tractors4u · Mar 18, 2004

Re: Why do helicopters \"chop\"?

I found a pretty cool link that shows a diagram of the swash plate and how a heli works.

Heli Page

RobertN · Mar 18, 2004

Re: Why do helicopters \"chop\"?

That makes sense. There is more to the answer than I might have expected. The descriptions given and the technical background make clear light of how it happens.

RobS · Mar 19, 2004

Re: Why do helicopters \"chop\"?

 BTW, my wife is getting suspicous about all of the questions I am asking her about helicopters. She was an engineer at Bell Helicopter 

Thanks much for the explaination Dave and also thanks for revealing the true source of your information /forums/images/graemlins/grin.gif Your wife must have done better than I in fluid dynamics. For some reason, I've always had a fascination with helicopters. Much like tractors I guess. Maybe helicopters are the tractors of the sky.

I'll have to take one of those tourist helicopter rides sometime. The pilot would probably think I'm nuts as I would be paying more attention to the machine and sounds than the scenery below /forums/images/graemlins/smile.gif

jinman · Mar 19, 2004

Re: Why do helicopters \"chop\"?

( The chop is caused by the the blade tips reaching super-sonic speeds. )

Dave, the only problem I have with the "sonic boom as chop" theory is that once you get a sonic boom, you don't continue to get sonic booms. Or said another way, when you reach enough speed to break the sound barrier, you don't continue to go boom, boom, boom. If this were true, wouldn't turbine engines constantly emit lots of booms/chops? /forums/images/graemlins/confused.gif

I'm not saying the boom theory is wrong, but I'm skeptical until I can figure out why the boom would be created with each rotation and why it's only once per rotation. Hmmm...

RobS · Mar 19, 2004

Re: Why do helicopters \"chop\"?

 Dave, the only problem I have with the "sonic boom as chop" theory is that once you get a sonic boom, you don't continue to get sonic booms. Or said another way, when you reach enough speed to break the sound barrier, you don't continue to go boom, boom, boom. If this were true, wouldn't turbine engines constantly emit lots of booms/chops 

I'll take a stab at this Jim, but we may need Dave's wife for the ultimate answer /forums/images/graemlins/wink.gif

An object moving at supersonic speed will produce a shock wave in the shape of a cone moving rearward from the object. A stationary listener will hear the sonic boom as this cone reaches his/her vantage point. For a supersonic aircraft, this is a single occurance as the plane passes once. For a helicopter blade it is producing a constant shock wave but each revolution of the blades introduces a new occurance to a stationary observer (thus my question about the sound IN the craft versus on the ground).

The chop chop sound distinctive to a helicopter has to do with the relatively low RPM of the rotor versus the high RPM of a propeller aircraft. The propeller may be doing the same thing but the occurances happen much closer together and produce a more constant sound.

Supersonic sound pressure waves can be quite damaging and in turbine engine design they must be taken into consideration. (to relate to tractors: internal shock waves are like cavitation in liquid systems) I had some classes in gas turbine theory but there is far more I don't know about this stuff than what I do /forums/images/graemlins/smile.gif

Wannafish · Mar 19, 2004

Re: Why do helicopters \"chop\"?

 "There is body interaction, so chop-chop." 

Big Dave,

Please explain what you meant by "body interaction".

Thanks

art · Mar 19, 2004

Re: Why do helicopters \"chop\"?

I'd have to say that with the blade pitch being changed with every rotation that it could be the cause. Different prop configurations might also cause it.

GaryM · Mar 19, 2004

Re: Why do helicopters \"chop\"?

I'm getting in on this late, but here's an explaination that I was given some time back. Most of the chop-chop comes from the sound of the tail rotor reflected of the bottom of the main rotor baldes! Think about it. Chopper heading toward you, lots of chop-chop. When it passes, the chop-chop is greatly reduced. A MD500 NOTAR, no tail rotor, has almost no chop-chop sound. A helicopter with an enclosed, ducted, tailrotor makes a different chop-chop than a helicopter with a conventional tail rotor.

Bird, if a helicopter is in a hover with the nose toward the observer do you get a chop-chop? If the pilot swings the nose away from you and has the tail toward you, is the sound different?