Why do helicopters "chop"?

shoppintractors · Mar 16, 2004

Re: Why do helicopters \"chop\"?

That That
was was
an an
amazing amazing
demonstration demonstration
of of
a a
Huey Huey
Helicopter Helicopter
/forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif

Don Don

RockyMnt1 · Mar 16, 2004

Re: Why do helicopters \"chop\"?

Standard rate of turn is 3 degrees/second (Thus the 2 minute turn to do a 360). Bank angle for a standard rate turn will vary based on ground speed, among other things. The faster an aircraft goes the greater the bank angle needed to achieve the standard rate. Some aircraft would need geater than 30 degrees of bank to attain the standard rate. So generally, pilots either use a standard rate or a max of 30 degrees of bank when manuevering for landing.

This is all needed so that controllers know when to turn pilots and pilots know how fast to turn.

Probably more than you ever wanted to know.

jdmar · Mar 17, 2004

Re: Why do helicopters \"chop\"?

Great post! The doppler effect and nearing speed of sound is correct...it is amazing these things work so well. I have been lucky enough to either fly or fly in several of our countries great aircraft (F18, F14, F5, S-3, A6, EA6B, H3, SeaHawk/BlackHawk, and a few others). I loved flying the fastmovers but a helo at low altitude (aka feet above ground/water/trees) is probably the greatest illusion of speed I have ever had. I have exceeded the speed of sound but it did not have the "feel" of the helo. The next closest illusion of speed is actually on a windsurfer...no kidding! **** I guess I have been lucky (an am really honoured) to have been given some of these opportunities...I love this country!

Peter

riptides · Mar 17, 2004

Re: Why do helicopters \"chop\"?

The sound of the air ambulance chop was the sweetest sound I have ever heard in my life.

AirCare 1 in Northern Virginia. I got the tee-shirt.

-Mike Z. /forums/images/graemlins/smile.gif

RobS · Mar 17, 2004

Re: Why do helicopters \"chop\"?

I'm sitting here listening to the sound of the Huey wondering this:

If you are in the aircraft and there is no other helicopter around you, do you still experience the "chop" sound?

Inquiring mind wants to know /forums/images/graemlins/smile.gif

EastTexFrank · Mar 17, 2004

Re: Why do helicopters \"chop\"?

"do you still experience the "chop" sound?"

I love this board. I've flown hundreds and hundreds of hours as a passenger in French, British, American and Russian helicopters and you know what? ... that question never occured to me. I'm trying to remember but it seems that the sound inside, if you CAN hear it, is a lot less noticable than outside. Personally, I find the sound reassuring. It means everything is still together and working. I would also add that I prefer helicopters to small planes anyday. Those things just scare me to death.

DAP · Mar 17, 2004

Re: Why do helicopters \"chop\"?

Ordinarily, we tend to think of sound as being a stationary phenom. Sound travels to be sure. And if you're in a helicopter, you can bet your bunyons that any sound made by the rotor blades is moving away from your ears at an extremely high rate of speed.

/forums/images/graemlins/shocked.gif

Jimbrown · Mar 17, 2004

Re: Why do helicopters \"chop\"?

Having spent several 100's of hours in a UH-1 I assure you that you do. You can actually feel the chop as it flies and you can hear it on the radio and the intercom. It is much worse if one of the blades is out of track with the other. In my expeierence the later UH-1's D and H models and cobras with the wider blades produce the loudest and most noticable chop.

RobS · Mar 17, 2004

Re: Why do helicopters \"chop\"?

Well now isn't this interesting /forums/images/graemlins/confused.gif There was some science behind my question. I would assume most of the "chop" was from the doppler shift in blades moving toward/away from a distant observer. In the vehicle, the blades aren't moving towards/away but rather around which is more steady state. If the chop is still present in the vehicle there must be another element going on? Something to do with the variation in pitch as the rotor revolves? Someone else mentioned effects over the tail versus over open air elsewhere?

Idle curiosity is killing me here /forums/images/graemlins/smile.gif

chucko · Mar 17, 2004

Re: Why do helicopters \"chop\"?

( Why do helicopters "chop"? )

So we can call them "choppers"

lamarbur · Mar 17, 2004

Re: Why do helicopters \"chop\"?

yyyouuu shureeee dooooo----------

Bird · Mar 17, 2004

Re: Why do helicopters \"chop\"?

( Personally, I find the sound reassuring. )

Yep, the worst sound in the world in any airplane (except gliders) is . . . SILENCE!

And did you ever notice the difference in the sound of a Bell 47 with wooden blades as compared to those with metal blades? /forums/images/graemlins/shocked.gif

lamarbur · Mar 17, 2004

Re: Why do helicopters \"chop\"?

Bird,
Wooden blades, you're giving away your age. Wooden blades came just, just after the leather helmets that were slipped on

Bird · Mar 17, 2004

Re: Why do helicopters \"chop\"?

/forums/images/graemlins/laugh.gifThere was a helicopter flight school at the same airport here in Dallas that the police helicopter section used, and they were still using a Bell 47 with wooden blades in '74. /forums/images/graemlins/laugh.gif

Tractors4u · Mar 17, 2004

Re: Why do helicopters \"chop\"?

I don't know why they go chop, but a buddy of mine has one and we went to Las Vegas this past weekend and went to the International Heli Expo while we were there. I kept accusing him of speaking in chop chop. You know, one pilot goes up to another and says "chop chop?", "Oh yeah chop chop chop chop!" While we were in Vegas we went on a heli tour of the Grand Canyon. We even landed in the canyon and had champagne. Attached is a picture of me and the Mrs beside the EC130 we flew in. And yes it is leaning. Notice the shirt?

Tractors4u · Mar 17, 2004

Re: Why do helicopters \"chop\"?

Here is another shot of the EC130. I really didn't notice the chop chop that much. Being a 3 blade it is quieter and with the fenestra tail you get a little bit of a high pitched whine.

Kelvin · Mar 17, 2004

Re: Why do helicopters \"chop\"?

I've always been told that helicopters don't fly; rather, they BEAT the air into submission.

Thus the "chop" sound?

Kelvin

Bird · Mar 17, 2004

Re: Why do helicopters \"chop\"?

I know nothing about the EC130 at all, but it sure is pretty.

BigDave · Mar 17, 2004

Re: Why do helicopters \"chop\"?

( Well now isn't this interesting There was some science behind my question. I would assume most of the "chop" was from the doppler shift in blades moving toward/away from a distant observer. In the vehicle, the blades aren't moving towards/away but rather around which is more steady state. If the chop is still present in the vehicle there must be another element going on? Something to do with the variation in pitch as the rotor revolves? Someone else mentioned effects over the tail versus over open air elsewhere? )

You're getting closer. /forums/images/graemlins/ooo.gif

I guess it is time to throw in my 2 cents. You are right in your observation about the doppler effect. The compression of the sound waves as an object is approaching will amplify the perceptipon of the "chop." As the helicopter moves away from you, the increased spacing between sound waves, caused by the moving acoustical source, force the doppler shift in the opposite direction and the chop appears to be decreased. While riding in the helicopter the chop is constant or at least the chop due to doppler is constant, since anyone who has ridden is helicopter knows the chop isn't constant.

Now for the second theory. The chop is not caused by blades hits turbulence or vortices from the other blade. The blades generate lift, thus pushing the air and it's wake downward. Even for a helicopter in hover, each blade hits clear air continously. There is some excellent visualization of this effect in high load, high humidity cases; you can see the vortices spiraling around the helicopter downward to the ground. These vortices are long lived, and just as in a commercial airplanes can cause real problems. Encountering wake was the problem with the V-22 that has crashed several times. Due to pivoting it's own props and descending too quickly, the V-22 encoutered it's own wake.

So what's the answer? The chop is caused by the the blade tips reaching super-sonic speeds. The sonic boom that is created is your chop. The sonic boom is a discontinuity in the fluid flow, which is basically a steep pressure wave. As you have started to deduce, change in pitch of the blades causes a change in chop. The blade pitch controls how much lift is generated. For supersonic flow, varying blade pitch changes the size & shape of the sonic boom and thus changing the intensity of the chop.

Okay, maybe that's a buck 50's worth. /forums/images/graemlins/wink.gif

lamarbur · Mar 17, 2004

Re: Why do helicopters \"chop\"?

First thing your told in flight crewchief school,
Angels, Hummingbirds and Helicopters can all hover, but,
none of them are aerodynamically correct.