Why do helicopters "chop"?

[lamarbur]

Re: Why do helicopters \"chop\"?

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

 

[Bird]

Re: Why do helicopters \"chop\"?

</font><font color="blue" class="small">( Personally, I find the sound reassuring. )</font>

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]

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]

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]

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]

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]

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]

Re: Why do helicopters \"chop\"?

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

 

[BigDave]

Re: Why do helicopters \"chop\"?

</font><font color="blue" class="small">( 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? )</font>

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]

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.