Why do helicopters "chop"?

RobertN · Mar 15, 2004

Why do helicopters \"chop\"?

Why do helicopters make that annoying "chop" sound?

We had one circling nearby this morning. It was rather annoying. I thought maybe it was an air amublance. It got me thinking though... Why do they maker the "chopper" sound? With turbine engines, there is no ignition impulse to affect the rotation of the blades; it should be a steady pressure. It seem they should make a swoosh sound instead.

Why do they make the "chop" sound instead of a swoosh sound? I've been thinking about it, but have not been able to figure it out /forums/images/graemlins/confused.gif

_RaT_ · Mar 15, 2004

Re: Why do helicopters \"chop\"?

Robert, a pilot on Tom Sullivans radio show mentioned the chop. Have you heard the more advanced helicopters. Much less noise. His comment was much shorter blades and better design. Apparently the long blades have tips that approach the speed of sound and in doing so cause so much air turbulence that it results in a lot of noise. I watched a special on copters and was amazed the whole thing holds together. The pitch of the blade is not only adjustable, but in motion actually is changing pitch during the 360 degree rotation, it's what gives the copter its forward speed. They showed it in slow motion with very high speed film. It makes the whole thing look unbelieveably unsafe. I want a ride in one. Rat...

RobertN · Mar 16, 2004

Re: Why do helicopters \"chop\"?

I rode in one on Maui, and years back at the Placer County fair in Roseville. It is fun! Didn't notice the "chop" so much when inside, especially the sight-seer type in Maui...

The shorter blade being quieter make sense. Still though, with the turbine motor is seems the rotation would be smooth. I gotta study this a bit /forums/images/graemlins/smile.gif

Cliff_Johns · Mar 16, 2004

Re: Why do helicopters \"chop\"?

Helicopter: 10,000 parts flying in lose formation.
Cliff

jinman · Mar 16, 2004

Re: Why do helicopters \"chop\"?

( Why do they make the "chop" sound instead of a swoosh sound? )

Disclaimer - This is a SWAG from my many years of radar and aircraft experience, but truly just an educated (uneducated?) guess.

I think it's the combination of the low speed blades, turbulent air, and the doppler effect. Since the blade is rotating at constantly changing angles to you, the pressure goes from high to low and the wavefront is perpendicular to your observation point only when the blade is pointing directly at you. At the same time, the doppler effect of the blade is going from positive to negative as the blade passes through that perpendicular point. I believe that is the "chop" sound creator.

Also, many turbo-prop airplanes have a chopping sound, but because their propellers rotate so much faster, the chop becomes a roar and we peceive it differently.

Okay, now someone with real science behind them may correct me, but I don't think I'm too far off. /forums/images/graemlins/blush.gif /forums/images/graemlins/crazy.gif

RobS · Mar 16, 2004

Re: Why do helicopters \"chop\"?

 I think it's the combination of the low speed blades, turbulent air, and the doppler effect 

You nailed it Jim. It's primarily the doppler effect with the blades moving both towards you and away from you. Some of the older military helicopters are very loud and very distinctive because of the size of the rotors.

I've never ridden in a helicopter but it seems like it would be fun /forums/images/graemlins/smile.gif

Wannafish · Mar 16, 2004

Re: Why do helicopters \"chop\"?

I always thought it was the change in the force of the downward air pressure that reaches the ground as the rotor blades crossed over the tail of the helicopter.

DAP · Mar 16, 2004

Re: Why do helicopters \"chop\"?

Moving away from the physics answer, suffice it to say that the chop sound is a result of the blades annihilating the air that they are flying through. People often take for granted the physical properties of our atmosphere here on earth. Here's a great example: when first learing to fly airplanes, one of the first things you learn is something called a standard rate turn. This maneuver is accomplished by entering into exactly 15 degrees of bank (boy, not sure about the 15 anymore) and holding that angle and exact altitude for exactly 2 minutes. If you can accomplish these few things, you will eventually return (with no wind) to your start completeing a relatively uniform circle. The point is, that I was amazed at when you return to your starting point some 2 minutes later, you run smack dab into the busted up air (prop wash) you made when you started the manuever. A very noticible thump. Way cool.

A lot of existing rotor designs still utilize (for lack of a better word) the concept of a "Jesus" nut.

An old friend of mine flew the Hughies in S.E. Asia. He explained that it got its name because if this nut went, the pilot would utter that same word.

While I've flown a couple of different fixed wing machines, I've only ridden in a heli a couple of times. They are kinda like Corvettes in that it is pure Brute Force technology.

Bird · Mar 16, 2004

Re: Why do helicopters \"chop\"?

( I've never ridden in a helicopter but it seems like it would be fun )

More fun than a barrel of monkeys. /forums/images/graemlins/wink.gif Especially in the little Bell 47s we had 30 years ago, with the doors off in the summer. I've only been up in a Jet Ranger once for a short ride; all the rest of my helicopter experience was the 47s. I don't know how it is for other folks, but for me, personally, it seemed to take a lot better coordination to fly the helicopter than it did to fly little fixed wing planes.

DocHeb · Mar 16, 2004

Re: Why do helicopters \"chop\"?

In the early 80's the Detroit Police & Fire Department had an old Vietnam-era Huey that I flew on a couple times carrying patients. That machine had a very loud and distinct "Whomp-Whomp-Whomp" that, to this day, I can tell when the Huey helicoptors are overhead, flying out of Selfridge Air Force Base.

Later we ran an air ambulance service flying a Bell BK117. That coptor had twin turbines, and apparently had a great power-to-weight ratio. I did ~60 patient transfers and scene runs. Every time we took off it was a rush - it was like flooring the pedal for a race car - but straight up!