Why do helicopters "chop"?

MossRoad · Mar 19, 2004

Re: Why do helicopters \"chop\"?

Rob, I didn't think the tip of the helicopter rotor or the tip of an airplane propeller ever get to super sonic speeds. I was always under the impression that if it did, terrible things would happen. That's why there are no supersonic airplanes powered by propellers. So the chop noise couldn't be a sonic boom, could it?

Interesting reading on supersonic propellers.

RobS · Mar 19, 2004

Re: Why do helicopters \"chop\"?

Neat website Dave. I can't answer whether or not a helicopter rotor travels at supersonic speeds only can guess what the shock wave might do. You are right though, shock waves can be very destructive.

Again, I'm recalling very vague memories of college but there are all sorts of strange phenomena that occur in trans-sonic flight. Wing shapes for sub-sonic flight don't work at supersonic speeds. Airflow can be quite different. The distinctive delta shape to fighter jets and the Concorde have to do this all of this. The swing wing fighters are an attempt to handle both sub-sonic and super-sonic aerodynamics.

The challenges continue inside the jet engines. There is a type of jet engine called a ram-jet that basically has no moving parts. Once moving through the air the compression of the air due to ram shape and super-sonic effects provide the the necessary compression for combustion. Fuel is dumped in which burns continuously and expands as it exits providing thrust. I have no idea if these engines actually exist, only that they make for challenging engineering class problems. For as poorly as I did in some of these classes, I'm surprised I have such interest /forums/images/graemlins/smile.gif

TresCrows · Mar 19, 2004

Re: Why do helicopters \"chop\"?

"Rob, I didn't think the tip of the helicopter rotor or the tip of an airplane propeller ever get to super sonic speeds. "

Actually they do, that is what causes the machine gun staccato of an airplane propeller at max output. The tips reach supersonic speed and the noise you hear is mini sonic booms, shock waves. Prop speeds (RPM) and propeller diameters on conventional aircraft are designed to limit it to efficient sub sonic speeds but during a dive or at full power tips speeds just barely reach super sonic speeds in SOME aircraft, most notably high speed prop aircraft like WWII iron etc. The whole propeller is not supersonic, just the very tip and the shock wave it produces is a sonic boom. Imagine a Mustang in a power dive at say 450 MPH and the prop with a tip speed in the range of 500 MPH, the combined speeds put it beyond supersonic, it is not direct additive, you have to do trig and what not I am not in the mood to do now or calculus
Active noise cancelling headsets are actually designed to cancel the propeller noise more than the engine noise. They are very effective. Light Speed is a well known brand of active headsets.
Most percieved noise from a passing propeller aircraft on the ground is from the prop, not the engine, same with a helicopter and you are correct about the doppler and phase shift causing the pulsating chopping sound. Helicopters are a flying nightmare. Their top speeds being limited by the speed of the advancing blade thus attempts of late to reduce the work load of the blades and allow them to operate at low RPM and use wings or other means to provide lift at higher speeds--the Carter Copter is an example--do a search for that subject.
Just came back from another P&W school, work on airplanes every day.

Well, that was my post for the week, got to get some primer on my recently sand blasted bumpers, the little Toy is getting some serious upgrades in preparation for high gas prices this summer. Line-X on the bumpers and on the side rock nerfs, on top of the bed cap and in the bed, a new TJM front bumper with Warn Winch, headers and exhaust, thinking on a supercharger but I would have to make it myself, new wheels and tires-slightly larger, heavy duty torsion bars and some other cool stuff just cuz such as an air locker from ARB upfront and an auto locker to replace the worn out thingy in the rear. It should still get it's 25-35 MPG so I cannot go huge on tires and I don't like that look anyway. Finally got Rancho to hand over 4 new RS9000 shocks since the others were long worn out and they just could not understand what Lifetime Warranty meant but I finally explained it to them and they agreed. Told them I would be back in touch for another set in 10 years or so /forums/images/graemlins/laugh.gif. I am going for 1 million miles, may not live long enough. Installed a diff drop to restore CV angles on my slightly lifted Tundra after a boot clamp came loose, looks like that cured the problem for the long term. Had to install a heavy duty LSD in the rear to control the new power to the ground, don't know if the wheels go supersonic or not Poor little 'botas are all lonely and forgotten covered in dust. Maybe they need propane injection /forums/images/graemlins/grin.gif. --or maybe not. Well, out'a here, take care /forums/images/graemlins/smile.gif. J

RobS · Mar 19, 2004

Re: Why do helicopters \"chop\"?

Sounds like you're having fun J, thanks for your insights /forums/images/graemlins/smile.gif

GaryM · Mar 19, 2004

Re: Why do helicopters \"chop\"?

The Hiller Hornet was a 50's era two place helicopter that had a ramjet mounted at the tip of each rotor blade. I don't know what was used to spin the rotor up to a speed where the ramjets would operate.

It had no tail rotor, just a rather large fin. http://avia.russian.ee/vertigo/hiller_hj-1-r.html

This one's a slightly later model. http://www.museumofflight.org/collections/craftdisplay.html?ID=59

MossRoad · Mar 19, 2004

Re: Why do helicopters \"chop\"?

Very neat. So they had to have fuel going up through the shaft and out through the rotor blade to the little engines? Alos, it said they had a small 1hp engine that would initially spin the rotor to get the tips up to 50MPH speed in order for the ram jets to start working. So, it really had 3 engines.

BigDave · Mar 19, 2004

Re: Why do helicopters \"chop\"?

( Please explain what you meant by "body interaction". )

I left the world of fluid dynamics 7 years ago, but here goes:

If you throw a rock in a lake, the you can see the concentric circles of water travels away from where the rock hit the water. Those ripples are actually the information, or the disturbance, propagating through the water. Now if you poke a stick int the water and move it slowly in one direction, you can see that the distance between the waves in front of the stick become compressed, while the distance between the waves behind the stick expands. This is the doppler effect. What happens if you move the stick faster than those waves can travel? Information from the distrubance from the stick can't travel faster than the stick, since this information travels at a same speed when the disturbance was stationary. So, all of the information travelling foward from the stick basically accumalates in fornt of the stick, since the disturbance in front of the stick is being pushed foward along with the stick. This what happens when you travel at supersonic speed, the air in front of the wing is undisturbed, but the wing is travelling faster than the information from the disturbance cause by the wing can travel forward. So, in a very short distance, the air goes from stationary to a supersonic disturbance, so somewhere in between the air is travelling at sonic speed, which means this is the maximum speed, given the current conditions, that information can travel in forward direction. This sonic region is marked by the sonic boom, where there is significant change in pressure, thus the noise, between the subsonic & supersonic regions. This mach wave not only sits in front of the wing, but radiates above and below the wing where it begins to spread as the transitition from subsonic to supersonic flow becomes less severe. Far away the the wing, all of the flow is subsonic, but the information of this disturbance radiates through the atmosphere, so the sonic boom can be heard at a long distance. Since the helicopter blade effectively travels slower as the blade get closer to the rotor, the mach wave also travels inward towards the subsonic region. Now much of this inward disturbance is dissipated blade istelf, but some of this wave still travels downward & inward to intersect the body(& boom) of the helicopter. At near subsonic speeds, the mach wave reflections won't be that severe, but will in general provide some interference waves to make life in the helicopter slightly more hospitable.

The misses is on the phone with her mother tonight, so I had to go it alone. I hope I didn't disappoint anyone. /forums/images/graemlins/grin.gif

 ... /
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| <==< ...... How shocking /forums/images/graemlins/ooo.gif
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P.S. For any fluid dynamists out there, the water example was for illustration purposes only, so no flaming. /forums/images/graemlins/wink.gif

BigDave · Mar 19, 2004

Re: Why do helicopters \"chop\"?

Interesting Story:

When we first moved to Texas, my wife told me she had no idea where here new job was located. So we took a look on the map, and I drove her to Bell Helicopter facility in Hurst, while she navigated. After we got back to our apartment, I declared that I had just taken her to Bell and back. /forums/images/graemlins/tongue.gif

Well, at least she thought is was funny. /forums/images/graemlins/wink.gif

jinman · Mar 20, 2004

Re: Why do helicopters \"chop\"?

( 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. )

Dave, after doing my research, I am going to say that helicopter rotors do not reach super-sonic speeds. They are in the transonic range and may approach .8 Mach, but no faster. The rotor tips are designed to reduce the pre-sonic shockwaves much like vortex generators on fixed wing aircraft. The following link to a short explanation by a Hughes Research Lab engineer is very specific in noting the blade tips are designed with an "ax-head" to reduce vibrations at "near sonic" speeds.

Helo Rotor Tip Design

I also talked briefly to our Sikorsky S76 lead instructor at our training facility who said a helo blade tip at supersonic speeds would probably self-destruct because of its simple airfoil shape.

As to some WWII prop airplane's props reaching supersonic speeds as Trescrows pointed out, that has happened, but it is not something done on a regular basis. Prior to reaching supersonic speed a simple airfoil like a prop will experience a severe loss of efficiency much like an aircraft wing losing lift as the sound barrier is approached. Since the prop's movement is perpendicular to the flow of air over the airplane, the speed of the prop and speed of the air cannot be added directly to produce a summative quantity exceeding the speed of sound.

I believe the chop of a helo rotor is nothing more than the peaks and valleys of air and sound waves which travel much like an electrical AC sinewave. The chops are the peaks passing our position and the changing pitch of the sound is the doppler effect. The volume of the sound depends on the pitch angle of the blades to our position (over us, to our side, approaching, or departing). The interference of the tail rotor and the volume of other interference waves certainly reduce the amount of chop we hear.

I just can't accept that sonic booms play any part in the sound we hear from a helicopter. /forums/images/graemlins/smirk.gif

RobS · Mar 20, 2004

Re: Why do helicopters \"chop\"?

 For any fluid dynamists out there, the water example was for illustration purposes only, so no flaming 

Hey, no problem if you'll stay off my case for comparing shock waves to cavitation /forums/images/graemlins/shocked.gif

Thanks for the information Dave, I may not sleep any better but it will have nothing to do with helicopter chop /forums/images/graemlins/smile.gif