Latest issue has some mention of a new panel looking into it (sound control and noise levels).

The AMA currently has two sound rules. 1.) you must have a muffler. and 2.) no flying before 9am.

Could you show us where those rules are? I was not aware of any such rules.

Found out an interesting piece of information yesterday, the county that my club is located in (lake county, FL) has a sound ordinance but it specifically exempts "aircraft" from it. Of course the bureaucrats intended this for full scale, but as our "model aircraft" are under FAA jurisdiction per AC 91-57 a good lawyer could make a great case that it also applies to our "aircraft".

Brad

Yeah, also mention by the AMA TD that "What you hear when the prop unloads in the air is the prop tips going supersonic." It's easier to repeat old wive's tales than do the math.

+1

is that 9am local? or is it Eastern/Pacific/Mountain time?

Paragraph535-B

But as I look at it again searching for the paragraph number for you folks, I realized it was actually an example of local club rules and NOT an an actual AMA rule. At least the 9am rule. However, I have seen the muffler rule before in another paragraph, (I think).

Ok, cj., I'll bite.
What do I hear when the prop "rips" (I assume that is what you speak of)?
And this better be good, 'cause supersonic prop-age makes for a good story.

804-

Dunno about the sound you describe, and wasn't me that spoke of it. Lots of things cause changes is prop-generated sound level, usually rapid attitude changes that disturb airflow through the prop.

Take a 12 inch prop of reasonable pitch turning at 12K rpm, typical for a decent .60 glow engine (takes about 1.5 bhp). A prop tip is traveling Pi feet each revolution, and at 200 revolutions/sec, the tip speed is Pi X 200 = abt 630 fps. Speed of sound (std conditions - dry air, barometer at sea level, etc) is 1125 ft/sec. Tip speed is nowhere near sonic. Increase the rpm to 21,500 and tip speed will be just up to sonic. That will take about 9 bhp. Got something hotter than nitromethane in that can?

Try it with other props turning at realistic revs and see what conclusion you arrive at.

More on supersonic props, just because I find them fascinating.

Look [link=http://en.wikipedia.org/wiki/XF-84H_Thunderscreech]HERE[/link] for a brief history of the brief but exciting career of the Republic XF-84H "Thunderscreech" which was AFAIK the only purpose-built aircraft ever with a supersonic propeller.

Also, consider the 1:1 scale Unlimiteds at Reno, in particular those with big round engines, e.g., Rare Bear. They are up against the 'sound barrier' and it's the prop rather than the engine or airframe that can't breach that barrier. As appears obvious, some of the prop blade must extend past the cowl to egest (spallchucker doesn't like that word....) air into the slipstream and so produce thrust. That means the prop tips have a limited minimum distance from the hub, and so travel faster than shorter blades used on planes with inline engines. The engines are more than capable of driving prop tip speed supersonic. When that happens prop efficiency goes to hell in a hand basket and the airplane slows down. As if the race pilot didn't have enough to do, he's got to have his mind and his hand on the throttle constantly to keep it on the edge between all that physics will allow and a tiny bit too much.

Thanks Frank for taking another look. That explains it.

I thought I looked closely myself and haven't found an AMA rule about mufflers... but how would I prove something doesn't exsist???

If you find such a rule, we will all learn something.


Again, thanks for taking the time to look...if more people would do as you have done this forum would be a lot more productive.

True. the static tip speed is far short of the speed of sound. But you forgot a couple of factors.
1.) The engine will unload in the air and operate at higher rpm. Much higher when the airplane is dove, as gravity provides the horsepower to accelerate the prop.
2.) Tip speed is not simply a function of Pi x D, you have to include the forward travel of the prop with the unloaded tip speed. For high rpm high pitch props, this is significant.
3.) The speed of sound varies with the airfoil moving through the air.

From my example, you need to gain about 500 fps from the static condition to be in the transonic realm.

Lets take your #2 and assume airspeed is 120 mph. Add that vector to rotational speed, which is orthogonal to it. So, say 176 fps in the axial plane and 630 fps in the rotational. I get a velocity vector resultant of 654 fps, not much increase and still way short of 1125 fps. Now add #1, rpm increase. I could see a few hundred or maybe even a thou or two increase due to prop unloading, but we're short on the order of 10K rpm. I'd have to see convincing data to believe an increase of that magnitude - my ears sure don't beleive it.

As for #3, I must admit to being totally unaware of that phenomenon. Can you expand on it?

Nearly any RC model engine will unload between 1 and 2 thousand rpm in the air. I've had racing engines on pylon models unload 5000 in level flight, verified with an audio tach and doppler shift. But when you put an airplane into a dive, the rpm will really climb due to potential energy turning into kinetic. Many airplanes can see an easy 25% increase in speed, and thus higher engine rpm with it.

I have flown many pylon planes that were above 175 mph in level flight, so forward velocity is a larger factor with them,

On 3.0 I meant to say that the localized speed of the air changes with the change in pressure over the surface of the prop.

Great that you fly pylon, as that represents the upper fringe of both speed and noise level produced by model airplanes and so is very much on topic with this thread.

So, what's the bottom line? At the most extreme, in an extended dive with a screaming 5-6 hp Nelson and 7 inch prop pushing a very slippery airframe, do the numbers add up to a prop tip speed in excess of 1100 ft/sec?

I believe the major concern with prop "ripping" comes from the 40% TOC guys spinning 25 to 30 inch props powered by large powerful engines (DA 120-160). And the general consensus is a prop will "rip" at around 60% of supersonic.

Brad

My 22" prop on 53cc will do it, straight and level full throttle.
It would be interesting to know how much "extra" noise this causes in the air, it won't do it on the ground when sound testing.

This question of model props going supersonic being the mechanism behind an increase in noise level has nagged at me.......found the following post in another RCU forum that gave me some data to work with.

Read more: [link=http://www.rcuniverse.com/forum/m_8936563/anchors_9767997/mpage_23/key_/anchor/tm.htm#9767997#ixzz0sUcTELxM]HERE[/link]

At 120 MPH a prop with 12 inch pitch is at "pitch speed" at 10,500 rpm. The angle of attack of the prop blade at this speed is 0 degrees. Talk about prop unloading with airspeed - you can't reduce the load more than this.

A 24 in dia prop at 10,500 rpm travels 2 pi feet/rev, so at 10,500 rpm (175 rev/sec) the speed at the prop tip is 1100 ft/sec. The vector sum of 1100 ft/sec in rotation plus 176 ft/sec on axis yields a resultant along the helical path of the prop tip of 1114 Ft/sec. Close enough to make the case for the prop tips going supersonic (>1125 ft/sec); with a prop of zero effective pitch I wouldn't hazard a guess at how much faster a DLE85 might turn, but if the reported figures for prop geometry and airspeed are accurate, it is at least enough to get the prop tips into the transonic realm.

The fix appears simple enough, assuming one wants to fix it: (1) if using a large dia/low pitch prop for 3D flying, throttle back when flying level or diving, or if speed is the objective (2) do as the author of the cited post said and switch to a prop with more pitch.

In prior posts I inferred a general rule that model airplane props don't go supersonic. As with most general rules, somebody will demonstrate an exception to it. So 804, it appears that indeed "supersonic prop-age makes for a good story."

Measure it. I assume you got a ground measurement at 3 meters, per AMA guidance. Due to spreading loss, the sound level diminishes 6 dB with each doubling of distance from the source. Ex: You ought to be able to fly a line down the runway 24 meters (approx, even several meters error won't matter much) away from the sound meter and have time enough for the meter to sample the sound level. Add 18 dB to the in-flight measurement and compare that to what you measured on the ground.