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R/C fliers, especially racing and pattern fliers, have a problem with noise. No, I'm not going to discuss, in detail, the fact that our problem is not so much with noise as with intolerance of our sport by non-participants. Nor will I say much about the mentally debilitating effects of staring at a dB meter for too long. I will simply say, that by any standard, including that of the dB, the use of a muffled tuned pipe significantly improves the noise situation with no power loss.

Tuned pipes, both muffled and non-muffled, have been around since the 1960s. The use of muffled pipes is now commonplace in R/C pattern. A muffled pipe with an appropriate header can be found to fit virtually any two stroke engine now on the market. The modern Schnuerle ported engine designs benefit most from the use of a pipe.

How much benefit can be expected? Experimenting on my Picco .40 powered Scat Cat racer, I measured a drop of over 11 dB simply by fitting a stock MACS #1240 muffled pipe over the open end of the header. At the same time I gained over 1,000 rpm. More significant than either of these changes, however, was the change in sound quality - the pipe changed the sound from a raspy, drag racer snarl to a pleasant, Saab turbo-style "whoosh." By contrast, the muffler that came with the engine reduced dB by only about half that much, had no noticeable effect on the rpm, and did nothing to smooth out the sound quality.

I am convinced that most complaints about noise have nothing at all to do with dB, but rather with the quality of the sound our models make. Why else do out non-modeler neighbors tolerate lawn mowers, motorcycles, and full size aircraft that vastly exceed the sound levels of model aircraft? Why else are giant scale models so often granted an unofficial exemption from dB limits at flying fields which have such limits? But I digress. Suffice it to say that the use of a muffled tuned pipe gives at least as much reduction in noise as the dB meter would indicate - and probably much more - without sacrificing power. I'll now explain how you can get these benefits with little or no tinkering.

Many fliers think of a tuned pipe as a cylindrical chamber filled with voodoo magic. They're scared to use one because they think you need a Ph.D. to understand it. Balderdash! If you've even seen a six-year-old take a bath you already know how a pipe works. Just watch. The kit sits in the tub, leans forward a little, back a little, and forward again. A small wave starts traveling from one end of the tub to the other, back and forth. The kid soon discovers that the wave always goes the same speed, so rocking faster doesn't help it along. But, by rocking at just the right speed, the kid adds a little energy each time the wave passes, building it up into a real tsunami that slams into the end of the tub, spraying water everywhere. The fun doesn't end until the tub is empty or an adult comes into the room, whichever happens first.

A tuned pipe is the bathtub in the above example, and your engine is the kid. Or goal is to find the point of maximum splat by varying the length of the tub or the speed of the kid, or both. The length of the tub (or pipe) determines it's resonant frequency, which is the speed the kid (or engine) needs to go to achieve maximum splat.

The beauty of the tuned pipe is that it takes an otherwise wasted portion of the engine's energy, noise, and returns some of it to the engine with each stroke of the piston. The shock wave from each little burp of exhaust travels down the diverging cone of the pipe, gaining momentum and pulling the spent fuel/air mixture behind it out of the engine. At the same time, it's also pulling some fresh fuel/air mixture completely through the engine from the carburetor. Then part of the shock wave hits the converging cone at the far end of the pipe and is reflected back toward the engine. The reflected wave actually pushes some of the fresh fuel/air mixture upstream into the exhaust port just before it closes, packing more of the fresh charge into the combustion chamber than would otherwise be there. The result is like supercharging, but with no moving parts and only a small weight penalty. The addition of a secondary expansion chamber diffuses the exhaust steam, reducing the noise further, but doesn't interfere with the supercharging effect.

By contrast, the ordinary untuned expansion chamber muffler simply absorbs and diffuses the engine's sonic energy; it's the equivalent of covering the bathtub in our example, kid and all, with a large plastic bag. And it's just about as healthy for your engine as the bag is for the kid.

How do we find the point of maximum splat? Start by mounting the pipe header, sans pipe, on the engine. Put a normal size propeller on the front and fire it up. (Don't forget your hearing protection!) Needle it to peak rpm, the back it off slightly rich. Shut the engine down.

Now, add the pipe. The pipe generally attaches to the header using a short piece of neoprene tubing, allowing the overall length the be adjusted over a range of 1 inch or so. For now, keep it long - don't insert the metal parts all the way into the neoprene tubing. You can use a pair of small automotive hose clamps as stops to set the length of this assembly.

Start up the engine at low throttle, then advance to high throttle. As the engine rpm approaches the pipe's resonant frequency it will start to "come on the pipe." You'll notice a very definite boost in power as the splat begins. If it doesn't either the propeller is too big or the pipe is too short. Switch to a lighter prop and try again. Manufacturers generally make their pipes and headers about right or, if anything, a little long. You should be able to get that nice boost without too much fiddling.

The next step is to push the ends of the pipe and header together until they almost touch inside the neoprene coupler. This shortens the pipe a little. Don't let the header and pipe touch or the aluminum will grind away and be sucked into the exhaust port. Now, see if the engine still comes on the pipe while sitting on the ground. If not go back to the original length. Set the needle valve as rich as you can without noticeably reducing the rpm. This ensures that the engine won't be running lean in the air; with the pipe on there, you won't be able to hear it sag until it's very lean.

Fly the airplane. If it comes off the pipe at the top of loops or in vertical climbs, and you're sure it's not because of a lean mixture, go to a smaller propeller. If you hear it waffling and warbling in dives, and you're sure it's not because of an overly rich mixture, go to a bigger prop or start shortening the header 3/8 inch at a time, using a saw or tubing cutter. After each cut, be sure to blow out any metal shavings before running the engine again or the pumping action of the pipe will inject them right into the exhaust port.

Once it sounds happy in the air, you're basically done. During flight testing you may have shortened the pipe to the point where it won't always come on while sitting on the ground, but this is OK. The engine generally unloads 10-20% in the air, so the ground rpm may be well below the pipe's resonant frequency, even though the system works well in the air.

If you want the highest possible power output, such as for FAI racing competition, there's one more fact to consider. Every engine has an rpm range - "power band" or "peak rpm" - in which it produces the most power. Manufacturers often include this information in the instruction sheet that comes with the engine. (Ha! You threw it away didn't you?)

In this case the trick is not only to have maximum splat occur, but to have it occur in the engine's power band and in the air. Recall that maximum splat depends on two factors: pipe length and engine rpm. If we know the engine's peak rpm, we can make ourselves a bench prop that will spin at about that speed to simulate conditions in the air. This allows us to control the other factor, pipe length, by gradually cutting the pipe shorter until it rings happily at the target rpm. When you do this, always balance the propeller and always wear eye protection.

As we zero in on the target rpm, the bench prop may spin a little too fast due to the assistance of the pipe. Also, as an inevitable result of cutting the pipe to match a specific rpm, the last cut will actually go a little too far. It's like the helpful lady on the bus who says, "Oh, I know where your stop is. Just watch me, and get off one stop before I do." So it's best to start with the bench prop a little large, say 2,000 rpm below the engine's actual peak. If at any time you go over the target rpm, make a slightly larger bench prop. Shorten the pipe 1/4 inch at a time until the rpm no longer increases with each cut.

When this is done, whittle a new prop so that it turns at 80-90% of the engines peak rpm (to account for 10-20% unloading in the air). Fly the plane, using trial and error as outlined above to find exactly the right propeller.

Muffled tuned pipes may not be the cure for all of our ills, but they do solve, in elegant fashion, the paradoxical problems of insufficient muffling where power is needed and insufficient power where muffling is needed. If more modelers used these readily available devices, perhaps we could quit bickering about noise and get on to the real business of our sport and hobby, which is to enjoy it. May the splat be with you.