The following article is taken from the AMA National Newsletter.
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This article is by Mark Gollattsheck.

TUNED PIPES

Tuned pipes are a means of boosting the power of two-stroke engines. When properly set up, they can be very effective. If you have ever played a note by blowing over the end of a piece of tubing, you are using the same principle. That is, any tube has a natural resonant frequency, usually dependent on its length, and the speed of sound in air. This means that some oscillations will die away quickly, but one in the right range will resonate, and be strengthened in force when the wavelength matches the resonant length of the tube. As a pressure wave in the sound reaches the end of the pipe, a reflection is set up, and moves back up the tube. This occurs at the end, whether open or closed, and at changes of section or taper.

Now, if we arrange a length of pipe as a muffler for a two-stroke engine, we will find that at a certain rpm, the pipe will resonate, and boost the engine's rpm up. This is because the reflected pressure wave arrives at the exhaust port just in time to push some fuel/air mixture that was about to be lost (due to timing overlap), back into the cylinder, where it will be burned, producing more power. All we have to do is arrange the length of the pipe so that the boost in rpm occurs at a specific range that is useful to us with the relevant load (propeller). It may be that the engine cannot produce enough power to turn the fitted prop at a useful speed. Some engines have port timing that cannot benefit fully from any pipe.

The major factor in setting up a pipe is the length, for any given propeller and rpm range. Some examples are given later. Different designs of pipes will produce different lengths, because of the effects of diameter, taper angle, and type of end reflector. Many pipes also have a muffled section that hides the rear cone or reflector's shape. Here are the basic questions to ask yourself before trying a pipe:
* Is the engine likely to benefit?
* Is the aircraft capable of handling extra speed?
* Is the pilot capable of handling extra speed?
* What propeller and rpm range are you targeting?

Let's get started. Record the static rpm on the prop of your choice with a muffler before doing anything else, so we know a starting point to make all comparisons. Try to get a starting point for the length from a similar set up if possible, and fit your pipe. If you have a choice, get a header that is a bit (one-inch) longer than you think you need-it is easier to shorten than lengthen the header. Now, start the engine and tune for slightly rich from peak rpm. Note that this may require a richer setting than usual, as we (hopefully) are producing more power than before. If we have fewer rpm than with a muffler, something is wrong-the pipe is probably too long. Try shortening the header (or pipe if more convenient) in 1/4-inch increments until the rpm starts to rise. If the pipe is too short, the engine will run harshly and the needle setting will be unstable. Add 1/4-inch spacers between the header and the pipe to resolve this problem.

Now to fly it. If it is not visibly faster in the air, try a shallow dive. If there is a distinct jump in rpm and speed, the pipe is too short, and the 'coming on' is caused by the prop unloading in the dive and coming up to a resonant rpm. If, however, the dive produces no change, but the vertical performance is better, the pipe is too long. Note that the references to 'short' and 'long' are relative-the pipe cannot improve the speed over all rpm ranges, and you will have to decide what the most appropriate compromise is for your case.

Most flyers do not need to have the engine speed up while descending, only to slow down in level and upward flight, so most adjustments will be aimed at improving level and upward flight.

From the Bee Line
Mark Gollattsheck, editor
11521 Malibu St.
Chester VA 23831