This article is from the Radio Control (R/C) Flying FAQ, by Shamim Mohamed email@example.com with numerous contributions by others.
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Tuned pipes are a means of boosting the power of two stroke engines.
They are not all things to all engines, but 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 principle involved. This is that any tube has a
natural resonant frequency, usually dependant 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 wave---length 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 out
the port (due to timing overlap), back into the cylinder, where it will
be burnt, producing more power than without the pipe. All we have to do
is arrange the length of the pipe so that the boost in rpm occurs at a
rev 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 timimg that cannot benefit
usefully from any pipe.
The major factor in setting up a pipe is the length for a given
propeller and rpm range---some examples are given later. Some 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 which 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---if it is a sport type engine, less
likely, but ask around. If it has a name for power (eg ROSSIs, YS, the
hotter OS) almost certainly.
is the aircraft capable of handling extra speed?
is the pilot capable of handling extra speed?
what prop and rpm range are you aiming at?
Let's get started. Record the static rpm on the prop of your choice
with a muffler before doing anything else, so we know where we are
starting from. 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 (1") 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 revs. Note that this may require a richer setting
than usual, as we (hopefully) are producing more power than before. If we
have fewer revs than with a muffler, something is wrong---if your mixture
is correct, the pipe is probably too long. Try shortening the header (or
pipe if more convenient) in 1/4" increments until the revs start to rise.
If the pipe is too short, the motor will run harshly, and the needle
setting will be unstable and critical---add 1/4" spacers between the
header and the pipe. Now to fly it. If it is not visibly faster in the
air, try a shallow dive. If there is a distinct jump in revs 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 for your case is
yourself. Most fliers 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.
Remember that pipes will vary in their boost and tolerance of
The lengths given below are from the exhaust port face to the high
point of the two cones of the pipe, or if muffled, usually to the point
where the muffled section joins the first cone.
Prop Length Rpm OS 46 SF MA 10x6 345 mm 14000 OS 45 FSR MA 9.5x6 305 mm 15000+ (10x6 cut down) OS 45 FSR MA as above 305 mm 16200 (exhaust port lifted 1.0mm) ROSSI .60 MK 11x7.5 375 mm YS .45 MA 11x7.5 320 mm