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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 non-optimum length.
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. | Engine | Prop | Length | Rpm | Remarks | | 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 | - | - |
These examples used a variety of pipe makes, but I have found that MACS pre-tuned pipes are hard to fault - i.e. they will come up straight away. Some other types and makes of pipes will differ - GRAUPNER pipes will give bigger boosts, but are MUCH more critical on almost every parameter - length, prop, plug, fuel etc.
Dont forget to record what you try so you don't repeat mistakes or dead ends in your trials. I have found good muffled pipes, properly set up, frequently are quieter than mufflers, especially when set up long with big props - the best result I have had was an OS 46 SF with a 12x6 and a pipe about 40mm(1.5") longer than for a 10x6, measured about 85db (at 3m/10ft) over grass, and in the air it was inaudible if there was anything else in the air.
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