Tuned Pipes

[wamco]

I have a webra speed 61 with a muffled tuned pipe on a UFO. Problem seems to be that the engine comes on pipe quickly then looses it. ie: on take off run it will blast off from idle and then seem governed in about 30 to 50 feet, if that, and remain there as long as the throttle is WO. I understand theory of these pipes but have not tried adjusting the length for proper rpm curve. My concern is making a bad guess and having to cut something longer. Any suggestions?

[wamco]

I found the formula......

Lt = Eo x Vs / N

Lt in inches (tuned length)
Eo is exhaust open duration in degrees
Vs = speed of sound in fps (1700 fps as an average)
N = engine speed in RPM

Lt is the distance from the edge of the piston to a point half way down the baffle cone, if we pretend the baffle cone continues to make a point.

Simple Huh?

[wmazz]

That is a very old formulae for Motocross bikes, that had a big flaw. The speed of sound was a constant for all engines. It especially doesn't suit model airplane engines.

The speed of sound is directly proportional to the temp in Kelvin. In your case an exhaust temp of 720 - 730' F, should be a good starting point.

The conversion to the speed of sound in Meters/Seconds is:

(TempF - 32)*.555 = TempC

Speed of Sound is = the Square root of ((tempC + 273) * 1.4 * 287)

Pipe length = (83.3 * Speed of Sound * Exhaust Duration) / RPM

This formulae was devised by the same person, GP Blair, as your formulae was, but it is a newer formulae (mid 80's vrs. mid 60's). Also, it measures from the piston to the end of the rear cone.

It would certainly help to know your unloaded rpm in the air. Weather conditions and prop size will effect that rpm.

Good Luck

Bill M.

[wmazz]

I have a webra speed 61 with a muffled tuned pipe on a UFO. Problem seems to be that the engine comes on pipe quickly then looses it. ie: on take off run it will blast off from idle and then seem governed in about 30 to 50 feet


Your pipe is too long, and is passing peak power when it's seems governed. Watch nitro content and glow plug selection, while the pipe is too long or else it may lead to an engine failure. Over-Rev without a pipe, isn't near the problem as with a pipe.

Another way to go, may be trying a larger prop first, but be conservative on fuel and plug choices until you are close to finding your right setup.