I'm pretty close to finishing my plane, just getting the paperwork
 assembled for registration and then inspection.

 I was up working on it when one of the other tennants stopped by to
 compliment me on the looks of the metal cowl. He said it looked really
 nice with a cowl like that, but that it should sound more "Macho". He
 said it was too quiet, when I'm running it all he can hear is the fan
 in front.

 It's got 2" diameter pipes into a "y" 32 inches back from the exhaust
 ports, then into a Hushpower muffler and then out to the side and
 bottom under the wing. It has the  jspec 13BREW motor with no exhaust
 port baffles.

 The prop is a Catto 74" dia x 88" pitch.

 Brian Trubee

 I wish there was a way to lower the tone. I am thinking hard on that
 problem.

 Paul Lamar



One way to take out the high frequency component of the exhaust is to
reflect the high frequency component back up the exhaust pipe. This can
be done by making a concentric
pipe muffler with a 90 degree cone at the end of the  exhaust pipe the
internal pipe looks at this cone. The external concentric pipe takes the
exhaust out by reversing the direction of the gas flow and having the
gas exit from this pipe via a right angle  tube. Does it work - yes - I
have built one and can demonstrate it. Basically all that is heard is
prop tip noise.
I have not measured back pressure so that I do not know if any
horsepower is lost. But it is sure quiet.
Chris Smale

Give me a couple of dimensions here Chris and I will do a 3D.

What is the diameter of the internal pipe?
External pipe diameter?

BTW here are some pictures of Chris's airplane.

Paul Lamar