Mark

Try the coax muffler. Less drag for an external muffler
and a lower note tone. The main problem with rotary noise is
the high frequency component.

It works a little bit like a ram jet, ejector or thrust augmenter
so it does not add too much to the aerodynamic drag.  Just like a P-51 cooling system. The tone of the rotary sound is much more pleasant.

Also it can be visually  inspected so it won't plug up if it ever
cracks. The outer casing is open at both ends. The inner tube
is cooled by the slipstream flowing between to the tubes..

Paul Lamar
--

I did try the coaxial muffler.  It was quieter, but the egt's went up and I lost 20+ mph on the top end.  I'm considering trying the muffler Tracy is running on his 3-rotor.  Its called an aero-turbine muffler.  Only problem is OD.  I'll have to build a new exhaust manifold in order to fit it inside the cowl.  Did I mention, for aesthetic reasons, I don't want a muffler hanging on the belly of my Lancair?

Mark


   How could the EGT's go up. Something is wrong. Got any pictures.


   It is just an long open 2.5 inch pipe with a lot of small holes in it. Did you weld the inner pipe closed at the end? If so your not supposed to do that. If you still have it drill a big hole with a step drill in the end cap and try it again. It probably has the lowest back pressure of any muffler. In theory it has very
   low drag as there is essentially no frontal area except for that of the center pipe. The slip stream is
   free to flow down between the inner pipe and outer pipe. This air is energized by the 300HP worth of
   hot air from the inner pipe generating some thrust. That is how augmenters work.

   It can be made to look like part of the airplane if you use a U shaped outer pipe.

   On your current muffler you can measure the back pressure with a water manometer.
   Weld  3 foot of brake line tubing to the exhaust pipe so it cools off before it gets to the water
   manometer.


   Paul Lamar


   I would think that there would be some benefit to having the exhaust discharge behind the passenger compartment, especially in the decibel department. However, like the Lanciar my plane is fiberglass and the close proximity of the hot pipe to the belly of my plane might be a cause for concern.

   One thing I do know is that the pushers that I have flown in seem to be much quieter, which leads me to the plane I would like to build next- the Velocity twin. A pair of 13B p ports would make her a smoking hot plane I tell ya!

   OK, back to reality;

   Paul, has there been any discoloration of the fuselage over the top of the pipe in the RV? That would give an indication of how hot it has been in this area.

   Mark:
   You could always make it shorter- like a couple of feet max.

   Kevin Alderman

   There is a layer of fiberfrax next to the skin and then some .020 SS.
   Then a layer of high velocity cooling air.  You can forgo the holes
   on the top of the center pipe. It gets a bit warm in the cockpit above
   the exhaust but not uncomfortable. You can always go to 2 layers
   of fiberfrax and SS. Not a lot of weight there.

   If you have an external muffler now make a test rig and monitor the temp
   with sensors .

   Paul Lamar


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