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
I think it would be wise to install an infrared reflector on the
fuselage, and then space the muffler assembly down approximately 1/2
inch to allow air flow between the shielding, at least in the case of a
composite airplane. With a bit of tweaking the right amount of air would
flow between the fuse and the muffler assy and keep that area cooler.
With the foam core between the composite layers, by the time it got hot
enough inside to feel the heat, the outside could be on fire.
Kevin Alderman
That far back it does not run cherry red :) Composites are good to at
least 200F.
I just bought some from McMaster Carr rated at 220 F.
SS sheet would make a good infrared reflector.
The interesting thing about SS. It does not conduct heat very well.
Paul Lamar
If you polish the surface of the SS it will make an excellent IR reflector.
C. Smith
----------------------
It doesn't stay polished. I have a KIS Cruiser -- fiberglass, low wing, 4
place running a 13-B with a spintech muffler mounted under the fuselage.
I built a layer of fiberfrax covered with SS sheet to protect the bottom of
the plane. Due to an oversite, the painter ended up painting the SS whereas
I wanted it left bare.
A short ground run rapidly burned off the paint over the Spintech, so I
removed it, and tried to polish the SS. It rapidly discolored with further
running.
I have ended up putting in a second layer of SS with air gaps as additional
protection.
The Spintech is a large flat plate, radiating to the underbelly of the
plane. During flight I don't think it is a problem, however when running on
the ground and you shut off, it clearly is very hot, and sits there
radiating and convecting heat to the body. The extra heat shield has helped
a lot.
By the way, you can see how the spintech works by looking at the case after
having been run -- there are dark discolorations on the upper and lower
surfaces where the 'spinning' is taking place.
Bill Schertz
Didn't Tracy's spin tech fail as I recall ?
Paul Lamar