Coax Muffler by Paul Lamar
Hooker's book "Not much of an engineer"
had some math
theory that explains the coax muffler and ejectors (AKA augmenters) in general.
The Coax muffler is basically a ram jet.
Page 183- 184
"In all gas-turbine jet engines the central core can be regarded as a 'boiler' producing hot gas under pressure. In a simple turbojet this flow is allowed to escape through the propulsive nozzle, producing thrust. The higher the pressure ratio and the higher the operating temperature, the greater the efficiency of the boiler provided, of course, that the efficiency of each compressor and turbine can be maintained. On the other hand, the higher the 'boiler efficiency', the greater the velocity of the jet, the kinetic energy of which is wasted on being released to the atmosphere. The propulsive efficiency therefore falls. "
"Propulsive efficiency is equal to 2Va divided by Vj plus Va, where Va is the velocity of the aircraft (relative to still air) and Vj is the velocity of the jet (relative to the aircraft). In round figures Va may be 850 ft/sec (580 mph) and Vj 2,000 ft/ sec, giving a propulsive efficiency of 60 per cent. For Concorde we do better because Va is much greater.
The only way to do better with subsonic aircraft is to reduce Vj. If we could get it down to 1,500 ft/sec, by augmenting the airflow, we could raise the propulsive efficiency to 72 per cent, thus other things being equal - cutting the fuel burn by at least 20 percent. "
The same thing is going on in the coax muffler. The rotary engine is the boiler.
The large outer pipe reduces the velocity of the exhaust jet while increasing
the mass flow.
Of course we are not going to get a 20% reduction in fuel burn but every
little bit helps.
It is just an long open 2.5 inch pipe with a lot of small holes in it.
The outer casing is open at both ends. The inner tube is cooled by the slipstream flowing between to the tubes.
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 waste hot air from the inner pipe generating some thrust. That is how augmenters work.
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.
Here is a version built by Brian Trubee for his rotary powered RV4.
Less drag for an external muffler and a lower note tone. The main problem with rotary noise is the high frequency component. The tone of the coax rotary sound is much more pleasant.
There would be some benefit to having the exhaust discharge behind the passenger compartment, especially in the decibel department.
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.
Also it can be visually inspected so it won't plug up if it ever cracks. Very important as almost all muffler used so far have cracked. One melted internally causing the air craft to crash.
It can be made to look like part of the airplane if you use a U shaped outer pipe riveted to the bottom sheet metal.
With fiberglass airplanes the close proximity of the hot pipe to the belly of the plane will be a cause for concern. Add 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. You can always go to 2 layers of fiberfrax and SS. Not a lot of weight there.
Super quit rotary powered Lockheed reconnaissance aircraft.