From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Lynn Hanover
Sent: Tuesday, September 08, 2009 09:49
To: Rotary motors in aircraft
Subject: [FlyRotary] Muffling

Its poor service life behind a rotary is due to the shock wave
> dropping to subsonic by applying its energy to the flat sides of the
> design. It is both a reflective and an impinging design.
Reflective?  Impinging?

I think I know what you're referring to, but could you define the two
terms more clearly?  Do all mufflers fall into these two categories?

I can add my feeble understanding of the problem. It may be completely incorrect............

The beast in the rotary is a supersonic shock wave that is traveling in a very fast moving superheated media. Say 1600 degrees. So there is energy that is a compressed portion of the gas (the sound), energy in the form of a high temperature gas and the energy represented by the velocity of the gas. 

So, you can take out energy by cooling the gas, by slowing the gas. And the sound may be removed by reflection, as in the noise cancelling headsets. Or, by homogenizing more uniformly throughout the gas.

To impinge on the flow is to make it change direction, and, or, velocity,  like the poor header system design or the square header/muffler idea. Where drag or just design helps homogenize the sound waves and spreads them time wise. So a muffler that dies quickly when mounted close to the engine, may last much longer when mounted at some distance from the engine.

The high pressure of each pulse spreads in both directions in the gas and its peak pressure is reduced. The only energy lost would be the heat radiated by the longer tube exposed to cooling air. But the peaks are reduced as a function of time, so stress seen by the muffler parts is reduced. Suppose the pipe were 100 feet long. The pulses would be nearly 100% homogenize, and most of the heat energy would have been radiated by the pipe. The sound energy left over could be easy to deal with.

To change flow direction close to the muffler like the Inconel splitters in the NA rotor housings requires exotic materials. The splitter slows a part of the shock wave and accelerates the rest of it, but spreads it so its peak pressure is reduced, and a less complex muffler can be effective.  The splitter impinges on  the flow.

Even the stainless Mazda racing muffler did not last long when mounted close to the engine. It has a room at the front where the shock wave can expand and slow to subsonic. Half of the battle is won.

Some muffler designs use part of the pulse energy to impinge on the main flow through the muffler. The glass pack or steel pack muffler (a Swiss muffler with packing) allows high pressure to bleed into the larger volume of the packing chamber near the front, and further along the muffler gasses at lower energy (after heating the media) are flowing out of the media to impinge on the flow exiting the central tube of the muffler. (like a big electrical capacitor) A relatively effective system so long as the packing remains in place. Once the packing is gone, the muffler may start to favor some frequencies over others and muffling effect will drop to near zero.

Conventional car mufflers are very advanced. They must outlast the warranty period (just barely) and meet some strict sound profiles. They use cooling, impingement, reflection and cavity resonance, to remove particular frequencies. They may be tailored to particular engines. Unfortunately they are also very large, heavy and hot. The tubes of various lengths inside the muffler cavities absorb offensivee notes peculiar to some Engines/systems. A rotary may sound great at speed while a Chevy odd fire V-6 may sound like

s---t. Never quite right.  

Reflective mufflers break the pulse into two or more parts and then direct the two parts into a head-on collision. This scheme may be used more than once in the same muffler. The energy expended in each of these events must be removed as heat, and in some cases as movement of the sides of the (soon to fail) muffler.

Lynn E. Hanover