Mailing List Message #65329
From: Charlie England <>
Subject: Re: [FlyRotary] Re: Exhaust setups
Date: Tue, 12 Nov 2019 12:41:15 -0600
To: Rotary motors in aircraft <>
On 11/11/2019 2:28 PM, Thomas Mann wrote:
I would really like to hear from someone like a gunsmith who has experience in the design and construction of suppressors.
They typically devoid of any packing to collapse/blow out.
As I recall, Al Geitzen had a similar setup on his 3-rotor.
T Mann
That video is a good 'listen', even though we don't get to hear it making real power. Notice how it has a deep, 'pleasant' sound, instead of the raspy snarl we normally associate with rotaries? I'll throw out some of my opinions about why, based on my experiences in a former life as a live sound engineer/loudspeaker designer.

While Al's was a 3rotor, I heard a very similar sound from the late Paul Conner's SQ2000 (also a canard pusher) during taxi tests. Paul used a stock 13B cast iron manifold to a very short pipe directly out the back of the cowl.  Pics of both are attached. Dennis Haverlah's Renesis powered RV-7A, when using the stock Renesis manifold feeding a small in-cowl muffler, was even quieter. My *opinion* is that what we perceive as quiet could be more accurately described as 'good', meaning the sound is pleasing, rather than irritating. Paul's 13B sounded like a small block V-8.

I believe that especially with Paul's and Dennis' systems, the mass of the stock manifold contributes greatly to damping the higher order harmonics (that raspy, 2-stroke-sounding snarl) of the rotary, leaving us with the pleasing, primary combustion noise. What all three have in common is the entire exhaust system is contained within the cowl. Having all the exhaust metal within the fiberglass cowl allows the cowl to damp the remaining high-order harmonics.

Note that high frequencies don't like to turn corners, but if they hit a barrier (exhaust manifold) that can 'ring' (resonate) near the same frequency, the energy may well get amplified* by the barrier itself. I strongly suspect that is happening in many belly mounted rotary exhausts. Many years ago at one of Tracy's rotary flyins, we played around with an audio spectrum analyzer monitoring rotary exhaust sounds, and saw *lots* of high energy high frequency harmonics. Since most of us fly metal a/c, we have big problems with both the exhaust system and the airframe itself resonating, transmitting and even amplifying* exhaust noise.

Try tapping the exhaust, and the airframe near the exhaust, with a metal object like a wrench, and listen to the sound. If you get a dull tap or thud kind of sound, that's great. But you'll likely get at least a slight ringing sound, like a bell. That's bad, especially if it's a harmonic of the engine's natural excitation frequency. Now, damping the airframe isn't that difficult; it just takes the right material (and added weight). But damping the exhaust itself will be tougher. It may be possible to alter the *frequency* of resonance by using things like stainless band clamps (worm drive hose clamps, etc) positioned to break up the muffler into unequal size segments, but any type of soft damping material will likely get shredded by the heat/vibration.

Don't forget the 'hangers' that attach the exhaust to the airframe. If they're too rigid, they'll directly transfer engine/exhaust vibration directly to the airframe, causing it to act like a loudspeaker.

Or, like Lynn H. says, 'I could be wrong'.

*I'm torturing the term 'amplify' above, to loosely describe the effect of the material making an even more efficient coupling to the air than the original sound source. Same idea of an acoustic horn, or cupping your hands in front of your mouth when shouting.

I hope the actual engineers will forgive my abuse of terms...

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