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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'.
Charlie
*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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