From: "Stephen Izett stephen.izett@gmail.com" <flyrotary@lancaironline.net>
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Subject: Re: [FlyRotary] Re: Exhaust setups
Date: Wed, 13 Nov 2019 06:48:02 +0800
References: <list-13168991@logan.com>
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
In-Reply-To: <list-13168991@logan.com>
Message-Id: <60EED3DC-AED2-4770-ACA0-EDA32B0722E7@gmail.com>

Thanks Charlie.
Sounds like some good wisdom in there.
Much appreciated.

Steve Izett.

> On 13 Nov 2019, at 2:41 am, Charlie England ceengland7@gmail.com =
<flyrotary@lancaironline.net> wrote:
>=20
> On 11/11/2019 2:28 PM, Thomas Mann tmann@n200lz.com 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.
>> =20
>> As I recall, Al Geitzen had a similar setup on his 3-rotor.
>> https://www.youtube.com/watch?v=3DzxIKDnYZE24
>> =20
>> 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.
>=20
> 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.=20
>=20
> 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.=20
>=20
> 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.
>=20
> 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.
>=20
> 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.
>=20
> Or, like Lynn H. says, 'I could be wrong'.
>=20
> 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.
>=20
> I hope the actual engineers will forgive my abuse of terms...=20
>=20
>=20
> <Exhaust1.jpg><exhaust manifold.jpg>--
> Homepage:  http://www.flyrotary.com/
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