Mailing List Message #65363
From: Andrew Martin <>
Subject: Re: [FlyRotary] Re: Mufflers
Date: Tue, 19 Nov 2019 10:44:00 +0800
To: Rotary motors in aircraft <>
Noise in cabin is acceptable. And I just use passive headset. Is a glass plane. makes big diff from aluminium so hard to compare with others Ive flown as to whether its airframe or engine related.
Original exhaust exited at 90deg to cowl and was quieter than this one but did not flow as well. This time I turned the outlet about 30deg back so can actually see inside of pipe while flying. reckon I get a bit more noise because of this.
Was quick to make but had to do in right sequence to ensure could get full welds everywhere needed. Just used 5mm mild steel for end plates, ms mig wire to weld 1.6mm 304ss pipes to manifold & end plates.
Inner section of original mazda renesis manifold was not welded. Is just slip fit to allow movement. 
Now wish I took pic's as we put it together but really did not expect it to work as well as it does.
Just the turbo by itself will tame the exhaust pulses lots on your plane if you use properly sized hot section rather than a wastegate.
Lots of things can be done to reduce noise. On your current setup just tack welding a washer into end of pipe will help lots. Just keep increasing hole size until backpressure is not an issue. About 25-30mm out is all thats needed. If your smart enough you end up with a tuned exhaust. But that is way beyond me.

On Tue, 19 Nov. 2019, 9:45 am Neil Unger, <> wrote:


                   all very interesting,  What noise do you get back in the cab?  bearable, loud or great?  Neil.

On 11/19/2019 11:31 AM, Andrew Martin wrote:
Neil, I stumbled on a system that seems to work quite well. In my head I think it works similar to an ANR headset where the noise is not muffled as such but reflected to cancel it out.
Important bit is exhaust enters & exits at same end. In my case I used a 65mm ss pipe inside a 100mm pipe about 150mm long, but longer would be better again as outlet on outer pipe does see some holes on inner pipe so noise has direct escape path. 65 mm pipe has 10mm holes with about twice area of pipe to try to minimize back pressure.
Theory is that exhaust pulses are broken into smaller bits as they see holes in inner pipe, but then hit the blanked off end and are reflected just like in ANR. Not sure if this is correct or not, but it seems to work in use quite well.
My exhaust system is quite short as tailpipe exits just behind prop similar to PT6 turboprop. I made exit pipe from 65mm also but think smaller may be better as I think the exhaust gas flows much better after the pulses are broken up. 

On Tue, 19 Nov 2019 at 5:20 am, Neil Unger <> wrote:
Attn Todd Bartrim,

                                       When in doubt check the
instruction book!!  I finally checked the archives to find that Todd you
are way in front of me.  I am definitely going to put a turbo on my
renesis, but am still looking for a muffler. Depending on what Lynn
suggestes, but your experience with the spintech or your latest version
is appreciated.  Do you have any tests with a decibel meter?  My exhaust
noise is just too loud, fine for me but passengers do not exactly que up
for another ride.

So design starts all over again, but I do have another engine to play
with to keep mine in the air while I experiment.    Here again Todd you
have the electric water pump --- thoughts?


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Regards Andrew Martin Martin Ag
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