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Mark,
Sorry, I lose track of who has what, is that a 13B
or a 20B. If 2" on a 20B has proven to be fine, then that's the real world proof
we need and indicates that the sound waves are sufficiently staged and
the size is enabling free flow of the exhaust gases. 2" is even good for
flowing a 13B, as I would have suggested 2.5".
At what RPM have you flown with this set-up and
what size ( volume) is your muffler.
George( down under)
My latest exhaust system uses a DNA muffler which has a 3" inlet and
3" outlet. When I first ran it, I was disappointed at how unacceptably
loud it was. Then I remembered Tracy's statement about most
guys using too large of an exit. So, I reduced the outlet
to 2". Sure enough, it reduced the noise level significantly and there
was no detectable loss of power. I'm still running it
with the 2" exit.
Mark S.
On Mon, May 18, 2009 at 10:14 AM, Tracy Crook <tracy@rotaryaviation.com>
wrote:
Your
mathematics are sound George and using that approach will certainly keep
back pressure to a minimum. But I think it is doomed to be noisy.
That's just my understanding about how mufflers generally work and I could
be wrong.
During a test of mufflers at a rotary fly-in some years
back I was amazed at how effective the muffler became and how small the
power loss was when we installed a 1" diameter muffler outlet pipe on the
test stand engine (13B). 1" is admittedly too small but we learn
by going to extremes sometimes.
Tracy
On Mon, May 18, 2009 at 2:40 AM, George Lendich <lendich@aanet.com.au> wrote:
Tracy,
The reason I ask is that I like to keep the
muffler outlet the same area size or a little bigger than the exhaust
outlet in the rotor housing which is usually about 2" dia.
I do understand the 2 and 3 rotors are staged
and the exhaust and the exhaust (shock) waves are in sequence,
but I like to keep the volume of the muffler and muffler exit to the total
of the rotor exits i.e. 3x Pi.r squared (3x Pi 1 x 1) = 9.2 sq" or Radius
of 1.71" = Dia of 3.42".
I thought 2.25 may have a little too much
back pressure, as the rotary doesn't like back pressure.
Probably to do this exercise properly would
be to calculate the volume of the heated and expanded exhaust gases and
calculate the timing of the pulses to see how a particular sized muffler
and exit size coped with the speed and volume of flow. It would be an
interesting exercise.
George (down under)
Too
soon to say really. I have not even had time to assemble the
builders log to get ready for the DAR. Lack of engine power is
definitely not what is keeping this thing grounded though :
)
Tracy
On Sun, May 17, 2009 at 6:01 PM, George Lendich
<lendich@aanet.com.au> wrote:
Tracy,
Any significant back pressure to speak
of?
George (down under)
-----
Original Message -----
Sent:
Monday, May 18, 2009 12:11 AM
Subject:
[FlyRotary] Re: 20b Exhaust
In general, the bigger the exit pipe, the louder the
exhaust will be. I used 2.25" which I think is about the
minimum for a 20B.
Tracy
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