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Sorry to keep everybody waiting. Since I got back,
I've been dealing with all the other obligations and hassles that most of us
wish would go away so we could get on with this airplane stuff.
The muffler we tested was one I designed to be as
small and light as I thought I could get away with. It was a 6in dia, 8.5 in
long, 7lb muffler. It had a small expansion chamber followed by a resonator with
interchangeable baffles. The muffler was a straight through design with 2 in
inlet and exit.
What we did:
Ran Tracy's test stand without the muffler. It was
predictably very loud. Tracy noted about 200 static rpm increase without the
standard restrictive automotive muffler.
Note on the test engine-it has exhaust splitters
and the stock cast iron manifold. This means this was a best case
scenario.
Put my muffler sans baffle on the test stand. It
was much quieter, but did not pass the Laura test. There was no static rpm drop
compared to no muffler.
Put what "theoretically" would be the best baffle
in the muffler. The engine was quieter, but the sound was qualitatively not as
pleasant.
Put in another baffle that "theoretically" would be
the worst. Sound was subjectively much more pleasant than any of the other
tests. Still did not pass the Laura test.
After a discussion that I had with Richard Sohn
about his muffler, we made a "field expedient" but stylish restriction from
vacuum cleaner pipe (aprox 1 in dia) and put it on the end of the muffler.
The result was a reduction in noise and a noticeable amount of thrust due to
high velocity exhaust gas. The muffler and pipe were bending forward and had me
worried that it might fail and get into the prop some how. Leaves and dirt were
flying everywhere from the exhaust jet. There was about 100 rpm static
drop due to the restriction.
We did not get to run a control test with an
exhaust pipe of the same length as the restriction to see how much noise
reduction was due to the restriction and how much was due to the tail pipe
length.
Unfortunately the mic on my PDA saturated and I
have no good numbers for noise. This is all subjective. Tracy could not find his
DB meter.
One thing I did notice is that as the engine revved
up the exhaust got much quieter and you could distinctly hear the prop. At the
point the muffler was designed to have the max attenuation, 5500-6500 rpm, an
ungodly screech would begin. I think the resonator was working, but the muffler
itself was ringing like a bell. I have yet to ring the muffler and measure the
frequency. If it coincides with the resonance that could be one problem to
address.
It seems that there were two groups of people.
Those that liked the sound, and those that didn't. Those that didn't (me and
others) were standing behind the engine. Those that thought it was great were
mostly standing in front.
The next step I believe is to design a muffler that accomplishes the
majority of expansion ahead of the resonator, so that the pressure waves are not
as strong and easier to modify-DUH!. A combination of expansion, a smaller
opening into the resonator (not so small as to restrict the flow) and perhaps a
small absorption silencer with stainless steel wool following the
resonator might pass the Laura test.
I still have 2 baffles I have yet to try and I need
to try different lengths of tail pipe. The baffles made more difference in the
sound quality than I had anticipated.
Basically I have gone as far as I can without a
test stand, which is my next project. (besides the kitchen, the shop, three
decks and work)
Other observations:
Tracy confirmed the laws of physics by
demonstrating that high speed cold air off a prop will chill a warm body. We are
all grateful/sympathetic to his sufferings. I think the picture Bob Tilley sent
illustrates this adequately.
George Graham is a man of many talents including
poetry recital. He did an excellent job of reading Robert Service's The
Cremation of Sam McGee, an aptly chosen poem considering the chill in the air
and the bonfire.
As usual a great time with good people. A big
thanks to Tracy and Laura for making this possible. I am once again struck
by the irony of two of the nicest and most honest people I know being a couple
of Crooks from Shady Bend. ;-)
Monty
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