X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from ispmxmta05-srv.alltel.net ([166.102.165.166] verified) by logan.com (CommuniGate Pro SMTP 5.0) with ESMTP id 813374 for flyrotary@lancaironline.net; Thu, 03 Nov 2005 10:59:03 -0500 Received-SPF: pass receiver=logan.com; client-ip=166.102.165.166; envelope-from=montyr2157@alltel.net Received: from Thorstwin ([4.227.99.144]) by ispmxmta05-srv.alltel.net with SMTP id <20051103155815.UBXG18995.ispmxmta05-srv.alltel.net@Thorstwin> for ; Thu, 3 Nov 2005 09:58:15 -0600 Message-ID: <001b01c5e08f$67d73770$9063e304@Thorstwin> From: "Monty Roberts" To: Subject: muffler/Shady Bend report Date: Thu, 3 Nov 2005 09:58:16 -0600 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0018_01C5E05D.1C853740" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2900.2180 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.2180 This is a multi-part message in MIME format. ------=_NextPart_000_0018_01C5E05D.1C853740 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable 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.=20 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.=20 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.=20 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.=20 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.=20 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 ------=_NextPart_000_0018_01C5E05D.1C853740 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
Sorry to keep everybody waiting. Since = I got back,=20 I've been dealing with all the other obligations and hassles that most = of us=20 wish would go away so we could get on with this airplane stuff. =
 
The muffler we tested was one I = designed to be as=20 small and light as I thought I could get away with. It was a 6in dia, = 8.5 in=20 long, 7lb muffler. It had a small expansion chamber followed by a = resonator with=20 interchangeable baffles. The muffler was a straight through design with = 2 in=20 inlet and exit.
 
 
What we did:
 
Ran Tracy's test stand without the = muffler. It was=20 predictably very loud. Tracy noted about 200 static rpm increase without = the=20 standard restrictive automotive muffler.
 
Note on the test engine-it has exhaust = splitters=20 and the stock cast iron manifold. This means this was a best case=20 scenario.
 
Put my muffler sans baffle on the test = stand. It=20 was much quieter, but did not pass the Laura test. There was no static = rpm drop=20 compared to no muffler.
 
Put what "theoretically" would be the = best baffle=20 in the muffler. The engine was quieter, but the sound was qualitatively = not as=20 pleasant.
 
Put in another baffle that = "theoretically" would be=20 the worst. Sound was subjectively much more pleasant than any of the = other=20 tests. Still did not pass the Laura test.
 
After a discussion that I had with = Richard Sohn=20 about his muffler, we made a "field expedient" but stylish restriction = from=20 vacuum cleaner pipe (aprox 1 in dia)  and put it on the end of the = muffler.=20 The result was a reduction in noise and a noticeable amount of thrust = due to=20 high velocity exhaust gas. The muffler and pipe were bending forward and = had me=20 worried that it might fail and get into the prop some how. Leaves and = dirt were=20 flying everywhere from the exhaust jet. There was about 100 = rpm static=20 drop due to the restriction.
 
We did not get to run a control test = with an=20 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=20 length.
 
Unfortunately the mic on my PDA = saturated and I=20 have no good numbers for noise. This is all subjective. Tracy could not = find his=20 DB meter.
 
One thing I did notice is that as the = engine revved=20 up the exhaust got much quieter and you could distinctly hear the prop. = At the=20 point the muffler was designed to have the max attenuation, 5500-6500 = rpm, an=20 ungodly screech would begin. I think the resonator was working, but the = muffler=20 itself was ringing like a bell. I have yet to ring the muffler and = measure the=20 frequency. If it coincides with the resonance that could be one problem = to=20 address.
 
It seems that there were two groups of = people.=20 Those that liked the sound, and those that didn't. Those that didn't (me = and=20 others) were standing behind the engine. Those that thought it was great = were=20 mostly standing in front.
 
The next step I believe is to design a muffler = that accomplishes the=20 majority of expansion ahead of the resonator, so that the pressure waves = are not=20 as strong and easier to modify-DUH!. A combination of expansion, a = smaller=20 opening into the resonator (not so small as to restrict the flow) and = perhaps a=20 small absorption silencer with stainless steel wool following the=20 resonator might pass the Laura test. 
 
I still have 2 baffles I have yet to = try and I need=20 to try different lengths of tail pipe. The baffles made more difference = in the=20 sound quality than I had anticipated.
 
Basically I have gone as far as I can = without a=20 test stand, which is my next project. (besides the kitchen, the shop, = three=20 decks and work)
 
Other observations:
 
Tracy confirmed the laws of physics by=20 demonstrating that high speed cold air off a prop will chill a warm = body. We are=20 all grateful/sympathetic to his sufferings. I think the picture Bob = Tilley sent=20 illustrates this adequately.
 
George Graham is a man of many talents = including=20 poetry recital. He did an excellent job of reading Robert Service's The=20 Cremation of Sam McGee, an aptly chosen poem considering the chill in = the air=20 and the bonfire.
 
As usual a great time with good people. = A big=20 thanks to Tracy and Laura for making this possible. I am once = again struck=20 by the irony of two of the nicest and most honest people I know being a = couple=20 of Crooks from Shady Bend. ;-)
 
Monty
 
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