Return-Path: Received: from [24.25.9.101] (HELO ms-smtp-02-eri0.southeast.rr.com) by logan.com (CommuniGate Pro SMTP 4.2.5) with ESMTP id 551948 for flyrotary@lancaironline.net; Tue, 30 Nov 2004 21:19:52 -0500 Received-SPF: none receiver=logan.com; client-ip=24.25.9.101; envelope-from=eanderson@carolina.rr.com Received: from edward2 (cpe-069-132-109-019.carolina.rr.com [69.132.109.19]) by ms-smtp-02-eri0.southeast.rr.com (8.12.10/8.12.7) with SMTP id iB12JJ4S019552 for ; Tue, 30 Nov 2004 21:19:20 -0500 (EST) Message-ID: <002501c4d74c$324b79b0$2502a8c0@edward2> From: "Ed Anderson" To: "Rotary motors in aircraft" References: Subject: Re: [FlyRotary] Re: New Muffler Design Date: Tue, 30 Nov 2004 21:19:32 -0500 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0022_01C4D722.48F719F0" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2800.1106 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 X-Virus-Scanned: Symantec AntiVirus Scan Engine This is a multi-part message in MIME format. ------=_NextPart_000_0022_01C4D722.48F719F0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable ----- Original Message -----=20 From: Bill Dube=20 To: Rotary motors in aircraft=20 Sent: Tuesday, November 30, 2004 7:03 PM Subject: [FlyRotary] Re: New Muffler Design At 04:02 PM 11/30/2004 -0500, you wrote: Well, folks, got the new muffler design installed today - and = initially results are encouraging. I designed the muffler based on what = I believe I have learned concerning Finite Amplitude Wave theory and = applied it to this muffler design.=20 Tell us more about Finite Amplitude Wave theory. Bill, I would rather not reveal just how little I understand about = the theory - except to say that it is a disturbance in the air = (generally not caused by a vibrating medium) that exceeds the strength = of a 125 dB sound wave by 1000 of times. They are formed in both the = intake and exhaust systems of internal combustion engines (as well as a = number of other environments). They can be used to enhance both intake = and exhaust performance (If used properly) - when they terminate they = can produce localized pressures of over 15 psi (short duration). The = FAW can either support macro airflow in the direction you want or = adversely affect it depending on many variables.=20 The bottom line is these waves interact in a different manner with = their environment that what we normally think of as sound waves. You = can find a bunch on the internet by searching for "Finite Amplitude = Waves" - some so mathematically esoteric that I certainly can't follow = it - but other explanations are more understandable. Try a search. =20 Here is a starter http://www.racetech.com.au/faqinertia.htm =20 look down about half way down the page for a layman's description of = some of their attributes. Here is another one - a bit too much math for me = http://www.ae.gatech.edu/~comblab5/Publications/RobEJASAApril2003.pdf The mufflers consider of two 2" dia tubes 4' long with sound = attenuating discs inserted. Each muffler hangs off of my two header = pipes and extend back under the fuselage with the exhaust exit dumping = behind the cockpit area. Pictures? Drawings? How many disks? What spacing? Sorry, I normally take photos of my projects, but the digital = camera's battery failed. I will come up with some drawings shortly - = but, want to fly with it first before doing so. The space between discs = is variable in order to make "traps" for different frequencies but is = basically harmonics of the basic rpm (so you have to sort of decide what = rpm you want the effect to be most dominate - but at least in my case it = seems to be pretty effective across the spectrum). I presume that is = because the difference spacing is doing a fair job of attenuating = several different frequencies. For instance, if you combine your two exhaust into one muffler you are = dealing with 200Hz at 6000 rpm but if you use two separate tubes (like I = do) then with each exhaust port feeding a separate tube the primary = frequency (in each tube) is 1/2 200Hz or 100Hz. That calls for a = different spacing of the discs. =20 I use two types of discs, one is nothing more than a 2" dia washer = with a 1" dia hole in the center. The other type disk is a 2" washer = with a 1/2" hole in the center. This one is modified to (hopefully) = present a solid disk to the FAW causing it to bounce back and = destructively interact with the next FAW wave - but still permitting air = molecules to pass with minimum restriction. I use 3 of the 2/1 = washers and 4 of the 2/1/2 washers in each tube with the spacing between = discs starting off small (to attenuate the higher frequencies) and = incrementally increasing to a larger distance between discs to attenuate = the lower frequencies. I also place a 2" dia SS wire mesh disc before = each of the three 2/1 washers that has an weave open area of 40%. I really want to fly with this set up to see if it survives. I = discovered a couple of years ago that the intensity of the rotary turbo = housing exhaust could actually pulverize SS tubing in a matter of a few = minutes of flight. These were "Fishtails" or squished down tubing ends. = I came back from one flight with the ends of the tubes looking like = something had bitten big chunks out of them. This due to the fatiguing = of the metal by the powerful pulse (not the heat) in a matter of = minutes. Besides, while there appeared to be no adverse affect on my power at = my static of 5600 rpm that may not necessarily be true when I get = airborne and attempt to reach 6400 rpm. Subjectively, two hanger bums standing by the wing tips told me that = the exhaust was no louder at WOT than at idle and that the sound of the = prop drowned out the exhaust at WOT. I also noticed that while = sitting in the cockpit with the radio on I heard ignition static over = the radio receiver for the first time! Did you do the test in the open or in the hangar?=20 The aircraft was in the hangar, but the hangar has no door and the = tail was pointed back outside the hangar. There little doubt that I was = getting some sound reflection from the back and sides of my qonset hut = type hanger - but I do not do run ups on our exterior hard surface = pad/runway due to its poor condition and the fact that my last prop = picked up two nasty nicks due to debrie lifted by the prop during run = ups.=20 For you sound experts, I presume if a 3DB increase represents a = perceived doubling of loudness does that mean a 10 DB decrease = represents an approx 3 fold decrease in perceived loudness?? It is a log scale. 3 dB is indeed 1/2 the sound intensity. The = 9 dB is not 3 x 0.5 it is 0.5 x 0.5 x 0.5 Thus it is 1/8th the sound = intensity.=20 Thanks, Bill. Well, I guess that may well be why the individuals = reported they could not hear the exhaust over the prop noise. It also = may mean I can eliminate some of the discs and still have good sound = supression if I do run into adverse effects on power production at high = rpms. =20 I'm surprised you haven't connected the two pipes after they = leave the engine. In cars with dual exhaust, they put a cross-over pipe = just after the header (or manifold). The cross-over pipe is typically = smaller than the main pipes. The cross-over effectively doubles the = frequency and halves the amplitude of the sound pulses in each side. The = higher frequency is more strongly attenuated by the low-pass filters = (mufflers). The effect of adding a cross-over pipe is dramatic. The = exhaust note goes from painful to pleasant.=20 Sounds like reason for another muffler design {:>). I am aware of the = reported benefits of a cross over tube - but attempting to keep the = underside of the aircraft from looking too cluttered I have not = attempted it. Right now all I have are two 2 1/8" tubes running = straight back from the bottom of the cowl separated by approx 12". It = does sound like it would be benefical to design one with such a = cross-over pipe. By doubling the frequency and halving the amplitude = that would appear to indicate the mufflers could be perhaps closer to = 24" rather than my 48". =20 If the cross-over pipe placed properly, there is no loss of = power. (Sometimes you get a small increase.) You want the header to = scavenge the combustion chamber properly, so you have to leave enough = header to do this. On a V-8 trying to make good low-end torque, this is = about 18" or so. I have no clue what header a rotary would like to make = max HP at high RPM.=20 Thanks for the questions and suggestions, Bill Ed Ed Anderson RV-6A N494BW Rotary Powered Matthews, NC >> Homepage: http://www.flyrotary.com/ >> Archive: http://lancaironline.net/lists/flyrotary/List.html ------=_NextPart_000_0022_01C4D722.48F719F0 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
 
----- Original Message -----
From:=20 Bill = Dube=20
Sent: Tuesday, November 30, = 2004 7:03=20 PM
Subject: [FlyRotary] Re: New = Muffler=20 Design

At 04:02 PM 11/30/2004 -0500, you wrote:
Well, folks, got the new muffler = design=20 installed today - and initially results are encouraging.  I = designed=20 the muffler based on what I believe I have learned concerning Finite = Amplitude Wave theory and applied it to this muffler=20 design. 

        T= ell=20 us more about Finite=20 Amplitude Wave theory.
Bill, I would = rather not reveal=20 just how little I understand about the theory - except to say that = it is a=20 disturbance in the air (generally not caused by a vibrating medium) = that=20 exceeds the strength of a 125 dB sound wave by 1000 of times.  = They are=20 formed in both the intake and exhaust systems of internal combustion = engines=20 (as well as a number of other environments).  They can be used = to=20 enhance both intake and exhaust performance (If used = properly) - when=20 they terminate they can produce localized pressures of over 15 psi = (short=20 duration).  The FAW can either support macro airflow in the = direction=20 you want or adversely affect it  depending on many variables.=20
The bottom = line is these=20 waves interact in a different manner with their environment that = what we=20 normally think of as sound waves.  You can find a bunch on the = internet=20 by searching for "Finite Amplitude Waves" - some so mathematically = esoteric=20 that I certainly can't follow it - but other explanations are more=20 understandable.  Try a search. 
 
Here is a=20 starter
 http://www.racetech.com.au/faqinertia.htm 
 
look down = about half way=20 down the page for a layman's description of some of their=20 attributes.
 
Here is = another one - a bit=20 too much math for me  http://www.ae.gatech.edu/~comblab5/Publications/RobEJASAA= pril2003.pdf

The mufflers consider of two 2" = dia tubes=20 4' long with sound attenuating discs inserted.  Each muffler = hangs off=20 of my two header pipes and extend back under the fuselage with the = exhaust=20 exit dumping behind the cockpit area.

        Pictures? Drawings? How many = disks? What=20 spacing?
        =20 Sorry, I normally take photos of my projects, but the digital camera's = battery=20 failed.  I will come up with some drawings shortly - but, want to = fly=20 with it first before doing so.  The space between discs is = variable in=20 order to make "traps" for different frequencies but is basically = harmonics of=20 the basic rpm (so you have to sort of decide what rpm you want the = effect to=20 be most dominate - but at least in my case it seems to be pretty = effective=20 across the spectrum).  I presume that is because the difference = spacing=20 is doing a fair job of attenuating several different=20 frequencies.
 
For instance, = if you combine=20 your two exhaust into one muffler you are dealing with 200Hz at 6000 = rpm but=20 if you use two separate tubes (like I do)  then with each exhaust = port=20 feeding a separate tube the primary frequency (in each = tube) is 1/2=20 200Hz or 100Hz.  That calls for a different spacing of the = discs. =20
I use two types = of discs, one=20 is nothing more than a 2" dia washer with a 1" dia hole in the = center. =20 The other type  disk is  a 2" washer with a 1/2" hole in the = center.  This one is modified to (hopefully) present a solid disk = to the=20 FAW causing it to bounce back and destructively interact with the next = FAW=20 wave - but still permitting air molecules to pass with = minimum=20 restriction.    I use 3 of the 2/1 washers and 4 of the = 2/1/2=20 washers in each tube with the spacing between discs starting off small = (to=20 attenuate the higher frequencies) and incrementally increasing to a = larger=20 distance between discs to attenuate the lower frequencies. I also = place a 2"=20 dia SS wire mesh disc before each of the three 2/1 washers that has an = weave open area of 40%.
 
I really want to fly = with this set=20 up to see if it survives.  I discovered a couple of years ago = that the=20 intensity of the rotary turbo housing exhaust could actually pulverize = SS=20 tubing in a matter of a few minutes of flight. These were = "Fishtails" or=20 squished down tubing ends.  I came back from one flight with the = ends of=20 the tubes looking like something had bitten big chunks out of = them.  This=20 due to the fatiguing of the metal by the powerful pulse (not the heat) = in a=20 matter of minutes.
 
Besides, while =  there appeared=20 to be no adverse affect on my power at my static of 5600 rpm that may = not=20 necessarily be true when I get airborne and attempt to reach 6400=20 rpm.

Subjectively, two hanger bums standing by the = wing tips told=20 me that the exhaust was no louder at WOT than at idle and that the = sound of=20 the prop drowned out the exhaust at WOT.    I also=20 noticed  that while sitting in the cockpit with the radio on I = heard=20 ignition static  over the radio receiver for the first = time!

        Did you do the test in the open or in = the hangar?=20

The = aircraft was in the=20 hangar, but the hangar has no door and the tail was pointed back = outside the=20 hangar.  There little doubt that I was getting some sound = reflection=20 from the back and sides of my qonset hut type hanger - but I do not do = run ups=20 on our exterior hard surface pad/runway due to its poor = condition=20 and the fact that my last prop picked up two nasty nicks due to debrie = lifted=20 by the prop during run ups.

For=20 you sound experts, I presume if a 3DB increase represents a perceived = doubling=20 of loudness does that mean a 10 DB decrease represents an approx 3 = fold=20 decrease in perceived loudness??

        It is a log scale. 3 = dB is=20 indeed 1/2 the sound intensity. The 9 dB is not 3 x 0.5 it is 0.5 x = 0.5 x 0.5=20 Thus it is 1/8th the sound intensity.
 
Thanks, = Bill.  =20 Well, I guess that may well be why the individuals = reported they=20 could not hear the exhaust over the prop noise.  It also may mean = I can=20 eliminate some of the discs and still have good sound supression = if I do=20 run into adverse effects on power production at high=20 = rpms.
       &= nbsp;
        I'm = surprised=20 you haven't connected the two pipes after they leave the engine. In = cars with=20 dual exhaust, they put a cross-over pipe just after the header (or = manifold).=20 The cross-over pipe is typically smaller than the main pipes. The = cross-over=20 effectively doubles the frequency and halves the amplitude of the = sound pulses=20 in each side. The higher frequency is more strongly attenuated by the = low-pass=20 filters (mufflers). The effect of adding a cross-over pipe is = dramatic. The=20 exhaust note goes from painful to pleasant.
Sounds like reason = for another=20 muffler design {:>).  I am aware of the reported benefits of a = cross=20 over tube - but attempting to keep the underside of the aircraft from = looking=20 too cluttered I have not attempted it.  Right now all I have are = two 2=20 1/8" tubes running straight back from the bottom of the cowl = separated by=20 approx 12".  It does sound like it would be benefical to design = one with=20 such a cross-over pipe.  By doubling the frequency and halving = the=20 amplitude that would appear to indicate the mufflers could be perhaps = closer=20 to 24" rather than my 48". 
 

        If = the=20 cross-over pipe placed properly, there is no loss of power. (Sometimes = you get=20 a small increase.) You want the header to scavenge the combustion = chamber=20 properly, so you have to leave enough header to do this.  On a = V-8 trying=20 to make good low-end torque, this is about 18" or so. I have no clue = what=20 header a rotary would like to make max HP at high RPM.
 
Thanks for the = questions and=20 suggestions, Bill
Ed
 
Ed Anderson
RV-6A N494BW Rotary = Powered
Matthews, NC

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