Return-Path: Received: from tomcat.al.noaa.gov ([140.172.240.2] verified) by logan.com (CommuniGate Pro SMTP 4.2.5) with ESMTP id 551812 for flyrotary@lancaironline.net; Tue, 30 Nov 2004 19:03:40 -0500 Received-SPF: none receiver=logan.com; client-ip=140.172.240.2; envelope-from=bdube@al.noaa.gov Received: from PILEUS.al.noaa.gov (pileus.al.noaa.gov [140.172.241.195]) by tomcat.al.noaa.gov (8.12.0/8.12.0) with ESMTP id iB103Ba5021629 for ; Tue, 30 Nov 2004 17:03:11 -0700 (MST) Message-Id: <5.2.1.1.0.20041130164241.0263e008@mailsrvr.al.noaa.gov> X-Sender: bdube@mailsrvr.al.noaa.gov X-Mailer: QUALCOMM Windows Eudora Version 5.2.1 Date: Tue, 30 Nov 2004 17:03:11 -0700 To: "Rotary motors in aircraft" From: Bill Dube Subject: Re: [FlyRotary] New Muffler Design In-Reply-To: Mime-Version: 1.0 Content-Type: text/html; charset="us-ascii" 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.

Tell us more about Finite Amplitude Wave theory.

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?

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?

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.

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.

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.