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From: "Richard Sohn" <res12@fairpoint.net>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
References: <list-3415451@logan.com>
Subject: Re: [FlyRotary] Re: Muffler design (was 20B isssues . . )
Date: Wed, 7 Jan 2009 15:54:20 -0600
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----- Original Message ----- 
From: "Ernest Christley" <echristley@nc.rr.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Wednesday, January 07, 2009 3:07 PM
Subject: [FlyRotary] Re: Muffler design (was 20B isssues . . )


> Tracy Crook wrote:
>> Agreed, this is definitely a new concept and I hope it works.
>>
>> The only thing that bothers me is that, as you pointed out, the
>> bandwidth needed for attenuation is about 0 to 12 Khz.  That
>> represents MANY octaves even if we bump the lower limit to a realistic
>> number ( only a closed pipe will work at zero :-).  This bandwidth is
>> mutually exclusive with the term "tuned".  And if by tuned they mean
>> low pass, then it is not a new concept since that is covered by
>> conventional mufflers.  There may be something to this new concept but
>> until this contradiction is explained, I would hesitate to start
>> cutting parts.
> The first parts I'm going to cut are wood sides and PVC pipe baffles.
> That'll be good enough to experiment with.  I've got a sound sample of a
> frequency sweep.  I can build a test muffler, play the frequency sweep
> in one end, and record it on the other.  Displaying both samples in a
> sound editor will tell me what, if any, frequencies are attenuated, and
> by how much.  Losses in the playback speakers and microphone can be
> documented by running a test without the intervening muffler.  I may
> have to spend money on good speakers and a good microphone (Oh,
> bummer!).  If that shows promise, I replace the PVC with stainless
> tubes.  If that works, I replace the wood with stainless sheet.
>
> Which frequencies need to be attenuated?  Remember that Monty Roberts
> did some work a while back showing a sonagram of a rotary recording?
> The sound energies are much higher around 6 and 12kHz.  With that in
> mind, look at the graphs on the top of page 4 of this report
> http://web.mit.edu/course/3/3.042/team1_08f/documents/complete-phononic-bandgap.pdf
>
> It shows very significant attenuation in frequencies from 4 to 8kHz and
> then 10 to 14kHz.  This is with 13mm cylinders set in a 30mm square
> array.  Hollow cylinders showed similar efficacy.  Close enough to 1/8"
> pipe (mcmaster-carr part# 44635K422 ... cheap!) to make a prototype
> almost mandatory.  So, my current plan of action is to produce some
> sonagrams of rotary exhaust noise, and verify the target frequencies for
> attenuation.  With the experimental jig, optimize for bandgaps around
> those frequencies while using the smallest filling fraction possible.
> At that point, I will build a matching prototype.  Two sections of steal
> sheet, match drilled with 1/2" holes (or whatever size was found to be
> optimum).  Fill the holes with 3" sections of tube.  A bunch of welding
> later, a 8.5" wide muffler with a .500 filling fraction will render
> about the same flow area as a 4" diameter exhaust pipe.
>
> Monty built a resonator a couple of years ago and brought it to Shady 
> Bend. Tracy ran it on his test stand. If I remember correctly, it took 
> some of the higher frquencies out, which seemed softening the engine sound 
> some. After that test I welded a piece of 1 1/4 "  pipe to the end of the 
> resonator, just as I have it on the single rotor engine. The result of 
> this narrowing the exit down was, at least subjective, a tremendous 
> reduction in exhaust noise. The power loss was, according to Tracy, 200RPM 
> at 6000. Obviously, there is no free ride.
If anyone has any pictures from this setup, could you put them up?
This was essentially a setup, as Tracy mentioned reasently, of an expansion 
volume with a narrowed down exit.
The way for us experimenters to utilize this configuration would be, build 
an expansion volume as big as practical for your project, and play with 
different sizes of  exit pipe sizes until you get what you want.
Most of the exhausts I have seen on pictures of rotaries have a way too big 
exit pipe.
FWIW

Richard Sohn
N2071U

> Homepage:  http://www.flyrotary.com/
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> http://mail.lancaironline.net:81/lists/flyrotary/List.html


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