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-----Original Message-----
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On
Behalf Of Ernest Christley
Sent: Tuesday, January 06, 2009 6:02 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: 20B install issues overcome
Mike Wills wrote:
At one point a few months ago I added a SuperTrapp to my existing exhaust just to see if it would tame it a little. Cost me about 500RPM on the top end with little or no improvement in the sound output.
Mike Wills
----- Original Message -----
*From:* wrjjrs@aol.com <mailto:wrjjrs@aol.com>
*To:* Rotary motors in aircraft <mailto:flyrotary@lancaironline.net>
*Sent:* Monday, January 05, 2009 11:49 AM
*Subject:* [FlyRotary] Re: 20B install issues overcome
Earnest,
I believe that the SuperTrapp muffler is an example of a band gap
device and it worked well, if properly tuned.
I think the large block idea has merit too, but you will have to
allow enough thickness to prevent the tubes that see the exhaust
first to fight the harmonic vibration and heat. Different size
diamonds in the exhaust stream might also help fight harmonics.
Bill Jepson
Looking at their website, the SuperTrapp isn't a phononic filter. In fact, it looks to me to be little more than an adjustable exhaust restriction. The secret to the phononic devices is that they use a crystal structure to maintain destructive interference of the sound waves within a narrow band of wavelengths. Increasing the size of the "crystal" will not only reduce restriction, it will improve the filtering/muffling properties, in direct contrast to what the SuperTrapp is doing.
The design problem is to first identify the frequencies that need to be filtered. I'm pretty sure, from work done early by Monty Roberts, that we're mostly concerned with frequencies from 0 to 12kHz. Referring to the second link below, it appears that a chamber of 1/8" stainless rods, arranged in a hexagonal matrix with a .907 filling fraction would result in a 60dB noise reduction. The inlet to the muffler would look like the diffusing duct to spread out exhaust heat and energy over as much area as needed to get good airflow.
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Unless I did the math wrong; the maximum filling fraction you can get
with these rods is about .8. So I think there would be a problem there.
Al G
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