Contrary to intuition – two parallel mufflers actually reduce the sound less than one of the same type.  If you have both rotor pulses going through a single muffler you have a dominant high energy frequency (at 6000 rpm) of 200 Hz to filter.  If you have a single rotor exhaust port with its separate muffler then the dominant freq for each muffler is now 100Hz rather than 200Hz.  The dimensions for two mufflers of equivalent effectiveness would need to be approx twice as large for the 100Hz  pulse of which you are required to hang TWO of these twice as larger hummers underneath to get the same effectiveness in sound suppression as using one muffler half the dimensions – or something along those lines.

 

So a single muffler (for sound suppression) appears to be a better way to go – now if you are looking for power instead of sound suppression then the story can be different.

 

Having experimented with several different approaches, the one that seemed to have the most promise was the one in which I placed 3” diameter 1/8” thick Stainless Steel discs.  The discs had vanes cut from the outer perimeter down to within 3/8 – 1/2” of the center.  Then each vane was bent approx 45 deg to the plane of the disc.  These then had a 3/8” dia hole drilled down the center and a rod on which 5 discs were strung and first locked in using jam nuts (don’t bother trying) and later welded to the rod.

 

The idea was that the pulse in the exhaust would in effect see (straight on) an almost solid metal disc and bounce back what pulse go through the first disc would find a 2^nd and 3^rd and 4^th etc, where as the exhaust gas would fairly easily flow around the blades and through the discs – relatively unimpeded (so the theory went {:>)). 

 

The first one I made I used a 2” dia tube and discs – it was tremendously effective in suppressing the sound – unfortunately it was equally effective in suppressing power.  So I concluded I needed a larger tube – so went to the 3” dia tube and six discs.

 

This one I thought was really the solution (and may have been if I had continued development) as it was very effective in suppressing the sound (although not quite as much as the 2” tube) and I got 6000 rpm static.  Ah ha! I thought – this is it!!! I have succeeded.  So I buckled up and ran the engine up to 6000 rpm and launched.  The only unfortunate aspect of the flight is that I never got over 6000 rpm static {:<(.  

 

I then cut the number of disc back to 4 and that appear to do the job.  However, my next flight was down to Tracy’s Crook.  I was later told by a witness to my take off that it sound like I had a turbine engine under the cowl (more on that later).  In any case, I noticed about 20 minutes into the flight that the EGT on one exhaust was up into the 1700 + range whereas my normal EGT was more like 1550 -1600 max.  After landing at Tracy Crooks, I decided to determine what the problem was.

 

It turns out that the discs in one tube  had broken loose of their jam nuts and were free wheeling like a turbine blade inside the tube. Not only were the tips of the discs burnished but you could clearly see the polishing effect of their rubbing against the inside of the 3” tube.   So that explained the high EGT on the one tube (and the turbine sound reported) the discs were acting like a freewheeling prop and impeding the flow of exhaust gas.

 

So I decided to reduce the number of discs down to 3 and Tracy was nice enough to weld the disc to the rod.  Well, that seem to be the answer.  I was getting good power and good sound suppression still.  However, the discs only stayed welded to the rods for approx 2-3 weeks, and then I began to find pieces of disc back by the end of the tube.  The pulse was too powerful and were slowly beating the stuffings out of the SS blades on the discs and they were breaking off and leaving the tube .

 

By that time the expense (and more telling - the work) of six experimentation with mufflers had taken their toll.  I decided to have my two tube header modified into one tube - turns out it cost twice to modify the header   as two hushpower mufflers. So I just kept my old 10 year old header of stainless steel pipe and put on two hushpower mufflers.  Not as quite as my experimental design – but they have lasted.

 

So good luck folks, I’m eagerly awaiting the magic muffler design.

 

Ed

 

Ed