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Larry LaBeau sent me some excellent data on work he's doing with his muffler. Enjoy
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Finally found a muff that works. Similar to the tangential muffs. Used a two cycle model airplane design program by Martin Hepperle called "Silencer Design Application". You can input the exhaust tempreture, engine rpm, type of fuel used, select the type of silencer and add them in series, and then add the dimensions in millimeters.
It plots out the DB vs Frequency with the peak rpm. I found relationships between the dimensions that gave a smooth curve without spikes. My thinking is this tunes the muff. Sizing of the muff was also driven by the volume I could put it in. The material is off the shelf automotive exhaust pipe of 16 gage. For the large can I used two 3" pipes split on one side longitudinally and welded the edges together.
The can has two chambers, one to expand and swirl the incoming gas, and the second to also expand and exhaust the gas. The dimensions of the center tube are critical as well as the dimensions for the exhaust pipe, its length, dia, and how far it is inside the can. Some of the dimensions are multiples of other dimensions. Like 204/102 = 2. This keeps the spikes down or tunes the muff.
Flight test shows a back pressure or loss of rpm of from 200 to 300. I am planning to modify the center tube to a large diameter of from 2" to 2-1/4". If the back pressure is still present then I will make the exhaust tube larger. Unfortinatly this will increase the noise. Took my headset off during flight and the greatest noise was air noise, not the muffler as before. Results:
rpm DB a DB c measured inside the cabin at pilots lap position.
Idle 2K 80 100
run up 3K 88 100.5
climb 5.5K 96.5 107
cruise 5.3K 96 104
cruise 5.4K 96 105
taxi 1.7K 78 97
I flew for 46 min and the muff did not fall apart. No real discoloration except on the can's ends. I thought I would see two discolorations where the two tubes come from the engine and blast on to the cans skin, but it did not happen. Its a please to fly the airplane without that Mazda beat.
The distance from the face of the engine to the cans edge is 12". To support the aft end of the can I added a tie rod from it to the engine mount (see photo). Weight of the muff is 11 pounds which is real good for a nose heavy Dyke Delta. Now to get it made of SS.
TNX
Larry La Beau
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