Attached is a (crude) drawing of my current muffler. It is the same one I've been flying for the last 25 - 30 hours with the exception of the added deflector plates near the inlet.
The muffler body is 4" in diameter and about 44" long. Material is .035" 321 stainless. There are about 400 3/32" holes in the bottom half which were cut with a plasma cutter (much easier & faster than drilling). The rear end of the muffler (not shown) is completely capped off so all exhaust gas must exit through the holes. Without the deflector plates the muffler was unacceptably loud and I was getting ready to install some "gun silencer" plates like Neil & Jim used in theirs. With the ends already blocked off with the inlet diffuser and end cap it was not going to be easy so I started thinking about alternatives. From past experiments and discussion I thought it was apparent that much of the challenge was breaking up the strong shock wave from the turbo exhaust ports so thought I would start with an easy to try fix.
The deflector plates shown in the drawing are made from .125" thick 304 stainless plate and measure 2"x 2" (nothing scientific here, just happened to have some) . Two slots were cut in the muffler with an abrasive cutoff saw at about a 45 degree angle where the plates were welded in. This was done for two reasons: 1. reduce the stress on the plates from the shock wave so they hopefully would not fatigue crack the muffler where they are welded in and 2. maybe induce a swirling flow inside the muffler to enhance silencing.
Considering the simplicity of the change, the results were astonishing. The in-cockpit noise went from just tollerable (with ANR headsets) to very acceptable (able to listen to music through the headsets). I left my sound level meter with the ground observers so I didn't get an onboard level to compare with the previously measured 110 DB cockpit noise level. Both observers (Laura & my brother Barry) immediately noticed a big improvement and a neighbor called up just after the flight and commented on the lower noise level so it wasn't all in my head.
Outside noise measurments: All measurements are peak reading which occures about 45 degrees either side of directly behind. Noise level falls off outside of this zone.
Full throttle takeoff measured from about 25 feet away was 104 DB compared to 124 DB before.
75% Cruise power flyby at 200 feet from meter: 90 DB (no previous data)
75% cruise at 1000 feet from meter: 70 DB (no previous data)
I'm not recommending that everyone run out and do exactly what I did in this experiment but I think it does indicate the direction we should go. I still want to try the gun silencer type but I will probably try installing another couple of plates like the first pair at 90 degrees from the first pair and slightly down stream.
Tracy Crook
tcrook@rotaryaviation.com
www.rotaryaviation.com
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