"Bass, Jerry" wrote:
I just tried two short (4.5") fishtails with 3/16 slot and 4.31 wide.
86
i/8" holes. No apparent improvement. I could hear some Wiz-ing under
my
feet, though. The extensions apparently cause some buzzing.
Jerry Bass
That is interesting. Are the lips on the fish tail vibrating? The P51's
have beefed up lips.
Are the holes in the fish tails themselves or in a pipe leading up to
the fish tails?
Jerry, are you willing to pop $80 for a Radio Shack sound (db) meter?
If you get one stand 30 feet away along side the engine while running it
up to static RPM and let us know what the RPM and the reading is on the
"slow" A scale (as I recall). I published some numbers back in
December for Tracy's airplane before his bullet muffler failed.
I'll be sure to bring my db meter to Sun & Fun so we can have
a little muffler competition :-)
Paul Lamar
Finn Lassen wrote:
RadioShack has their analog sound level meter for sale at the moment. $30.
Finn
Good news. Cheap enough for everybody to get one, essential tool if we are
going to make progress on this muffler problem.
Paul Lamar
"Bass, Jerry" wrote:
The holes are on the fishtails. You may be right about the lips vibrating.
As the material is quite thin. I will bend an edge on the lip to create a
stronger edge.
I Made them in two halves. Drew a drawing on Auto Cad for the hole locations
and drilled one half with 1/8" holes. Then dimpled them with a mandrel and
punch outward. I actually drilled 3/32 holes and cleaned them with a 1/8"
after dimpling.
The Sound meter may be the only way, but when my helper standing along side
the plane during with/without tests, and saw no discernable change. Seems to
be a good first opinion. I am looking for at least half reduction.
Jerry Bass
That is unique Jerry. I don't think that is what the authors of Scientific Design of
Exhaust & Intake Systems had in mind when they suggested fish tails :-)
Paul Lamar
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