Tracy Crook wrote:

Attached are a couple of shots of my 304 SS Edelbrock muffler after about 24
flight hours.   It cracked around the entire circumference near the front
bulkhead.  Only a small strip about an inch wide near the top kept it from
separating while in-flight.  I landed at Tampa Exec and sawed it the rest of
the way off and flew home.  The sound level was so high on takeoff it
overloaded the noise canceling Bose headset!  Fortunately the noise was
proportional to throttle setting so I flew back at 25% power and saved my
ears.

It was interesting to note that almost all the ceramic packing was gone and
I think this happened before it cracked (the muffler was getting louder
every flight).

My theory on why it cracked:    The internal baffle directs the exhaust out
to the sides soon after entering the muffler.  The front bulkhead does not
get any direct exposure to exhaust AND is directly exposed to the 200 mph
slipstream of cool air under the plane.  Since the coefficient of expansion
for stainless steel is fairly high, this causes a high thermally induced
stress immediately behind the bulkhead which resulted in the crack.

The good news is that this gave me a chance to see how the plane flew
without the muffler hanging underneath.  Cruise speed was up by 5 mph :)
but I was unable to test for top speed because I backed down when it got to
212 mph which is over Vne (210 mph).  Have not done flutter testing above
this speed so thought it best to wait until I had.  Dave Anders has flown
his RV-4 to 264 mph after carefully flutter testing (and four years of work
on speed mods to the airframe).  He did run into flutter at around the 250
mph point.  Fortunately, it was only a wheel pant that fluttered but he said
it felt like the plane was coming apart!  Ballancing of the pant eliminated
the problem.  I'm slowly catching up on backordered redrives and EC2s so the
aerodynamic cleanup of my -4 will start soon.

I'm working on my low drag  replacement muffler now and will post something
when I see how it works.

Tracy Crook
tcrook@rotaryaviation.com
www.rotaryaviation.com

I am reading this book called;
The Scientific Design of Exhaust and Intake Systems. by Philip Smith and John
Morrison.  $22.95.

Looks like the bible on this subject.
Got some stuff in there on mufflers too.

Paul Lamar

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