Finn Lassen wrote:

Had my first real engine problem today at 22 hours on the airplane and
probably some 35 hours on the engine.

I'd just added another aileron trim tab and climbed up above the clouds
(7000') when I smelled something melting or burning. A few seconds later
I woke up to the fact that the exhaust note had changed. Pulled power
all the way back and spiraled down to a nice landing at the airport,
which happened to be just below me (Brooksville, Hernando Co.). Somewhat
an inconvenient location (some 60 miles from home and 27 miles from my
airport).

Turned out the decision to land was justified. Muffler cracked around
one of the header pipes (well hidden from inspection because of the
surrounding heat shields).

Part of one of the aluminum heat shields had melted to the point of
evaporation. Fortunately the crack was on the engine side and not on the
cowling side (where the muffler wall actually is double).

This failure is not surprising, as it was a known potential stress
point.

I guess I'll just patch it up for now and fly it home, but I'm
considering to change my heat shields to stainless shim material and
also cutting the exhaust pipe where it exits the muffler and adding a
piece of stainless flex conduit/pipe there.

Ain't life exciting?

Finn

This brings up a thought that some of the muffler systems may be subject to
short life.  I don't know Finn's configuration, but it sounds like the two
exhaust pipes come out of the engine, directly into a tangential entry into a
larger piece of tubing. Type "A"

This is contrasted to Tracy's where the tubes coming out of the engine bend
down and the larger muffler tubing is below the engine. Type "B"

A Type "A" muffler will have a possibility of fatigue-stress cracking at the
junction of the exhaust header tubes into the muffler, due to repeated
thermal expansion and contraction of the muffler, with the header tubes being
short and rigidly held. The muffler will expand more than the inter tube
spacing, each time the engine is fired up.

A Type "B" muffler has lots of header tube length to absorb the relative
motion. Therefore would be less susceptible to fatigue cracking.

A Type A muffler where the headers wrap around the muffler and then are
welded might not suffer this behavior.

Bill Schertz

 
The AirCraft Rotary Engine NewsLetter.              Powered by Linux.

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