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Date: Sun, 16 Mar 2014 09:41:50 -0400
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From: adam@validationpartners.com
Subject: Prime suspect
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During January's cold snap I had to attempt an engine start after the 
plane had been sitting outside in 20degF weather for 4 hours. After many 
minutes of cranking, priming, and more cranking I had to throw in the 
towel and cancel the flight. I had the FBO move my plane to their heated 
hangar. When I was finally able to retrieve it 4 days later I noticed 
significant heat damage on belly just aft of the right exhaust pipe. I 
don't know how long the damage was there but I strongly suspect it was 
from raw fuel spitting out the exhaust and catching fire.

You can see pictures of the damage and repair here:
https://plus.google.com/photos/102798913343718743387/albums/5991210570436247697?authkey=CIK38uXuxNOOJg

I believe this ties in with recent discussions about vibration isolation 
for exhaust systems. The header pipe attaches to the tailpipe with a 
ball joint which allows about 3" of movement in any direction. A stiff 
rubber strap bolts to the firewall and extends down to support the 
tailpipe and attaches with a clamp.

The problem is that the rubber strap allows too much movement. I set the 
strap length so that the tailpipe is angled slightly down. But if I push 
the tailpipe up so it's parallel to the fuselage or even angled up 
slightly it stays there after I let go. The stiffness of the rubber 
strap can't overcome the friction in the ball joint.

I'm not sure about the solution yet. Certainly changing from stock 
baloney-slice tip to a turn-down tip would help. Changing from a rubber 
strap to a metal bar would also help, but I'm only just starting to 
think about the geometry and required freedom-of-movement issues.

On the bright side, the plane is still in primer so at least I didn't 
ruin a nice paint job!

-Adam Molny
Legacy N181AM 220 hours