The mod 72 has one good side benefit, it gives us the chance for a real
close inspection of the back of the engine and underneath. I had to remove
the silencer for better access to the firewall and two bottom frame bolts.
Here is what I discovered :
The bottom of the silencer , starboard side, an oval area of 30x90mm was
badly pitted with strong signs of having melted. The aluminum duct directly
underneath is similarly affected. I tapped the affected area with a hammer
to look for any weakness, but it appears to be still okay. The aluminum is
paper thin and needs to be patched.
My installation is a standard 912S fwf kit with stainless exhaust system and
Singleton firewall. I never experienced any temperatures (according to the
EIS) above the permissable ranges. The EGT probes are on the exhaust stubs
of cylinders 3 and 4. I never noticed anything unusual.
There is no other heat damage. The coolant pipe directly behind it to the
radiator shows no sign of anything unusual.
My questions are:
Has anyone ever experienced or heard of anything like this before ? What
could be the possible cause ? What is the melting point of stainless steel ?
There is no direct fresh air to the bottom of the engine. Some time ago I
had taped off the starboard NACA duct, as it was supposedly designed for the
914. There was no detrimental effect to the engine temperatures. I have now
opened up the precut flap in the starboard side of the alu duct, which will
divert some fresh air to the silencer. (I believe this flap was also
designed for the 914).
One very long shot : Could there be any electrical sparking between the two
surfaces, acting like a capacitor ? I just don't see how those high
temperatures could come from the exhaust gases without also burning the
outside of the fuselage etc.
I wonder if anyone knows. And any help greatly appreciated.
Karl
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