The Murphy Builders Site

Mike - couldn't you fabricate some small covers to place over the exhaust
once you land? I wouldn't think that all that hot air would rush DOWN out
of the pipes. I assume that you are concerned about this after you shut the
engine down.

Brian

-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of Mike
Kimball
Sent: Monday, September 11, 2006 10:35 PM
To: Rebel Builder's List
Subject: [rebel-builders] Engine shock cooling on the ground

Everyone knows that shock cooling can be a problem with some aircraft
engines during rapid descents but how about shock cooling the engine on the
ground by shutting it down when it's very, very cold outside? More than one
person has expressed concern over my short exhaust due to the short path to
the heads for cold air to meet hot heads and cause a crack. I would think
that if it's a problem for my aluminum Chevy with aluminum heads it would
also be a problem for aircraft engines that typically have pretty short
exhaust. I'm interested in opinions, and possible solutions if it is a
problem. My plane is finally ready to fly and, of course, my luck timed
that with the onset of winter in Fairbanks, Alaska.

Mike
044SR




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Air cooled cylinders crack during rapid cooling due to different parts
of the cylinder cooling at different rates.
ie. large temperature differences at different parts of the cylinder.
That is generally not an issue with liquid cooling due to the much
higher thermal masses involved, especially with a thermostat installed.

However even in moderate temperatures, I have heard of problems with any
exhaust valve that stays open after shutdown with very short exhaust
stacks allowing the valve head to cool (not the cylinder head) very
quickly. I'd guess that you are correct and that is more of a problem
with aircooled engines as liquid cooled engines will have a cooler valve
by the time you taxi in and shutdown... Liquid cooled engines can take
a lot of abuse and the valves are usually smaller. Imagine flooring it
to pass a car on a hill then taking your foot off to go down the other
side of the hill which then cuts the fuel and lets unburnt air through
on a modern injected engine - no big deal generally.

I've also heard of similar exhaust valve issues on a running engine
where it was felt that reversion was causing cold air to hit the valve
between exhaust pulses and initiating cracks. I don't doubt that these
can be real issues but I do doubt that the difference between say +10 C
and -30 C is significant... I don't like short stacks but if you know
of others flying your engine with similar stacks down South without
problems, I probably wouldn't worry about the cold temperature much...
If you are concerned, is there any way to attach temporary stack
extensions? Don't know if it would help but perhaps even tapered down
(flattened ends) with holes drilled as pictured in the Bingelis book.
Flexible stainless extensions grouped and tied together???

Ken

Mike Kimball wrote:

Everyone knows that shock cooling can be a problem with some aircraft
engines during rapid descents but how about shock cooling the engine on the
ground by shutting it down when it's very, very cold outside? More than one
person has expressed concern over my short exhaust due to the short path to
the heads for cold air to meet hot heads and cause a crack. I would think
that if it's a problem for my aluminum Chevy with aluminum heads it would
also be a problem for aircraft engines that typically have pretty short
exhaust. I'm interested in opinions, and possible solutions if it is a
problem. My plane is finally ready to fly and, of course, my luck timed
that with the onset of winter in Fairbanks, Alaska.

Mike
044SR

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At 06:34 PM 9/11/2006 -0800, you wrote:

Everyone knows that shock cooling can be a problem with some aircraft
engines during rapid descents but how about shock cooling the engine on the
ground by shutting it down when it's very, very cold outside? More than one
person has expressed concern over my short exhaust due to the short path to
the heads for cold air to meet hot heads and cause a crack. I would think
that if it's a problem for my aluminum Chevy with aluminum heads it would
also be a problem for aircraft engines that typically have pretty short
exhaust. I'm interested in opinions, and possible solutions if it is a
problem. My plane is finally ready to fly and, of course, my luck timed
that with the onset of winter in Fairbanks, Alaska.

Mike
044SR

I wouldn't worry Mike Liquid cooled engines don't normally suffer from
shock cooling. Unless you're flying it into the hangar it should cool down
nicely even on a short taxi from the runway.
Drew



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Comforting comments guys. Thanks. It's just that I'd like to keep flying
down to 20 below. Maybe I will make some simple covers for when it's that
cold. It gets as low as 50 below but I think I'll stay home on those days.

Mike
044SR

-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of Drew
Dalgleish
Sent: Tuesday, September 12, 2006 5:00 PM
To: rebel-builders@dcsol.com
Subject: Re: [rebel-builders] Engine shock cooling on the ground

At 06:34 PM 9/11/2006 -0800, you wrote:

Everyone knows that shock cooling can be a problem with some aircraft
engines during rapid descents but how about shock cooling the engine on the
ground by shutting it down when it's very, very cold outside? More than

one

person has expressed concern over my short exhaust due to the short path to
the heads for cold air to meet hot heads and cause a crack. I would think
that if it's a problem for my aluminum Chevy with aluminum heads it would
also be a problem for aircraft engines that typically have pretty short
exhaust. I'm interested in opinions, and possible solutions if it is a
problem. My plane is finally ready to fly and, of course, my luck timed
that with the onset of winter in Fairbanks, Alaska.

Mike
044SR

I wouldn't worry Mike Liquid cooled engines don't normally suffer from
shock cooling. Unless you're flying it into the hangar it should cool down
nicely even on a short taxi from the runway.
Drew



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