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Walter, Gary,
First let me thank you for all the great posts that
I've found so informative over the years.
And thank you Marv. The benefits can't be
overstated.
Some of the reasoning behind my choice of warming
up comes from the information available concerning starting a cold
engine.
Here in the northeast, preheating is many times
required. ( I use 32 deg F as a guide but others differ. )
Without this preheat, cylinder walls can be scored
because the aluminum piston will expand more and faster than the steel
cylinders, which are distorting under the stress of the rapid heating.
Clearances are reduced not allowing enough oil for the rings or even binding the
pistons.
This scenario is obviously worse for a tight
engine.
Walter, your concerns on keeping the CHT's cooler
seem to be more of a summer operations concern. I couldn't agree more and
do rush to the end of the runway in warm weather. Temp is always at least
100f. Your reference to Lycoming's position on this eludes me. My
Lycoming Operators Manual specifically calls for a "warm up" as part of the
ground check and has instructions for doing so. However, no temperature
information is given other than in "operating conditions" where there is a note
not to run below 140F for continuous operation.
Gary, The cooling effects of the oil are more
limited with less flow. There is a squirt tube that directs an oil on the
bottom of each piston and excess flow to the valve mechanism helps cool the
cylinder heads. I know the oil carries a fair portion of the
cooling load but have no data or numbers.
As an engineer, I'm assuming that the faster the
cylinders are heated the more internal stress occurs. (Which leads to
cracks)
Most of us aren't looking at the CHT's during to
first part of takeoff roll but I know the temps are climbing really
fast.
I'll bet some data might be provided by anyone with
a data recorder ( Like Greyhawk for instance) who could quantify the rate
of change of CHT's on takeoff as opposed to other phases of flight. My bet
is that it's the fastest changing part of the flight.
I've chosen 100F as a reasonable starting
point. The warmer the engine is before full power is added the slower will
be the rate of change.
If anyone can show me how to do it better, I'd
appreciate it.
What temperature would you hang your hat on Walter
- as ideal?
Mark Ravinski
N360KB 1320 hrs
If you have any data to support that
contention, I would really appreciate seeing it. I know of no such data and do
not share your concern. As a matter of fact, waiting until the cylinders are a
higher temperature before takeoff, only serves to end up with a hotter CHT
during initial climb. I'd rather keep the cylinders cool. TCM, Lycoming, Pratt
& Whitney, and Wright all seem to agree with that. I think they're
right.
Walter
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