Engine out practice

Matt Whiting wrote in
:

Bertie the Bunyip wrote:
Matt Whiting wrote in
:

Bertie the Bunyip wrote:
Matt Whiting wrote in news:foeQi.309$2n4.18956
@news1.epix.net:

Stefan wrote:
Matt Whiting schrieb:

And Lycoming benefits if your engine lasts fewer hours.
So avoiding shock cooling actually lowers its life span? Wow.
You have no evidence that following Lycoming's recommendations
avoids the mythical shock cooling demon or that it lengthens
engine life. My experience is that the engines that are run the
hardest also last the longest. I'm basing this on everything from
chainsaws to lawnmowers
to
motorcycles to cars to trucks to off-road heavy equipment (dozers,
skidders, etc.) to airplanes (trainers, air taxi operations,
cargo).

I'm personally not convinced that Lycoming's recommendations
lengthen engine life.

Matt


Shock cooling isn't mythical. It's a fact. It's a physical law.
A physical law, eh? I've had 8 years of engineering school and
haven't seen this law. Can you provide a reference to the law of
shock cooling?
I searched for the "law of shock cooling" in Google and came up
empty...


Any component subject to heating is subject to this law. If you
take a piece of metal and heat it rapidly on one side, that side
will expand more rapidly than the other. This gradient of temp will
cause a difference in physical size one side to the other. The
elastic stress induced by this is cyclically compounded and the
resultant locked stress points that build up in the material,
particularly if it's a brittle material like cast iron, will
eventually fail, given time. The speed at which these stresses are
imposed are critical. Speed because if you introduce the heat
gradually (decrease the speed of the overall temp change), it's
given a chance to get to the other side and expand the other side
at a rate not quite so dramatically different as the side the heat
is applied to. Simple eh? The quicker you insert heat on one side
of the material, the greater the load on the opposite side and the
more likely minor damage events (cracks on a near molecular leve)
are occuring. These tiny bits of damage will become stress risers
for the next time th ematerial is loaded and the cracks will
continue to expand until a failure of the component occurs.
Yes, I'm well aware of thermal expansion and its affects. When an
engine is pulled to idle, the cylinders and heads are getting cooled
from both sides, the outside via airflow and the inside via airflow
through the engine. The far greater differential is under full
throttle during the first take-off when the engine has not yet
reached thermal equilibrium and you are heating it intensely on the
inside and cooling it on the outside.

If people wanted to talk about shock heating, then I'd be much more
willing to believe them and this fits the physics a lot better in my
opinion. Shock cooling is much less an issue from both a physics
perspective and an experience perspective.


It's the same either way. Cooling and heating are two sides of th
esame coin. It takes time to disapate heat and it's not so much the
passage of heat from one area to another (or the disappation, it's
irrelevant) but the speed at which the cooling or heating is taking
place and thus the gradient across the material.
In short, you take a frozen lump of metal and apply a torch to one
side you have a problem.
Take a cherry red pice of metal and put some ice on side and you have
the same problem (more or less, and disregading crystalisation)

It is the same if the same delta T is present, but my point is that it
is easier to heat something quickly than cool it quickly. Even at 250
C, you are only 523 degrees above absolute zero. So, this the
absolute largest delta T you can induce for cooling, and it is very
hard to get absolute zero, so you are more likely to have a cool temp
closer to 0 C yielding a delta T of only 250 degrees.

On the hot side things are more open-ended. It isn't too hard to get
450 C exhaust gas temperatures. For an engine that is started at say
20 C ambient temperature, you now have a delta T of 430 degrees which
is much greater than the 250 likely on the cooling side of the cycle.

That is one reason why I suspect that "shock heating" is more likely
to be an issue than "shock cooling." I suspect you can induce a
higher delta T during a full-throttle initial climb than you can
during an idle descent from a cruise power setting.


Right, I'm with you now. yeah, I can buy that. Froma strictly clinical
viewpoint it absolutely makes sense. My experience with damage says
otherwise, though I can offer no explanation why that should be the
case. Years ago I towed gliders with Bird-dogs and we cracked a lot of
cylinders when we just closed the throttle after release. When we moved
to gradual reduction to ultimately 1500 RPM the problem disappeared
completely. Later, when I flew big pistons,the procedures for cooling
down the cylinders on the way down. You were almost gaurunteed a crack
if you yanked the taps closed. Can't see how we went from cold to hot
any more than you would just starting up and taking off.
I've just bought an aerobatic airplane with a Lycoming. We're not
expecing to get to TBO with the engine because we'll be doing aerobaics
with it, but of course we're prepared to live with that.
I suppose the point I'm making is that even if shick cooling is over-
rated, it certainly does no harm to observe trad practices as if it did.