Matt Whiting wrote in
:

Bertie the Bunyip wrote:
Matt Whiting wrote in

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.

I suspect, as with most "real world" problems, that there is more in
play than delta T induced stress. Probably geometry and other
factors.
Maybe having the thin fins on the outside vs. thick metal on the
inside is making a big difference in the stress profile.


I think that has more to do with the gradient along the cylinder as the
combustion chamber expands and the gasses cool. There's a lot more heat
produced up top, thus the intricate finning all over the head. In fact,
in the early days , it was improved casting techniques that alowed this
finning which in turn gave large horsepower boosts to the engines back
then. This was particulalry true in the 20s and thirties, but it still a
widely putsued goal today. the better the cooling, the more fire you can
make and the more fire..
I'll still hold to my original thoughts on it, though. I think the
difficulty in getting heat away from some parts as opposed to others
makes the temp gradient across the cylinder walls uneven in spots and
since I consider I've seen the proof of the pudding I can't shake the
habits of a lifetime as easily as al that!


I've not had experience with the larger engines or with radials.
However, my experience with O-470 and smaller engines is that shock
cooling just isn't an issue and many folks are paranoid for nothing.

I'm not paranoid about it, I just don;t think it;s a myth.

Operating the engine as if shock cooling was an issue is probably not
a problem in most cases, but if it causes you, as it has with Jay, to
not practice essential emergency procedures, then I disagree that it
causes no harm. This may be very harmful should Jay experience an
engine failure for real.



I agree and I don't subscribe to that stance in any way shape or form. I
was only picking a nit about shick cooling being a myth.
You have to do what you have to do in an airplane. You have to have some
respect for the engine, but you don;t have to go nuts!
UI mentioned earlier a place I worked did ab initio training in a J-3
(BTW, with no radios, starter or intercom) and, as you might imagine the
engine was up and down a lot.
Standard practice in airplanes like that is to chop the power on
downwind opposite the touchdown point and regualte your approach by
varying the size of your pattern from that point. Now, with some regard
towards rapid cooling we reduced to about 1200 rpm initially and then
chopped it a bit later.
Needless to say the students had very little trouble doing forced
landings when it came to that time in their training.
I've also taught just the same in Cherokees and Cessnas, although
teaching relatively recently within flying clubs I've had to go with the
flow because somewhere some asshole back in the '70s got it in his head
that since airliners do power stabilised approaches it;s a good idea in
a lightplane as well. "Makes the whole trianing experience more
professional" you know.
Now there's a new thread!


Oh, and the J-3? Last time I saw it it had over 4,000 hours on the
engine and hadn;t even had a top.
I think it;s stil flying, though hopefuly it's had a bit of work since
then. Poor old thing!


Bertie