Engine out practice

Ernest Christley wrote in
:

Matt Whiting wrote:
Ernest Christley wrote:
Bertie the Bunyip wrote:

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.


With the heating, you only have the few hundred CFM of air passing
through the engine to heat it. With the cooling, you have all of
the great outdoors to do the trick. To tie it into your anology,
you have a butane lighter to heat the metal, and the Atlantic Ocean
to cool it.

The heat doesn't come from the air, but from the fuel.

Matt

- Heat comes from the reaction of the fuel vapors with the oxygen in
the air.
- Once the fuel is vaporized, isn't it also part of the air.

Semantics aside, the point is, you have a limited amount of BTU
available from the fuel-air mixture. Since some of those BTU's are
carried away even as the engine is warming up, the heating will be
gradual. It will heat until a dynamic equilibrium is reached between
the heat from combustion and the cooling from air flow. Hopefully at
350 degrees F or less.

Pull the heating part of the equation out, and all you have is
cooling.
All the air around you is a really large heat sink to dump into.
Push
that engine through the air at 100mph, and the heat will come out
FAST!!

When you cut the power, you cut the heat, but the pistons are still
moving. The cylinders cool quickly. They're exposed to the air, and
have lots of vanes designed to give up that heat. The piston is
insulated...by the cylinder, coatings of oil, etc. The cylinder
shrinks, clamps the moving piston, and parts give up shortly
thereafter.
I'm not brave/fool (you pick) enough to test this, but the engine
might never crack if you stopped the windmilling when you chop the
power.

Your welding torch example is not germane.


You have to pump pure
oxygen into an acetylene flame to get welding temps. Acetylene gives
up more BTUs that gasoline, and it won't work with normal atmosphere
which is mostly nitrogen. You won't ever be able to reach the 6000
degree max temp of a welding flame inside a normal combustion engine.
Even then, try to weld a dirty piece of metal sometime. Even the
thinnest coat of crud is enough to insulate the metal enough to make
welding a frustrating experience.




That's beside the point.


Bertie

"Bob Moore" wrote in message...
I wear my "Slips with Flaps" T-Shirt proudly!


Mine's totally worn out, even though I only wore it to fly-in events. I
wonder if Tina will ever resurface and run another batch???

In article ,
Matt Whiting wrote:

Steve Hix wrote:

So they went out one morning, got plenty of cushion between themselves
and the ground, set the 172 into a landing configuration with full
flaps, and slipped it.

It shook a bit and then went inverted on them. They recovered and came
back home.

Got to love those urban legends!

I was sitting at the FBO front desk when Lynn and Fred went out to see
what would happen, and was there when they returned about a half hour
later.

They told me then what had happened, and agreed that they weren't
inclined to try it again with our 172. They weren't sufficiently
interested to see if it was affected one way or another by changing
payload, CG location, etc.

Not an urban legend, sorry if that disappoints you.

Matt Whiting wrote:
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'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.

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.

Matt

Shock cooling damage is merely the effect of different rates of thermal
dimensional change between the aluminum cylinder head and the steel
valve seats and possibly between the head and the barrel where its
threaded on. When contracting, the head shrinks faster than the valve
seats and barrel and huge tension stresses are built up in the area of
the head between the valve and the nearest hole, which is usually the
spark plug. Most shock cooling damage is a crack between the exhaust
valve and the nearest plug hole.

It's not a problem when heating because the head expands faster than the
steel parts so the stress effects are reversed. This is why the
manufacturers have no problem with going from idle to full power as soon
as the engine will take it without stumbling. There is no such thing as
shock heating...

Shock cooling is generally a problem when at the extremes, going from
full power to idle. You won't get enough stress to cause damage going
from cruise power to idle, so for airplanes not used for towing, or
aerobatics or some flight training scenarios, it's not a problem.

John

"J.Kahn" wrote in
:

Matt Whiting wrote:
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'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.

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.

Matt

Shock cooling damage is merely the effect of different rates of
thermal dimensional change between the aluminum cylinder head and the
steel valve seats and possibly between the head and the barrel where
its threaded on. When contracting, the head shrinks faster than the
valve seats and barrel and huge tension stresses are built up in the
area of the head between the valve and the nearest hole, which is
usually the spark plug. Most shock cooling damage is a crack between
the exhaust valve and the nearest plug hole.

It's not a problem when heating because the head expands faster than
the steel parts so the stress effects are reversed. This is why the
manufacturers have no problem with going from idle to full power as
soon as the engine will take it without stumbling. There is no such
thing as shock heating...

Shock cooling is generally a problem when at the extremes, going from
full power to idle. You won't get enough stress to cause damage
going from cruise power to idle, so for airplanes not used for towing,
or aerobatics or some flight training scenarios, it's not a problem.



I can buy that..

Bertie

"J.Kahn" wrote

Shock cooling damage is merely the effect of different rates of thermal
dimensional change between the aluminum cylinder head and the steel valve
seats and possibly between the head and the barrel where its threaded on.
When contracting, the head shrinks faster than the valve seats and barrel
and huge tension stresses are built up in the area of the head between the
valve and the nearest hole, which is usually the spark plug. Most shock
cooling damage is a crack between the exhaust valve and the nearest plug
hole.

It's not a problem when heating because the head expands faster than the
steel parts so the stress effects are reversed. This is why the
manufacturers have no problem with going from idle to full power as soon
as the engine will take it without stumbling. There is no such thing as
shock heating...

Shock cooling is generally a problem when at the extremes, going from full
power to idle. You won't get enough stress to cause damage going from
cruise power to idle, so for airplanes not used for towing, or aerobatics
or some flight training scenarios, it's not a problem.

Yours is the first explanation that I can believe. Thanks.

I can also believe that cruise power to idle is not a huge problem, and that
perhaps it is even more true if the airspeed is kept down a bit, while the
power comes off.
--
Jim in NC

Steve Hix wrote:
In article ,
Matt Whiting wrote:

Steve Hix wrote:

So they went out one morning, got plenty of cushion between themselves
and the ground, set the 172 into a landing configuration with full
flaps, and slipped it.

It shook a bit and then went inverted on them. They recovered and came
back home.
Got to love those urban legends!

I was sitting at the FBO front desk when Lynn and Fred went out to see
what would happen, and was there when they returned about a half hour
later.

They told me then what had happened, and agreed that they weren't
inclined to try it again with our 172. They weren't sufficiently
interested to see if it was affected one way or another by changing
payload, CG location, etc.

Not an urban legend, sorry if that disappoints you.

I don't for a second believe that a slip with flaps turned them over. I
suspect they STALLED it and went inverted, but that wasn't due to a slip
with flaps.

Matt

J.Kahn wrote:

It's not a problem when heating because the head expands faster than the
steel parts so the stress effects are reversed. This is why the
manufacturers have no problem with going from idle to full power as soon
as the engine will take it without stumbling. There is no such thing as
shock heating...

What about the aluminum piston in the steel cylinder?

Matt

In article ,
Matt Whiting wrote:

Steve Hix wrote:
In article ,
Matt Whiting wrote:

Steve Hix wrote:

So they went out one morning, got plenty of cushion between themselves
and the ground, set the 172 into a landing configuration with full
flaps, and slipped it.

It shook a bit and then went inverted on them. They recovered and came
back home.
Got to love those urban legends!

I was sitting at the FBO front desk when Lynn and Fred went out to see
what would happen, and was there when they returned about a half hour
later.

They told me then what had happened, and agreed that they weren't
inclined to try it again with our 172. They weren't sufficiently
interested to see if it was affected one way or another by changing
payload, CG location, etc.

Not an urban legend, sorry if that disappoints you.

I don't for a second believe that a slip with flaps turned them over. I
suspect they STALLED it and went inverted, but that wasn't due to a slip
with flaps.

Kewl. Believe whatever floats your boat.

They were both experienced CFIs, one with a good deal of A-26 and T-28
time. If they'd stalled it, I would think they'd said so.

Steve Hix wrote:
In article ,
Matt Whiting wrote:


Steve Hix wrote:

In article ,
Matt Whiting wrote:


Steve Hix wrote:


So they went out one morning, got plenty of cushion between themselves
and the ground, set the 172 into a landing configuration with full
flaps, and slipped it.

It shook a bit and then went inverted on them. They recovered and came
back home.

Got to love those urban legends!

I was sitting at the FBO front desk when Lynn and Fred went out to see
what would happen, and was there when they returned about a half hour
later.

They told me then what had happened, and agreed that they weren't
inclined to try it again with our 172. They weren't sufficiently
interested to see if it was affected one way or another by changing
payload, CG location, etc.

Not an urban legend, sorry if that disappoints you.

I don't for a second believe that a slip with flaps turned them over. I
suspect they STALLED it and went inverted, but that wasn't due to a slip
with flaps.


Kewl. Believe whatever floats your boat.

They were both experienced CFIs, one with a good deal of A-26 and T-28
time. If they'd stalled it, I would think they'd said so.

It did not go inverted from slipping with flaps.