Engine out practice

On Oct 15, 8:49 am, Bertie the Bunyip wrote:

The examiner wouldn't allow him to slip because he reckons they are
dangerous with the flaps out and that he should wiggle the ailerons back
and forth to lose height. He didn't even want him to slip clean.

Jesus wept.

This examiner had had a fright in a 172 (this was an archer anyway) and
did not alow anyone to slip with flaps out.
While I am firmly in the camp that says some cessnas can get a litle
fuzzy in pitch with full flaps, this is just stupidity incarnate.

Shoot. We do slips with full flaps all the time in 172s, have
done so for years, and never had a scare. I wonder if that "Avoid
Slips With Flaps Extended" applied to some earlier models? I'll have
to check the TCDS sometime.

Dan

Matt: At least in my experience shock cooling did exist. I flew sky divers
in a Skylane and had taken over after another pilot who would climb hard and
chop the throttle and descend to the ground. There were frequent low hour
Top Over hauls, and cracked cylinders. When I began flying the bird the
owner asked me to be aware of cooling it down to fast. My method was to
climb to the drop and then close the cowl flaps, carry 15" MP and spiral
tightly down. It stopped the low hour top overhauls.
My descent rate could be pretty high and the engine was kept relatively
warm.

Stu Fields
Experimental Helo Magazine.
"Matt Whiting" wrote in message
...
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

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

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

Matt Whiting wrote in
:

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?


That can happen too! But the clearances are cgrater ther than it would be
in a valve stem or head to cylinder seat.

Bertie

On Fri, 19 Oct 2007 11:05:32 +0000 (UTC), Bertie the Bunyip
wrote:

Matt Whiting wrote in
:

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?


That can happen too! But the clearances are cgrater ther than it would be
in a valve stem or head to cylinder seat.

If we had that kind of piston to cylinder clearance in an automotive
engine we'd think it was worn out. :-)) At least in the "old days".

Roger (K8RI)


Bertie