Engine out practice

"Jay Honeck" wrote in message
ups.com...
As previously noted (in the thread about Paul's wife getting scared),
Mary and I had virtually stopped doing this kind of flying for fear of
harming our (very expensive) engine. A lively debate ensued as to
whether or not repeated high-to-low-to-high power applications would
wear out your engine any faster than would normal operations.


Jay, the cylinder barrels on your engine are steel with air-cooling fins
attached. Your pistons and cylinder heads are aluminum alloy. The two
metals will expand and contract at different rates when heated or cooled.
Think about the way the thermostat in your house works. A bi-metal strip
(strip of metal composed of one type of metal on one side and another type
on the other), when heated or cooled bends because of the different
expansion rates of the two metals. When I think of shock cooling I think of
the sudden removal of the heat source (abrupt power reduction) along with
the different metals contraction rate (steel with air blowing over it's
cooling fins versus aluminum inside the barrel with hot oil being sprayed on
it). It is easy for me to visualize the scuffing that can occur because of
the reduced clearances as the barrel contracts onto the piston. The same in
reverse would hold true for shock heating.

Whether any of this is true I don't know, but I am with you that gradual
increase or reduction of power seems less likely to cause damage to an
aircooled engine.

--
*H. Allen Smith*
WACO - We are all here, because we are not all there.

"Jay Honeck" wrote in message
ups.com...
As previously noted (in the thread about Paul's wife getting scared),
Mary and I had virtually stopped doing this kind of flying for fear of
harming our (very expensive) engine. A lively debate ensued as to
whether or not repeated high-to-low-to-high power applications would
wear out your engine any faster than would normal operations.

I eventually agreed that gradual power changes would not unduly harm
an air-cooled engine, and vowed that I would endeavor to practice this
most-important skill on our next flight. And we did.

We were on a flight back from Galesburg, IL when I started the
procedure, and very gradually began a power reduction whilst in cruise
flight at 3500 feet. I took a full minute to reduce the power to
idle, watching our (newly reinstalled) JPI EDM-700 engine analyzer for
signs of stress.

As RPMs dropped below 1000, the "shock-cooling alarm" suddenly went
off, flashing its dire warnings that EGTs had dropped beyond (and
faster) than recommended limits. (I can't remember what the threshold
is for that alarm -- it's preset.)

This despite my most careful power reduction, which (obviously) wasn't
slow enough.
....

Thoughts?

You're worrying about virtually nothing!
http://www.avweb.com/news/maint/182883-1.html

Shock Cooling: Myth or Reality?

Powerplant management guru Kas Thomas of TBO ADVISOR examines the physics
and metallurgy of "shock cooling" and concludes that, contrary to the
conventional wisdom, it is not a major contributor to cylinder head
cracking.

What were your CHTs?

Matt Whiting wrote:
Jay Honeck wrote:
The issue with shock cooling isn't the rate of cooling per se, but
rather stress induced by differential cooling.
Actually, I think it is the rate of cooling *and* the differential
cooling -- if it exists at all. Like you, I am skeptical -- but am I
willing to bet $25K on it? Nope.
How does the rate affect things? I have a masters in structural
engineering and work for a materials company so don't be afraid to get
technical. :-)

How 'bout this: It's the disparate rates of cooling in some parts of
the engine (versus others) that causes the differential cooling that
induces stress?

Yes, that is what I said originally. It is differential cooling that
causes the problem, not the rate of cooling itself. If you could cool
the entire engine uniformly, I don't think it would matter much how fast
you cooled it.

It isn't the rate itself that causes a problem, it is the difference in
rates from one location to another. However, I still think that the
greatest thermally induced stress occurs during the initial heat-up from
a cold start, but I don't have any data to confirm that and I don't have
an instrument airplane with which to collect the data.

Matt

I would think the greatest thermally induced stress occurs when you fly
into rain.

--
Jim Pennino

Remove .spam.sux to reply.

Matt Barrow schrieb:

You're worrying about virtually nothing!
http://www.avweb.com/news/maint/182883-1.html

Shock Cooling: Myth or Reality?

Powerplant management guru Kas Thomas of TBO ADVISOR examines the physics
and metallurgy of "shock cooling" and concludes that, contrary to the
conventional wisdom, it is not a major contributor to cylinder head
cracking.

Lycoming says otherwise:
http://www.lycoming.textron.com/supp...Operations.pdf

Lycoming
Sudden cooling is detrimental to the good health of the piston aircraft
engine. Lycoming Service Instruction 1094D recommends a maximum
temperature change of 50° F per minute to avoid shock-cooling of the
cylinders.

Operations that tend to induce rapid engine cooldown are
often associated with a fast letdown and return to the field after
dropping parachutists or a glider tow. There are occasions when Air
Traffic Control also calls for fast descents that may lead to sudden
cooling.

The engine problems that may be expected when pilots consistently
make fast letdowns with little or no power include:

1. Excessively worn ring grooves accompanied by broken rings.
2. Cracked cylinder heads.
3. Warped exhaust valves.
4. Bent pushrods.
5. Spark plug fouling.
/Lycoming


Be aware that "powerplant management guru Kas Thomas" won't buy you a
new engine if you happen do damage yours by following his recomendations.

It's every operator's choice whether he prefers to believe the engine
manufactorer or some guru.

Stefan

Jay:
In the future I think we'll practice slow flight (which mimics this
whole engine management procedure) before practicing engine-out
stuff. That should prevent the whole shock-cooling problem,
methinks.

Dale:
Slow flight might increase the problem. You're mushing along with poor
flow through the cowling, low airspeed and using power...perhaps you're
going to increase engine temp. over cruise.

That was my first thought, that slow flight would increase temp and,
therefore, how would it prevent the shock-cooling problem (if indeed it
is one)?

"Stefan" wrote in message
...
Matt Barrow schrieb:

You're worrying about virtually nothing!
http://www.avweb.com/news/maint/182883-1.html

Shock Cooling: Myth or Reality?

Powerplant management guru Kas Thomas of TBO ADVISOR examines the physics
and metallurgy of "shock cooling" and concludes that, contrary to the
conventional wisdom, it is not a major contributor to cylinder head
cracking.

Lycoming says otherwise:
http://www.lycoming.textron.com/supp...Operations.pdf

Lycoming
Sudden cooling is detrimental to the good health of the piston aircraft
engine. Lycoming Service Instruction 1094D recommends a maximum
temperature change of 50° F per minute to avoid shock-cooling of the
cylinders.

Thomas offers data and evidence, Lycoming offers anecdote and legend.

Takes yer picks.

Matt Barrow schrieb:

Thomas offers data and evidence, Lycoming offers anecdote and legend.

Lycoming offers running engines. Thomas offers words.

"Stefan" wrote in message
.. .
Matt Barrow schrieb:

Thomas offers data and evidence, Lycoming offers anecdote and legend.

Lycoming offers running engines. Thomas offers words.

Try something other than "Argument from Authority", such as EVIDENCE.

Or, if you can show that Lycoming HAS NOT been shown to frequently be FOS,
then you can make their case.

Matt Barrow schrieb:

Thomas offers data and evidence, Lycoming offers anecdote and legend.
Lycoming offers running engines. Thomas offers words.

Try something other than "Argument from Authority", such as EVIDENCE.

Evidently, Lycoming knows how to build engines. Evidently, Lycoming has
a lot of experience by looking at used engines while overhawling them. I
don't know how many engines Thomas has built or overhauled. I don't even
know where his data comes from and how it was collected.

Or, if you can show that Lycoming HAS NOT been shown to frequently be FOS,

No idea what a FOS should be. Please write in a language I understand.

then you can make their case.

I'm not making anyone's case. In fact, I couldn't care less, as I'm
happy enough to operate a liquid cooled engine with 21th century technology.