Cylinder Cracking

[Colonel Sanders]

I know people love their "inch MP per minute"
rule during descent, which is actually completely
irrelevant.

Take a look at this crack:



That's from someone shoving the mixture forward
in a descent.

Don't do that. Mixture lean in the descent.

[Beefitarian]

But they kept telling me to select full rich. It was even on the checklists. I'm pretty sure it's like doing 1000s of bell blocks in karate. Now it's a reaction.

[jg24]

CS,

You have to explain to me then why leaning in the descent. I am totally confused. Just like the other poster, i have been taught since the beginning to mixture full rich prior to descent ( and altitude changes). If i'm all trimmed out at say 6500ft and leaned out perfectly, if i descend and lean as i go down, i'll eventually get to close to ICO. Please explain the proper procedures for leaning in a climb/descent. I'd really appreciate it. Would be good for me to know now before my current habits become engraved in stone.

Thanks

[photofly]

Unless you're descending at full power, don't touch the mixture: you won't get close to ICO in a descent.

Go rich before a go-around.

[TopperHarley]

At altitude, your temps are low and your likely at a very low power setting, so your engine will be cool. Going full rich right away will cause it to cool further. Leaning it will keep the temps warmer.

[jg24]

Pretty straight forward. With fuel injected i usually do en-route descents and adjust my mixture every thousand feed or so and would go full mix short final, but wih traditional engines, i'd usually go full mixture as i descend. I have slowly started transfering the habits from the injected engines to the traditional carb'd engines that i fly for better engine managment. Sadly it's not really a thing that's taught properly these days. It's stuff that you would learn on your own from self teaching/ research/ discussions etc...

[Big Pistons Forever]

For a fixed pitched prop and carburated engine if you maintain the same RPM that you leaned your engine for at cruise, as you descend there is no reason to adjust your mixture. However as you descend the RPM will rise as the air density increases so you will have to periodically reduce your throttle in order to keep the same RPM.

For low time pilots I suggest you go to full rich as part of your prelanding check. The engine power will be reducing anyway and you will not hurt the engine.

(NOTE: For high density altitudes, at or above around 5000 feet, it is essential that the mixture remained leaned as full rich mixture will not allow full power to be developed in the event of a go around. The easiest way to do this just prior to entering the ATZ go to a high cruise setting and lean to best power (ie highest RPM) then enrichen a bit more and leave the mixture where it is until you are on the ground.)

[iflyforpie]

I usually have to richen the mixture going from 10,000 to 3,000. All of my aircraft will start running rough if I don't.

But I just richen a bit and re-lean the same I would in cruise. No point in going rich on final.... even with six levers it's pretty easy to push them all forward if I need to.

[willow burner]

Whats relevent is how fast or slow the engine cools. Yanking power off, going full rich at 10000 feet, or suddenly increasing airspeed at low OAT will all shock cool things. A CHT guage will tell you clearly what you can and cant get away with. Without one, you have to go by something. Inch a min is a start at least.

[photofly]

why then is nobody concerned with shock heating? Full power suddenly applied on takeoff!!? How damaging! No more than 1" per minute or damage will occur!

[Colonel Sanders]

why then is nobody concerned with shock heating?

Precisely. As we can see above, people stubbornly
cling to the belief that the throttle cracks cylinders,
when in fact it's the mixture.

In accordance with the publications of both Lycoming
and Continental, for the big engines (eg GTSIO-520
in 421) I reduce MP FIVE inches at a time for the
descent. With the mixture lean, to keep the engine warm.

People are so far off, on the "inch a minute" silliness,
which NO engine manufacturer recommends. Like
oversquare. Another old wives tale.

At our airport here, we have four Lyc AEIO-540 engines
in identical two-seat Pitts. How we fly and approach is
wide open - all the knobs forward, all the time. I like to
see 200 mph on downwind. Abeam the numbers, throttle
all the way back - which is probably an instant TWENTY
inch MP reduction - and 20 seconds later, 3000 fpm on
an arcing base and final, touch down.

http://www.youtube.com/watch?v=dHE503Rz ... lpage#t=23

Not a single cracked cylinder in the bunch of them, and
they have the best compressions on the field.

Another airplane, has the fearsome TCM IO-360. Original
cylinders after 45 years. Not a single crack, because I've
flown it for the last 42. You give that engine to one of the
experts here, after one year it would be trash:

[Colonel Sanders]

Part Duh:

You will notice, in the 20 second power-off Pitts approach,
that the engine makes this wonderful, gnarly afterfiring
sound. Sometime, see if you can watch that happen at
night.

It would actually be better for the engine, for me to pull
the mixture to idle cutoff, during that 20 seconds, to
completely get rid of any fuel passing through the engine.

But it sounds so cool on the ground, and looks so neat,
as you approach so ridiculously close and steeply.

Curtis would be proud

I am weak - I'm willing to admit it.

[digits_]

why then is nobody concerned with shock heating? Full power suddenly applied on takeoff!!? How damaging!

I don't have a very detailed knowledge about this, but the way I interpreted it, was that the maximum heating difference you generate with your full power is less than the maximum cooling difference you can generate with idle power, high airspeed in cold air. Similar to: "you can't accelerate to death in your car, but you can decelarate to death if you hit a tree"

After reading the other info in this topic, it's probably a flawed analogy, but if you could clarify why, I'll have learned another thing or two

[Colonel Sanders]

That's depressing. You can easily hurt your engine
with full power. Just raise the nose, cut off the
airflow, and cook it.

You want to avoid cracked cylinders? Here's
another rule: avoid high CHT's. Doesn't matter
how fast you get to them, or how fast you come
down from them. The "shock", or rate of change
of temperature with respect to time, is irrelevant
if you cook the CHT's.

You run CHT's over 400F, you will use up the
cylinders faster. Metal doesn't last as long if you
run it hot. It's that simple.

Keep your CHT's below 400F and lean the mixture
in the descent. Is that really so hard to do?

The engineers (eg wrt the GTSIO-520) say that
the reason for slower throttle change (eg 5 inche
at a time) is for that big turbo. They say that the
cylinders couldn't care less if they had 50 or 25
explosions per second in them.

[digits_]

You can easily hurt your engine
with full power. Just raise the nose, cut off the
airflow, and cook it.

But doesn't the propeller itself keep generating quite a big airflow into the engine, since it's mounted right in front of it ? Even when your airspeed is close to zero ?

[Colonel Sanders]

You're pulling my leg, right?

EDIT -- no hard feelings, digit, but you scare the crap
out of me.

Is it really typical for a CPL/MIFR/class 3 instructor to
not know that you can cook a piston engine if you climb
too steeply?

[digits_]

You're pulling my leg, right?

EDIT -- no hard feelings, digit, but you scare the crap
out of me.

Is it really typical for a CPL/MIFR/class 3 instructor to
not know that you can cook a piston engine if you climb
too steeply?

Yes I know you can overheat your engine. My question was related to the shock cooling during descends and the reverse, let's call it shock heating at takeoff.

To make sure we use the same terminology: with shock cooling, the problem is not the cool temperature in itself, but more the quick change in temperature which might cause damage because it causes excessive thermal stress in the materials.

Now, I got the idea from photofly that he was implying that shock heating might also be a problem. I haven't encountered that, and I am wondering why. At take off , you do generate a lot of power in a short time, but I find it hard to believe this would create more stress than shock cooling. One of the arguments is that, even though your airspeed is low, you still have your propeller generating quite a bit of airflow over your engine. That's not really a waterproof explanation, so I was hoping to get an explanation that was a bit more scientific.

No worries, I'm not the moron you might think I am from reading a select sample of my posts. But I am touched I have such an effect on you

[Beefitarian]

Hey I have a bigger problem digits. Often Hedley flys as much as he posts so he's current.

I think I blew up an imaginary muffler the other day. I can't type, think, visualize controls that I have not seen in seven months, wish I had a hamburger, watch a monty python movie, vacuum, etc. all while staring into a lighted screen for too many hours a day.

maybe I just can't think.

Another airplane, has the fearsome TCM IO-360. Original
cylinders after 45 years. Not a single crack, because I've
flown it for the last 42. You give that engine to one of the
experts here, after one year it would be trash:

Betcha $25 I could fly it 100 hours and not hurt it.

[iflyforpie]

I guess I am not one of the experts... because I've got a TCM IO-360 coming up to TBO with all original jugs.

[Shiny Side Up]

Yes I know you can overheat your engine. My question was related to the shock cooling during descends and the reverse, let's call it shock heating at takeoff.

"Shock heating" as one might imagine it, doesn't happen. One has to imagine how an engine gains heat, and how it loses heat. Overheating is different than one might imagine a heating "shock". Cylinders separate when overheated since they're made of both steel and aluminum and at the point of redline temperatures both start to lose their strength (Mike Busch suggests that both Continental and Lycoming have optimistic redlines). Metal has a greater ability to absorb heat than it does to lose heat - remember that when it loses heat it shrinks. If everything is expanding in the engine, stuff gets looser (and more malleable) so stuff never breaks when you're heating it (unless you heat it too much to essentially melt it). Cooling and shrinking however are a different matter. As the Colonel's picture shows, your chief danger is cooling the engine at a specific point. Unburnt fuel makes a terrific coolant, escpecially when it is suddenly applied.

Something to think about: Even if you throttle back and nose down suddenly, the rate of cooling is still not going to be near as great as if when we suddenly dump a bunch of cold fuel on something (mixture full rich).

That said though, I guess you can "shock heat" an engine when one starts it from cold, but that has more to do with friction heating, which will tend just to cause more wear to the rubbing surfaces (which is still a really bad thing) but unlikely to crack a cylinder. "Shock heat" wouldn't really be an accurate term in this case.

[sidestick stirrer]

". They say that the
cylinders couldn't care less if they had 50 or 25
explosions per second in them."
Whoa there, Big Fella!
Explosions, you say?
That would be detonation methinks, certainly not good for cylinders or pistons.
Perhaps "combustion events" would be more appropriate.

[photofly]

Metal has a greater ability to absorb heat than it does to lose heat.

I have no idea how to interpret that. Could you elaborate?

Heating and cooling is a symmetrical activity - rate of cooling or heating is proportional only to how far away from equilibrium you are.

I don't think the alleged damage caused by shock cooling has anything to do with clearances, it's to do with fatigue cracks generated from the cycling of unrelieved stresses set up when one end (or the inside) of the metal (aluminium) object cools and shrinks faster than the other end (or the outside) setting up a steep thermal gradient.

Those stresses should be equal and opposite in situations where the cylinder head is suddenly cooled from the outside compared to situations where it's suddenly heated from the inside, like when full power is applied to an idling engine, or even when the engine is started from cold.

Again, that's nothing to do with clearances or friction.

I'm open to explanations of why shock heating isn't an issue but they need to be more precise.

[jump154]

OK, I'll have a go:

Specific Heat Capacity does not change with direction of energy flow -- takes the same amaount of energy to heat somethign up 1 degree as it looses to cool 1 degree, so the statement about "ability to absorb heat" is wrong.

What is important is Coefficient of Expansion - how much a metal expands (or contracts) with temperature. You have an Iron barrel inside aluminum fins, with an aluminum head bolted on. Aluminum expands and contracts at a higher rate than the iron with a given change in temperature. The fins around the barrel are not that much of an issue - they are heat shrunk into place, so are under stress. as the barrel heats this stress will increase, but then be relieved somewhat as the aluminum heats as well - all can be predicted at design time and are nicely balanced due to beign concentric cylinders.

The issues come at the junction between the barrel and the head, where the forces are not balanced as you have a flat (ish) head bolted to the end of a cylinder. Why then is shock cooling and issue, and not shock heating? The difference is heat load distribution. The heat is generated internally to the cylinder, and although there is some variation can be thought of as a constant load - therefore expansion is constant in both the barrel and the head. Yes, the heating may not be even and some thermal stresses are set up - but again this can be calculated at the design stage. The issue with shock cooling via fuel (rich mixture) is this cooling is only at a relatively small place on the head. This sets up unbalnced stresses as the aluminum around the inlet valve is cooler and shrinking relative to the rest of the head. Do this many times fatigue sets up, then failure cracks start.

Makes sense to me, but if someone wants to pick it apart go ahead!

[photofly]

So your argument is that shock cooling occurs only around the inlet valve as a function of sudden fuel introduction, and has nothing to do with cooling airflow, is that right?

In that case wouldn't a sudden application of full power (with rapid increase in fuel flow) have the same effect?

I really thought it was a "cool fins" vs "hot cylinder heat *suddenly* being less hot" issue. Anything published by the engine manufacturers on the issue?

[Colonel Sanders]

Hedley flys as much as he posts

Thanks, Beef. I think. Actually, rather unusually, I
didn't fly today. Normally I like to fly at least once
or twice a day.

Instead I was taking groundschool. For what, shall
remain nameless, to avoid widespread depression
amongst the fair denizens of AvCan


Anyways, back on topic. If I could simply convince people:

1) to not run CHT's greater than 400F
2) to lean in the descent

there would be a whole lot fewer cracked cylinders
out there. Do whatever the hell you want with the
throttle - doesn't matter.

My job is to help people not wreck airplanes, and
not wreck engines. Be part of the solution.

Note that my motivation is the preservation of the
hardware. Not to make people feel good about
themselves.