Cylinder Shock Cooling

[Nev]

Many years ago I flew a Skydiving aircraft. We were taught to descend with the power at 20"/2000RPM, and a spiral descent in order to prevent shock cooling. I never had any issues with cylinders cracking etc.

Recently I've read a couple of articles that challenge the advice I was given all those years ago. Here's one such ARTICLE. Dynon also used to include a shock cooling alarm on their displays. The newer displays don't have it, and the reason given is that the engine manufacturers don't consider shock cooling to be an issue.

So with my attempts to further control Cylinder Head temperatures, I got to wondering.......WHY. Am I chasing a unicorn ?? If I can already keep the CHT's from exceeding 205c during normal operations, and 215c on an extended warm climb, do I need to be worried about them running cooler ? Typically in a cruise they're around 160-190c anyway.

Lycoming's position on the topic is :

At all times, caution must be taken not to shock cool the cylinders. The recommended maximum temperature change should not exceed 50F (10c) per minute.

[Nev]

Specifically interest to know if anyone has cracked a cylinder on a Bearhawk?

Edit to add:

If anyone can add what change in pressure differential they are seeing between cowl flaps OPEN and CLOSED this would be very helpful.
If the change in pressure differential isn't available, what typical change in CHT's do you observe say in the cruise between cowl flaps OPEN and cowl flaps CLOSED ? This information would be very helpful for those of us adding cowl flaps.

[Utah-Jay]

Nev

I have read a bit about shock cooling and watched some videos, it seems current belief is it’s a non-issue.

One example was there is no shock cooling when an engine is shut down in a cold climate after a flight.

I read enough to believe it’s a non-issue

I’m interested in other opinions

[dramsey]

The folks at GAMI have long contended its not an issue. The only source that I have that had evidence of it being an issue was the test pilot for Mooney aircraft at the time they were certifying the turbo charged "231". He was cracking cylinders during dives until they put a limit in place. So perhaps if very hot engine in a high speed aircraft that can route a lot of air through the cowl, then cracking might be an issue. Otherwise I would never worry about it.

Not exactly your question but there may be side effects due to running too cold all the time. The maker of camguard says that the chemistry in the combustion process to convert lead to lead oxide is an 8 step reaction whose rate is controlled by temperature. The higher the temperature the more likely for the reaction to complete to lead oxide, the intermidiary lead-oxybromide will plate out on "cooler" spots in the combustion chamber, namely valve stems. So running cooler may result in faster valve stem buildup and the associated problems. Its more a Lycoming issue that a Continental issue because of the sodium filled valves. Continental separately warns of running below 300 CHT in decent. I asked them the consequences of violating that, since I never ran above 300 except in the climb. They said the oil rings would seize. Damn if they didnt call it in advance. My oil rings seized on multiple cylinders within a very few hours of one another. I since took action to warm up my engine.

[Nev]

Very insightful thanks !

[Battson]

Specifically interest to know if anyone has cracked a cylinder on a Bearhawk?

Edit to add:

If anyone can add what change in pressure differential they are seeing between cowl flaps OPEN and CLOSED this would be very helpful.
If the change in pressure differential isn't available, what typical change in CHT's do you observe say in the cruise between cowl flaps OPEN and cowl flaps CLOSED ? This information would be very helpful for those of us adding cowl flaps.

It's about 15*C difference flaps open to flaps closed - taking at least 3 minutes to change temp and depending on airspeed a lot.

[Battson]

I question the validity of that ARTICLE. Here's a couple of example:

Thomas also pointed out that flying through rain reduces CHTs by nearly as much as a 50 percent power reduction

The engine manufacturer that has published data on the potential for shock-cooling damage—Lycoming—said to avoid the risk of damage, pilots should limit CHT reduction in flight to 50 degrees F per minute. The good news is that, even assuming such a rate of cooling will damage an engine—Lycoming said that damage potential existed only if done “consistently”—it’s nearly impossible to cool an engine that fast in flight even by shutting it down.

Clearly - both those statements are highly questionable, if not plain wrong:

• I have flown in rain for long periods of time, the temperature change is not that dramatic. Now, imagine dropping from 75% to 25% power, then nothing I have seen would corroborate that statement.
• I have a shock cooling alarm in my Dynon, which I originally set up for 50*F/min - however I had to relax it, because it was too easy to meet that cooling rate!

I know of a Cessna 185 that does short hops for a living, the maintenance team tell me it reliably cracks a cylinder approximately every 600 hours. Clearly there are many other examples. So we have evidence, and the physics are clear, it's definitely possible if the engines are mishandled consistently.

I have long been told that shock cooling is more common on TCM-styled engines, and this seems to be widespread common knowledge, but I work hard to avoid exposing my Lycoming to the risk all the same.

[Bissetg]

I think it might be a little confusingly worded. What I think he is saying is that if you are flying at 65% power and then reduce that by 50% to 32.5% power you would expect to see your CHT drop by about 10% or from say 390 F to 350 F, and that flying in rain would produce nearly that much effect.

[svyolo]

My piston experience is minimal, and in the distant past. But don't Lyc/TCM engines heat up faster than 50 deg/s after startup, and initial climb out? Why is "shock heating" any better/worse, than shock cooling?

[Battson]

Metal is a very good conductor. If the whole engine starts at room temperature and heats up together steadily - even if it heats up quickly - then everything expands at a similar rate. There is limited cooling airflow. The difference in temperature from the outside to the inside of the cylinder is not that large. It changes shape relatively uniformly, roughly speaking, based on its thermal coefficient of expansion. This means there is little strain within the metal, so there's not enough stress to crack the metal. Typically, we heat our engine up over 5 or 10 minutes from cold, allowing the heat to soak through the whole thing before applying full power.

When shock cooling happens we have a very hot cylinder inside, where the combustion and friction is happening, with all the cooling originating around the fins. The cooling at 130kts is very efficient, but the engine is still producing a lot of power / heat. So the same part is very hot inside and relatively cold on the outside, across the space of an inch or less. Remembering we don't measure the temperature at the outside edge of the cooling fins. This creates a huge thermal gradient across the cylinder, and some parts of the cylinder try to change shape in accordance with the material's thermal coefficient. That creates strain within the metal. That strain creates intra-material stresses. The stress is proportional to the thermal gradient, and can be powerful enough to cause crack growth if the thermal gradient is large enough, and if the stress is applied often enough (regular shock cooling). This is kind of similar to low cycle fatigue, except in this case it's caused by thermal strain. This force is felt most keenly at any sharp corners in the material - especially corners with sharp edges, like a thread - or where different materials (with different thermal coefficients) are forced together tightly. That is why the spark plug holes, with a torqued metal part inside the aluminium threaded hole, are one of the main areas prone to cracking. We borescope ours every 100 hours.

Because the metal is relatively hot, certain kinds of crack formation can occur more easily. This is quite a different cracking process at a microscopic level, compared to the cracking we are more familiar with on the airframe - such as high cycle fatigue cracking or stress corrosion cracking.

[svyolo]

Yeah. Plus add in the steel cylinder liner inside an aluminum cylinder can't help when there is temp gradient. I also think I remember from the distant past that # of thermal cycles affects air cooled cylinders more than water cooled ones.

[whee]

The Continental IO360 is known for being somewhat hard to cool and temperature sensitive. When my forged pistons failed I had to send out the cylinders for overhaul. 5 of them were cracked. No telling when it happened but the cylinder shop figured it was done prior to me. He couldn’t say it was caused by shock cooling but did say it’s a Continental thing, the IO360 is even worse but Lycomings do it too.

The cracks start at the spark plug hole. It makes since they start there; steel insert in an aluminum head with lots of heat cycling. Makes sense to me that abusive shock cooling could crack a cylinder.

Been too long since I paid attention but as I said in the other thread. My cowl flaps cool CHTs by 16-20F.

And an entertaining note to stir the pot; A friends flies charters in the Idaho mountains. He hates flying the turbo TSIO540 C206 and picks the IO550 C206 as often as he can. Idk what he hates about the Lycoming other that he said it eats cylinders. On one of his last flights in the TSIO540 plane he blew a head off a cylinder just as the wheels left the ground at a short, now where to go, strip. Even though the engine got him out of there on 5 cylinders, he really didn’t want to fly that plane after that. Maybe he’d rather unknowingly crack cylinders than lose one completely.

[TimTall]

Specifically interest to know if anyone has cracked a cylinder on a Bearhawk?

Edit to add:

If anyone can add what change in pressure differential they are seeing between cowl flaps OPEN and CLOSED this would be very helpful.
If the change in pressure differential isn't available, what typical change in CHT's do you observe say in the cruise between cowl flaps OPEN and cowl flaps CLOSED ? This information would be very helpful for those of us adding cowl flaps.

I'm seeing a 15 - 20 F difference in CHT's with the cowl flaps open in a climb. I've never measured the pressure differential but it is significant. At speeds over about 130 mph the top skin of my cowl gets sucked down by the vacuum created. That's a 0.032" skin with stringers 18" apart. It takes quite a bit of force to push it down by hand. My cowl flaps are 8" long by 6" wide and open to 30 degrees measured from the line of the lower cowl skin. Operated with a push-pull cable.

[Nev]

Tim, would it perhaps be the external air pressure pushing the top cowl down? My understanding is there should be a high pressure inside the top cowling, with the lower pressure being lower in the cowl.

[Battson]

I see the same. The 0.032 skin is sucked down by the flaps when open in the cruise. Huge suction force.

Nev - this suction is behind the top cowl plenum, above the accessory case.

[Nev]

Right - that's a lot of force then. I wonder what that equates to in inches of pressure differential. I don't see that on mine with the fixed exit lip even in a fast descent. The cowl flaps must be adding even more differential. I'm wondering if the location they are installed is having more effect than I'd anticipated.

[Bentmettle]

My piston experience is minimal, and in the distant past. But don't Lyc/TCM engines heat up faster than 50 deg/s after startup, and initial climb out? Why is "shock heating" any better/worse, than shock cooling?

Not specific to engines, it usually has something to do with the stack up off thermal masses and coefficient of expansions of the various parts. Also, the heat source from combustion is arguably very different than the heat sink of pulling heat out of the cylinders via cooling air, and the oil cooling system, etc. I can envision the sort of heat transfer circuit, but I don't have the experience or knowledge to speak beyond very general heat transfer concepts you'd use to start the analysis.

[Nev]

I have a shock cooling alarm in my Dynon, which I originally set up for 50*F/min - however I had to relax it, because it was too easy to meet that cooling rate!

I know of a Cessna 185 that does short hops for a living, the maintenance team tell me it reliably cracks a cylinder approximately every 600 hours. Clearly there are many other examples. So we have evidence, and the physics are clear, it's definitely possible if the engines are mishandled consistently.

This is interesting. So even with cowl flaps it's difficult to reduce the cooling rate, and in the case of the C185 it's still getting cylinder cracks. Do you think the cowl flaps are improving things in some way ? I'm trying to decide what to do next with mine, perhaps try another installation location.

[Battson]

I definitely rely on the cowl flaps to get that initial heat out of the cylinders pre-approach. As TimTall says, the cowl flaps create a huge suction force, compare to the smooth cowl with flaps shut. With the cowl flaps shut, there is very little cooling suction, we rely on the ram air from the prop.

With cowl flaps shut, and not having a cooling lip, I have to reduce power a loooooong way out to cool the engine slowly, which makes for a slow last 10 miles - it just feels laborious chopping power that far out. I am sure the ATPLs will laugh at that!

It is possible to supplement cowl flaps by leaning the mixture further (if LOP), or for the hottest summer days when I cruise with cowl flaps open - then mixture is all I have left. I am sure you are already familiar with that trick. In NJB, I cannot open cowl flaps and lean the mix at once without great care, otherwise I get the shock cooling alarm in a big way (i.e. across 5 or 6 cylinders). More often, it's just #2 or #5(?) which shock cool, being the hotter cylinders.

It would be interesting to rig a manometer so we can compare, however I won't have time to learn how to do that, and then do it, around holiday season.

I think the crux of the issue is: We are comparing a smooth cowl, to open cowl flaps. That is like comparing night and day. I believe you are comparing a cowl with a huge lip, to a cowl with a huge lip + open cowl flaps. The effect of the flaps is lost compared to the lip. More like comparing a day and a cloudy day, it is still bright outside.

[Nev]

Yeh the turbines are different for sure, ATC aside a normal descent profile is to reduce the power to idle at top of descent, effectively a glide approach until spool up when gear and flaps are out. Shock cooling is not an issue!

[kestrel]

One piece that seems to be missing from this conversation is the absolute cylinder temps. I've read numerous articles about how Lycoming's 500F limit is crazy because the aluminum loses some large percentage of its strength when that hot. The more common recommendation these days is more like 400F or maybe 420F in climbs. It has been my understanding that a hot cylinder is much more likely to crack from rapid cooling than one that is not so hot.

[TimTall]

Makes sense. The gradient between the super hot internal parts of the engine and the cooling air will be steeper the hotter the engine is.

[dramsey]

I think thats a good point. I believe its why the turbo engines are more prone to the issue. They run the hottest at altitude then you push them over and cool that irregular shape quickly and introduce all the subsequent stress in the metal. The starting point would seem to matter a lot. My Buddy who set a lot of crazy records flew over the North pole in January, The hottest he could get his cylinders was 150 F and the oil was 75 F. That engine was seeing a heck of a gradient, But it did not start from a high temp and cool. The engine now has over 2000 hrs on it and is still doing well.

[TimTall]

My original reason for installing the cowl flaps had as much to do with keeping the engine warm as it did with keeping it cool. Bear with me for the logic.

I live in the mountains, so I often must climb out steeply to clear terrain. If I'd built the cowl outlet large enough to handle the cooling demand of the steepest climb outs, I'd be way over cooled in cruise. The downside to that is unnecessary drag which costs speed and/or fuel. Not ideal but not damaging anything. Descent would be where the problem is. It would mean painfully slow descents to avoid shock cooling and dangerously low CHT’s at low power settings. Sometimes steep descent profiles are unavoidable in the mountains so I wanted to give myself as much as possible to work with in terms of control.

I didn’t reinvent the wheel for this. I crawled under all of the planes at my airport and copied what I thought would work. I’m basically running the same size, shape, and opening angle as a Cessna 206. I figured if it works in that thing with 300 hp it should work for me.

It works very well. I may write up an article or post explaining exactly how I did it and why if there was interest.

[Nev]

There's definitely interest on that article Tim.

[kestrel]

Waiting to read it!