Shock Cooling a myth?

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Dronepiper
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Shock Cooling a myth?

Post by Dronepiper »

I am gonna open a can of worms.

We shut down engines every day and temps drop quicker than any rapid throttle closure would ever cause.

Thoughts?
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ahramin
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Re: Shock Cooling a myth?

Post by ahramin »

Data data data. We cannot make bricks without clay.

If you're saying that the CHTs come down faster after shutdown than at idle in flight, where are your graphs?
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telex
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Re: Shock Cooling a myth?

Post by telex »

Image
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Re: Shock Cooling a myth?

Post by digits_ »

Dronepiper wrote: Sun Jan 31, 2021 3:24 pm I am gonna open a can of worms.

We shut down engines every day and temps drop quicker than any rapid throttle closure would ever cause.

Thoughts?
If you are flying, you have high speed cold air flowing over the engines.
If you shut down stationairy, the warm air stays in the engine cowling. Also, here is no extra stress (compression) in the cylinders, trying to blow it apart.
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ahramin
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Re: Shock Cooling a myth?

Post by ahramin »

Here I'll help.

Here's pulling power to idle.
Idle Before.jpg
Idle Before.jpg (595.34 KiB) Viewed 3050 times
Idle After.jpg
Idle After.jpg (590.46 KiB) Viewed 3050 times
Smallest Δ°F/min 13, highest 30

Here's shutdown.
Shutdown Before.jpg
Shutdown Before.jpg (476.71 KiB) Viewed 3050 times
Shutdown After.jpg
Shutdown After.jpg (478.42 KiB) Viewed 3050 times
Smallest Δ°F/min 0, highest 5
Dronepiper wrote: Sun Jan 31, 2021 3:24 pm I am gonna open a can of worms.

We shut down engines every day and temps drop quicker than any rapid throttle closure would ever cause.
So do you actually have a can? Please justify your statement.
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Curiousflyer
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Re: Shock Cooling a myth?

Post by Curiousflyer »

Dronepiper wrote: Sun Jan 31, 2021 3:24 pm I am gonna open a can of worms.

We shut down engines every day and temps drop quicker than any rapid throttle closure would ever cause.

Thoughts?
Shock cooling is very subjective and you’re statement might be true for some aircraft but certainly not all.
Pressurized twin engine aircraft not so much. Top of descent from the flight levels CHT’s would drop dramatically if you pulled the throttle off even 20% you’d see a CHT drop of 100 degrees F within a few seconds. On a cold day just starting your descent was enough to drop CHT’s by 50 degrees from the increase in airspeed alone.
You’re premise is also completely flawed. Lycoming and Continental recommend leaving the engine run at idle, until temperatures stabilize before cutting the mixture off to shutdown. So to spare yourself sitting on the ramp with your engine running, just taxi ridiculously slowly if needed.
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digits_
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Re: Shock Cooling a myth?

Post by digits_ »

Cool!

Do you have any graphs for mixture usage?

For exmaple: going cruise lean to full rich at altitude

vs

going cruise lean descend and then going full rich at circuit altitude
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rookiepilot
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Re: Shock Cooling a myth?

Post by rookiepilot »

Shut down a turbo, car or plane, right after running it at high power enough times, and soon the very expensive answer to your question will be revealed....
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TrilliumFlt
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Re: Shock Cooling a myth?

Post by TrilliumFlt »

Ahramin, those graphs are fantastic very cool intel thank you for sharing it.
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ahramin
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Re: Shock Cooling a myth?

Post by ahramin »

digits_ wrote: Sun Jan 31, 2021 5:04 pm Cool!

Do you have any graphs for mixture usage?

For exmaple: going cruise lean to full rich at altitude

vs

going cruise lean descend and then going full rich at circuit altitude
Not really. EGT takes up to 30 seconds to stabilize and I can't think of anytime I would ever go full rich from cruise. I'm sure there's an example here and there of leaning in a step climb and then going rich before resuming climb but the power is going to be added as soon as the mixture is full rich so I don't know what the data would show. Here's the reverse though, fairly rich climb to lean cruise. The EGT increase is anywhere from 151°F to 215°F rich to lean.

leaning.jpg
leaning.jpg (600.67 KiB) Viewed 2956 times

Similarly I leave the mixture and prop back in the circuit and only go full rich full rpm on base or final after pulling power so it doesn't really show in the data. Is the EGT drop from the mixture, or the throttle?

Mixture Rich.jpg
Mixture Rich.jpg (497.88 KiB) Viewed 2956 times
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digits_
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Re: Shock Cooling a myth?

Post by digits_ »

Interesting! I was expecting a much bigger effect on the EGT gradient from the mixture than the throttle.
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Re: Shock Cooling a myth?

Post by altiplano »

It's been a long time, but in my day flying pistons stage cooling was largely about protecting the turbos, not so much the engines themselves.
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matt foley
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Re: Shock Cooling a myth?

Post by matt foley »

Most "data" is that handed down and repeated. "Fuel is cheaper than cylinders" and "1 inch per minute power reduction" and our all time favourite "Never run over square...that's what started the war of 1812 don't ya know!?". Blah blah blah. Maybe these myths all held water when they couldn't be contradicted because we ran with a stock Cessna EGT gauge circa 1930s technology but times have changed. As you see with the afore posted graphs, we have better technology to make ACCURATE power applications/reductions schedules, mixture leaning and power setting decisions. Personally my piston engine cooling is done via the engine monitor and varies tremendously from season to season and with sector profile. To say emphatically that "this will work all the time" is asinine. How many hours do we here have collectively flying a C-185 or 206 around repeating to ourselves like an insane version of Rain Man "15 gallons per hour in cruise, never over square, never never over square."? At any rate, I digress. To the original post: If the machine you fly doesn't have an engine monitor be it JPI, MVP50 etc then get one. And if it's not your airplane then get the owner on board. Failing that, 1 inches every 1 minute in decent and slowly rich to maintain smooth engine running will have to do.

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Re: Shock Cooling a myth?

Post by photofly »

What is the purported damage to cylinders from rapid cooling?
FTU airplanes go from full power to idle and back to full power repeatedly, many flights per day. There's no great prevalence of cooling-related issues that I know of.
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Re: Shock Cooling a myth?

Post by Kaykay »

photofly wrote: Mon Feb 01, 2021 5:36 am What is the purported damage to cylinders from rapid cooling?
FTU airplanes go from full power to idle and back to full power repeatedly, many flights per day. There's no great prevalence of cooling-related issues that I know of.
Because most FTUs don’t use airplanes where it matters quite so much. Beat the crap out of a Navajo at an FTU and you’d have plenty of issues.
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Re: Shock Cooling a myth?

Post by rookiepilot »

photofly wrote: Mon Feb 01, 2021 5:36 am What is the purported damage to cylinders from rapid cooling?
FTU airplanes go from full power to idle and back to full power repeatedly, many flights per day. There's no great prevalence of cooling-related issues that I know of.
0-320's don't run nearly as hot as an 0-540 or 550, never mind a turbo ---
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Re: Shock Cooling a myth?

Post by digits_ »

photofly wrote: Mon Feb 01, 2021 5:36 am What is the purported damage to cylinders from rapid cooling?
FTU airplanes go from full power to idle and back to full power repeatedly, many flights per day. There's no great prevalence of cooling-related issues that I know of.
You'd expect no issues when going full power to idle on a -35C day in a C172?

Most FTUs in the prairies pay attention to it. I know of at least 2 cracked cylinders on such cold days. It might not have been shock cooling, didn't know the details, but it sounded very plausible.
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Re: Shock Cooling a myth?

Post by fleetcanuck »

You can also induce shock cooling just going from climb power back to cruise power too quickly.
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Re: Shock Cooling a myth?

Post by CpnCrunch »

fleetcanuck wrote: Mon Feb 01, 2021 10:29 am You can also induce shock cooling just going from climb power back to cruise power too quickly.
You can see in ahramin's graph's above, he went from climb to cruise within 15 seconds or less, and no shock cooling. I've never seen any shock cooling warnings when leveling off and reducing power with an O-360. It is only when reducing power to idle at high speed that I have noticed warnings (50F+/min), which I don't do any more. Lycoming recommends avoiding more than 50F/min of cooling:

https://www.lycoming.com/content/how-av ... 0cylinders.
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Re: Shock Cooling a myth?

Post by photofly »

rookiepilot wrote: Mon Feb 01, 2021 9:33 am
photofly wrote: Mon Feb 01, 2021 5:36 am What is the purported damage to cylinders from rapid cooling?
FTU airplanes go from full power to idle and back to full power repeatedly, many flights per day. There's no great prevalence of cooling-related issues that I know of.
0-320's don't run nearly as hot as an 0-540 or 550, never mind a turbo ---
Not talking about turbochargers, we are talking about cylinders. My IO-470 runs at the temperature I want - 350. So does my O-320. Plenty of small engines have higher CHTs. I fly one O-360 that gets above 380 if you don’t take care.
You'd expect no issues when going full power to idle on a -35C day in a C172?
so now we’re putting a box around it. Do you still have to worry about it if you’re flying at +10? 0?

It’s hard to practice forced approaches or if you’re going to take four minutes to close the throttle, especially if you’re an advocate of “surprise”. Or power-off stalls. Or spins. I’m just curious why it’s not a huge enormous deal, considering FTUs do this dozens of times per day. Multiple times per flight. You might expect to see cylinders lined up a dozen at a time. Waiting to be replaced.
Because most FTUs don’t use airplanes where it matters quite so much. Beat the crap out of a Navajo at an FTU and you’d have plenty of issues.
I don’t really understand this comment. If the airplane is used for revenue it doesn’t really matter what type it is, or where: having it out of service matters. The metal the cylinders are cast from and the temperatures are the same, and they need to replaced for all the same reasons.
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