Do you believe in shock cooling?
Thread Starter
Joined: Nov 2003
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From: Brazil
Do you believe in shock cooling?
http://www.avweb.com/news/maint/182883-1.html
I'm not certain if shock cooling is harmfull.
What do you think?
I'm not certain if shock cooling is harmfull.
What do you think?
Joined: Mar 2003
Posts: 71
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From: Chessington, UK
The author manages to make the non-existence of shock cooling sound plausible; however I suspect there is some subtlety in his carefully-chosen words.
For example, he says near the end: "Sudden cooling of an air-cooled engine does not contribute in any dramatic way to cylinder-head cracking" (my emphasis added).
I suspect his use of the word "dramatic" means that shock-cooling isn't going to crack your cylinder heads in an instant, catastrophic way. And he's probably right there, otherwise students in 152's would be falling out of the sky every day.
But fatigue cracking is very real over a long-ish period of time, and the more thermal cycles you give your engine the sooner this cracking will happen. Suddenly chopping the power will result in a "miniature" temperature cycle, and if you make a habit of it they'll all add up. I think there's little doubt that if you keep your engine's temperature as constant as possible it'll thank you in the end.
The author also asks: "Does anyone complain that repeated shutdowns are causing head cracking? Of course not."
Well actually, I would claim that repeated shutdowns contribute to cracking eventually. But shutting down the engine is unavoidable - you simply have to bear that pain. If you ran the engine continually without ever shutting it down, I bet it would keep going for at least double its TBO.
For example, he says near the end: "Sudden cooling of an air-cooled engine does not contribute in any dramatic way to cylinder-head cracking" (my emphasis added).
I suspect his use of the word "dramatic" means that shock-cooling isn't going to crack your cylinder heads in an instant, catastrophic way. And he's probably right there, otherwise students in 152's would be falling out of the sky every day.
But fatigue cracking is very real over a long-ish period of time, and the more thermal cycles you give your engine the sooner this cracking will happen. Suddenly chopping the power will result in a "miniature" temperature cycle, and if you make a habit of it they'll all add up. I think there's little doubt that if you keep your engine's temperature as constant as possible it'll thank you in the end.
The author also asks: "Does anyone complain that repeated shutdowns are causing head cracking? Of course not."
Well actually, I would claim that repeated shutdowns contribute to cracking eventually. But shutting down the engine is unavoidable - you simply have to bear that pain. If you ran the engine continually without ever shutting it down, I bet it would keep going for at least double its TBO.
Joined: Dec 2002
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From: London
Correct me if I'm wrong: When you reduce the power rapidly and subsequently, the engine cools rapidly, the engine may be damaged. So it obviously makes sense to reduce the power slowly. However, if you consider the drop in temperature that's incurred when switching off the engine, it is just as great, if not greater and occurs over a shorter period of time, so presumably this would cause the most damage and make shock cooling unavoidable if it really was a serious problem.
Joined: Mar 2003
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From: Chessington, UK
The issue isn't really one of "damaging" the engine - it's more like shortening its life (and I think this is the point that the author misses in his article). Eventual fatigue cracking is inevitable in practice, but with good handling (and a bit of luck) it shouldn't happen until after your engine has reached TBO.
Sudden cooling of the engine is unavoidable sometimes (like shutting down). It doesn't cause instant damage, but it does cause cumulative fatigue. You can't avoid the "damage" caused by switching the engine off, but you can avoid the damage caused by poor engine handling in flight.
The author uses some fairly twisted logic - he uses the fact that engines don't "fall apart" as proof that shock-cooling doesn't cause any damage. In reality, I think most engineers would agree that it's a case of gradual fatigue.
Sudden cooling of the engine is unavoidable sometimes (like shutting down). It doesn't cause instant damage, but it does cause cumulative fatigue. You can't avoid the "damage" caused by switching the engine off, but you can avoid the damage caused by poor engine handling in flight.
The author uses some fairly twisted logic - he uses the fact that engines don't "fall apart" as proof that shock-cooling doesn't cause any damage. In reality, I think most engineers would agree that it's a case of gradual fatigue.
Last edited by dmjw01; 28th December 2003 at 00:28.
Joined: Aug 2001
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From: UK Work: London. Home: East Anglia
I am not an engineer and have no claim to great mechanical wisdom, so have no answer to what will / won't damage any specific engine in any specific installation. But l'd like to point out a couple of undisputable facts about the comparison between and airborne throttle-back and a ground shutdown.
These two are not the same case, and inevitably have somewhat different effects on your oily bits, even though certain of the headline figures are similar.
Two significant differences:
1) In the airborne case there is a great deal of cooling airflow being forced over the fins, and that air is colder (often MUCH colder) than the ambient air on the ground. On the ground, there isn't. Furthermore, a nice cozy fug may build up in your cowling after you shut down, the warm engine will heat the air immediately around it. In the airborne case, the cold air constantly sweeps out any such warm air and replaces it with cold.
2) In the ground shutdown case, the engine is at rest, the bearing surfaces are not moving. In the airborne case, the engine may cool rapidly and all the parts are obliged to keep whirring around, going up and down, or whatever they do.
All other things being equal, Sod's Law and engineering realities seem to combine - the larger, more complicated and expensive an engine installation, the more prone it is to damage through improper handling.
These two are not the same case, and inevitably have somewhat different effects on your oily bits, even though certain of the headline figures are similar.
Two significant differences:
1) In the airborne case there is a great deal of cooling airflow being forced over the fins, and that air is colder (often MUCH colder) than the ambient air on the ground. On the ground, there isn't. Furthermore, a nice cozy fug may build up in your cowling after you shut down, the warm engine will heat the air immediately around it. In the airborne case, the cold air constantly sweeps out any such warm air and replaces it with cold.
2) In the ground shutdown case, the engine is at rest, the bearing surfaces are not moving. In the airborne case, the engine may cool rapidly and all the parts are obliged to keep whirring around, going up and down, or whatever they do.
All other things being equal, Sod's Law and engineering realities seem to combine - the larger, more complicated and expensive an engine installation, the more prone it is to damage through improper handling.
Joined: Nov 2000
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From: Pewsey, UK
I can't remember where I read it but a gliding club with a Cub or Pawnee or some other such type reduced its cylinder replacement rate drastically by modifying the technique of throttle and airspeed reduction at the end of a a tow - I think it may have been in Pilot or Flyer this last year.
Shock cooling - definitely alive and well. In piston engined helicopters a period of time after touchdown is required to allow the engine to cool and non-catastrophic but important problems have been observed if this is not done.
Shock cooling - definitely alive and well. In piston engined helicopters a period of time after touchdown is required to allow the engine to cool and non-catastrophic but important problems have been observed if this is not done.
Joined: May 2001
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Shock cooling is very real.
I have seen cracks down cylinders due to it.
It works on the same priciple of sticking a cold glass into hot water.
End in the airflow cools much faster than the other setting up stress through out the body of the engine. Glass is brittle so it goes with a bang. But the steel of the engine is alot more ductile so you get away with it the first few times.
Gliding tugs are famous for it.
Yes it does occur and it really shortens the usefull life of the engine.
MJ
I have seen cracks down cylinders due to it.
It works on the same priciple of sticking a cold glass into hot water.
End in the airflow cools much faster than the other setting up stress through out the body of the engine. Glass is brittle so it goes with a bang. But the steel of the engine is alot more ductile so you get away with it the first few times.
Gliding tugs are famous for it.
Yes it does occur and it really shortens the usefull life of the engine.
MJ
Joined: Jun 2003
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From: EuroGA.org
The only plausible evidence I have seen on this subject is the article probably referred to by TNF where they found that shock cooling did not matter so long as the CHT was below a certain value to start with.
Joined: Mar 2000
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From: TL487591
The impact of cooling has at least as much to do with even-ness as magnitude.
If it were to be possible to ensure that the entire engine cooled at a rapid, but uniform rate, the impact of shock cooling would be lessened. It is this, combined with the relatively even and low starting CHT that avoids cracking when the engine is shut down at the end of the flight.
By way of contrast, a large engine, throttled back at the top of a steep descent would bring about enormous differences of temperature between front and back cylinders, and a high probability of cracking.
The Avweb article is a nice bit of argumentativewriting that regularly surfaces on bulletin boards, but disbelieving in shock cooling is something I am only happy to do with somebody else's engines.
2D
If it were to be possible to ensure that the entire engine cooled at a rapid, but uniform rate, the impact of shock cooling would be lessened. It is this, combined with the relatively even and low starting CHT that avoids cracking when the engine is shut down at the end of the flight.
By way of contrast, a large engine, throttled back at the top of a steep descent would bring about enormous differences of temperature between front and back cylinders, and a high probability of cracking.
The Avweb article is a nice bit of argumentativewriting that regularly surfaces on bulletin boards, but disbelieving in shock cooling is something I am only happy to do with somebody else's engines.
2D
Joined: Mar 2000
Posts: 1,639
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From: TL487591
What exactly do you mean?
Disbelief in shock cooling is for fools, and those with unfeasibly large wallets. I would rather that anybody practised their disbelief on somebody else's engines than on my C421's rather delicate pair of geared Geared Continentals. I am not advocating the abuse of engines belonging to a third party...
Alles klar?
Joined: Apr 2000
Posts: 165
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From: UK
Mad Jock you say
" It works on the same priciple of sticking a cold glass into hot
water "
So what your saying is applying full power before a sufficient warm up time will cause the damage. I agree with you.
ie Shock Heating
Floppy
" It works on the same priciple of sticking a cold glass into hot
water "
So what your saying is applying full power before a sufficient warm up time will cause the damage. I agree with you.
ie Shock Heating
Floppy
Joined: Dec 2002
Posts: 14
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From: Crowthorne
You better believe in shock cooling damage or pay the price.
Lasham gliding club UK carried out research in this area and can prove it pays.
Basicaly always keep cylinder head temps under 400f and no sudden power reductons untill under 380f.
We have the artical on our web site to prove it.
http://www.horizonflyers.co.uk/Shock%20cooling.pdf
Lasham gliding club UK carried out research in this area and can prove it pays.
Basicaly always keep cylinder head temps under 400f and no sudden power reductons untill under 380f.
We have the artical on our web site to prove it.
http://www.horizonflyers.co.uk/Shock%20cooling.pdf
Joined: Oct 2001
Posts: 2,517
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From: Vancouver Island
Two donkeys:
I know you did not mean that you abuse engines...
I just didnt want it to look bad for you.
By the way just another note on this subject, in the engine manual from Wright for the R1820 they are very clear that power reductions be made slowly to prevent cylinder choking resulting in ring flex damage and piston scuffing. and caution you should exercise care in maintaining cyl temp and avoid rapid cyl. head temp. fluctuations.
Proper care of any engine is manditory, and thermal shock does damage engines....period...
Chuck
I know you did not mean that you abuse engines...
I just didnt want it to look bad for you.
By the way just another note on this subject, in the engine manual from Wright for the R1820 they are very clear that power reductions be made slowly to prevent cylinder choking resulting in ring flex damage and piston scuffing. and caution you should exercise care in maintaining cyl temp and avoid rapid cyl. head temp. fluctuations.
Proper care of any engine is manditory, and thermal shock does damage engines....period...
Chuck
Joined: Aug 2000
Posts: 2,681
Likes: 10
From: Just South of the last ice sheet
Here we go again........
Anybody who has owned an aircraft will attest to the wallet emptying powers of cracked cylinders.
So what cracks cylinders?
Numpties (muppets for those left of the Atlantic) who have been in a fast cruise (or other high cylinder head temperature regimen) and then close the throttle thus lowering the temperature in the cylinders and even worse, sometimes compound the situation by shoving the nose down and increasing the airflow over the, by now, rapidly cooling cylinder barrels. Unfortunately, the engine block is still nice and hot and the cylinder heads are relatively warm as well so you get a pair of steep temperature gradients across the cylinder barrel which meet in the middle. This causes fatigue. Indisputable. Ask anybody who has studied metallurgy. It's a fact. It doesn't go away. It's real. Do it brutally enough (it ain't hard to do) often enough and the cylinder(s) WILL CRACK.
It might not crack the first time you do it, it might not crack the next time you do it. It will crack sooner than it should though. It might even crack when you are asking for max power on the
climb out ......
PS Shutting down the engine on the ground is a whole different ballgame:
a) The lighter components like the cylinders soak up heat from the heavier and thus slower cooling chunks like the engine block so the whole mass of the engine is cooling at a relatively uniform rate. This assumes that there is a cowling over the engine.
b) Unless you have parked nose into a howling wind, there will be very little airflow through the cowling to change a) above.
c) You will have taxiied in (at low revs) and let the engine sit for a few minutes at 1,200 ish rpm to burn the lead off the plugs after the low rpm taxiing and also to equalise the temperatures in the engine.
Thus when the mixture is pulled or the mags switched off, there should be no part of the engine that is so much hotter than it's neighbour that it will create thermal shock between them.
Rant mode OFF,
Tablets taken,
Time for the darkened room
Anybody who has owned an aircraft will attest to the wallet emptying powers of cracked cylinders.
So what cracks cylinders?
Numpties (muppets for those left of the Atlantic) who have been in a fast cruise (or other high cylinder head temperature regimen) and then close the throttle thus lowering the temperature in the cylinders and even worse, sometimes compound the situation by shoving the nose down and increasing the airflow over the, by now, rapidly cooling cylinder barrels. Unfortunately, the engine block is still nice and hot and the cylinder heads are relatively warm as well so you get a pair of steep temperature gradients across the cylinder barrel which meet in the middle. This causes fatigue. Indisputable. Ask anybody who has studied metallurgy. It's a fact. It doesn't go away. It's real. Do it brutally enough (it ain't hard to do) often enough and the cylinder(s) WILL CRACK.
It might not crack the first time you do it, it might not crack the next time you do it. It will crack sooner than it should though. It might even crack when you are asking for max power on the
climb out ......
PS Shutting down the engine on the ground is a whole different ballgame:
a) The lighter components like the cylinders soak up heat from the heavier and thus slower cooling chunks like the engine block so the whole mass of the engine is cooling at a relatively uniform rate. This assumes that there is a cowling over the engine.
b) Unless you have parked nose into a howling wind, there will be very little airflow through the cowling to change a) above.
c) You will have taxiied in (at low revs) and let the engine sit for a few minutes at 1,200 ish rpm to burn the lead off the plugs after the low rpm taxiing and also to equalise the temperatures in the engine.
Thus when the mixture is pulled or the mags switched off, there should be no part of the engine that is so much hotter than it's neighbour that it will create thermal shock between them.
Rant mode OFF,
Tablets taken,
Time for the darkened room
Thread Starter
Joined: Nov 2003
Posts: 52
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From: Brazil
My SOP
My Standard Operational Procedure is:
For Bonanza A36:
At Top of Descent:
- I gently reduce the MP to 16 inches;
- reduce the RPM to 1800 RPM;
- maintain altitude while reducing speed;
- at 110 knots I put 500fpm on the VSI and let the speed increase slowly.
That works fine for a 500fpm and 130KIAS descent. For a 140-145 KIAS descent I reduce the RPM to 1900-2000, instead of 1800.
I think that's the least harmfull procedure for my engine.
For Bonanza A36:
At Top of Descent:
- I gently reduce the MP to 16 inches;
- reduce the RPM to 1800 RPM;
- maintain altitude while reducing speed;
- at 110 knots I put 500fpm on the VSI and let the speed increase slowly.
That works fine for a 500fpm and 130KIAS descent. For a 140-145 KIAS descent I reduce the RPM to 1900-2000, instead of 1800.
I think that's the least harmfull procedure for my engine.
Joined: Jul 2001
Posts: 486
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From: Belgium
Shock cooling is only possible if the cylinders get too hot to start with.
Good baffling will provide proper cooling at cruise airspeeds, properly set full rich mixture will provide cooling at take-off power and climb airspeeds.
I had a very good demonstration of this in my friends A36. He installed the GAMI Liquidair baffles. I feel these are the most engineered baffles you can find. In-flight testing was done by GAMI with tufted cylinders and under the hood videocamera as well as circumferential thermocouples measuring the temps at eight different points on the cylinder.
Result of the new baffles: dramatic drop in cruise temperatures.
Still remained the unacceptable high climb temps even at full rich, full MP. Switching the fuel pump on and then pulling the mixture knob as required solved the problem.
Previously the CHTs were climbing about 1°F per second as recorded on the JPI. With the increased full rich fuel flow, the climb temps stabilize at a given CHT temp and stay there. Simply the engines as they come from the factory are set too low for maximum full rich fuel flow. The redline on the fuel flow instrument is a minimum not a maximum.
On my own A36, CHT rarely go over 300°F, 320° maximum. Everything over 400°F is to be avoided, the redline CHTs the mfgrs give (460 TCM and 500 LYC) are just criminal.
If you are running the cylinders too hot (CHT) then cracked cylinders are to be expected.
By the way, EGT is NO indication of exhaust valve temperatures, CHT is.
Good baffling will provide proper cooling at cruise airspeeds, properly set full rich mixture will provide cooling at take-off power and climb airspeeds.
I had a very good demonstration of this in my friends A36. He installed the GAMI Liquidair baffles. I feel these are the most engineered baffles you can find. In-flight testing was done by GAMI with tufted cylinders and under the hood videocamera as well as circumferential thermocouples measuring the temps at eight different points on the cylinder.
Result of the new baffles: dramatic drop in cruise temperatures.
Still remained the unacceptable high climb temps even at full rich, full MP. Switching the fuel pump on and then pulling the mixture knob as required solved the problem.
Previously the CHTs were climbing about 1°F per second as recorded on the JPI. With the increased full rich fuel flow, the climb temps stabilize at a given CHT temp and stay there. Simply the engines as they come from the factory are set too low for maximum full rich fuel flow. The redline on the fuel flow instrument is a minimum not a maximum.
On my own A36, CHT rarely go over 300°F, 320° maximum. Everything over 400°F is to be avoided, the redline CHTs the mfgrs give (460 TCM and 500 LYC) are just criminal.
If you are running the cylinders too hot (CHT) then cracked cylinders are to be expected.
By the way, EGT is NO indication of exhaust valve temperatures, CHT is.
Joined: Dec 2001
Aviation Qualifications: ATPL
Posts: 3,766
Likes: 424
From: GA, USA
For all the infidels out there
Take a walk to your nearest engine shop/maintenance facility.
Take a good HARD look at an engine being taken apart.
All of the different components of the engine are of a different metal alloy because of the alloy properties better suited to whatever job the component does. They all contract and expand at different rates.
Pick up a prop and be amazed at how heavy it is.
Now this weight is swinging of the end of a very short crankshaft.
Imagine all the twisting forces when you jockey the throttle either open or closed.
That engine is keeping YOU from making a big smoking hole in the ground. It needs a little bit of respect and common sense.
I'm sorry but I cannot put it any simpler.
"Fly it like you own it" does not even work with some people..
Take a good HARD look at an engine being taken apart.
All of the different components of the engine are of a different metal alloy because of the alloy properties better suited to whatever job the component does. They all contract and expand at different rates.
Pick up a prop and be amazed at how heavy it is.
Now this weight is swinging of the end of a very short crankshaft.
Imagine all the twisting forces when you jockey the throttle either open or closed.
That engine is keeping YOU from making a big smoking hole in the ground. It needs a little bit of respect and common sense.
I'm sorry but I cannot put it any simpler.
"Fly it like you own it" does not even work with some people..