Shock Cooling our Gas engines.....
02-01-2010, 04:46 PM
#1
Thread Starter
Shock Cooling our Gas engines.....
On full scale engines, we are very careful about chopping the throttle when the cylinder head temperature is at normal operating temperature. Jerking the engine back to idle from cruise does indeed shock cool the engine, and does have some ramifications to the engine itself. Most of us are hesitant about simulating an engine out procedure on our aircraft. Are we doing this to our model engines as well (with most of us never knowing what we are doing)?
Bill, Waco Brother #1
Bill, Waco Brother #1
02-01-2010, 05:06 PM
#3
RE: Shock Cooling our Gas engines.....
Keep in mind that the small trainers that they do simulated engine out proceedures have low performance engines which will not shock cool at lower altitudes and non winter weather.
What brought this up? I don't normally post in the gas section. I thought this was in glow.
What brought this up? I don't normally post in the gas section. I thought this was in glow.
02-01-2010, 05:36 PM
#4
Thread Starter
RE: Shock Cooling our Gas engines.....
ORIGINAL: Sport_Pilot
Keep in mind that the small trainers that they do simulated engine out proceedures have low performance engines which will not shock cool at lower altitudes and non winter weather.
What brought this up? I don't normally post in the gas section. I thought this was in glow.
Keep in mind that the small trainers that they do simulated engine out proceedures have low performance engines which will not shock cool at lower altitudes and non winter weather.
What brought this up? I don't normally post in the gas section. I thought this was in glow.
Bill, Waco Brother #1
02-01-2010, 05:44 PM
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RE: Shock Cooling our Gas engines.....
I chop the throttle all the time on my planes. both glow and gas power. especially my aerobatic planes, I haven't noticed any problems.
02-01-2010, 05:47 PM
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RE: Shock Cooling our Gas engines.....
In any engine when hot, and the cylinder cools faster than the piston, there is a risk of piston seizure. I have however never seen reports of it concerning model airplane engines, though they should be prone due to prop windmilling. Probably the problem does not existbecause of two educated guesses:
1) when the engine gets very hot, it is due to lack of proper cooling, and there will be no "shock cooling" when the throttle is cut.
2) with proper cooling, the engine stays quite cool, and the risk is small, maybe even nonexistant. When the prop is not driven anymore, cooling air extraction by the propwash and plane speed stops quite quickly as well, and the cooling down is retarded enough to avoid seizure.
1) when the engine gets very hot, it is due to lack of proper cooling, and there will be no "shock cooling" when the throttle is cut.
2) with proper cooling, the engine stays quite cool, and the risk is small, maybe even nonexistant. When the prop is not driven anymore, cooling air extraction by the propwash and plane speed stops quite quickly as well, and the cooling down is retarded enough to avoid seizure.
ORIGINAL: Stickbuilder
On full scale engines, we are very careful about chopping the throttle when the cylinder head temperature is at normal operating temperature. Jerking the engine back to idle from cruise does indeed shock cool the engine, and does have some ramifications to the engine itself. Most of us are hesitant about simulating an engine out procedure on our aircraft. Are we doing this to our model engines as well (with most of us never knowing what we are doing)?
Bill, Waco Brother #1
On full scale engines, we are very careful about chopping the throttle when the cylinder head temperature is at normal operating temperature. Jerking the engine back to idle from cruise does indeed shock cool the engine, and does have some ramifications to the engine itself. Most of us are hesitant about simulating an engine out procedure on our aircraft. Are we doing this to our model engines as well (with most of us never knowing what we are doing)?
Bill, Waco Brother #1
02-01-2010, 06:41 PM
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RE: Shock Cooling our Gas engines.....
My personal belief on this is that we run our engines at significantly lower CHT's than full scale planes. In full-scale, you have MUCH better readings of your CHT (ie: in flight, constantly, and always assumed to be correct). You can change your mixture constantly to get the best combination of fuel efficiency and safety possible, even though the full-scale CHT range for optimum performance is higher than our engines. I think that's due to a higher precision used when manufacturing full-scale engines compared to ours. Also, I believe that full-scale runs at a much leaner level than R/C generally does (for fuel efficiency), and full scale pilots can get away with it due to (several things, but mostly) better cooling systems (and better cooling from higher speed). It also may be a difference between 2-stroke and 4-stroke engines. Also, our models normally fly for 10-15min....whereas full-scale engines run for hours on end.
I don't think there's a single answer to why, but a lot of little factors. I also think that Pe and everyone else in here have contributed plausible explanations for, even if only part of, the reason why we don't worry about shock cooling.
I don't think there's a single answer to why, but a lot of little factors. I also think that Pe and everyone else in here have contributed plausible explanations for, even if only part of, the reason why we don't worry about shock cooling.
02-01-2010, 09:11 PM
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RE: Shock Cooling our Gas engines.....
It must not be a major problem with 2 strokes, ever chainsaw etc goes from WOT to idle all day everyday. The same applies to IMAC and 3D.
02-01-2010, 09:18 PM
#10
RE: Shock Cooling our Gas engines.....
It is not just going to idle it is doing so with a high speed cold air blast on the engine, hense the shock. The fronts of the cylinders cool faster than the rears. Our model engines have a much higher thickness to size ratio verses a real aero engine. The jugs on them are built very light weight and the walls are made as thin as possible while still being reliable. It is the light construction combined with suddenly going from hot to freezing cold that harms them..something we will never have to worry about with our little two strokes. There is a good short explaination here:
http://www.ramaircraft.com/Maintenan...ck-Cooling.htm
http://www.ramaircraft.com/Maintenan...ck-Cooling.htm
02-01-2010, 09:47 PM
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RE: Shock Cooling our Gas engines.....
Yeah, I guess the saw example wasn't too good. Our (my) little 2 strokes go from a short WOT up-lines to a short idling down lines many times each flight.. I know our baffling is not nearly as efficient as on full scale, which might have some affect on prolonged descents.
02-01-2010, 10:31 PM
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RE: Shock Cooling our Gas engines.....
The OP has an interesting thought. The common problem with shock cooling in full scale is with high performance airplane that can fly at high altitutes (turbo normalized). The problem is that you are cruizing along at a high power setting in very cold air and if you don't plan your descent well you have to pull too much power out and "glide" down with no power thus shock cooling the motor and decrease it's life expectancy. The shock cooling doesn't happen immediately, but over time throughout the descent.
I think in general we abuse the crap out of our rc engines, however I don't think it's the shock cooling that does it. For one reason is that you can't pull back the power very long before you will have to reunite with terra firma. You can even take a look full scale aerobatic airshow perfomances. How do they treat their engines? It doesn't seem to me that they are worried about shock cooling. It is not like the full scale which a descent from 12000ft can put 20 minutes of shock cooling on the engine. Having said that if we could start our engines and slowly bring them up to temp and fly them for hours while slowly reducing power and finally landing my bet would be we could put a ton more hours on these engines than we do.
One of the thing I see at my field that I think destroys engines is the guys that take thier cold engines and start them and go to full power immediately.
There are tons of things we do that shorten the life of these engines. I don't worry about it because I fly RC for fun and it would not be fun flying RC with the same discipline as flying full scale. It would be purpose defeating.
I think in general we abuse the crap out of our rc engines, however I don't think it's the shock cooling that does it. For one reason is that you can't pull back the power very long before you will have to reunite with terra firma. You can even take a look full scale aerobatic airshow perfomances. How do they treat their engines? It doesn't seem to me that they are worried about shock cooling. It is not like the full scale which a descent from 12000ft can put 20 minutes of shock cooling on the engine. Having said that if we could start our engines and slowly bring them up to temp and fly them for hours while slowly reducing power and finally landing my bet would be we could put a ton more hours on these engines than we do.
One of the thing I see at my field that I think destroys engines is the guys that take thier cold engines and start them and go to full power immediately.
There are tons of things we do that shorten the life of these engines. I don't worry about it because I fly RC for fun and it would not be fun flying RC with the same discipline as flying full scale. It would be purpose defeating.
02-02-2010, 07:48 AM
#13
RE: Shock Cooling our Gas engines.....
http://en.wikipedia.org/wiki/Shock_c..._%28engines%29
There are a few interesting links at the bottom of the page.
There are a few interesting links at the bottom of the page.
02-02-2010, 08:45 AM
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RE: Shock Cooling our Gas engines.....
Seems to me (and this is just a wild guess here) that there are a couple of variables at play that differ greatly:
1) The outside ambient temp is considerably lower when we pull this stunt in a full scale bird
2) Our models spend considerably less time in a "in flight at idle" condition than their the full scale counterparts do.
I would GUESS that both of these conditions would make it far less likely that a significant temperature difference between piston and cylinder could ever develop.
1) The outside ambient temp is considerably lower when we pull this stunt in a full scale bird
2) Our models spend considerably less time in a "in flight at idle" condition than their the full scale counterparts do.
I would GUESS that both of these conditions would make it far less likely that a significant temperature difference between piston and cylinder could ever develop.
02-02-2010, 10:10 AM
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RE: Shock Cooling our Gas engines.....
agreed. PLUS, our engines have a closer piston fit that allows better heat transfer and so equals the temperature differences. We also need not worry about cracking of parts with our small capacity two stroke cylinders.
Myth busted?
Myth busted?
02-02-2010, 11:02 AM
#16
RE: Shock Cooling our Gas engines.....
I wouldn't worry 'bout it on model engines for reasons already mentioned.
I will say that in all the years I worked on full scale piston stuff I don't recall ever seeing a normally aspirated engine come in the shop with evidence of shock cooling damage, i.e. cracked cylinder heads.
You do see 'em with evidence of folks trying to get too busy with the red knob either through a desire to economize or lack of attention and they pay for it at annual or 100 hour inspections if those are done.
Now turbocharged piston engine aircraft are a whole different story; higher engine output at altitude where it's much colder air, tighter/better cooling systems and engine baffling contribute to the issue. The most sensitive engines I was ever around for this shock cooling issue are the big GTSIO-520 Continentals used on Cessna 421's. Geared engines so they turn higher RPM's, lots of boost out of the turbos and you have to handle those puppies with kid gloves if you want the cylinders to live. What really gets most of these is cruising along at enroute altitude and requesting lower as you approach the destination airport and Center won't comply. With a 421 you better be thinking about that 80 to 100 miles out and Center doesn't want to cooperate, they keep you high until you're 20 miles out and then want you to drop like a turbine.
Doesn't work out well....
I will say that in all the years I worked on full scale piston stuff I don't recall ever seeing a normally aspirated engine come in the shop with evidence of shock cooling damage, i.e. cracked cylinder heads.
You do see 'em with evidence of folks trying to get too busy with the red knob either through a desire to economize or lack of attention and they pay for it at annual or 100 hour inspections if those are done.
Now turbocharged piston engine aircraft are a whole different story; higher engine output at altitude where it's much colder air, tighter/better cooling systems and engine baffling contribute to the issue. The most sensitive engines I was ever around for this shock cooling issue are the big GTSIO-520 Continentals used on Cessna 421's. Geared engines so they turn higher RPM's, lots of boost out of the turbos and you have to handle those puppies with kid gloves if you want the cylinders to live. What really gets most of these is cruising along at enroute altitude and requesting lower as you approach the destination airport and Center won't comply. With a 421 you better be thinking about that 80 to 100 miles out and Center doesn't want to cooperate, they keep you high until you're 20 miles out and then want you to drop like a turbine.
Doesn't work out well....
02-02-2010, 01:49 PM
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RE: Shock Cooling our Gas engines.....
ORIGINAL: ndb8fxe
The OP has an interesting thought. The common problem with shock cooling in full scale is with high performance airplane that can fly at high altitutes (turbo normalized). The problem is that you are cruizing along at a high power setting in very cold air and if you don't plan your descent well you have to pull too much power out and ''glide'' down with no power thus shock cooling the motor and decrease it's life expectancy. The shock cooling doesn't happen immediately, but over time throughout the descent.
I think in general we abuse the crap out of our rc engines, however I don't think it's the shock cooling that does it. For one reason is that you can't pull back the power very long before you will have to reunite with terra firma. You can even take a look full scale aerobatic airshow perfomances. How do they treat their engines? It doesn't seem to me that they are worried about shock cooling. It is not like the full scale which a descent from 12000ft can put 20 minutes of shock cooling on the engine. Having said that if we could start our engines and slowly bring them up to temp and fly them for hours while slowly reducing power and finally landing my bet would be we could put a ton more hours on these engines than we do.
One of the thing I see at my field that I think destroys engines is the guys that take thier cold engines and start them and go to full power immediately.
There are tons of things we do that shorten the life of these engines. I don't worry about it because I fly RC for fun and it would not be fun flying RC with the same discipline as flying full scale. It would be purpose defeating.
The OP has an interesting thought. The common problem with shock cooling in full scale is with high performance airplane that can fly at high altitutes (turbo normalized). The problem is that you are cruizing along at a high power setting in very cold air and if you don't plan your descent well you have to pull too much power out and ''glide'' down with no power thus shock cooling the motor and decrease it's life expectancy. The shock cooling doesn't happen immediately, but over time throughout the descent.
I think in general we abuse the crap out of our rc engines, however I don't think it's the shock cooling that does it. For one reason is that you can't pull back the power very long before you will have to reunite with terra firma. You can even take a look full scale aerobatic airshow perfomances. How do they treat their engines? It doesn't seem to me that they are worried about shock cooling. It is not like the full scale which a descent from 12000ft can put 20 minutes of shock cooling on the engine. Having said that if we could start our engines and slowly bring them up to temp and fly them for hours while slowly reducing power and finally landing my bet would be we could put a ton more hours on these engines than we do.
One of the thing I see at my field that I think destroys engines is the guys that take thier cold engines and start them and go to full power immediately.
There are tons of things we do that shorten the life of these engines. I don't worry about it because I fly RC for fun and it would not be fun flying RC with the same discipline as flying full scale. It would be purpose defeating.
Several years ago I read an article I think was on the EAA site or AOPA site or maybe Light Plane Maintenance, I can't remember which, by several engineers which proved mathematically that it is impossible to shock cool an engine. The math went way over my head but it was the consensus of those involved that it could not be done. They were too many btu's even at idle to accomplish any sort of "shock" cooling.
Not trying to start a fuss just passing on what I had read.
02-02-2010, 01:53 PM
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RE: Shock Cooling our Gas engines.....
Indeede some consider it a myth.
Also, some engineers have calculated the bumblebee's inability to take off in flight. So I hardly ever say never.
Also, some engineers have calculated the bumblebee's inability to take off in flight. So I hardly ever say never.
02-02-2010, 02:39 PM
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RE: Shock Cooling our Gas engines.....
ORIGINAL: 82blackbird
Several years ago I read an article I think was on the EAA site or AOPA site or maybe Light Plane Maintenance, I can't remember which, by several engineers which proved mathematically that it is impossible to shock cool an engine. The math went way over my head but it was the consensus of those involved that it could not be done. They were too many btu's even at idle to accomplish any sort of ''shock'' cooling.
Not trying to start a fuss just passing on what I had read.
ORIGINAL: ndb8fxe
The OP has an interesting thought. The common problem with shock cooling in full scale is with high performance airplane that can fly at high altitutes (turbo normalized). The problem is that you are cruizing along at a high power setting in very cold air and if you don't plan your descent well you have to pull too much power out and ''glide'' down with no power thus shock cooling the motor and decrease it's life expectancy. The shock cooling doesn't happen immediately, but over time throughout the descent.
I think in general we abuse the crap out of our rc engines, however I don't think it's the shock cooling that does it. For one reason is that you can't pull back the power very long before you will have to reunite with terra firma. You can even take a look full scale aerobatic airshow perfomances. How do they treat their engines? It doesn't seem to me that they are worried about shock cooling. It is not like the full scale which a descent from 12000ft can put 20 minutes of shock cooling on the engine. Having said that if we could start our engines and slowly bring them up to temp and fly them for hours while slowly reducing power and finally landing my bet would be we could put a ton more hours on these engines than we do.
One of the thing I see at my field that I think destroys engines is the guys that take thier cold engines and start them and go to full power immediately.
There are tons of things we do that shorten the life of these engines. I don't worry about it because I fly RC for fun and it would not be fun flying RC with the same discipline as flying full scale. It would be purpose defeating.
The OP has an interesting thought. The common problem with shock cooling in full scale is with high performance airplane that can fly at high altitutes (turbo normalized). The problem is that you are cruizing along at a high power setting in very cold air and if you don't plan your descent well you have to pull too much power out and ''glide'' down with no power thus shock cooling the motor and decrease it's life expectancy. The shock cooling doesn't happen immediately, but over time throughout the descent.
I think in general we abuse the crap out of our rc engines, however I don't think it's the shock cooling that does it. For one reason is that you can't pull back the power very long before you will have to reunite with terra firma. You can even take a look full scale aerobatic airshow perfomances. How do they treat their engines? It doesn't seem to me that they are worried about shock cooling. It is not like the full scale which a descent from 12000ft can put 20 minutes of shock cooling on the engine. Having said that if we could start our engines and slowly bring them up to temp and fly them for hours while slowly reducing power and finally landing my bet would be we could put a ton more hours on these engines than we do.
One of the thing I see at my field that I think destroys engines is the guys that take thier cold engines and start them and go to full power immediately.
There are tons of things we do that shorten the life of these engines. I don't worry about it because I fly RC for fun and it would not be fun flying RC with the same discipline as flying full scale. It would be purpose defeating.
Several years ago I read an article I think was on the EAA site or AOPA site or maybe Light Plane Maintenance, I can't remember which, by several engineers which proved mathematically that it is impossible to shock cool an engine. The math went way over my head but it was the consensus of those involved that it could not be done. They were too many btu's even at idle to accomplish any sort of ''shock'' cooling.
Not trying to start a fuss just passing on what I had read.
I agree with Zeeb that the 421 engine were particularily suseptable to throttle imputs though not necissarily for shock cooling, but the gearing setup made it "sensitive" to large/fast throttle movements.
As far as the reason our engines get much less time than full scale, I believe this is for several reasons:
1. Quality of design/enginering/ 2s v. 4s, etc.
2. Run times. I bet we could quadruple(or more) the hours we could get out of our engines if our flights were 4 hrs at a time rather than 10 mins at a time.
02-02-2010, 02:42 PM
#20
RE: Shock Cooling our Gas engines.....
ORIGINAL: pe reivers
Indeede some consider it a myth.
Also, some engineers have calculated the bumblebee's inability to take off in flight. So I hardly ever say never.
Indeede some consider it a myth.
Also, some engineers have calculated the bumblebee's inability to take off in flight. So I hardly ever say never.
I vote for a can of compressed liquid nitrogen
02-02-2010, 03:14 PM
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RE: Shock Cooling our Gas engines.....
ORIGINAL: Super08
So now I guess the question is....How do you shock cool a bumblebee?
I vote for a can of compressed liquid nitrogen
ORIGINAL: pe reivers
Indeede some consider it a myth.
Also, some engineers have calculated the bumblebee's inability to take off in flight. So I hardly ever say never.
Indeede some consider it a myth.
Also, some engineers have calculated the bumblebee's inability to take off in flight. So I hardly ever say never.
I vote for a can of compressed liquid nitrogen
02-02-2010, 03:57 PM
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RE: Shock Cooling our Gas engines.....
Cost me over $13,000.00 to rebuild the engine in my 152 after a cracked head was discovered. Until it was used a rental on lease back it never had a problem, but all the student pilot renters and their instructors always went from wot cruise to 1,300 rpm for the descent when they flew it. None of them knew how to push the nose over and only slightly reduce power to keep the cht's and speed up. Not a high performance engine but the consensus among the A&P's and I/A's was shock cooling. 1,100 hours on the engine when it was found during a 100 hour inspection.
02-02-2010, 03:59 PM
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RE: Shock Cooling our Gas engines.....
ORIGINAL: Super08
So now I guess the question is....How do you shock cool a bumblebee?
I vote for a can of compressed liquid nitrogen
ORIGINAL: pe reivers
Indeede some consider it a myth.
Also, some engineers have calculated the bumblebee's inability to take off in flight. So I hardly ever say never.
Indeede some consider it a myth.
Also, some engineers have calculated the bumblebee's inability to take off in flight. So I hardly ever say never.
I vote for a can of compressed liquid nitrogen
Now I like that!!
Another thing to consider, real airplanes drop thousands of feed at a lower power setting at wide temperature differentials while models reduce power and drop only hundreds of feet at basically the same temperature.
02-02-2010, 04:25 PM
#24
RE: Shock Cooling our Gas engines.....
but the consensus among the A&P's and I/A's was shock cooling. 1,100 hours on the engine when it was found during a 100 hour inspection.
02-02-2010, 04:33 PM
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RE: Shock Cooling our Gas engines.....
Zeeb, I know what you mean. We had a 421 and shock cooling was a huge fear of my Dad's (its primary pilot). We had to plan our approaches before take off to make sure it would be okay. The gearbox was also a consideration. Regardless, they both contributed to the huge amount of concern with CHT and throttle settings.