Shock Cooling - Myth Busted!!
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With the first air cooled cylinders visible through the front of the engine cowls in Chieftains and their ilk, I often wonder why flying through heavy rain does apparently not cause shock cooling as the rain hits the cylinders.
Of course, radial engines such as those in the DC3, Sea Fury, Beaver, DC4 and similar would also be affected by rain impinging on the first row of cylinders causing significant cooling and quickly at that. Would that be termed `shock cooling`? Were cylinders cracking after flights in wet weather or large water droplets found in towering Cu?
Of course, radial engines such as those in the DC3, Sea Fury, Beaver, DC4 and similar would also be affected by rain impinging on the first row of cylinders causing significant cooling and quickly at that. Would that be termed `shock cooling`? Were cylinders cracking after flights in wet weather or large water droplets found in towering Cu?
Last edited by A37575; 2nd Sep 2012 at 02:13.
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A37575
It does do things to the F/A ratio a bit.
You would also be surprised as to where the air entering the cowls actually goes, and does not.
Hard to describe in words but the bulk air travels in around the edge of the inlet, travels down the back and returns to the front, trying to escape behind the spinner. One of the reasons why a small amount of oil spilled soon ends evenly spread across your windscreen.
Basically, what you think would happen doesn't and in fact quite the opposite. Of course airframe to airframe variances apply so get some tufts of wool out and a tiny camera or 4 and video away!
It does do things to the F/A ratio a bit.
You would also be surprised as to where the air entering the cowls actually goes, and does not.
Hard to describe in words but the bulk air travels in around the edge of the inlet, travels down the back and returns to the front, trying to escape behind the spinner. One of the reasons why a small amount of oil spilled soon ends evenly spread across your windscreen.
Basically, what you think would happen doesn't and in fact quite the opposite. Of course airframe to airframe variances apply so get some tufts of wool out and a tiny camera or 4 and video away!
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I think your point was missed indeed.
I will have to search through a pile of data to find some better examples.
Thermal inertia is hard to overcome and hence why shock cooling is very hard to achieve, if in fact it exists.
The point to cylinder cracking is all about pressure and temperature. So it should be considered that all the damage reffered to is when the pressure pulses are high and temperature is high.
This issue of temperature difference is rather funny. The temperature at the tip of a fin is way less than that at the base, but the fins do ot all fall off on the first engine start do they?
So the worst things are high temp and high ICP. I have seen this on George Braly's dyno and lets just say, we made jokes about hiding behind not just the wall, but the Control Console as well!
So when do you think the slightly less abusive instances of high pressure and temperature happen? When the ag guys are taking the load OUT, when the grass darts are being taken up.
Most pilots flying most planes do not do this. And that is why they do not have the problems.
I was sent recently pictures of a G36....yes G36 bonanza engine with 800 hours on it. Lots of cracks, all around the plugs, and while the early guess is plug torque, it is most likely a manufacturing defect in all of them.
My bet is if I get my boroscope onto this beast some time soon, the LAME's and the owner will be shocked.....not shock cooled, but shock heated into new cylinders! And not the cracks, the likely crappy valve guide fitting! Just a hunch, but I bet a carton on it.
Anyway...as clinton points out, the data after shut down is not often considered. I have data which varies from 12F/min to a lot less. This will depend on ambient, airflow, the airframe etc. Some will be higher, some not so much, but of course these are at nil ICP
The bottom line is a start up the Delta T is often at 60F/min and there is mild ICP. So perhaps that is worse by far, but I never hear anyone get bent out of shape over SHOCK WARMING
I have hours and hours of really cool data, and I can assure you that folk like John Deakin love it. Why you ask? no not because it comes from a land upside down, but it backs up all they have been saying for years. It also combines flight data not just engine data. And that further proves what John Deakin, George Braly and Walter Atkinson have been teaching for the last 12-13 years.
So lets not get bogged down in Jaba's data files, lets learn from the guys who have been teaching this stuff for years. My data is just icing on the cake
Any sensible questions greatly appreciated.
I think your point was missed indeed.
I will have to search through a pile of data to find some better examples.
Thermal inertia is hard to overcome and hence why shock cooling is very hard to achieve, if in fact it exists.
The point to cylinder cracking is all about pressure and temperature. So it should be considered that all the damage reffered to is when the pressure pulses are high and temperature is high.
This issue of temperature difference is rather funny. The temperature at the tip of a fin is way less than that at the base, but the fins do ot all fall off on the first engine start do they?
So the worst things are high temp and high ICP. I have seen this on George Braly's dyno and lets just say, we made jokes about hiding behind not just the wall, but the Control Console as well!
So when do you think the slightly less abusive instances of high pressure and temperature happen? When the ag guys are taking the load OUT, when the grass darts are being taken up.
Most pilots flying most planes do not do this. And that is why they do not have the problems.
I was sent recently pictures of a G36....yes G36 bonanza engine with 800 hours on it. Lots of cracks, all around the plugs, and while the early guess is plug torque, it is most likely a manufacturing defect in all of them.
My bet is if I get my boroscope onto this beast some time soon, the LAME's and the owner will be shocked.....not shock cooled, but shock heated into new cylinders! And not the cracks, the likely crappy valve guide fitting! Just a hunch, but I bet a carton on it.
Anyway...as clinton points out, the data after shut down is not often considered. I have data which varies from 12F/min to a lot less. This will depend on ambient, airflow, the airframe etc. Some will be higher, some not so much, but of course these are at nil ICP
The bottom line is a start up the Delta T is often at 60F/min and there is mild ICP. So perhaps that is worse by far, but I never hear anyone get bent out of shape over SHOCK WARMING
I have hours and hours of really cool data, and I can assure you that folk like John Deakin love it. Why you ask? no not because it comes from a land upside down, but it backs up all they have been saying for years. It also combines flight data not just engine data. And that further proves what John Deakin, George Braly and Walter Atkinson have been teaching for the last 12-13 years.
So lets not get bogged down in Jaba's data files, lets learn from the guys who have been teaching this stuff for years. My data is just icing on the cake
Any sensible questions greatly appreciated.
I tend to think fast cooling, rather than fast warming, is an issue due to the mix of metals used in the cylinders ie aluminium vs steel. Different metals expand & contract at different rates. Is it not the case that aluminium's rate is greater than steel's? A typical cylinder has aluminum surrounding a steel liner and also the steel spark plugs (valve seats too? Can't remember what they're made from). Warming the engine has the aluminum structure expand faster than the steel, leaving the steel to 'catch up' into the minute gap. Cooling, however, has the aluminium contract around the slower shrinking steel structure. Over many cycles it seems reasonable to me that the stress will lead to fatigue & cracking.
Reduce the rate of cooling (and possibly the starting temp ie very hot vs hot vs warm vs cool) and the difference in shrinkage would be reduced.
Reduce the rate of cooling (and possibly the starting temp ie very hot vs hot vs warm vs cool) and the difference in shrinkage would be reduced.
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You need to remember that as all these items expand and contract, they are sinking heat from one to the other, they are not acting independently. So what you might think is happening is not.
Sure things change at different rates, however the mass and shape of the object affect the rate of expansion. A piston is aluminium, the cylinder is steel, the head aluminium. The cylinder is thin, the piston and head not.
There are folk who for some strange reason have done many tests, like multi probing turbochargers and cylinders to see what is actually happening. I am sure you can guess which mad engineer/lawyer this guy is? Whats more he has no axe to grind, just education.
The fruits of his labours can be found at Advanced Pilot
Need I say more.
You need to remember that as all these items expand and contract, they are sinking heat from one to the other, they are not acting independently. So what you might think is happening is not.
Sure things change at different rates, however the mass and shape of the object affect the rate of expansion. A piston is aluminium, the cylinder is steel, the head aluminium. The cylinder is thin, the piston and head not.
There are folk who for some strange reason have done many tests, like multi probing turbochargers and cylinders to see what is actually happening. I am sure you can guess which mad engineer/lawyer this guy is? Whats more he has no axe to grind, just education.
The fruits of his labours can be found at Advanced Pilot
Need I say more.
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Thereason why you never hear about these issues with radial engines, at least the single row and double row engines ( no comment about the corn cob radials), is in engine handling.
Big engines deserve and get respect, no rapid throttle movements, if you can see your hand moving it is too damn fast.
It is about thermal cycles and keeping the changes in thermal states nice and slow so the engine metal bits can keep up with the power developed.
Also radial engines are NEVER allowed to be windmilled by the propellor this is an engine killing practice and most large radials are geared so positive manifold pressure is always applied with the one exception in the landing roll out
Big engines deserve and get respect, no rapid throttle movements, if you can see your hand moving it is too damn fast.
It is about thermal cycles and keeping the changes in thermal states nice and slow so the engine metal bits can keep up with the power developed.
Also radial engines are NEVER allowed to be windmilled by the propellor this is an engine killing practice and most large radials are geared so positive manifold pressure is always applied with the one exception in the landing roll out
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Also radial engines are NEVER allowed to be windmilled by the propellor this is an engine killing practice and most large radials are geared so positive manifold pressure is always applied with the one exception in the landing roll out
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It is aself evident fact that those of us priveliged enough to own one of these today are very careful with them, the evidence on which we rely came from the WW 2 veterans who flew aircraft with these into battle.
The RAN had a run of failures in theTrackers which had an up rated version of the Wright 1820.
The RAN had a run of failures in theTrackers which had an up rated version of the Wright 1820.
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So no hard evidence then? Were the RAN failures gearboxes? I have heard the mention of the prop pushing the engine being bad.
From the geared and direct drive radials I've worked I can't remember (Could be bad memory) any mention of special proceedures in flight manuals apart from standard temps and pressure stuff. As I said I could be wrong, it strikes me as being one of those tails.
From the geared and direct drive radials I've worked I can't remember (Could be bad memory) any mention of special proceedures in flight manuals apart from standard temps and pressure stuff. As I said I could be wrong, it strikes me as being one of those tails.
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In the late 80's/early 90's I believe that the geared R-1340 in VH-WIR had the prop driving the engine, as it needed a gearbox rebuild fairly early. 50/100 hours-ish?? The bloke who flew it the most had no skin in the game and was generally abusive to equipment.
Once that fella was off the scene, the poor old girl also had a main bearing failure around the mid-90's. I have no idea what triggered that though.
Once that fella was off the scene, the poor old girl also had a main bearing failure around the mid-90's. I have no idea what triggered that though.
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Jaba, the link you provided to Advanced Pilot says in part
You and Clinton and Jas told me in the 'Oversquare/LOP' thread, that was not possible. (said with tongue planted firmly in cheek)
And because it saves gas and emits fewer unburned hydrocarbons, it's also greener.
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Back in the Sixties, the RAAF had a Dakota stationed at Butterworth. Over a period of time (don't know how long because this is going from memory but I was in the RAAF when it happened), there was a marked increase in the number of engine changes when previously the serviceability was good. The problem was tracked down to the engine handling technique of a new captain who had recently arrived at the unit.
I think he was the qualified flying instructor (QFI) allotted to the unit to conduct training, instrument rating tests and conversions to type. His method of simulated engine failure after take off was to harshly close the throttle and have the pilot drag the Dakota around the circuit with its propeller windmilling.
The result being the propeller was in effect driving the engine by means of very low manifold pressure and high windmilling RPM. This was extremely damaging to the engine in the long run due I think to reversal of bearing loads. In fact I vaguely recall he left the windmilling prop pitch lever at full fine.
In terms of figures, I would guess that at 100 knots IAS with full fine pitch and throttle against the idle stop, it would likely give less than 12 inches manifold pressure and RPM windmilling around 2300 RPM.. I know that take off power in the Dakota was 48"HG and 2700RPM and typical cruise power 30.5"HG and 2050 RPM. Anyway an engineering investigation nailed the engine failures to the habit of this particular pilot because as unit QFI he was doing all the throttle yanking on "training".
He was "counselled" and given a kick up the arse and from then on engine failures were the thing of the past. Note: This interested me at the time because I too was a Dakota QFI although at another squadron and I recall we received a letter from the RAAF detailing what had happened. A slow smooth closure of the throttle to simulate engine failure was used after that and we also pulled back the pitch lever to around 1500 RPM I think to minimise the gap between manifold pressure and RPM. Of course we had to pull back the throttles on base leg to around 15"MP for the landing approach and Pratt& Whitney advised us by letter that it would not cause a problem at the slow IAS on base and final (90 knots slowing to 80 knots over the fence). P&W went on to say that a fully throttled back engine and high IAS such as could happen in a descent from altitude would eventually cause long term engine damage.
I think he was the qualified flying instructor (QFI) allotted to the unit to conduct training, instrument rating tests and conversions to type. His method of simulated engine failure after take off was to harshly close the throttle and have the pilot drag the Dakota around the circuit with its propeller windmilling.
The result being the propeller was in effect driving the engine by means of very low manifold pressure and high windmilling RPM. This was extremely damaging to the engine in the long run due I think to reversal of bearing loads. In fact I vaguely recall he left the windmilling prop pitch lever at full fine.
In terms of figures, I would guess that at 100 knots IAS with full fine pitch and throttle against the idle stop, it would likely give less than 12 inches manifold pressure and RPM windmilling around 2300 RPM.. I know that take off power in the Dakota was 48"HG and 2700RPM and typical cruise power 30.5"HG and 2050 RPM. Anyway an engineering investigation nailed the engine failures to the habit of this particular pilot because as unit QFI he was doing all the throttle yanking on "training".
He was "counselled" and given a kick up the arse and from then on engine failures were the thing of the past. Note: This interested me at the time because I too was a Dakota QFI although at another squadron and I recall we received a letter from the RAAF detailing what had happened. A slow smooth closure of the throttle to simulate engine failure was used after that and we also pulled back the pitch lever to around 1500 RPM I think to minimise the gap between manifold pressure and RPM. Of course we had to pull back the throttles on base leg to around 15"MP for the landing approach and Pratt& Whitney advised us by letter that it would not cause a problem at the slow IAS on base and final (90 knots slowing to 80 knots over the fence). P&W went on to say that a fully throttled back engine and high IAS such as could happen in a descent from altitude would eventually cause long term engine damage.
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I have very little knowledge of the radials construction and design, however they are not designed to carry thrust loads in the reverse direction and the same applies to some geared engines, although not all.
Engine braking is not what the design criteria of a big radial.....it was in fact to look and sound awesome!
Trent....They are not combusted or burned in a power producing way.....but they do not remain as fuel drops that one can condense (and reuse) in the exhaust either. They get a little black and crispy, over exposed to heat you might say. George is not interested in them once they have gone into the exhaust aft the turbo. Nice try
Engine braking is not what the design criteria of a big radial.....it was in fact to look and sound awesome!
Trent....They are not combusted or burned in a power producing way.....but they do not remain as fuel drops that one can condense (and reuse) in the exhaust either. They get a little black and crispy, over exposed to heat you might say. George is not interested in them once they have gone into the exhaust aft the turbo.
Nice try
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Cecil as long as you don't come near my 1820 life will be good.
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I didn't want to hi jack this excellent thread to have a discourse on Planatery gearboxes or the wonders of the geometry of radial engines and the importance of protecting the master rod bearing no matter what is happening to the flight profile.
My point in raising radial enines is/was that they don't suffer from cracking through thermal cycling to anywhere near the degree that the flat engines do, and that is primarily a result of the way radials are managed in flight.
The mid sized radials are generally in the 1500 H.P. area so their thermal loads are significant, but with good disciplined handling they are a really reliable power plant.
the1820 cyl head temp runs 170 celciusin the climb cowls open and 150 celcius in cruise cowls closed, very benign temps.
So lets get back to Thermal Cycles and engine management
My point in raising radial enines is/was that they don't suffer from cracking through thermal cycling to anywhere near the degree that the flat engines do, and that is primarily a result of the way radials are managed in flight.
The mid sized radials are generally in the 1500 H.P. area so their thermal loads are significant, but with good disciplined handling they are a really reliable power plant.
the1820 cyl head temp runs 170 celciusin the climb cowls open and 150 celcius in cruise cowls closed, very benign temps.
So lets get back to Thermal Cycles and engine management
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I'd like to see some figures on descent rates, EGT/CHT and forward slips too.
I'm a big fan of forward slips (when flying solo - pax don't like it too much).
(Beware some aircraft types don't like slips when flaps are out beyond a certain amount. Read the POH.)
I'm a big fan of forward slips (when flying solo - pax don't like it too much).
(Beware some aircraft types don't like slips when flaps are out beyond a certain amount. Read the POH.)