Radial vs liquid cooled engines

[CobberKane]

It is not only what we think or what we think we know now but what was thought at the time. The US army in the 1930s bought the argument that air-cooled engines were less vulnerable and specified only air cooled engines for "attack" aircraft. Aircraft intended for ground support and more likely to be hit by ground fire.

While only a few radials may have made it home with cylinders missing many more made it home with damaged cylinders, dented or broken cylinder fins or damaged cylinder heads. Hits that would have caused leaks on a liquid cooled engine. While a liquid cooled engine won't stop immediately with a cooling hit and may even run at low power settings for 15-20 minutes getting "home" all depends on how far you are from "home" ( friendly air field or friendly farmer's field) when the damage occurs. Some pilots may have nursed an ailing liquid cooled engine even further.

There is little doubt that the size of the liquid cooling system offers more target area than an equivalent air-cooled engine.

Exactly. Examples of radials surviving the kind of damage that deprivers them of a cylinder are rare, but they did happen and serve to illustrate an extreme of the air colled engines resistance to battle damage. At the end of the day the cooling of any given cylinder of a radial is independent of the the others; a hit from, say a .30 or .50 cal round that would smash fins off a radial cylinder or even crack the cylinder itself has no effect on the airflow to the other cylinders. In contrast a similar hit to the cylinder of an LC engine will very probably breach the water jacket, and once this occurs in short order you will loose cooling to the entire engine (yes I know, some designs allowed isolation of half the system, but this still left multiple cylinders vulnerable to a singel hit). How thick is the alloy water jacket of an LC engine? 10mm maybe?
Regarding the length oftime an LC engine will run with a punctured cooling system, I'd suggest we are talking tens of minutes, tops. A cooling system is pressurised and any puncture will depressurise it, lowering the boiling point of the colant and turning it to gas. Gas escapes through holes very quickly. Try this for an experiment: take your car out on to the highway and run it up to operating temperature. Stop and take the radiator cap of. Keep driving (no crawling, if you were in an aircraft you would probably need fifty percent of throttle to stay in the air). How far do you think you will get?

 

[gumbyk]

Try this for an experiment: take your car out on to the highway and run it up to operating temperature. Stop and take the radiator cap of. Keep driving (no crawling, if you were in an aircraft you would probably need fifty percent of throttle to stay in the air). How far do you think you will get?

Now, try this same experiment, but with a hole in the block to an oil gallery.
One thing I haven't seen mentioned here is that you would not be able to sustain significant damage to an air-cooled engine without damaging the oil path, and as the oil is used as a coolant, it would have a significant effect on the cooling and operation of the engine.

 

[davparlr]

Try this for an experiment: take your car out on to the highway and run it up to operating temperature. Stop and take the radiator cap of. Keep driving (no crawling, if you were in an aircraft you would probably need fifty percent of throttle to stay in the air). How far do you think you will get?

Do not do this! There is a warning on the cap. Automobile engines operate pressurized and can be at a temperature higher than boiling. Opening a radiator cap when hot can cause immediate boiling of coolant and a violent surge of coolant out the cap scalding any body part it touches.

 

[wuzak]

Losing a cylinder on a radial isn't all about cooling. There is the oil issue, as mentioned by Gumbyk. There is also the small matter of parts flailing about without their normal constraints. Lose the master rod cylinder and you've lost that row - because all timing will be lost.

It's not just oil that will be pumped out - the fuel/air mix will be escaping to atmosphere, and possibly onto hot parts as a potential ignition source. This will also screw up the buddy of the lost cylinder, as for many radials two cylinders shared the same intak pipe (4 cylinders in an R-4360).

So, all is well, we have oil spewing out, bits and pieces flailing about, fuel/air mix being pumped out but hey, we still have cooling to all the other cylinders.

 

[fastmongrel]

So Liquid cooled engines arent quite as fragile as some would have us believe and Radials cant really run properly with a cylinder missing and all its oil disappearing fast.

My conclusion is getting hit with a supersonic lump of metal is generally a bad thing no matter whats hanging off the front of your plane.

Does that about sum it up

 

[buffnut453]

Brilliant! Simply brilliant. Pithiest comment I've seen in a long time!

 

[CobberKane]

Losing a cylinder on a radial isn't all about cooling. There is the oil issue, as mentioned by Gumbyk. There is also the small matter of parts flailing about without their normal constraints. Lose the master rod cylinder and you've lost that row - because all timing will be lost.

It's not just oil that will be pumped out - the fuel/air mix will be escaping to atmosphere, and possibly onto hot parts as a potential ignition source. This will also screw up the buddy of the lost cylinder, as for many radials two cylinders shared the same intak pipe (4 cylinders in an R-4360).

So, all is well, we have oil spewing out, bits and pieces flailing about, fuel/air mix being pumped out but hey, we still have cooling to all the other cylinders.

Come on Wuzak, you're being selective. As I said, loosing an entire cylinder is an extreme example, and as Shortround pointed out a hit that might dent the fins or knock a couple of valves off a radial' head would probably easily puncture the thin alloy of an LC engine's water jacket - which is the most robust part of the LC system. Regarding oil fuel and air being pumped everywhere, sure - the same thing is just as likely to happen to an LC engine it is no more or less vulnerable in this respect - the achilles heel of the LC engine was always cooling
Gumbyk, the role of oil in cooling a radial engine is very much secondary to that of air, which is why aircooled engines are called air-coled and oiled cooled engines are called oil-cooled. The fastest effect of depriving either type of engine (LC or AC) of oil would be seizure due to lack of lubrication, and in this they are equally vulnerable And oil is a viscous (thick) liquid that will never in an engine whereas water/glycol has vastly lower viscosity and higher volatility - it will escape far more quickly through a hole of a given size, depriving the LV engine of ALL its cooling.

I guess we could all go on with this forever (which is part of the fun!), and I wouldn't like to try to justify selecting either the AC or LC engine as 'better' overall, but in summary I just can't see any reason to resile from my original view that air cooled radials are significantly more resistant to battle dammage than LC Vs and Inlines. The great preponderance of available experience and commentary seems to support me.

 

[wuzak]

Come on Wuzak, you're being selective. As I said, loosing an entire cylinder is an extreme example,

Am I?

It is an extreme example, but nonetheless it is the example you put forward to prove the air cooled engine's relative invincibility.

a hit that might dent the fins or knock a couple of valves off a radial' head would probably easily puncture the thin alloy of an LC engine's water jacket - which is the most robust part of the LC system.

How do you "knock a couple of valves of a radial's head"? The valves are captive in the head, and if they aren't constrained the only way they can go is into the cylinder and piston, which could potentially cause all sorts of mess inside the engine.

Regarding oil fuel and air being pumped everywhere, sure - the same thing is just as likely to happen to an LC engine it is no more or less vulnerable in this respect

There is truth to that, but it isn't the complete truth. A radial engine expels more heat through oil for a given power level than does a liquid cooled engine. This requires a larger oil cooling system (ie radiator). Radials also burn more oil, so need a larger system overall.

And oil is a viscous (thick) liquid that will never in an engine

Oil is less viscous at higher temperatures, such as in operating temperatures, than at room temperatures.

whereas water/glycol has vastly lower viscosity and higher volatility - it will escape far more quickly through a hole of a given size, depriving the LV engine of ALL its cooling.

True.

I just can't see any reason to resile from my original view that air cooled radials are significantly more resistant to battle dammage than LC Vs and Inlines. The great preponderance of available experience and commentary seems to support me.

Define significantly?

How do you explain this:

the P-38 had the worst record of a.) destruction of German aircraft per fighter lost in the air, and b.) the destruction of German aircraft on the ground.

The latter environment was the most hazardous light flak concentration fighter encountered in WWII. Why did the single engine Mustang achieve far superior ratios?

The 8th AF P-38 destroyed ~161 a/c for the loss of ~ 109 P-38s while strafing at low altituded
The 8th AF P-47 destroyed ~740 a/c for the loss of ~ 200 P-47s " " " " "
The 8th AF P-51 destroyed ~ 3204 a/c for the loss of 569.

By contrast the 8th AF Mustangs destroyed 3315 air for the loss of 322 in air combat.
the P-47 destroyed 1562 for loss of 214
the P-38 destroyed 281 for loss of 101.

Sure the P-47 has better ratios than the P-38, but it has worse ratios than the P-51 with its "significantly" more vulnerable liquid cooling system.

Of course we can't know the reasons that the aircraft were lost - and most probably had nothing to do with the engine.

 

[Shortround6]

Some radials were more "robust" than others and some liquid cooled engines were more "robust" than others.

I don't think anybody is talking about "relative invincibility"but rather the ability to keep running at 30-50% or so of full power in order to get home, not keep fighting at 90-95% power.
The R-2800 and some others had three main bearings so a missing cylinder or two would certainly throw the engine out of balance both in firing order ( although not much worse than a couple of bad plugs) and physically due to flopping pistons but in reality the pistons may have gone with the cylinder or been battered to pieces in short order banging against the crankcase. It rather depends on how far down in the cylinder the skirt went at the bottom of the stroke.
Some radials ( most/all service French engines, the Russian M-88 and a few Italian ones) had two main beings for a two throw crankshaft and were operating at the limits of crankshaft flex to begin with. Throwing them out of balance might have resulted in excessive crankshaft flex and failure (broken crank) in short order.
Other variables are the type of bearings, ball, roller, plain and of the plain bearings you had a variety of materials. Ball/Roller bearings will actually run for quite a while on little or no oil pressure.
Differences in radial engine damage can be entire cylinder barrel gone and piston (or remains) flopping about, cylinder head gone but cylinder remaining so piston is riding in the bore ( keeps engine in balance), damage to head causing valves either to stay closed (rockers or push rods shot away) or valve retainers damaged allowing valves to fall inward and hit pistons. Usual result is bent valves and dings in piston top. It is possible to hole the piston. But you can do that by severe knocking or pre-ignition. Worst case in a holed piston is (in addition to loss of power) is pressurizing the crank case and blowing oil (or oil vapor) out the breathers. Small particles in the oil aren't good long term. A large amount of debris can block oil pick up/return tubes.

Engines of both types can continue to run (obviously not at full power) with miss-firing cylinders or cylinders with damaged valve trains. I once had a timing chain jump on a V-8 engine, 16 bent push rods and 8 bent intake valves, exhaust valves were smaller and if bent slightly could be reground. the engine "ran" but would not make enough power to move the car on any kind of slope.
Plenty of engines of both types have continued to run with broken valves, valve springs, rocker arms, or followers.
On the other hand a hit to the carburetor could spell the end of some engines no matter what type they were. Hispanos and their Russian off-spring being a bit better in this regard with 6 carburetors, one for every two cylinders, but then you are much more likely to hit a Hispano carburetor.

http://www.enginehistory.org/Museums/SNECMA/157 H-S 12Y.jpg

V-12s have larger areas of vulnerability to small arms ( non-cannon)fire . How each particular engine responds to such damage may very well be different but there seems to be little doubt that the radials, in general, were less vulnerable ( but not invulnerable) to combat damage. I repeat IN GENERAL as one or two radials were rather notorious for not taking combat damage well.

 

[CobberKane]

Am I?

It is an extreme example, but nonetheless it is the example you put forward to prove the air cooled engine's relative invincibility.

--I withdraw my assertion that the radial engine is invulnerable. I can't remember saying that, but obviously I must have.





How do you "knock a couple of valves of a radial's head"? The valves are captive in the head, and if they aren't constrained the only way they can go is into the cylinder and piston, which could potentially cause all sorts of mess inside the engine.

--Why couldn't a hit destroy or incapacitate valves and cams without damaging the cylinder and piston. The valves and cams are outside the cylinder surely?


There is truth to that, but it isn't the complete truth. A radial engine expels more heat through oil for a given power level than does a liquid cooled engine. This requires a larger oil cooling system (ie radiator). Radials also burn more oil, so need a larger system overall.

---but oil follows a distant second to air in cooling an air cooled motor.




Oil is less viscous at higher temperatures, such as in operating temperatures, than at room temperatures.

---And at whatever temperature is is far more viscous and less volatile than water




True.




Define significantly?

I would define 'significantly' as 'to a degree that makes a useful difference in practice'.

How do you explain this:



Sure the P-47 has better ratios than the P-38, but it has worse ratios than the P-51 with its "significantly" more vulnerable liquid cooling system.

Of course we can't know the reasons that the aircraft were lost - and most probably had nothing to do with the engine.

---Looks like you answered your own question. Probably most of the losses had nothing to do with the engine? Sticking 'probably' in frony of an unsubsttianted opinion is a pretty lazy way of disguising it as a fact.

 

[wuzak]

---Looks like you answered your own question. Probably most of the losses had nothing to do with the engine? Sticking 'probably' in frony of an unsubsttianted opinion is a pretty lazy way of disguising it as a fact.

Didn't say it was a fact.

From the start of this whole discussion I acknowledged that radial engined aircraft are less susceptible to battle damage than liquid cooled engines. What I disagreed with was the degree of difference.

 

[wuzak]

Didn't notice the other replies:

--I withdraw my assertion that the radial engine is invulnerable. I can't remember saying that, but obviously I must have.

I was being a bit sarcastic. You have portrayed liquid cooled engines as easily damaged and highly susceptible to battle damage, therefore air cooled engines must be relatively invincible.

--Why couldn't a hit destroy or incapacitate valves and cams without damaging the cylinder and piston. The valves and cams are outside the cylinder surely?

The head of the valve is on the piston side of the head.

The rocker and pushrod (on most WW2 radials) could be damaged or shot away, and the retaining collet on the valve may break so that the valve is no longer constrained. But the valve head will not pull through the port.

---but oil follows a distant second to air in cooling an air cooled motor.

Yes it does. That's not the point. The point is that an air cooled engine has a larger oil system than a liquid cooled engine, and thus is more vulnerable in that area.

I would define 'significantly' as 'to a degree that makes a useful difference in practice'.

So is that 1/1,000,000? 1/100,000? 1/10?

 

[CobberKane]

Spot on with the valve gear, brain fade on my part. Regarding everything else, I just watched the All Blacks do the Haka on top of the Wallabys and none of it seems to matter. To all American contributors, it's an Antipodean thing...

 

[wuzak]

Spot on with the valve gear, brain fade on my part. Regarding everything else, I just watched the All Blacks do the Haka on top of the Wallabys and none of it seems to matter. To all American contributors, it's an Antipodean thing...

Eden Park?

No suprise that the All Blacks beat the Aussies. Two reasons:

1. I undertsand the Aussies have never won there.
2. The Wallabies are hopeless at the moment, and have been for some time.

 

[CobberKane]

Eden Park?

No suprise that the All Blacks beat the Aussies. Two reasons:

1. I undertsand the Aussies have never won there.
2. The Wallabies are hopeless at the moment, and have been for some time.

No, the Wallabys have had some good wins there, just not for twenty five years or so. And don't be too hard on them, beating the ABs at home is a big ask for any team. Now, if they'd just had radial engines...

 

[Timppa]

A few anecdotes I could find quickly:

James A Goodson, 4th FG:

I remember an early mission to Kiel, which brought home to me again the dependability of the P-47. The bombers were hitting the German U-boat base and naval base. As usual, we were giving them close escort over the target area. Suddenly, the heavy flak opened up, and almost immediately there were black woolly clouds with bright flashes in them floating by. One hit my engine, rocking the plane and filling the cockpit with the acrid smell of cordite. I heard the crump. Black oil hit the windscreen and I stated to lose power.
...
[After a forced landing near Martlesham Heath]
...
I climbed out onto the wing and looked at the smoking, stattered engine, covered in oil. The top of the front cowling was torn to bits and two cylinders were completely torn apart!

David McCampbell:

My wingman, Roy Rushing, came back aboard the Essex one day after being shot up by anti-aircraft fire and you could see the no. 9 piston pumping up and down. Roy sat in the cockpit with the "thumbs up" signal meaning the aircraft was ready in all respects for the next flight.

Ilyich Kardopoltsev:

I remember the engines on Lavochkins were really good. I got hit by flak in the engine, and two lower cylinders were shattered to pieces… But I made it home. Mech opened cowls, and we saw some bits falling out. And I can't say that I lost power drastically.

Kardopolysev

 

[drgondog]

How do you explain this:

Sure the P-47 has better ratios than the P-38, but it has worse ratios than the P-51 with its "significantly" more vulnerable liquid cooling system.

Of course we can't know the reasons that the aircraft were lost - and most probably had nothing to do with the engine.

My theory about P-38 vs P-51 strafing losses (since both have liquid cooled engines) is that the P-38 was a.) a far bigger target, and .b) had twice as much vulnerability to engine fires. The P-38 did have the ability to casually shut down one engine if the problem was a slow coolant leak but a leaky fuel line could flash before the pilot would notice a problem.

But still, just a theory.

 

[CobberKane]

A few anecdotes I could find quickly:

James A Goodson, 4th FG:


David McCampbell:


Ilyich Kardopoltsev:

Kardopolysev

Along with Clayton Gross acount of flying back to base with a cylinder missing that makes four accounts of radial engines surviving this kind of damage in three different models of fighter, all getting home under power. It seems reasonable to expect there were more. Still waiting for an account of a liquid cooled engine getting home with a smashed cylinder and still running.

 

[Shortround6]

only two different engines though.

 

[drgondog]

Sumner Wiliiams Scout Force (Experimental) was hit by a B-17 and lost' three cylinders - left bank - over Paris and made it back to Steeple Morden. September Engineering Report 355th FG.