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#1
Old 06-08-2004, 11:21 PM
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Oil circulation in radial engines

Usually in a reciprocating piston engine the valves are at the top of the engine and the oil pump is at the bottom, within the oil pan/sump.

However, in a radial, some of the cylinders are upsidedown...at rest, oil would end up in the underside of the piston, possibly lealing past the gap in the compressipon rings and thus floodding the combustion chamber. Also, oil would collect in the valve covers...unless the covers each has its own pump.

How do you keep oil from building up where its not supposed to be?
#2
Old 06-08-2004, 11:28 PM
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You don't. It will sometimes seep past the rings in the lower cylinder, filling it with oil. That's why it's standard procedure to turn the prop over by hand to make sure the lower cylinders are clear and you won't bend a rod when you engage the starter. It's called hydraulic lock.

If you encounter resistance when hand turning the prop before starting, you pull one of the lower spark plugs and let the oil drain out. Some people try turning the prop backwards, but that can lead to filling the intake pipe with oil and a partial lock on the start.
#3
Old 06-08-2004, 11:33 PM
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Quote:
Originally Posted by Enola Straight
Usually in a reciprocating piston engine the valves are at the top of the engine and the oil pump is at the bottom, within the oil pan/sump.

However, in a radial, some of the cylinders are upsidedown...at rest, oil would end up in the underside of the piston, possibly lealing past the gap in the compressipon rings and thus floodding the combustion chamber. Also, oil would collect in the valve covers...unless the covers each has its own pump.

How do you keep oil from building up where its not supposed to be?
Radial engines use a full pressure system with the oil being collected in a small sump st the bottom where it is picked up by a scavenging pump and returned to the main tank. The oil for the bottom valves collecting in the covers is a problem and is one reason why Wright radials tended to throw oil. Pratt and Witney engines didn't and I'm damned if I know why.

One of the prestart procedures for radials is to turn the engine through several revolutions by hand to check for oil having leaked into the combustion chambers of the bottom cylinders. If the prop comes to a sudden stop and won't move you take out a spark plug in the bottom cylinders to let the collected oil out.

I never had the Pratt and Witneys in a plane have an oil block, but a co-worker of mine was a B-25 mechanic and he said they had that trouble all of the time. They would still be draining oil out of their engines after the nearby Douglas A-26's had already been started and were in the preflight check routine.
#4
Old 06-08-2004, 11:38 PM
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All my radial time (what little I have) is in a DHC-2 Beaver with a Pratt Wasp Jr. I've never found oil in the cylinders, and neither have the guys who fly it a lot more than I do.

That's a baby radial compared to what David Simmons flew. It's a measly 985 cubic inches and 450hp.
#5
Old 06-08-2004, 11:40 PM
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Good article on hydraulic lock in radials.
#6
Old 06-09-2004, 12:02 AM
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Since David was kind enough to poke his head into this thread, I'm hoping that he'll have an answer to something that G. Gordon Liddy always said about radial engines: Namely, that once you got them going, they tended to stay going. So while a Vee engine might seize up immediately upon losing oil pressure, a radial would keep going for much longer (of course, there wouldn't be a heckuvalot left of that radial engine when you shut it off, but it'd tend to stay running). Any truth to that, Dave? Or is Liddy telling another one of his stories?
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#7
Old 06-09-2004, 12:39 AM
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Radial engines have shown phenominal durability even with severe damage. It wasn't unknown for a few cylinders to be shot away but the engines kept running with connecting rods hanging in the breeze.
#8
Old 06-09-2004, 02:03 AM
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Quote:
Originally Posted by Tuckerfan
Since David was kind enough to poke his head into this thread, I'm hoping that he'll have an answer to something that G. Gordon Liddy always said about radial engines: Namely, that once you got them going, they tended to stay going. So while a Vee engine might seize up immediately upon losing oil pressure, a radial would keep going for much longer (of course, there wouldn't be a heckuvalot left of that radial engine when you shut it off, but it'd tend to stay running). Any truth to that, Dave? Or is Liddy telling another one of his stories?
People say all sorts of things. I don't think there is anything magical about radial engines that allows them to run very long without oil. Radial engines run at a pretty high temperature, 250-300 C and the oil circulation is a fairly large factor in cooling them.
#9
Old 06-09-2004, 04:18 PM
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WELL!

I posted this question in order to find out what engineering solution existed to avoid the problem...and I find out that the problem HAS no solution; you must simply endure.
#10
Old 06-09-2004, 05:48 PM
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Quote:
Originally Posted by Enola Straight
WELL!

I posted this question in order to find out what engineering solution existed to avoid the problem...and I find out that the problem HAS no solution; you must simply endure.
Actually, the problem could be solved, but the question is: Would the cost of the solution outweigh the benefits gained? I don't know if they're still being built, but I can't imagine that there's much market for them, and during WW II, there probably wouldn't have been much incentive to correct the problem, since you would have had to have done a lot of testing to make sure the solution didn't create more problems, and then train the mechanics who would be working on the planes. Remember, it was known before the war that Sherman tanks were too lightly armored and that powering them with gasolene wasn't a good idea (tankers nicknamed them "Ronsons" after the cigarette lighter with the slogan, "Always lights the first time." ), but the designs were pressed into service since many mechanics were familiar with gasolene powered engines (Sherman's used a Cadillac engine, IIRC), and changing the armor would have delayed getting the tanks into the field where they were sorely needed.
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