How dose a radial engines oiling system work?

[chuntington101]

Hi all,

Just wondering if you guys and girls could shead some light on how the oiling system on Radial engiens works. Obviously in a normal engine you mount the pump in (or as close to) the sump and you can just suck the oil out. However how dose that work in a Radial engine?

Thanks,

Chris.

[Simpo Two]

Radial or rotary?

[Eric Mc]

Rotaries are quite different to radials (although old style rotaries were "radial" in design).

[freddytin]

Dry sump lubrication system, with the return oil being fed to a separate tank.

[davepoth]

Eric Mc said:

Rotaries are quite different to radials (although old style rotaries were "radial" in design).

If it's a rotary I think it's total loss (Biggles was always smeared in oil after flying his Camel!)

[Eric Mc]

Yep - centrifugal force carried the castor oil lubricant to the top of the cylinders and then out into the airstream.

WW1 pilots were very regular - I've heard.

[dr_gn]

chuntington101 said:

Hi all,

Just wondering if you guys and girls could shead some light on how the oiling system on Radial engiens works. Obviously in a normal engine you mount the pump in (or as close to) the sump and you can just suck the oil out. However how dose that work in a Radial engine?

Thanks,

Chris.

Radials usually have an odd number of cylinders (per row), and are usually mounted with one cylinder pointing vertically upwards (12:00), therefore there is a corresponding gap between cylinders at 6:00. This space is often filled with a small 'sump', from which oil is collected by a scavenge pump to be eventually put through through the engine. When running, the oil doesn't collect in the lower pistons, it just gets chucked around inside the crankcase, but ends up in the sump eventually.

If the engine is left stationary however, oil can leak past the piston rings and into the cylinder heads of the lower cylinders where it could cause a hydraulic lock and damage the engine. This is why radials are usually turned by hand before starting to check that everything is free.

[jr6yam]

Eric Mc said:

Yep - centrifugal force carried the castor oil lubricant to the top of the cylinders and then out into the airstream.

WW1 pilots were very regular - I've heard.

That may have had more to do with their circumstances than the castor oil!

[chuntington101]

dr_gn said:

chuntington101 said:

Hi all,

Just wondering if you guys and girls could shead some light on how the oiling system on Radial engiens works. Obviously in a normal engine you mount the pump in (or as close to) the sump and you can just suck the oil out. However how dose that work in a Radial engine?

Thanks,

Chris.

Radials usually have an odd number of cylinders (per row), and are usually mounted with one cylinder pointing vertically upwards (12:00), therefore there is a corresponding gap between cylinders at 6:00. This space is often filled with a small 'sump', from which oil is collected by a scavenge pump to be eventually put through through the engine. When running, the oil doesn't collect in the lower pistons, it just gets chucked around inside the crankcase, but ends up in the sump eventually.

If the engine is left stationary however, oil can leak past the piston rings and into the cylinder heads of the lower cylinders where it could cause a hydraulic lock and damage the engine. This is why radials are usually turned by hand before starting to check that everything is free.

Thanks for the info dr_gn.

IS the oil colecting in the cylinders also the reason why Radials often kick out plumbs of black smoke on startup?

Also I have heard of radials with even numbers of cylinders, but i gues these would still need a small sump for the oil to collect. Fianlly could a serveral dry sump pumps (i.e. say a four stange pump inbetween each bank) not provide enough suction to remove the oil from the crankcase? Or would there not be enough oil for that to work?

Thanks agian,

Chris.

PS sorry if this sounds like ramblings, i have been awake for over 31 hours now!

[dr_gn]

chuntington101 said:

Thanks for the info dr_gn.

IS the oil colecting in the cylinders also the reason why Radials often kick out plumbs of black smoke on startup?

Also I have heard of radials with even numbers of cylinders, but i gues these would still need a small sump for the oil to collect. Fianlly could a serveral dry sump pumps (i.e. say a four stange pump inbetween each bank) not provide enough suction to remove the oil from the crankcase? Or would there not be enough oil for that to work?

Thanks agian,

Chris.

PS sorry if this sounds like ramblings, i have been awake for over 31 hours now!

You'll find many even cylinder number radials, but that would probably be total cylinders - the number of cylinders per row is always odd for a conventional four stroke (eg an 18 cylinder would be a double row 9 cylinder).

The problem with trying to suck oil out of a circular crankcase from all around is aeration. You'd be sucking more air than oil, and that would require far more elaborate separation equipment to cope with it.

I guess oil will always collect in a radial more than an upright inline or vee engine, but many aero engines of all types are pretty smoky on startup. Might be becasue of the priming fuel being oil-rich to start with.

[Eric Mc]

I was reading an accident report recently on an air cooled radial powered aircraft and the accident was caused by engine failure due to insufficient lubrication of the pistons. This had been caused by the fact that the propellor had not been hand turned sufficiently enough before start-up to ensure that oil that had collected in the bottom cylinders had been scavanged properly.

[freddytin]

Each row on radials is an odd number of cylinders to allow a smoother firing order.

It's a while since I was trained on these ,but I believe there were many designs for the scavenging of oil.

The lower pots were prone to oiling if left standing, and took a while to clear on startup.

[eharding]

Eric Mc said:

I was reading an accident report recently on an air cooled radial powered aircraft and the accident was caused by engine failure due to insufficient lubrication of the pistons. This had been caused by the fact that the propellor had not been hand turned sufficiently enough before start-up to ensure that oil that had collected in the bottom cylinders had been scavanged properly.

Which one was that Eric?

A generic problem with the radials used in the Yak-52 is that the non-return valve through which oil is passed from the sump back to the tank can tend to stick open - not a problem in flight, but when the aircraft is left parked the oil will tend to drain out of the tank and back into the sump, so when bloggins checks the oil level before the next flight the measuring stick is under-reading, hence the tank is topped up above the recommended 10 litre level, and is promptly dumped over the side during aerobatics. Even without this, the Yak will tend to get through between 500ml and 1 litre an hour, and most of that ends up on the airframe. We must have been mad to paint the underside duck-egg blue.

The prop needs to be pulled through initially to clear any hydraulic lock caused by oil pooling in the lower cylinders, and then subsequently to prime a cold engine. One problem with the Ivchenko is that in addition to having oil pooled in the cylinders - which can be cleared by pulling it through - oil can also collect in the inlet manifolds, which won't generally be cleared by pulling through. In the soviet days, it was SOP to decowl the engine and manually pull plugs out of the inlet manifolds in the engine hadn't been run for 7 days to clear this oil - which is a ball-ache to do, but if you have a ground-crew to do it for you not really a problem - most Yaks in the West nowadays have a mod which fits a drain to the inlet manifolds which is left open when the aircraft is left standing for any period.

[Eric Mc]

I think it was the accident to Piston Provost G-AVWF which seemed to involve a hydraulic lock of one of the pistons.

[eharding]

Eric Mc said:

I think it was the accident to Piston Provost G-AVWF which seemed to involve a hydraulic lock of one of the pistons.

Particularly nasty one that - the accident report should be required reading for anyone operating a radial

The immediate cause of the accident wasn't lack of lubrication, it was the catastrophic failure of a piston gudgeon pin, which in turn destroyed the engine and led to an intense in-flight fire. The suspicion was that years of 'micro' hydraulic events caused by small amounts of oil in the cylinders had caused fatigue in the gudgeon pin - but the fact the engine hadn't been overhauled for decades probably didn't help.

[chuntington101]

dr_gn said:

chuntington101 said:

Thanks for the info dr_gn.

IS the oil colecting in the cylinders also the reason why Radials often kick out plumbs of black smoke on startup?

Also I have heard of radials with even numbers of cylinders, but i gues these would still need a small sump for the oil to collect. Fianlly could a serveral dry sump pumps (i.e. say a four stange pump inbetween each bank) not provide enough suction to remove the oil from the crankcase? Or would there not be enough oil for that to work?

Thanks agian,

Chris.

PS sorry if this sounds like ramblings, i have been awake for over 31 hours now!

You'll find many even cylinder number radials, but that would probably be total cylinders - the number of cylinders per row is always odd for a conventional four stroke (eg an 18 cylinder would be a double row 9 cylinder).

The problem with trying to suck oil out of a circular crankcase from all around is aeration. You'd be sucking more air than oil, and that would require far more elaborate separation equipment to cope with it.

I guess oil will always collect in a radial more than an upright inline or vee engine, but many aero engines of all types are pretty smoky on startup. Might be becasue of the priming fuel being oil-rich to start with.

How about the junkers Jumo 222? Or was that basically 6 x 4 pot engines?

See what you mean about the oil. Would be pointless. May as well let it settle in the sump and then deal with it from there.

[dr_gn]

chuntington101 said:

How about the junkers Jumo 222? Or was that basically 6 x 4 pot engines?

See what you mean about the oil. Would be pointless. May as well let it settle in the sump and then deal with it from there.

'Fraid I've no idea about that.

[Mr Dave]

chuntington101 said:

How about the junkers Jumo 222? Or was that basically 6 x 4 pot engines?

See what you mean about the oil. Would be pointless. May as well let it settle in the sump and then deal with it from there.

Its more like 3 V8s around a central crankshaft rather than a radial engine if its the one I am thinking of.

[eharding]

Mr Dave said:

Its more like 3 V8s around a central crankshaft rather than a radial engine if its the one I am thinking of.

Sounds like a nightmare - suspect it ended badly.

[dr_gn]

Mr Dave said:

Its more like 3 V8s around a central crankshaft rather than a radial engine if its the one I am thinking of.

But if you sliced the rear 18 cylinders off, you'd effectively have a single row radial. The difference I think between a multi-row radial and the Junkers or the Armstrong Siddeley Deerhound is how firing order balance is achieved.