Oil in radial engines?

Discussion in 'Engines' started by engguy, Sep 17, 2012.

  1. engguy

    engguy Member

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    So how does someone know how much oil is in a radial engine?
    Being a dry sump engine, and if they sit for a long time, even with a fire wall shut off valve closed.
    I'm sure some of the oil leaks into the crankcase. This would give the one checking the oil the wrong quantity, and if more is added then
    when the engine is started the extra would have to go some place. Is motoring the engine with the starter enough to scavenge the oil from the crankcase?
     
  2. razor1uk

    razor1uk Well-Known Member

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    #2 razor1uk, Sep 17, 2012
    Last edited: Sep 17, 2012
    I am no aero engine expert, more of motorbike knowledge, but assuming the starter system is working near enought to stat but with the plug leads un-attatched or no fuel, the scavengers should work as normal under starting conditions.
    ...unless one or most are are out of tolerances, or any residual oils have previously dried to a clogging nature, also dependant on oil, 'resting'/surrounding air and engine/casing temps, viscoity, air and/or crankase pressure(s) etc, play some part on oil flows as you'd naturally expect as per (usual)...
     
  3. gumbyk

    gumbyk Well-Known Member

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    I have heard of draining the sump into a clean container and putting that oil back into the oil tank if that is suspected of happening. But from memory, there is often an oil overflow, which would create a mess, but not create any damage.
    If any oil has dried, then you're in trouble...
     
  4. msxyz

    msxyz Member

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    I seem to remember that radials engines had to be turned daily when aircrafts were not in use to prevent accumulation of oil. If a engine was left too long without running, the mechanics would proceed to remove the spark plugs from the lowest cylinders and spin the crank a few times to drain the cylinders of any oil that possibly dripped into them.

    Note that many radial engines had the cylinder walls continuing into the crankcase (look at this "pot", for example: http://www.rustys.net.au/assets/uploads/IMG_02671.jpg : the bottom of the cylinder extends past the collar). With this arrangement, the oil splashing on the crankcase walls wouldn't end in the downward facing cylinders but would be collected at the bottom of the cranckase. Between the lowest two cylinders there is a conduit which leads to a small reservoir and a pump.
     
  5. engguy

    engguy Member

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    Draining sumps not always an option. In some cases ie not on wing, they are very very hard to access. And the mess would be huge. Hmmm maybe rocker box drains.
     
  6. FLYBOYJ

    FLYBOYJ "THE GREAT GAZOO"
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    From the radials I've been around, you clear hydraulic lock from the lover cylinders, then check sump capacity. There is usually a sight glass or dip stick used for this. Add oil as required and you're good to go. If it's over serviced most of the time the engine will naturally blow the excess oil out of the crankcase breather.
     
  7. gumbyk

    gumbyk Well-Known Member

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    If its not on the wing, its not plumbed in to the oil system, and nothing to drain back. But, yeah, you could remove the rocker cover, but it would end up just as messy as removing the sump dain.
     
  8. R Pope

    R Pope Member

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    The ground crews always pulled the engine through to clear any oil from the cylinders. On a B-29, I think it was 9 blades on each engine.
    Talking about a mess, have you ever walked under a plane with round engines? A B-17 will have oil dripping everywhere, every rivet head out past the prop circles will have a drop of oil hanging from it. B-24, too. B-25, not so bad, maybe because they have only half the drippers!
     
  9. gumbyk

    gumbyk Well-Known Member

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    There is a difference between clearing the oil in the lower cylinders, which is standard procedure on radials before every flight, and the oil that drains from the oil tank back into the oil sump, which is what I thought engguy was talking about.

    The only time that I have heard of it happening, was where there was a problem with a non-return valve.

    I had not heard about sight glasses on the sump, but it makes sense.
     
  10. N4521U

    N4521U Well-Known Member

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    I flew as a passenger in a TBM and we had to add 5 gals to bring it up to minimums!
     
  11. engguy

    engguy Member

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    #11 engguy, Sep 19, 2012
    Last edited: Sep 19, 2012
    I'm not talking about hydraulic locking or oil in the combustion chambers. I was talking about oil draining from the oil tank into the crankcase. Read the original post.

    So now think. If all the oil has drained into the engine, and the tank is empty. If more is added, and it is run, then the scavenge system will fill the tank to overflow.
     
  12. tyrodtom

    tyrodtom Well-Known Member

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    The oil tank on most radial holds anywhere from 12 to 20 gals of oil, if all of that drained into the crankcase there'd be no way the engine would start, because the crankcase would be almost completely full of oil.
     
  13. FLYBOYJ

    FLYBOYJ "THE GREAT GAZOO"
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    Speaking in very general terms, I really dont think you can have all the oil drain into the engine in a dry sump system.
     
  14. tyrodtom

    tyrodtom Well-Known Member

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    I doubt it too, I don't see how it could get past the gears or vanes in the oil pump, since they wouldn't be moving with the engine not turning.
     
  15. engguy

    engguy Member

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    The same way it leaks past rings or valve stems and guides, and gets into the bottom cylinders. The oil pumps are not liquid tight, they do have running clearances, else they would instantly seize up. I think even a sliding fire wall valve will leak in time, not sure how they are designed, but I'm thinking there is no rubber or such for a positive seal, as they would be doing a good job if they sealed at the rate of allowing a gallon to leak every month in warm weather. I'd like to know more about their construction.
     
  16. engguy

    engguy Member

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    #16 engguy, Sep 19, 2012
    Last edited: Sep 19, 2012
    Why not if the crankcase is below the tank, and has the volume to hold it? Well yeah not all the oil since at what ever level in the crankcase there would be hoses and oil coolers etc. but the tank can be empty and all oil in the system, or close to it, till it levels out with maybe a few inches left in the tank.
     
  17. FLYBOYJ

    FLYBOYJ "THE GREAT GAZOO"
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    I believe you may have a pump between the sump and engine that will prevent oil flow into the crankcase, again this is speaking in very general terms.
     
  18. gumbyk

    gumbyk Well-Known Member

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    From my experience, it only really happens when something is wrong, either with the scavenge pump, or non-return valve. I've only heard of it happening once.
    WW1 aircraft have a manual oil shut-off valve to prevent this exact thing from happening, as pump clearances weren't as tight as more modern systems, and there wasn't a non-return valve plumbed in
     
  19. jimh

    jimh Active Member

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    The oil capacity of the B-17 and B-24 is alittle over 30 gallons per engine. We keep them around 24 to 26 gallons on a daily basis. As stated previously any more oil is just blown out the breather. Before the first flight of the day or after 8 hours the engines are pulled through 9 blades. I've been flying behind 1820's and 1830's for 10 years and I have never seen one lock. The B-25 on the other hand is terrible...it has locked in an afternoon and leaks like a pig when pulled through.

    The amount a radial leaks is a function of maintenance. We pride ourselves in the fact that the 24 leaves virtually no mess...only when somebody spins the turbos, the 17 will leak when the props are pulled through. It is vital that pushrod tubes and rocker gaskets are maintained to prevent 90% of all leaks. The 25 is the worst and will leak a fair amount of oil when pulled through...if it hasn't locked. The 1830 is by far the driest radial I've ever seen....that said we recently got a Skyraider...I've never seen an engine leak as much...but that is just at start up :)

    jim
     
  20. GregP

    GregP Well-Known Member

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    The rings on most radials are tapered. That is, they have an angle to the side surface that runs along the cylinder wall.

    The angle is supposed to taper from touching at the bottom edge of the ring (in the top cylinder) to not quite touching at the top edge of the ring (in the top cylinder). That way, when combustion happens, the oil is wiped back into the crankcase. If the rings are tapered backwards (installed upside down, I've seen that a LOT of times!), the eninge fairly eats oil and the whole airframe is wet. If YOUR radial uses a LOT of oil, check the tapered rings! Some are backwwards, most likely. That happens when mechaincs don't even realize the rings are tapered. All the compression rings sould be tapered so the smaller diameter is toward the top of the cylinder.
     
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