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Thread: Oil Temperature Probe Blast Tubes.

  1. #1
    Administrator Steve Pierce's Avatar
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    Default Oil Temperature Probe Blast Tubes.

    There was a thread a while back on the SWPC website about blast tubes on the oil temperature probe and if it actually dropped the oil temperature or fooled the oil temperature indicator. The Clipper has this blast tube arrangement on the oil temperature probe. I can't remember if the Vagabond or Colt has it or not. A friend of mine did some tests in his O-200 powered J3 that he competes in STOL contests with. Interesting results.

    Oil Temperature Test.26 Jan 2010

    OAT. 37deg F.Alt. 1000MP. 23.5MPH. 92.

    A two inch scat tube was used between the outside ram air and the oil screen shroud. It was modified with an air shutoff for the tests.
    The oil galley on the left rear of the engine where the oil cooler mounts was modified to accept a temperature probe.
    Two cameras were used to monitor the screen oil temp and the galley oil temp.
    These were fed to a common screen on tape and uploaded to computer to do the final comparison.
    10 minute runs were used as the temps seemed to normalize at that point.

    Time. Galley temp Screen temp Cold ram air
    10 min 207 183 Off
    10 min 207 152 On
    10 min 207 180 Off
    10 min 207 152 On

    Land approach 202 150 On
    Touchdown 195 152 On
    Park 193 162 On

    Slow flight
    1 notch flaps 200 180 On

    Oil temp in the galley seems to be affected mainly by power setting and not much else. The cooling of the screen had no effect what so ever on the galley oil temp.

    Jerry
    Attached Images Attached Images

  2. #2
    d.grimm's Avatar
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    Default Re: Oil Temperature Probe Blast Tubes.

    Steve,
    That's it, still like looking at the lower temperature on the gauge,
    and it's probably all my wishful thinking but I do
    believe my oil pressure is a little higher.
    Dave

  3. #3
    Gilbert Pierce's Avatar
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    Default Re: Oil Temperature Probe Blast Tubes.

    I am glad Jerry did that test. The last time this came up I threatened to do the heat transfer calculations to quiet some doubters that thought it actually worked. Cooling the temp probe was probably done to prove to the FAA that oil temps were in their spec.

    I would have had to do some (a lot) boning up to do the calculations as it has been a long time.
    Last edited by Gilbert Pierce; 07-01-2012 at 12:00 PM.

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    BrettL's Avatar
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    Default Re: Oil Temperature Probe Blast Tubes.

    It makes me wonder why they used the shroud on the PA-17 at all. My oil temps are darn near perfect with the shroud removed (120F on a 30F day, 160F on a 70F day, 180F on a 90F day, etc. and at most 15 degrees higher in an extended full power climb). I do otherwise have the original PA-17 baffling. I do recall the PA-16 having a reputation for high oil temps.

    Dave,
    The Cessna 140 baffling seems to be the "choice" replacement baffling for Continental engines. My Culver had it's baffling replaced with Cessna 140 baffling.
    Brett

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    Ed Brown's Avatar
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    Default Re: Oil Temperature Probe Blast Tubes.

    If cool air is blown on hot metal, the metal is cooled. If hot oil comes in contact with cool metal, the oil is cooled.

  6. #6
    andya's Avatar
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    Default Re: Oil Temperature Probe Blast Tubes.

    Quote Originally Posted by Ed Brown View Post
    If cool air is blown on hot metal, the metal is cooled. If hot oil comes in contact with cool metal, the oil is cooled.
    I would tend to agree but Steves data looks genuine. I'm just wondering how the oil temp probe can get cooled from the baffle arrangement and not cool the oil. The oil temp in that test that steve shows was taken not that far from the probe, close enough that I don't think it could have heated up in that short of distance, just by passing thru passages in the accessory case or case?

    Be interesting to know what the flow rate is, Gilbert?
    "Progress is our most important problem"

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    BrettL's Avatar
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    Default Re: Oil Temperature Probe Blast Tubes.

    Quote Originally Posted by andya View Post
    I'm just wondering how the oil temp probe can get cooled from the baffle arrangement and not cool the oil.
    With the shroud being attached to the base of the oil temp probe, is it possible the heat is being transferred from the oil temp probe through its base to the shroud/baffle itself, bypassing the oil and the accessory case?

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