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  • #31
    Do NACA ducts provide ram air or just passive offtake?
    If you made a diffuser and everything lined up right, you would almost certainly get away with a short 3" duct without any bends. Refer to Mark's first post on this thread. A good diffuser would be important, and a large cooler. If the cold air is going nice and slow over a large cooler, it will be more effective (in theory) than ramming air across a smaller cooler.

    I've got about 11" of 4" diameter SCAT duct these days, with a 90 degree bend in it. With EI the temps are only just acceptable on a hot summer day. I need to lean the engine more aggressively (LOP) to keep temps at 95*C, else they creep up above boiling point. I am sure the sharp bend and long duct are the issue. The coarse mesh screen at the duct intake doesn't help either.
    Last edited by Battson; 05-10-2020, 04:56 PM.

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    • #32
      Originally posted by Battson View Post
      Do NACA ducts provide ram air or just passive offtake?
      If you made a diffuser and everything lined up right, you would almost certainly get away with a short 3" duct without any bends. Refer to Mark's first post on this thread. A good diffuser would be important, and a large cooler. If the cold air is going nice and slow over a large cooler, it will be more effective (in theory) than ramming air across a smaller cooler.

      I've got about 11" of 4" diameter SCAT duct these days, with a 90 degree bend in it. With EI the temps are only just acceptable on a hot summer day. I need to lean the engine more aggressively (LOP) to keep temps at 95*C, else they creep up above boiling point. I am sure the sharp bend and long duct are the issue. The coarse mesh screen at the duct intake doesn't help either.
      They don't capture as much air as say putting a scoop external to the cowl would. But they still get a large percent of the velocity of the airflow. The big advantage they provide is they don't increase drag very much.
      Last edited by zkelley2; 05-10-2020, 10:13 PM.

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      • #33
        Good point Jon, they're not designed for ram air intake air. Installing them in engine cowl area should provide a higher positive pressure area though, particularly when mounted towards the mid/front part of the engine cowl. On the boot cowl should be sufficient for fresh air vents on the instrument panel.

        A search of the Vans forums indicates that to be successful I’d need to vent the oil cooler discharge well into the low pressure outflow area. Overall it appears to have mixed results when vented into the lower engine compartment.
        Nev Bailey
        Christchurch, NZ
        Bearhawk Bravo, 4 place Quick-build #25B.

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        • svyolo
          svyolo commented
          Editing a comment
          Yeah I would guess venting a NACA ducted cooler to the inside of the cowl, the higher pressure inside the lower cowl might limit airflow from the NACA duct.

          I wonder if the same would be true with tapping air from the baffling. Maybe have a separate oil cooler exit, with "something" causing a negative pressure. Louvers?

        • Nev
          Nev commented
          Editing a comment
          My limited understanding at this point is that when the oil cooler is ducted from the rear baffle area, it is effectively ram air, which I think may be Jon’s point. Louvers could well be a valid concept. The Vans guys that used a NACA intake had to add a discharge duct down to the engine compartment outlet area to get the pressure differential. These are all things I had absolutely no idea about in my GA days!

        • Nev
          Nev commented
          Editing a comment
          PS John I bet we’ve crossed paths in a bar around Asia at some point
          Last edited by Nev; 05-11-2020, 09:00 AM.

      • #34
        I was looking at some engine installation pictures and I came across a few oil coolers that were mounted with both the inlet and the outlet facing down. I thought that might be a problem but then I realized I really didn't know the internal layout of my oil cooler. You really can't tell by looking at it. I mounted mine so the outlet it as the high point, so air can not get trapped.

        What is the internal layout of an oil cooler? A single tube, snaking back and forth throughout? A parallel flow arrangement, where there is a manifold on either side with interconnecting tubes?

        Mine is a Positech. I think it is similar to the AeroClassic style. I couldn't find any info on my particular brand, or any other brand. Their was some info on their type of construction.

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        • Bcone1381
          Bcone1381 commented
          Editing a comment
          Boy, thats a great question!

      • #35
        In the Rotax world, where there is a radiator to cool the cylinder heads, getting the radiator to completely fill with coolant can be an issue. Several aircraft cowl designs have the radiator in a horizontal (or near-horizontal) position. There are all sorts of stories about the gyrations they go through to get those radiators to completely fill... Some go so far as to drill a small hole near the top of the radiator housing to allow air to vent, then insert a screw with LockTite to close the hole.

        With that as background, I would think that ensuring the oil cooler actually completely fills with oil after an oil change would be pretty important to effective cooling, and that svyolo's idea of mounting with the outlet on top would be a good idea. You may be able to see through the fins to determine the orientation of the internal tubes, and if so, it seems logical that you'd want those to run horizontally so as to not trap air where they turn 180 degrees.

        The one thing that's working in our favor with filling the oil cooler is that oil is, in fact, much thicker than water, and may "flush" the air out of the cooler more easily. But why make it fight for it?
        Last edited by JimParker256; 05-24-2020, 10:29 AM.
        Jim Parker
        Farmersville, TX (NE of Dallas)
        RANS S-6ES – E-LSA powered by 100 HP Rotax 912ULS

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        • svyolo
          svyolo commented
          Editing a comment
          I have done a lot with cooling systems, including a few "difficult" ones. With the oil cooler it depends on the configuration. Does it functions as a tank (two manifolds joined by cross tubes) or is it a continuous tube snaking back and forth. The latter will self purge the air. The former will not without a vent at the top, or a specific procedure to purge the air when you fill it.

          I was looking at different engine installations and for some reason a couple of them caught my eye for having the cooler installations "upside down". Some builders have perpetual oil temp issues, even after installing ever larger coolers.

          Just wondering.
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