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Thread: Engine Cooling Fixes

  1. #11
    andya's Avatar
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    Default Re: Engine Cooling Fixes

    # 3 is usually the hotter cylinder. what kind of probes do you have, bayonet or spark plug washer?
    I had a #3 that ran 60-70 hotter that the next highest temp. Put a bayonet probe on it and it was
    much more normal. The rule of thumb is the spark plug washer sensors run 50 degrees hotter than
    the bayonet, but mine was much bigger delta.

    Agree with most the other suggestions, don't think you'll do any damage to scat tube, I would cover up
    and go fly to see if there is a diff. Seems like you don't have much exit in the aft bottom compared
    to the normal cowl and the tab doesn't look all that big either for generating enough low pressure
    to get air flow. Not sure how much affect it would have, but the RV cowl was probably designed
    for having more dynamic pressure at the intake due to higher indicated airspeed.
    "Progress is our most important problem"

  2. #12
    CamTom12's Avatar
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    Default Re: Engine Cooling Fixes

    Quote Originally Posted by andya View Post
    # 3 is usually the hotter cylinder. what kind of probes do you have, bayonet or spark plug washer?
    I had a #3 that ran 60-70 hotter that the next highest temp. Put a bayonet probe on it and it was
    much more normal. The rule of thumb is the spark plug washer sensors run 50 degrees hotter than
    the bayonet, but mine was much bigger delta.

    Agree with most the other suggestions, don't think you'll do any damage to scat tube, I would cover up
    and go fly to see if there is a diff. Seems like you don't have much exit in the aft bottom compared
    to the normal cowl and the tab doesn't look all that big either for generating enough low pressure
    to get air flow. Not sure how much affect it would have, but the RV cowl was probably designed
    for having more dynamic pressure at the intake due to higher indicated airspeed.
    I've got bayonet probes.

    I agree there doesn't appear to be as much exit size as a standard pacer cowl. I think the intake size is smaller as well, but I need to compare with someone.

    I can't maintain higher speeds to keep pressure above the baffles higher or the temps climb like crazy. My only current option is to throttle way back and accept what speed that gives me. Improving cooling should give me the ability to cruise fast again.

  3. #13

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    Default Re: Engine Cooling Fixes

    Well FWIW,

    Two of my neighbors have RV 6s. Their oil temps tend to run hot. One added a second cooler which helps some. The other has to do step climbs.
    Your cowl is made for fast planes and even at that it has minimum air inlet.
    If you have a tunnel at the firewall a lip isn't required. My Starduster Two doesn't have one. If you do a little research you can find the formula for the relation of inlet air and outlet air. It has been to long since I did my cowl and don't remember
    What are your cyl. heads running that is more important than oil temp. They should not go above 380. 400 for a shot time max. Yea I know what the red line states. They will crack just when
    As far as the oil breaking down at 230-240. From what a friend of mine with a Dr. Engr.that worked for Shell Oil in the research lab says not to worry about oil break down at temps below 300.
    Also you need to have air flow inside the muffler shroud. If not you will cook some of the parts due to the high heat build up inside. Anyway I did, you might get lucky
    Jim

  4. #14
    Bruce's Avatar
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    Default Re: Engine Cooling Fixes

    Generally the air inlet and outlet should be the same size. You don't want to create positive or negative pressure in the cowl.

  5. #15
    Administrator Steve Pierce's Avatar
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    Default Re: Engine Cooling Fixes

    I agree with Andy, doesn't apear to be much exit air. I went through this with a Super Cub last year. I learned a lot working with the Ken at LyCon on it and what it all boiled down to was airflow. The pressure difference between the top of the engine and the bottom. You can try this and that but if you really want to get to the bottom of it quick I would suggest me sending you my manometer and you put one hose below the engine and one on top and see what it reads. Lycoming likes 5". We ran the hoses through the heater ox and to the gauge in the cockpit.
    20140819_081150.jpg

    20140819_081156.jpg


  6. #16
    Gilbert Pierce's Avatar
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    Default Re: Engine Cooling Fixes

    The top cowl is a plenum, not much air movement but a pressure increase. Basically you want to convert velocity pressure to static pressure to increase the pressure on the top of the engine, slow the air flow and gain static pressure then you want generate low pressure on the bottom. Hence the lip that generates a vacuum behind it. This aids in pulling air thru the cylinders with minimum friction loss.
    As Steve pointed out, Lycoming says you need a pressure differential of at least 0.5 inches of water difference between the top and bottom of the cowl to properly cool the engine.

  7. #17
    CamTom12's Avatar
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    Default Re: Engine Cooling Fixes

    I measured approximately 49 sq in of inlet and approximately 73 sq in of exit. I took a bunch of pics today and will post more findings after dinner.

    Sent from my LG-D850 using Tapatalk

  8. #18
    taildraggerpilot's Avatar
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    Default Re: Engine Cooling Fixes

    It looks like two issues exist: 1) the inlet area is too small for the aircraft speed and 2) the exit area is too small. Generally, the exit area should be 1.5 times the inlet area. With such small inlets all bets are off, especially since the cowling was designed for an aircraft with almost twice the cruise speed. Adding a lip will create a pressure differential, but with a small exit area it's like adding a lip to an orifice.
    Is your exit area accounting for the exhaust and other items of area obstructing the exit airflow?

  9. #19
    CamTom12's Avatar
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    Default Re: Engine Cooling Fixes

    Quote Originally Posted by taildraggerpilot View Post
    It looks like two issues exist: 1) the inlet area is too small for the aircraft speed and 2) the exit area is too small. Generally, the exit area should be 1.5 times the inlet area. With such small inlets all bets are off, especially since the cowling was designed for an aircraft with almost twice the cruise speed. Adding a lip will create a pressure differential, but with a small exit area it's like adding a lip to an orifice.
    Is your exit area accounting for the exhaust and other items of area obstructing the exit airflow?
    Not accounting for exhaust pipes. Those and the gascolator are the only obstructions down there, though.

    This plane will cruise around 140 mph around 2400 RPM.... if I can keep the temps down. Didn't have a problem with it in Alaska, even in the summer. I'm betting something changed in combination with the hotter weather down here.

    Pictures to follow in my next post.

  10. #20
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    Default Re: Engine Cooling Fixes

    I found a couple of places in the rubber seals of the baffles that look like air could be leaking past. Two are near #1:





    The one directly to the left of the prop wasn't positioned correctly the last time I put the cowl back on. You can see where a notch was torn by the flywheel teeth as the high pressure air pushed it over.

    Also, this corner by the oil cooler looks like it might be letting some air past:



    Here's some more pictures of the cowling that make it easier to see the dimensions:







    I straightened the tab out (1.5 in x 10 in) before I put the cowling back on.



    And here's a better set of pictures of the ramps:







    I bought some foil tape today, so I'll do a partial plug of the heater tube holes, but leave enough open to keep air circulating through the heat muffs (thanks, Jim). I also bought some RTV and I'll plug the few holes I found (not many. 3 small ones, I think).

    Steve, I'd love to borrow your manometer. I'll PM my address in a second!


    Pardon the bugs, she's in need of a bath.

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