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Rebuilding Lower Cowl - Cowl flaps

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  • Rebuilding Lower Cowl - Cowl flaps

    I did something rather stupid that has caused me to rebuild my lower cowling. In the lower corners I had used a metal shrinker to form the metal to hug the curve. I had wanted to use all flush screws so dimpled the screw holes. I soon realized that was a big mistake as after a couple times on and off with the cowling I was noticing incipient cracks in some of the dimples. I tried drilling small holes to stop the cracks, but one cracked through to the edge. If it was just one I would just try to patch it, but there is a total of 6 screw holes I dimpled in the shrunk metal and will probably crack through. Remaking the lower cowl is about twice as hard as the initial build. All the screw holes on the firewall, cowl doors and nosebowl need to line up. As it turns out while remaking it, I discovered that I had interference between my cowl flaps and exhaust mufflers. I modeled my cowl flaps after Battson's and didn't have the exhaust system on the engine when I built the first cowling. I believe I know why I have interference and he didn't. Due to my cold air intake, Clint built me an exhaust system with the two back cylinders dumping into the pipe that sweeps back from the front cylinder to fit the cowling. Looking at pictures of the 3 into 1 system I now realize that my exhaust moves the mufflers back 5 to 6" in comparison. Consequently, I will be relocating my cowl flaps up the side of the cowling versus in front of the tunnel. I measure about 90" of outlet area at the tunnel, with a lot of that partially blocked by the mufflers. Planning on about 40" of cowl flap outlet, will see how that works before enlarging the tunnel outlet. I'm glad I found this out now before I had everything painted, put together and thought I was close to flying.

    DSCF1552 (2).jpgDSCF1554.jpg

  • #2
    Good catch. Not fun to deal with but better to find it now than later.

    I wonder if the shrinking jaws caused the cracks. My cowl is fastened with flush screws and don’t have any cracks in the dimples. I did find a crank where my cowl flap hinge ends. There must be some flexing going on there and the stiff spot where the hinge ends caused a crack.
    Scratch Built 4-place Bearhawk. Continental IO-360, 88" C203 McCauley prop.

    Comment


    • rodsmith
      rodsmith commented
      Editing a comment
      I'm sure it was the combination of shrinking and dimpling. Could you provide some pictures of your cowl flap installation? I think I will have to move them further up the side than I would like to avoid curvature in the cowling.

    • svyolo
      svyolo commented
      Editing a comment
      I don't think 2024T3 shrinks or stretches very well but I am sure it has been done. it works better on 5000 series not heat treat AL. I tried it a few times without success.

    • rodsmith
      rodsmith commented
      Editing a comment
      It takes more work with the Lancaster shrinker/stretcher, but you can shrink 2024T3.

  • #3
    Rod;

    I can understand the stresses at the dimpled hole as you describe it. The material work hardens when its shrunk, and dimpled and the hole is stretched when dimpled. Yet I like the way a dimpled countersunk attachment nest into each other and see significant strength over non dimpled attachment.

    Should I dimple this when I make my cowl? Are you going to dimple next time, or use Pan Head screws?
    Brooks Cone
    Southeast Michigan
    Patrol #303, Kit build

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    • #4
      There should be no problem dimpling for countersunk screws on the cowling, unless you did like I did and shrunk the metal first. The picture shows the lower corner at the firewall, which needs to be shrunk. It involves 3 screw holes. I will use pan head screws there, and flush screws elsewhere. That area actually needs to be slightly bent and shrunk to conform to the firewall shape.

      DSCF1557.jpg

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      • #5
        I dimpled my first cowl and found cracks around the dimples. I used that cowl as a template and made another. I didn't dimple this one, and have used dome head screws, which I'm very happy with.

        My post thread on it is HERE.
        Nev Bailey
        Christchurch, NZ

        BearhawkBlog.com - Safety & Maintenance Notes
        YouTube - Build and flying channel
        Builders Log - We build planes

        Comment


        • rodsmith
          rodsmith commented
          Editing a comment
          That's interesting. I just checked all the dimples in my old cowl with a magnifying glass and there are no cracks, except where I had shrunk the metal. I do use a drill size that allows the nipple on the die to fully enter the hole.

      • #6
        Dimpled holes also cause scratching on the boot cowl paint, if you ever slip when re-installing the cowl (p.s. this happens a lot when working alone).
        Countersinking isn't great either.

        I used a thick anti-chafe tape to seat the cowl onto the boot cowl, which has prevented some of the damage recurring. But it's not a fool-proof solution.

        Comment


        • #7
          I ran into another problem while relocating the cowl flaps. The inlet and outlet on the heat muffs on my Vetterman mufflers both point in the same direction. Turns out due to the location of my heater boxes, I need them offset about 30 degrees. I thought it would be a big deal to get that changed. Called Clint this morning. He said mark them up, send them back and he will make what I need for $20 each. Talk about service!

          Comment


          • Battson
            Battson commented
            Editing a comment
            Clint has always been so helpful and provides a fantastic product, I really hope everyone continues to support him.

        • #8
          It has taken me over a month to complete the new lower cowl with relocated cowl flaps. I permanently mated the lower nose bowl to the lower cowl.using 1/8th soft flush rivets. I faired over the intersection so now it looks like one piece. I spent considerable time getting the upper and lower nose bowl to fit together nicely. Lots of filling and sanding involved. Now I can work on laying out the paint scheme.

          DSCF1583.jpg

          Comment


          • Linusbear
            Linusbear commented
            Editing a comment
            Please advise what type of air cleaner assembly you used. I have noticed several BH's with this design and would like to modify/re-do my lower cowling. Thank you.

            Yours looks great.

            Scott Williamson
            924PL
            dinahdogaz@gmail.com
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