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Bonded door skins and nut-plates

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  • #31
    Rob, I’d be interested to see where your lexan is cracking and how.

    We used 3mm (basically 1/8”) plexi and IMO it is just thick enough. My doors and my big rear windows just barely don’t drum. The left side rear window is the most prone to drumming, it is big and totally flat, but it hasn’t done it bad yet.

    We used acrylic bits, drilled oversized and were careful about how tight we made the screws. The curve in the doors created quite a bit of stress at the screws near the curve and that is where ours has cracked. I’ll see about getting some pictures when go to the airport next.
    Scratch Built 4-place Bearhawk. Continental IO-360, 88” C203 McCauley prop.

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    • James
      James commented
      Editing a comment
      When we drill off and install the windows in at my work, the rule of thumb is torque the screws until you can just see the light reflected off the transparency bend with surface deformation, then back off until there's no localised "dimple". But this is much thicker lexan, probably about 150 thou thick, and we put down layers of padded tape under the window to torque against. All these are missing with Bearhawk doors, so I'm not sure how tight to do screws.

      Again, more speculating, but what happens if you did a bonded layup with some sort of wrap-around tape? Fibreglass? Or what if we glue the windows to the frame, then put oratex or something over the top with a cut out to see through, and shrink wrap the lexon to the frame like that??

      James

    • robcaldwell
      robcaldwell commented
      Editing a comment
      I'm pretty sure I know what my problem is... I used very large head pull rivets to attach the lexan to my door frame. Between the lexan and the oversized hole, I inserted a neoprene washer. I'm sure the washer took up too much space and when I pulled the rivets, the tension was too much, causing the cracking. Where James is saying they consciously adjust the torque on the screws to a desired tension, pull rivets just squeeze as tight as possible until the mandrel breaks. I will probably try rivets again this time without the neoprene washers. I can't use screws since my door frame is carbon fiber. Well, I guess I could... I would prefer not.

  • #32
    My flight display in my carbon fiber panel, is secured with sheet metal screws, direct into the CF! No nut plates etc. apparently needed.

    In the Rans kitplane world....we use Lexan, for our skylights and door windows, but plexi for the wind shield. "Soft"/easy pull, big flange rivets and over size holes seems to work just fine, but we also use a thin trim strip of aluminum between the Lexan and the rivet. At least our skylights, that works great. Can't recall the thickness, but just under 1/8 " is about right. Then again you big motor guys have more drumming
    to deal with.

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    • #33
      Holes in thin composites can wear away quickly. You need fasteners with a wide bearing area or wide washer. The bottom of my instrument panel (CF) is hinged with a bonded aluminum. I haven't done it yet, but I will use a few wide base SS pop rivets from the back side of the hinge, and in the front either just a large washer, or maybe a tinnerman washer, supporting the back side (but the front of the panel) of the rivet. Something that looks like it supposed to be there.

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      • #34
        Originally posted by svyolo View Post
        Holes in thin composites can wear away quickly. You need fasteners with a wide bearing area or wide washer. The bottom of my instrument panel (CF) is hinged with a bonded aluminum. I haven't done it yet, but I will use a few wide base SS pop rivets from the back side of the hinge, and in the front either just a large washer, or maybe a tinnerman washer, supporting the back side (but the front of the panel) of the rivet. Something that looks like it supposed to be there.
        That was my thought also, but to my surprise, the half dozen times I've removed and replaced the screws, they torque down the same. It is an experiment in progress, for well over 1000 hours now. Safe enough, as it won't drop out in my lap if all the screws fell out. I'll eventually install some nut plates probably, but so far so good.

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