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Thread: Screw directly into metal vs. Tinnerman clips or nutplates?

  1. #1

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    Screw directly into metal vs. Tinnerman clips or nutplates?

    I'm building a Zenith CH750 and it has a central "tunnel" between the seats with control runs inside, etc. The plans call for the top or lid to be riveted in place. The tunnel is not a critical structural member and I have seen others use screws instead of rivets so they can access the area if the need arose. However, I don't know how they secured the screws.

    The tunnel sides are 0.025 6061-T6 aluminum. Would it be permissable to drill a hole and attach the top directly with AN sheet metal screws? It would be rare that they would have to be removed, so I'm not concerned about wearing the holes out, etc., ... I'd just like to preserve easy access to the area "just in case".

    I know that Tinnerman clips would be an alternative, but I thought one could always resort to that if the holes did strip out too easily. Of course, nutplates would be the ultimate, but it would be pretty labor-intensive to install them ... it's a loooonnnngggg tunnel! HAHA!

    Regards,
    John

  2. #2
    Aaron Novak's Avatar
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    Personally I would use the tinnermans. Once tightened down they have better resistance to backing out from vibration than just using the aluminum itself. Plus remember that "wear and tear" on a sheetmetal screw comes not just from removal and installation, but from vibration as well.

  3. #3
    FlyFast's Avatar
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    You said that this is not a structural member, I hope you've checked with Zenith on that. It could be that the riveted structure serves as a stiffener. On the other hand, if it really is just a cover, non-structural fasteners would be okay. It is always best to check with the designer on these things.
    Where will the back of the fasteners be after completion? If they will stick out the bottom of the airplane, you'd probably want rivets.
    I agree with Aaron, that putting some sort of nut on a screw will greatly reduce the wear on the sheet metal.

  4. #4
    EAA StaffEAA Staff / Moderator Charlie Becker's Avatar
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    On my Sonex, I attached the wing & tail tips with aluminum rivets instead of the normal stainless steel rivets used for structure. I did this because if I need to remove them, the aluminum rivets are pretty easy to drill out.
    Sonex flight testing complete. Building a Super Cub clone, check it out at www.facebook.com/piratecub

  5. #5

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    Quote Originally Posted by FlyFast View Post
    You said that this is not a structural member, I hope you've checked with Zenith on that. It could be that the riveted structure serves as a stiffener. On the other hand, if it really is just a cover, non-structural fasteners would be okay. It is always best to check with the designer on these things.
    Where will the back of the fasteners be after completion? If they will stick out the bottom of the airplane, you'd probably want rivets.
    I agree with Aaron, that putting some sort of nut on a screw will greatly reduce the wear on the sheet metal.
    I believe the Zenith demo 750 at AirVenture used screws instead of rivets, but I'll check with them to be sure!

    The screws hold the cover on the top of the tunnel, so they would protrude inside the tunnel, just under the cover - can't be seen and nothing is up at the top that the screws could interfere with.

    Thanks,
    John

  6. #6
    Eric Witherspoon's Avatar
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    They don't specify small screws anywhere else in the design? On the Sonex, they use #4 x 3/8" long screws directly into the external skins to secure cover plates. These are on the outside of the airframe, and they don't loosen up in flight. In the plate, drill #30, then a very small hole in the base part. I forget what the small hole size was - maybe 1/16". Use your favorite vendor, but here's a Spruce part number to look at:
    T4X6 4X3/8-A-TR-PH-SS SHT MET SCREW

  7. #7

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    How about using rivnuts. The best of both worlds. Easy to install and reusable. Try this http://www.amazon.com/Professional-H.../dp/B0021CK4L0. Can't go wrong for 20 bucks.

  8. #8
    Eric Witherspoon's Avatar
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    The issue with the rivnut is the flange - the cover plate you are attaching is going to sit up on the flanges of the rivnuts and not be truly flush to the structure below. Though one of the members of my local chapter demonstrated a dimple die he made just for this purpose - rather than the angled dimple we are familiar with for countersunk rivet heads, this countersinks a washer-shaped circle of constant depth into the surrounding material. Install the rivnut into the countersink and the panel being attached gets held down flush to the surrounding metal. I'm not sure if a dimple die like this is available commercially.

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