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Rivet nuts in tubes?

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piepermd

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Oct 22, 2019
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Quick question- Would there be any significant reduction in the structural strength of a 1” diameter steel tube longeron if a series of small holes were to be drilled, say 6 or 8 mm holes along a line at 12-18 inch intervals? This is for placement of rivet nuts for attachment of cockpit skins, etc. Thanks!
 

BBerson

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Holes can start fatigue cracks.
Might be better to drill the hole then weld all around a regular nut to reinforce the hole instead of rivets.
 

piepermd

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One problem with welding tabs is that the tubes are already set in fiberglass with only about 45 degrees of arc exposed inboard and I’m concerned about welding damaging the fiberglass.
 

Peter Anson

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Stress adjacent to a hole is about 3 times as high as nominal stress in surrounding material, hence the danger of fatigue cracking, especially in the top longeron which is normally loaded in tension. Would bonding the skins using an appropriate adhesive be practical?
 

mcrae0104

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Quick question- Would there be any significant reduction in the structural strength...
The answer depends on how the longeron is loaded. It is probably subjected mostly to axial loading, so the reduction in strength shouldn't be hard to calculate (although we may not know the loads). How is it typically done for this type of aircraft, and what does the designer say?
 

rv7charlie

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The question you didn't ask: Would I use a rivnut if there were any other option? ABSOLUTELY, NO. I've never met a rivnut that didn't eventually spin in its hole.

If there's already fiberglass there, why not bond in an attachment hard point? I promise that if you use rivnuts, *someone* will live to regret that you did.

Charlie
 

BBerson

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Adel clamp around the longeron tube, with Tinnerman Clip-Nut, Nut-Plate, etc. to attach the skin

Zero stress or fatigue on the longeron, skins removable for maintenance/inspectio, and the skin mounting tab can be rotated any direction to match the skin.
I think a regular U shape hardware store type pipe clamp with a screw on both sides would hold firm and simple.
 

piepermd

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Oct 22, 2019
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I have considered using Click Bond studs bonded to the fiberglass but will be attaching switch boxes, throttle quadrant, and trim wheel controls to these points as well as the skins and am concerned about the strength of this method.
 

piepermd

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The problem with that is the longeron tubes are mostly encased in fiberglass and it is not possible to wrap clamps around them.
 

piepermd

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Oct 22, 2019
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So the skin is bonded....
No need for clamps. I guess I am confused :dead:
The fiberglass skin is already attached to the tubes, but I’d like to attach faux longerons, aluminum skins, and the various switch and control boxes to make it the most visually accurate replica of the P-51 cockpit as possible. If I could use structural adhesives such as the epoxy or acrylic used in Click Bond fasteners to attach to the fiberglass skin that would be great, but I don’t know if that would be as strong as the more problematic approach of attachment points on the steel tubes.
 

crusty old aviator

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Looking at the photo: you've already installed hard points: just install more where ever you need them, they’ll hold a quadrant fine. Use a 2x2 or larger phenolic block instead of another 1x1 if you’re concerned about strength.
 

piepermd

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Oct 22, 2019
Messages
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Looking at the photo: you've already installed hard points: just install more where ever you need them, they’ll hold a quadrant fine. Use a 2x2 or larger phenolic block instead of another 1x1 if you’re concerned about strength.
By “hard points” are you referring to the little threaded inserts glassed in to the fiberglass skin?
 
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