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Skin gaps in aluminum aircraft

Skin gaps in aluminum aircraft

Skin gaps in aluminum aircraft

(OP)
I normally do all composite aircraft, but I'm currently helping a coworker with his design on an aluminum skinned aircraft so I don't have a lot of experience with metal.  What are typical gap thicknesses between skin sheets for butt splices?

Thanks for the help!

RE: Skin gaps in aluminum aircraft

i'd use something between 0.1" and the skin thickness.

are you planning to file the gap with PRC, or are you going to leave it ?

RE: Skin gaps in aluminum aircraft

Lots of variation here.  For a small, unpressurized airplane, I'd say about 1/16" would be the max.  Depends on what's behind the butt splice - sheetmetal strap or frame - and how the load is transmitted across the joint.

RE: Skin gaps in aluminum aircraft

(OP)
It is high altitude but unpressurized.  Right now we have about a .032" gap and the joint is a double row of fasteners onto a frame.
We are planning to fill the gap so the finished assembly will be smooth when painted.

RE: Skin gaps in aluminum aircraft

may be a little small for the PRC to seat well.

practically, i don't think i'd dimension as 0.032" ... do you need to control the gap that closely (standard drawing tolerance would be +- 0.01")

RE: Skin gaps in aluminum aircraft

(OP)
No, I don't plan on dimensioning it that tightly at all.

RE: Skin gaps in aluminum aircraft

cool, just that here designers tend to throw in decimal places ('cause the CAD package prints it that way) without thinking ... clearly you're thinking !!

 

RE: Skin gaps in aluminum aircraft

I was going to ask if you're thinking primarily from a structural point of view or a tolerance point of view.

The smaller the gap you want the tighter the tolerance will be and hence the difficulty/cost will go up.

Within reason I'd go for a larger gap to allow relaxed tolerances.  Remember it's not just the size tolerance of the panels & structure but also the hole size & location.  I'm guessing you may be match drilling or using a jig which will reduce the hole size/location issues but keep it in mind.

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RE: Skin gaps in aluminum aircraft

I just checked the 747 CMM.  For the inlet cowl the gaps between panels is .02 to .08 with a sealant applied in the gaps after installation.

Just a reference.  If this is unmanned and high altitude and high speed this would probably be a fair rule of thumb.  If it is lower speed I think the tolerance could be even looser.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

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