Skin gaps in aluminum aircraft 1

[john9786]

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!

 

[rb1957]

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 ?

 

[bf109g]

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.

 

[john9786]

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.

 

[rb1957]

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")

 

[john9786]

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

 

[rb1957]

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 !!

 

[KENAT]

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.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[KirbyWan]

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.

 

[rerig]

See attached for 737-900 Skin gaps, Pg 6, "E".