Booster tips over aluminum tip ribs

Hello, everyone.

I'm fabricing a PA-12, and have run into a problem with its wingtips. The tips are a brand new pair of fibreglass booster tips from CubCrafters (replacements for the booster tips originally fitted on the aircraft in the 1970s), and they are going on aluminum tip ribs.

The booster tips fit perfectly over the ribs if the trailing edge skin is left off, but they will not fit on if the trailing edge skin is attached. The trailing edge skin has a wider radius bend at the trailing edge than the inside of the wingtips, and the skin over the wingtip (by itself) thickens the wing more than the wingtip will tolerate.

My AME and I have come up with a solution: putting the trailing edge skin over the wingtip, and the wingtip over the rib. It's a neater look than putting the trailing edge skin inside the wingtip, and I can put a spacer between the rib and the trailing edge while fabricing.

Does anyone see a problem with this approach? Can you think of a better solution?

Wingtip images at:https://photos.app.goo.gl/bHzoEFTNRQyzwWxp6

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Steve Pierce said:

You need to activate sharing on your photo album.

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Sorry about that. Corrected link is here:
https://photos.app.goo.gl/bHzoEFTNRQyzwWxp6

I would file the trailing edge of that rib down and get the skin to slide inside the tip. Nice on occasion to be able to remove those tips.

Or just cut the trailing edge off for the length that it would be under the wing tip. Notch it up about 1/2" to clear the tip trailing edge.

Steve Pierce said:

I would file the trailing edge of that rib down and get the skin to slide inside the tip. Nice on occasion to be able to remove those tips.

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Filing down the trailing edge was the first solution we tried, the problem is that the trailing edge stock has a wide curve radius, while the wingtip's inside has a long shallow taper. Even after a lot of filing, the trailing edge won't go inside.

Another idea we came up with was to do a butt joint between the trailing edge and the last rib. Put a piece of sheet metal inside the trailing edge to serve as a doubler, slide the doubler under the edge of the outer rib, then screw it in place..

Thank you for lending your experience to this problem.

dgapilot said:

Or just cut the trailing edge off for the length that it would be under the wing tip. Notch it up about 1/2" to clear the tip trailing edge.

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Thanks for making the suggestion. We had a similar idea: make a butt joint between the trailing edge skin and the edge of the rib, held in place by a piece of sheet metal serving as a doubler. The sheet metal could be screwed in place through the rib & the trailing edge, leaving the trailing edge flush with the outside of the rib.

Trim end rib so trailing edge can be moved forward. 1/2 " maybe. tip can fit over top.

I had the exact same problem trying to put a set of PA-12 Dakota Cub fiberglass tips on a Pawnee. I set them aside for a while. Let us know what you figure out.

don d said:

Trim end rib so trailing edge can be moved forward. 1/2 " maybe. tip can fit over top.

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Thank you for the suggestion, but I don't think it's practical. the trailing edge would have to be moved forward 1" (1" from the inside edge of the wingtip, 1-1/4" from the outside edge), and I'd still have to file the rib cap down by 3/4 in order to make allowance for the thickness of the trailing edge..

I would try notching the trailing edge like this.