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Old Oct 18, 2012, 01:14 PM
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Help!
Obechi Wing Repair

I recently acquired a sailplane with obechi over foam wings.

One of the wings has a couple of spanwise creases in the top sheeting about 1/4" from the leading edge piece. The longer crease is probably 6 inches, the other maybe 4". They are probably 1/16" to 1/8" deep, so the the wing is a little "flat" from the leading edge to the crease, and then it rises up quickly to the normal contour.

My first inclination is to just fill the crease with some sort of sandable material and then use a sanding bar to bring it down to match the rest of the wing. Ultimately I'll be covering the wings with Ultracote/Oracover, so it doesn't have to be pretty.

I'm open to suggestions as to what to use. I've thought about an epoxy/micro-balloons mixture, or maybe even just gluing in some soft sheet balsa with Titebond, either of which should be sandable.

Any other suggestions?

Jim
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Old Oct 18, 2012, 03:46 PM
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Gently pull the skin up to were it should be. then tape any holes or cracks .Then inject some gorilla glue.It should expand up to the skin.
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Old Oct 18, 2012, 06:28 PM
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Gently pull the skin up to were it should be. then tape any holes or cracks .Then inject some gorilla glue.It should expand up to the skin.
I would expect that to expand beyond the skin level, creating a bump.
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Old Oct 18, 2012, 07:24 PM
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I mixed up some epoxy and micro-balloons tonight. Applied it to the crease and then went over the area with a credit cart to smooth it out. I think I'll need to add a little more tomorrow as it looks like I didn't bring it out far enough. I think it'll be ok.
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Old Oct 19, 2012, 07:31 AM
Red Merle ALES
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Quote:
Originally Posted by GliderJim View Post
I mixed up some epoxy and micro-balloons tonight. Applied it to the crease and then went over the area with a credit cart to smooth it out. I think I'll need to add a little more tomorrow as it looks like I didn't bring it out far enough. I think it'll be ok.
Perfect!
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Old Oct 19, 2012, 11:39 PM
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Although the epoxy and microballoons ought to do a decent job, I should think the first choice would be to put an iron over a damp rag to steam the foam back out. Then, if there was any sign of delamination, I'd try to get some glue under the obechi.

I think I would have gone with the balsa, since it sands so easily.

I find that Monokot or Ultracote over wood is just a set of wrinkles waiting to happen. And it's hard to peel off for repairs. I think in your place I'd try to get things as smooth as possible and then use water based polyurethane, put on sparingly in very light coats with a paper towel as per DJAerotech. (Would rather use Sig sanding sealer, but afraid of hurting the foam). Or else I might use 3/4 oz. glass, with some effort to prevent too much epoxy. Could weigh about the same as the Ultracote.
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Old Oct 20, 2012, 08:11 AM
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I thought about trying to steam it out, but was worried about what it might do to the wood. There were no cracks or holes in the obechi, so I'm not sure if the moisture would've made it underneath, plus I didn't have a way to inject any glue underneath anyway.

The epoxy/microballoons is working out well, although the area that needed it turned out to be much larger than I realized, so I have quite a bit on there. And I agree that balsa would've been easier to sand and have questioned if that might not have been a better choice.

Sometimes impatience wins.

As far as covering goes, the wing was covered when I received it, and it was indeed a wrinkly mess, and as such, it came off very easily.

I have two other obechi covered planes, one natural and one with Econokote covering. The only reason I'm considering the film covering is to hide the repairs. We'll see. I'll try to post picture of the repairs so far later today.
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Old Oct 20, 2012, 01:18 PM
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Wing Repair

I added some lightweight spackling compound to finish off the repair as I had a few ridges here and there and I didn't feel like adding more epoxy. It turned out well and I can only hope that I'm at least close to the original airfoil.

It's a Ruby 2M by the way.

Jim
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Old Oct 20, 2012, 02:57 PM
Red Merle ALES
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You will never know it inflight.
Nice work!

A friend of mine accidentally stepped on a very similar wing and I repaired it the same way. I liked the nice obeechi grain and finished it with clear Deft. I then put in small letter next to the repair "No Step".
It was a good conversation piece.

Curtis
Montana
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Old Oct 21, 2012, 10:04 PM
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I'm just starting to do my own balsa repairs to a wing, and man, what a pain to sand epoxy without gouging the softer wood around it! Even with a non-conforming flat tee-bar the sandpaper still eats away the adjoining wood and leaves the epoxy higher.

I've never gone way overboard with microballoons to make a "soft" mixture, but the label says it's sandable. I've gone about 2:1 balloons:epoxy and it was still quite hard, albeit noticeably more brittle.

What was your ratio? Did it need sanding or did it squeegee smooth enough with the credit card?
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Old Oct 22, 2012, 06:44 AM
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I don't know my exact mixture, but I added about as much as the epoxy would readily absorb. It did need sanding, and I used a t-bar also. However, I was blending it into obechi, which isn't as soft as balsa.

You can protect the balsa around the repair with masking tape while you're sanding, but that's only going to get you most of the way there. From that point you'll need something softer like lightweight spackling compound to fill the transition areas from the epoxy to the balsa.
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Old Oct 22, 2012, 10:31 PM
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It will probably also help to use fairly fine, but fresh paper, to apply very little pressure, and to be PATIENT. Also helps if the epoxy has had a week (or a warm place) to set up really hard.

For my last repair job, I picked up some sandpaper which looks purple. Somehow it clogs up less. I was actually impressed with it. However, I'm sure it will still clog if you try to sand epoxy that isn't totally hard yet.
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