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Old Dec 22, 2010, 06:02 PM
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How to repair balsa sheeted foam wing?

Hi,

I have just acquired a 1.5 meter glider (Gerasis Bella VE sold by Hobby Lobby) with a damaged balsa sheeted foam wing.

The sheeting on the top is creased along a line (left picture) and the bottom sheeting does not appear damaged (right picture; the broken covering is over the servo bay). The crease is on the opposite side of the servo bay opening. When I check inside the bay, the foam in the bay area is buckled, but the foam in surrounding area seems fine.

Can anyone suggest how I should fix it?

By the way, I am new to sheeted wings and they seems to be much heavier than build-up wings. Are sheeted wings much stronger than build-up ones?

Thanks.
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Last edited by smau1; Dec 22, 2010 at 06:55 PM. Reason: clarifications
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Old Dec 23, 2010, 12:02 PM
k2k
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I would cut two 3" long span-wise 1/8" slots from the bottom, one infront of the servo box at the maximum thickness of the wing and one behind the servo box. Cut down to the top sheeting but not through it. Cut two 1/8" lite ply pieces to fit in the slots and glue them in with the wing straight. My preference would be to use polyurethane glue (Gorilla glue), but epoxy or other glues will work. Make sure whatever glue you use does not melt foam (Goop, regular CA).

There are a lot of options to fixing the top sheeting depending on the result you want. If you don't intend on flying the plane real hard, you could just fill in the defect with light spackle and sand flat. A stronger repair needs to tie the skin back together. A layer of 2 to 4 oz fiberglass is quick and easy if you don't mind a slight cosmetic defect. If you want better results, then dish out the balsa before glassing and do some filling and sanding. I usually cut away a band of the damaged skin and glue in a bridge of 1/64" ply, then glue balsa on top of the ply, sand, fill, sand, etc until satisfied, then recover.

I would say that sheeted foam core wings are always heavier than built up but there will undoutedly be a thousand opinions to the contrary, so I won't say that. The advantage is that they are faster to build, strong, and have a truer airfoil. Every building method has its pro's and con's.
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Old Dec 23, 2010, 04:41 PM
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Sheeted foam core wings aren't always heavier, but it's easy to make them that way. Depends on a whole lot of things like material selection, how much glue you use, etc. Lots of times the wood can separate from the foam pretty easily. So you could cut away enough sheeting to scarf in some replacement sheeting. http://upload.wikimedia.org/wikipedi...oint-scarf.gif Instead of doing the scarf through the thickness as shown, I'd do it the other way. In other words, you could cut out a long skinny diamond (parallel to the wood grain) and then replace it with a piece that fits very well. Or maybe two or three diamonds that look like teeth. With the joint slanted this way, the glue doesn't have to be much stronger than the strength of the wood across the grain. I've done this on wing sheeting and fuselages, but not over foam. It's tricky to get the fit right. The old piece might work as a template.

OTOH, on a 1.5 meter glider, if the sheeting is reasonably thick you might just steam the dents out, glue it with something (thin, foam safe CA?) and let it be. Or use a bit of glass as someone else said. If the sheeting really is that thick, (1/16"/1.5mm?) then the damage was probably done on the ground and not in the air.

If the foam is crunched between the sheeting, that's going to be a weak spot and you have to replace it or include other support. I like k2k's idea but I bet it could be 1/16" balsa on a glider this small.

A trick I've seen for creases is to make vertical slots maybe 1/8" deep and very narrow, and glue in a bunch of little carbon fiber laminate pieces edgewise.
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Old Dec 24, 2010, 01:50 AM
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More details

Thanks for your detailed suggestions.

The damage is certainly not due to flying as the plane was never assembled. The sheeting was measured to be 1/16".

I have opened up the covering more and attached a photo of the entire crease. There is no balsa loss. The front spar appears to be fine except for a slight bent.

The foam is probably slightly crunched as the foam in the servo bay is cracked.

Do these pieces of information change any of your suggestions? Perhaps a reasonable plan is to keep the wing straight, glue in the ply supports from the bottom per k2k, then apply foam safe CA over the crease. I don't know if I can get away with only CA the sheeting.

I found that if I try to bend the wing opposite to the bent that caused the crease, the crease would rise from being a slight valley to almost flat.

By the way, is the sheeting and front spar simply glued on to the foam? There is no rear spar. Can you point me to some instruction on how to build a sheeted wing?

Thanks again.
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Old Dec 24, 2010, 02:30 PM
k2k
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The spar is undoubtedly significantly damaged even if it looks only slighty bent. Under positive G-loads it is the top sheeting and top of the spar that buckle first and lead to wing failure. Since there is a spar, I would put the forward plywood reinforcement piece along the spar, not in a separate groove. It may not need second reinforcement. Glue in the main one first and see how it feels compared to the other wing.

The spar and sheeting are usually glued to the foam with epoxy, although I prefer polyurethane glue. Contact cement can also be used.

If you do a Google search on "foam core wings" you will be able to find everything you ever wanted to know about foam core wing construction.
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Old Dec 24, 2010, 07:30 PM
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Clarification

It sounds like extra support is necessary.

By spar, I mean the long rod-like wood (leading edge) at the wing entry. It's not near the CG.

Should I cut a slot at the bottom sheeting right behind the leading edge "rod" and insert the ply support?

Thanks again.
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Old Dec 24, 2010, 08:19 PM
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That sounds like a leading edge, not a spar. So I don't think it changes my opinion much. How far out on the wing is it? If it's in the center, that's 4 times as much stress as halfway out, and even more if tapered. The strength is almost all in the thickest area of the wing section, i.e. near 33% of chord or so.

1/16" sheet is pretty heavy for a wing that size, which is probably why it doesn't need a spar.
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Old Dec 24, 2010, 11:33 PM
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The crease is at 26 cm from the center. Half of the wing span is 75 cm. The wing is tapered.
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Old Dec 24, 2010, 11:58 PM
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Ok, so the load is less than 40 percent. Of course, the section there is smaller. OTOH, the lift distribution trails off at the tips. So I'm thinking you don't have to make the joint quite as strong as the sheeting. At least if you make sure the foam under it is solid. You may be able to steam out some of the dents, once you peel back covering, with a wet paper towel and an iron.
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Old Dec 29, 2010, 04:31 PM
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I appreciate all the help.
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