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Apr 01, 2017, 04:20 PM
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Carbon fiber repair

So my left wing tip is damage and I need some help in deciding how to repair. The damage starts at the root and extends to here end of yellow stripe. The black color identifies the area damaged. It's soft to touch and is depressed. Should I cut out the entire area and replace with balsa and layers of carbon sheets or ? The width of the depression is maybe 1 inch wide. I really don't know how else to describe damaged area and most pics I take don't really show damaged area. Any and all help will b appreciated. I have repaired many balsa build up but never carbon fiber hollow wing.
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Apr 02, 2017, 01:07 AM
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Knowing what caused the damage and/or what kind of forces were applied to the wing would be helpful.

(Before reading the following, please note that I'm an obsessive-compulsive perfectionist and familiar with full-size composites repair procedures. There probably are less labour-intensive, but good enough ways to fix this.)

What I'd do is some careful explorative surgery. Take a Dremel with cutoff wheel and cut around the damaged area, just deep enough to penetrate the carbon layer (which is maybe a hundreth of an inch thick). Then carefully peel off the damaged carbon.

Now you can see the core material, most likely some kind of foam, maybe balsa. See if that is damaged. If not, thank the deity of your choise, and sand the area around the damage. If you manage to make a consistent chamfer at least half an inch wide in the thickness of carbon skin, so much the better. Laminate a patch of new carbon in place, paying attention to fiber orientation.

If the core material is damaged, sand or scrape it off as well, then inspect the inner fiber layer. If it is damaged, laminate a patch over it. Don't fret too much about getting the airfoil shape exactly right, it will be done later with the core material.

Next, prepare a patch of core material, slightly thicker than necessary, and glue it into place. A vacuum pump would be helpful here - the vacuum "bag" film taped onto the wing so that air pressure inside the wing is pushing the inner fiber layer outwards and the rest of atmosphere pushing core material inwards.

When the inner fiber layer and core are fixed, the structure is stiff enough to be sanded into shape (that's why the core patch had extra thickness). There's likely some voids between edges of core patch and original core material, fill them with microballoon slurry. Prepare the outer skin around damage area and laminate new carbon in place.

When finishing the repair, do not damage the fibers of the patch or the original structure by sanding into them. Apply some lightweight filler around the patch and flair it smoothly onto the original surface. There will be a slight bulge in airfoil shape (because there is double thickness of outer skin fibers where the patch overlaps original structure), but that can't be helped.
Apr 02, 2017, 10:53 AM
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Jkettu. Thanks for the reply. I understand the steps you outlined and can perform all with exception of vacuum bag. I don't have a vaccum pump so is there an alternative to that step? From my examination of the wing it appears that all layers are damaged. So with that in mind and that the damage was caused by blunt force contact with the ground in a lost of transmitter control would I remove all damaged material and place some balsa strips under good section of wing and then lay new patch material to build up surface. I hope that my explanation is clear.
Apr 02, 2017, 09:15 PM
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I understand your idea and it will work fine. In fact, that will provide a stiffer surface to work on, than trying to fiddle with a single inner layer of carbon. In this case vacuum is not necessary, you can just tape the core material down while the glue cures, and it won't try to bulge the inner fiber layer because balsa is holding it in shape. The extra balsa can be left in place, the extra weight will be neglicable.

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