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Sep 28, 2013, 05:07 PM
Duane, LSF IV
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Build Log

Repairing a hollow Fr3aK wing


My hollow Fr3aK wing suffered minor leading edge damage in a midair collision at BASS. At the time, I just taped over the damage and continued flying. I showed the damage to a couple expert builders, and the consensus was that it should be repaired, since the damage was on the throwing peg side. Now that the contest season is over, I've commenced with the repairs.

First, I felt for soft spots and taped around the damaged area so the cuts would bisect the masking tape. The tape should help stabilize the fibers on the surface of the skin, to minimize splitting. I used a Zona razor saw to make two parallel cuts, roughly perpendicular to the spar. Since it wasn't practical to make the cut parallel to the spar with the saw, I used multiple light stroke of a snap-off razor knife, cutting straight down through the top and bottom skins.

After the damaged area was removed, I used the edges of the cut out area to mark a block of scrap foam to fit. I cut the foam to fit with the saw and knife. I used 5-minute epoxy to glue the foam block in, filling the removed skins. After waiting for the epoxy to fully cure, I carved the foam to rough shape using the knife. I removed the original tape and re-taped to protect the undamaged, adjacent surfaces, and sanded the foam flush with the wing. The epoxy at the edges didn't sand well, so I used the razor knife to carve the slight epoxy bead flush. There were very small indentations and gouges, along with a low area where I had mispositioned the sanding bar, so I filled with lightweight spackle, and waited for it to dry. After re-taping again, leaving 1/2 inch of the original skin exposed around the damaged area, so it would be roughened for better adhesion of the repair patch, I sanded it flush again. I used a 3M foam sanding block for final sanding.

Next, I will use transfer tape to apply a length of braided 1K carbon sock, split lengthwise (like a small piece of bias fabric), over the leading edge of the foam, to protect it from impact damage. Then, I'll laminate skin patches cut from scraps of bias ST carbon fabric, with freezer bag plastic taped over to yield a smoother finish.
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Sep 29, 2013, 12:27 AM
Always in the Glue
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Congratulations on a well thought out and skillfully executed repair. I'll certainly keep this in mind should I have a similar challenge in future.
Sep 29, 2013, 06:25 AM
nigelsheffield's Avatar
Thanks for sharing, I would not have had a clue how to repair a hollow moulded wing(my blaster 2) should such damage occur, I have had a few very small l/e dinks but so far I have just wicked in some CA and taped over.
Looks quite scary to be cutting so much of the wing away.
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Sep 29, 2013, 09:27 AM
Duane, LSF IV
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The section removed probably looks bigger in the close-up shots. It's about 1.5 inches (38 mm) wide. After removing the damaged section and seeing the inside, I might have been able to remove less, but I wanted to make sure all the damaged area was removed. The wing feels stiffer in torsion with just the foam than it did with the break in the leading edge. I expect it will be even stiffer with the skin patches in place.
Oct 03, 2013, 04:15 PM
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FD Pilot's Avatar
I commend you for your fortitude! However, you have verified why I stay away from hollow molded. I prefer foam core molded....Akcent 2, Validol, Predator II etc. I'm still learning so robustness is a requirement for me
May 13, 2014, 11:00 PM
formerly Juddkleinman
Mountain Soarer's Avatar
Doing a very similar repair to a Supper Fr3aK. I have the foam part almost done. Can you elaborate for me on applying the carbon skin. I have some scrap spread tow that I was going to epoxy directly to the foam. I looks like you added another layer? Why?
Did you over lap the old skin at all? did you use a top and bottom pice or did you fold one pice over the LE?

Thanks for the help and for turning me on to the thread via the Fr3aK thread.
Last edited by Mountain Soarer; May 13, 2014 at 11:00 PM. Reason: spelling
May 13, 2014, 11:32 PM
Duane, LSF IV
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I actually haven't finished this repair yet, and I do need to get the plane ready for flying season. I'm not sure what other layer you're seeing. The photos only show the foam block, the original spread tow carbon skins, and some blue painters tape to protect the skins. I've since applied the braided carbon to the leading edge with 3M 924 transfer tape. I plan to cut two pieces of spread tow carbon, one each top and bottom, to overlap the original skin 1/2 inch on all sides around the repair. The fabric will probably be difficult to fold over the sharp leading edge. Braided carbon on the leading edge will be wet out, and skin patches will be wet out on heavy plastic film (freezer bag cut open), excess epoxy blotted with paper towel and roller, then carefully placed over the repair. The plastic film will be taped to the wing, the wing placed between layers of foam padding, and a small sand bag and weights placed on top to press the skin against the foam. Will have to be careful not to use too much weight and deform the hollow wing.

The Super Fr3aK is a solid core wing and won't need a foam block inserted, unless the foam core is badly damaged. I believe it's Rohacell, which doesn't steam out dents like polystyrene foams.
May 14, 2014, 10:05 AM
formerly Juddkleinman
Mountain Soarer's Avatar
I did have to cut out chucks as you did. I hit one of those big light towers at a little league field and although there was not spar damage ( and I bounced and flew away from the impact!) the Rohacell disintegrated beneath the carbon.
Tell me about the braided carbon. Where do I get it? that is the pice I am missing.
I just picked up these killer waited bags from a dive shop that will be perfect for that part of the process.
As far as the freezer bags, are they just ziplock bag material or like turkey bags, or mylar?
May 14, 2014, 10:26 AM
Duane, LSF IV
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I bought the braided carbon tube from CST. I think it was the 1K 0.3" diameter. It's a lot cheaper and easier to get than 1K fabric and works for narrow bias pieces, as I am using on the leading edge. I slide the tube over a popsicle stick, smooth the braid flat, apply transfer tape to both the wide flat sides of the carbon, and carefully use a razor knife to slice along the thin edge to remove the tube from the stick. Then I use scissors to cut between the two pieces of transfer tape and trim the edges to the transfer tape. You could leave it as one piece if you wanted it wider.

I filled a heavy zip lock bag with play sand. Works well for conformal weighting, and I can stack an old UPS SLA battery on top if I need more weight.

I use zip lock freezer bags, cut open, with the zipper cut off. I think they're polyethylene. Thin Mylar would work also, but the .014" sheet typically used for bagging would be too stiff to wrap around the leading edge. You'd need two pieces for that, and would probably need to thin it along the leading edge, as you'd do for bagging. Heavier sheet will leave a better finish on the repair.
May 14, 2014, 11:10 AM
formerly Juddkleinman
Mountain Soarer's Avatar
Great info. Thanks.
May 14, 2014, 11:59 AM
formerly Juddkleinman
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What is your source for the transfer tape? What width?
May 14, 2014, 12:21 PM
Duane, LSF IV
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I got my transfer tape from ULine. I think it's 1/2" but I'd have to check. I had to buy the minimum six rolls, and I think I've still got four left. PM if you're interested in a single roll.
May 19, 2014, 08:57 PM
formerly Juddkleinman
Mountain Soarer's Avatar
So my repair is curing now. Do I let it go the full 8 hours before peeling off the plastic?
May 19, 2014, 09:53 PM
Duane, LSF IV
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Quote:
Originally Posted by Mountain Soarer
So my repair is curing now. Do I let it go the full 8 hours before peeling off the plastic?
Depends on the epoxy you used, but I'd leave it at least 24 hours if you can. Epoxy shrinks as it cures, but the plastic helps stabilize the surface. If you remove the plastic before the epoxy has fully cured, shrinkage can cause fabric print-through, leaving a textured surface. If you used a spread-tow patch, this is probably less of an issue, but longer is better. Also, keep it some place warm - at least 80-90F if possible.
May 19, 2014, 10:39 PM
formerly Juddkleinman
Mountain Soarer's Avatar
Cool. thanks. I used West Systems 105/205 and it states that it should be fully cured at 6-8 hours.
It will be about 20hr by morning. I have two areas to do. I started with the small one. I'll post pictures tomorrow.


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