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Old Nov 29, 2012, 12:03 PM
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Build Log

Molded wing repair with urethane foam


I've seen a lot of guys give up or do a crappy job on hollow wing repairs. It's really not that difficult with the right stuff.

Materials:
Dixi cup
mixing stick
Sandpaper
plastic (old bottle will do)
Two part Urethane (not the squirt foam)
Tape
Catalog or Phone book (only for large holes)
CA
Sucker (big type)
White putty
Paint (some auto paint stores will scan your wing for a perfect match and put in regular spray can)

Picture is of tree impaled wing tip. All the way to spar, but no structural damage.
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Old Nov 29, 2012, 12:07 PM
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CA parts


CA in what parts you can. This was a fairly hard hit, but I managed to find a couple pieces.
Old Nov 29, 2012, 12:16 PM
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Bend Plastic


Bend some plastic or mylar to match the airfoil.
I sometimes use an undamaged wing to bend it with a little heat. Not too much heat or you can damage the other tip.

NOTE: be sure to wax or use mold release. You can also use the silcone release paper from your covering. I used to throw a lot away, but now I keep it.

Remember, the leading edge is not necessarily sharp, so don't just fold it.
Old Nov 29, 2012, 12:21 PM
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Sucker


Sometimes if I have to get up into the middle of a wing, I use a flavor injector. With the foam, you have to drop it in a jar of lacquer thinner immediately after use. This one has made it through a number of repairs and is still like new.

The other tool I use is a Sucker. The larger holed ones work great. For the narrow repair I used it, but for the larger hole, I just poured the foam in.
Old Nov 29, 2012, 12:27 PM
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Mix the foam


Once you have everything ready, you can mix the foam. Equal amounts of A and B.
NOTE: It is better to use too little than too much. A tiny bit goes a long way.

I opted to do one hole at a time, since you only have a minute or so to work.

I did a quick mix (a couple of seconds) and sucked it up into the tube. It will start coming out right away, so work fast.
Old Nov 29, 2012, 12:32 PM
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Cover with plastic


Once you have the foam in, tightly attach the plastic. For the first hole, I just taped it.
On the second hole, I used a catalog the help secure it. Again, be sure it is waxed (on the inside). You can do a simple test. If masking tape won't stick to it, it is okay.
Old Nov 29, 2012, 12:44 PM
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A little white putty


After it sets, remove plastic. I usually wait about 30 min, but it only takes about 10. The main thing is to not take off the plastic too soon, or it will pull off some of the foam. If done right, you have an almost perfect surface.

A little white putty and block sanding and it's almost done.
Old Nov 29, 2012, 12:56 PM
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Prime


I primed, sanded, primed, both top and bottom. I didn't have any gloss black, so have to wait for the final picture of the whole job. Needs a tiny bit more sanding on the leading edge.

Weight (with servos) before repair, 7.5 oz
After repair, 7.7 oz (could probably have used a bit less putty if I had waited a little longer for the foam to set on the big repair. Could also have used some lightweight filler before using the white putty.) Two tenths of a ounce isn't too bad for such a large repair.

I have the foam kit for sale here:
https://www.rcgroups.com/forums/show...5#post23524420

Believe me, a little goes a Long way.
Last edited by tbroeski; Dec 14, 2012 at 08:12 AM.
Old Nov 29, 2012, 10:48 PM
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Just a cosmetic patch? Fibers still broken and therefore structural integrity compromised?

Old Nov 30, 2012, 06:31 AM
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Quote:
Originally Posted by jkettu View Post
Just a cosmetic patch? Fibers still broken and therefore structural integrity compromised?

Not much "structure" in front of the spar. You want the airfoil shape to match. Since it is solid foam, it has as much structure as you would need.
Old Dec 03, 2012, 07:51 AM
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It is also great for those leading edge dings. If done right, it has a hard smooth surface.
Old Dec 06, 2012, 07:24 AM
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The foam needs to be contained to have much structure. Coming out of a dixie cup actually gave it structure. Here's a video of the stress test after 20 or more hits (trying to make a video). I've burned it, spray painted it and hammered it.

I will stress again to make sure there is a place for the foam to go. Inside a hollow wing is fine, but if you put it between ribs or something, it will need a release hole, or lot of weight and a good form.

hamer foam (0 min 11 sec)
Old Dec 06, 2012, 04:23 PM
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Quote:
Originally Posted by tbroeski View Post
The foam needs to be contained to have much structure. Coming out of a dixie cup actually gave it structure. Here's a video of the stress test after 20 or more hits (trying to make a video). I've burned it, spray painted it and hammered it.
burned It? it IS highly toxic???? YOU OBLIVIOUS NEED A BIGGER HAMMER, come on.... SWING
Old Dec 06, 2012, 05:06 PM
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If brute force doesn't work, you're not using enough.
Old Dec 06, 2012, 05:33 PM
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Quote:
Originally Posted by IHAVAWDY View Post
burned It? it IS highly toxic???? YOU OBLIVIOUS NEED A BIGGER HAMMER, come on.... SWING
Don't eat it, smoke it or sleep with it.....


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