How do YOU stiffen foamies' wings? - RC Groups
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Jul 30, 2012, 11:39 PM
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Kurt's Avatar

How do YOU stiffen foamies' wings?

Just maidened my 1450mm FMS P-51 and couldn't be happier, but looking ahead I'd like to avoid any cracking/excessive flexing in the 2-piece wing. Even though there's a beefy carbon rod joining the wing halves (goes about half way into each wing), and fairly solid screws attaching the wings to the fuse, after a number of flights it's inevitable the rod-holes will expand and the wings will wobble more and more.
There are several excellent descriptions and photos in the FMS P-51 thread by those who have joined the wing halves permanently by various methods, but since this is an issue with many foamies thought I'd ask everyone what method worked best, was easiest to perform, and cheapest, for them.
I've leaning towards Gorilla Gluing the stock central CF tube in each wing to lock them together, and epoxying a CF strip along the top (or bottom?) of each wing half to virtually stop any flexing.
It's down to either 7mmx1.2mm (~$15 for 2 pieces shipped to my door)
or 10mmx2mm (a bit over $20)

1.2mm thick strip should go into a carefully cut slit, made along a metal rule and to a set 7mm depth (mark the exacto blade with electrical tape?). Would cut the strip to length then slightly rough up the surface with fine sandpaper, drag the edge of the strip through the length slit to widen it a bit, pull the strip through 5 minute 2-part epoxy to give the entire surface a thin coat, then as-neatly-as-possible (while wearing latex gloves) sink it into the channel.
2mm CF rod might require a wider channel, which I'm not sure how I'd neatly cut, and intuition tells me 1.2x7mm would provide sufficient structural strength. Or?

The only downside I see to this method is an inevitable, slightly ugly smear of epoxy on the wing surface, and of course the visible black edge of the strip unless painted.

Unless there's a fool proof method I don't know about to make a very level hole and not punch through the wing surface, I'm not comfortable boring another hole for a second rod inside of the wings.

Anyone have other methods that worked well in their experience? Or at least a cheaper source for decent CF strips and a clean way to glue the strip in the wing?

Happy flying
Last edited by Kurt; Jul 30, 2012 at 11:45 PM.
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Jul 31, 2012, 12:57 AM
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KaptnKurt's Avatar
The trick for me was to add another tube spar in the aft section of the wings and make it run from the middle of one wing and though the fuse to the other middle of the next wing. Then cut a channel and insert flat carbon in the middle for the length of each wing. Then just hit the black with some paint,they don't flex at all now.
Jul 31, 2012, 08:11 AM
Upstanding Member
Kurt's Avatar
hey Kurt -
How did you go about boring the holes for the new spar?
The P-51 doesn't have much dihedral but I'm struggling to picture how one would make the holes in the left and right wing line up fairly perfectly, without drilling/boring an oversized hole.
Do you use 2-part epoxy for the flat carbon strip?
Jul 31, 2012, 08:15 AM
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Spitfiresmacka's Avatar
Killerplanes crash proof....
It's on you tube ....
Jul 31, 2012, 11:13 PM
Upstanding Member
Kurt's Avatar
I have no doubt that a full Killerplanes kit would stiffen the hell out of the entire airframe, but I'm only interested in stiffening up the wings so $53 incl shipping is overkill for my purposes. Cheers to Steve for putting in a lot of time and effort into his business, and I really enjoy his videos too.

Just ordered enough 7.0x1.2mm and 4.0x1.0mm strips to do both the P-51 and un-built JPower P-38 wings, horiz and vert stabs for $33 from RCFoam.
Jul 31, 2012, 11:57 PM
Most people are stupid!
dambit's Avatar
There's a special blue wing spar for that now
Aug 04, 2012, 11:36 AM
Upstanding Member
Kurt's Avatar
Last night installed 7x1.2mm CF strips in each of the P-51's wings, roughly triangulated with an existing square fiberglass piece that came installed from by the factory.
Didn't measure the before/after flex in any way, but can say that they are MUCH stiffer than before. I'd bet at least twice as stiff.
In case anyone wants to do this on a foamie but has never done it, here's my method:
1) Have a good look on top and bottom of the wing and find an imaginary line at least an inch or two away from the leading or trailing edge that you can run the strip from root to near the tip.
- Be mindful of servos and various in-wing wires, then try to find a line that ideally will be relatively unobtrusive (panel lines are best!).
I put mine on the wing bottom mostly for aesthetics (though you'll generally see more of the bottom wing more in flight, unless you're hovering you'll never notice it), for overall stiffness I don't think it matters much if it's on the top or bottom. Both would be ideal.
2) Using a long metal straight edge and a brand new blade (I used 2 utility knife blades taped together to cut a wider slot), mark the blade depth according to the size CF strip you have. For small planes 3mm wide is plenty, for the 1450mm span Mustang 7mm is the minimum I'd use.
3) Taking a few mm at a pass, very carefully cut into the wing along the straight edge, moving your hand to hold the straight edge firmly down so the cut is straight. You want to cut just deeper than the width of the CF strip, but even if it's 2 or 3mm deeper in places the eopxy will fill this in.
4) Sand one end of the CF strip to make a sharpish edge 5) then starting at one end push the CF into the slot, firmly pressing with your finger tip to make sure it'll seat at least flush with the wing surface.
- If it's a very tight fit, or you find that the wing's chord is in any way changed due to the CF then heat up a utility blade with a MAP torch (not red hot) and VERY carefully swipe it down the exact same line. It'll only stay hot for about 8", then reheat and repeat. You're only going for a slight melting of the cut's edges. A slip of the hand here can seriously damage a wing!
6) After you're happy with the CF's dry fit take it out, slightly rough up the entire strip with fine sandpaper, then clean well with soapy water or a bit of alcohol on a rag.
7) Mix up 2-part epoxy, then using latex gloves (not an option, unless you want to destroy your hands) drag the entire length of CF through the epoxy on both sides, trying to get a thin, even coating over the whole thing (except for an inch close to one edge for a firm grip). After I get the entire strip covered I lightly drag the front and back along the edge of a clean sheet of cardboard to roughly even out the epoxy to a thin, even coat.
8) With the wing on a steady surface, start at the very edge of the cut and push the CF in all the way down the length.
It will take a fairly firm push every 1" or so. Then slide you finger firmly down from one end to the other. Epoxy will push out the top edge and be smeared a bit on the wing. Unless you use a hypodermic needle to squirt epoxy or CA into the bottom of the cut I don't know how to avoid this, but smearing the epoxy to a thin coating on the strip before installing will minimize this. It dries virtually clear, and with a coat of clear, matte or satin finish paint will just about disappear.
9) I hit the visible black edge with silver and white to roughly match the paint scheme.

There you go, within a few hours it'll be ready to fly with MUCH less wing flexing than before.

Although there's a nice big CF tube running between the wings, there is still some flex where the roots meet the fuse so will probably use the same 7x1.2mm CF and method to tie the two wing-halfs together, on the bottom through the central fuse foam. This means they won't be removable but that's ok.
Last edited by Kurt; Aug 04, 2012 at 11:48 AM.

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