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Joining the Wings
Author: Clay Ramskill


by Clay Ramskill

At one time or the other, we all have seen or will see an RC plane go in after the wing has folded and/or fell off. The flight path, wingless, is in scientific terms, a parabola - and results in a VERY messed up airplane!

The usual area where a wing breaks is at the wing joint in the middle. That's where the highest forces are concentrated, and in most cases, left and right wings were fastened together there in the first place. And, of course, that general area is where the wing is fastened to the rest of the plane.

In the figure, on top, is a fairly typical wing structure, but with a marginal joint between the two wings. The narrow strip of fiberglass or nylon tape with no other special support is sufficient in theory - but in practice, it depends too much on the glue joint between the sheeting and the spar, in a very small area. Note that foam wings use this method very successfully, where the wing skin itself provides the strength.

Considerably better is to extend the fiberglass on out past the next rib, at least in the area of the spar, providing much more area to bond glass, skin and spar together. Equally good is to use spar joiners in concert with the narrow glass strip - aircraft ply is far stronger for this job than lite ply. And perhaps the very best is to use a larger piece of ply to cover the whole area, as in the "best?" section of the figure. In this case, you're replacing the balsa webbing with ply, also. And ply, with less chance of splitting, is a far better webbing material. Also, note that you now have, with the ply, a perfect place to drill for wing dowels!

However, there are some qualifications here, hence the question mark. The one large sheet of ply means possible problems with seeing what you're doing when you glue it in, as is often the case with pre-constructed or ARF planes. The construction sequence should be arranged so that you can fit this piece and see the joints, and preferably be able to clamp them, as you do it!

And hence, the problem with some ARFs - they come precovered, so all you see is one or two spar joiners sticking out of one wing, and a hole or two in the other. And you're supposed to daub some glue on them and plug them together. So you have no way to know that the joiners are even touching the spars, let alone whether there's glue all along the joint.

Note that even with spar joiners you should still use glass or nylon tape to join the sheeting together. If there is 3 inches of 1/16" sheeting between the leading edge and the spar, that is the same amount of wood as would be in a 3/8" x 1/2" spar!! That's a lot of extra strength to be gained, just by properly joining the sheeting together!

Just a quick point about keeping the wing on the plane. The usual problem is with dowel(s). It/ they must be of sufficient size - 5/16" for one, 1/4" for two on a .40 size plane is usually sufficient. And they must be anchored strongly in the wing, go through a hole in a strong bulkhead, not have any appreciable slop in that hole, AND have negligible exposure between the wing and the fuselage bulkhead. The idea is that those dowels are very strong as long as they are only exposed to shearing action; but if they have to undergo any appreciable bending stress, they're weakened considerably.

Lets face it - the wing joint and the dowels or other wing fastenings are critical to the life of your plane. This is one place where you don't want to be asking "where's the beef?".


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