In the process of trying to skin some some thin, long foam wings. These are for a 104" U2 variant-the TR1. They are scale and therefore quite thin. One fellow suggested using Kevlar fishing line-as he had done-under the 'skins', drawn taut between ply ribs at each end. I was considering more or less convential spars, wood or carbon fiber rod/tube. The kevlar apparently has zero stretch. The question is: should wings of this type have the ability to flex, or are they best made as rigid as possible. The kevlar idea certainly has merit as to its ease of installation. Any thoughts? Bill

From my experience, glider wings cannot be made strong enough to not flex. So don't have that as a goal.

Also, if you're building an existing design, or even your own original but it uses the same construction method and components from an existing design, I wouldn't worry too much about trying for a major increase in strength.

There are a lot of construction ideas that sound great, but wind up having an application flaw. For example..... the thread. You're hoping for it to provide a significant increase in spanwise strength and rigidity. If you think about it's diameter a minute, it might not seem so magic. If it does provide rigidity, any at all, it's going to have to "distribute" it. So what's it going to be glued to that can buffer that increase in rigidity to the rest of the wing? There aren't too many things that can take what is basically a zero size point that has huge rigidity and not simply be sliced and diced by that tiny "stress riser". What you're basically asking the support structure to do is to stand all that force in what? .05" of it's own area? What materials have that strength?

So you have a root rib out of say aluminum and a tip rib the same. What does the rigid thread do to hold the wing inbetween from flexing? So you plug in some more aluminum ribs. See where this is going.

Surrounding structure has to be matched to whatever stiffens or supports it. I think a spanwise stiffener for a glider wing would have to be somewhat wider and not so rigid. A tape of glasscloth along the spar has been adequate for years. If you don't have a spar, then a wider tape between the foam and skin has worked for years.

Hey, I just waxed eloquent. I should have simply said that your idea to use a conventional spar will be way more than adequate. It will be very easy to execute. And it's proven.

I've seen and done a "T" spar and considered it the most elegant solution. Simply run your wing spanwise through a saw along the spar line. Glue the pieces back together sandwiching a strip of wood. Orient the grain of the wood vertically. Sand the wood to match the foam. Lay a strip of glass reinforcing tape to cap the T of the spar. You can layer the strip center span. The foam will make handling the tape easy. The epoxy will turn it into a very nice, sufficiently wide and rigid spar.

That does seem to be a simple method. It would not have occurred to me to saw my wings in half! Yet makes sense, in terms of ease. Now, how do you finish the tape off? I assume you use epoxy to skim over the tape.

Yes, epoxy. Glass cloth provides it's strength when it's locked in solidly. The epoxy does that. And it bonds it to the foam as well. Although, the bond to the foam is only as strong as the foam. So..............

If you're going to skin the wings, epoxy makes a good job of that. And to simplify the entire process, lay the tape with a thin application of epoxy, the same batch you're about to glue the skin on with.

I've built more than one (many more than one) big balsa-skinned-foam glider wing. A couple had no spar at all. Just skinned foam. It is amazing how much strength the skin imparts when it's got a chance to soak in some epoxy from the attachment process. And for most glider wings, I would be perfectly happy with a sheeted foam wing with just the spar for added strength. If your sheeting is sufficiently thick, and decent balsa, you'd be amazed at just how strong it is by itself. The camber provides a great deal of that strength. The foam is just strong enough to provide support to keep the two skins from buckling inward. Add any kind of spar and it's really good. And think about it..... you're going to stick the spar wood in the foam sandwich with what? epoxy..... which then does for the wood fibers what it does for fiberglass cloth.

Way back in the day that foam was a new method, we'd cut the blank from root to tip on a line where the spar would go. We then cut some balsa sheet into many pieces and run them out the wing. We wanted the grain going up and down, not spanwise. We'd stick the pieces between the foam pieces and press them so all the excess was sticking out the top surface. When the glue was dry it was simple to use a sanding block to bring that one "fence line" down to the foam. Then we painted the inside of the skins and etc etc etc.... BTW, paint the wood with the epoxy, not the foam. And once the wood is coated, use a playing card to scrape the epoxy to uniformity. The time it takes gives the epoxy just the right amount of time to penetrate the wood. And you'll have more than enough epoxy to stick the wood to the foam.

The toughest aspect of any epoxying job is putting on no more epoxy than needed. We almost always wind up with too much.

Good luck on the wing no matter what you do. Post some pictures later.