I always glass my center section of my wings, but I wonder if it is over-kill. During HIGH SPEED straight and level flight, what forces are being exerted on my wings??? Are the leading edges wanting to be pushed straight back to the tail feathers??? do high speed turns want to fold the wings. I have only had one structural failure...wing failure (nowhere near the glass joint)

I always glass the center wing join of all of my ARFs, too. This would help keep the wing from breaking in half on hard landings. Especially, when you have 2 aeleron servos and/or retracts, the wings are more prone to breaking in half. When I first learn to fly, the wings on my trainner keep comming apart on hard landings, I'd learned to glass center wing join of all of my ARFs, both sides, and never have any problem with the wings again. I have instant where the fuselage broke in half on landing but the wings are still in tack. I think it is worth the time to glass the wing center.

About the force acting on the wing. You bet that there are a lot of forces trying to break the wing apart, especially, on rolls and recover from steep dives.

This is a simplified approach full of inaccuracies but fairly intutive I think. Assume an airplane that weighs 10 pounds. Put it into a sharp pull up that causes it to pull 10 gs, 10 gs is 10 times its weight and very easy to do. The wings then have to support 100 pounds, 10gs times 10 pounds. Each wing panel has to support 50 pounds. For simplicity you can assume that the 50 pounds is applied about 1/3 of the way out from the root to the tip.

Practically it means if you hold one wing panel on a table with the center on the edge of the table and with the other panel over the floor and push with your flat hand 1/3 of the way out on the wing about 50 pounds worth it will give you an idea of how strong the root needs to be.

Glassing the root of the wing will never hurt. It adds strength exactly where useful (at the outer surface of the wing) with minimal weight. It will also stop the fuselage from dinging the wing in a crooked landing.

Just to take Ben's good example a step along the way. If the span in his example is 60 inches then the semi span is 30 inches and a third of that is 10 inches. Ten inches time 50 pounds is a bending moment of 500 inch pounds. Assuming the wing is two inches thick then the compression force to be resisted by the top skin and upper sparcap at the center of the wing is 500/2= 250 pounds. There is a tension force at the center of the wing that the bottom skin and spar cap must carry of 250 pounds, also.

When glassing the center of a wing the edge of a heavy glass layup produces a stressriser that can become a point of failure. To avoid this it is a good idea to use several plies of light glass of varying width.

The main stresses will always be concentrated in the spar and the spar center joint. As such I see the center sheeting adding more to the torsional rigidity in the center section to help support the leading and trailing edges. I don't really think a couple of short peices of sheeting is actually adding much to the spar's load bearing capability. Oh sure, it adds something around the center joint but it's very minimal.

So assuming a good spar design I really don't think there's a lot to be gained by adding a full fiberglass wrap. All it needs is a well crafted butt glue joint. For larger models there may be some advantage to adding a light band of glass tape but I don't see any reason for multiple layers. Once it's as strong as the balsa anything extra is just added weight.

There is one exception to this. In some designs I've seen the wings are built as two separate panels using a full D box with thick center ribs. These panels are joined like a big handlaunch glider with no carry through joiners. This sort of method uses the skin strength of the D box to carry the loads. In this case a good center joint wrap of light wieght cloth using a narrow and wide layer to avoid the stress risers as Olllie points out is the way to go. The sheeting IS the spar in this case and the wrap IS the joiner.