... | > But of course you specifically asked how you would determine the flight | > load capabilities of a given design. Well, I think the only way to do | > it would be to build a prototype, place the wings between two chairs | > with the support around midspan on either side
Alas, that's only an approximation of the load on the wing in flight. The reality will be different.
| > then pile weight on the center section until it breaks. Then you | > divide the weight by the weight of the airplane. This will give | > you a rating, i.e. "This plane will withstand 12 Gs."
... of course, other parts of the plane may fail before you reach 12 G's. Your wing supports may break off first, with a lawn dart hitting the ground, then a really strong wing flutters down later, probably undamaged.
And having the wing break is not the only possible problem. If it bends a lot but doesn't break, that'll affect how the plane flies. I've got a spad that the wings bow a whole lot in a loop. It doesn't break, but it makes the plane fly like ass. Some carbon fiber fixed that up, but if I ever crash it the plane won't be so resilient.
On the bright side, most wings bend before they break, so if you're paying attention and see your wings bend, you know to ease off the stick somewhat. Alas, this is most noticeable on large aspect ratio planes like gliders, and not always so obvious on planes with smaller wings.
| 2. What formula gives me the forces exerted on my wing.
I believe it's relatively complicated. If you were in a loop, and knew the radius of your loop and your airspeed, you could calculate the force there easily enough (but actually knowing these values might be hard), but to go beyond that, especially without flight testing, will require a whole lot of math.
The usual way that most models deal with this is to make really really strong wings. Sure, they're heavier, but it's better than the alternative. And really, they're not that much heavier if done right.
I'll bet the dynamic soaring guys know a whole lot more about this than most of us -- they fly their gliders in loops, over and over and over, often well over 100 mph -- the world record is 302 mph.