how do you figure out how much force the wings will have to soak up in a tight turn or loop? to get some idea if they will fold on the first flight?

If i can remember right, in a really tight turn (90 degree bank) the plane will pull be around 3 g's, in a 60 degree bank about 1.5 g's.
1 g = weight of plane.

To help with the memory, in a level, i.e. not climbing or decending 60 deg bank turn, the airplane will "feel" 2 g's, or twice its weight. However, as the bank angle approaches 90 deg of bank (for a non-climbing or decending turn), the g's approach infinity, as it would be impossible to attain any portion of the lift vector to counter gravity if it were aligned perfectly with the horizon. Hope this helps..

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Here you go, posted by Don Stegall, another RCU member...

http://www.rcpro.org/rccalc/GForce.aspx

Sample: 100 mph, and a 100 foot radius turn is 6.72 G's. Interesting, because when you distribute even these "high" loads over the surface of a reasonably-sized wing, they're not that terrifying. I always build with the idea of at least 10 G's, on most planes, and that's not really too extreme.

The recent Dynamic Soaring record of around 262 mph shows that if the circle's radius was about 200 feet, (my guess) then the plane was pulling 23.07 G's. And that's sustained, too, not just momentary. I've heard that severe winch launches can put 30 or 40 G's on these things.

As Mike James above says, the G load increases with the square of the velocity assuming all other factor remain the same. Remember F=1/2 MV^2 ?