wanna build a very strong spar on any wing, build a Tri Beam spar...picture below. This spar features shear webs on both sides of the horizontal rails, but also one in between them. This increases the strength of the spar dramatically. Be sure all three shears touch the wing ribs on either side of them and glue them at all points. I built a spar like this for the outer halves of a Corsair wing and I stood on it to test it. 230 pounds and not one sign of crushing!