I think that Ive opened a can of worms on the spar cap issue, that has evolved to bagged wing stuff. i.e., what is the best lay-up on a bagged wing?
For simplicity, I’ll not mix different stiffness materials. That is, no partial glass/carbon lay-ups. From numbers and experiment, the best lay-up on a strength/weight standpoint is with the upper surface being 2-2.5 times thicker than the lower surface. This is highly dependent on the camber and thickness used. The sample for the above is with an Eppler 374 airfoil. This is not the best stiffness/weight, but the best from a most bending moment in the positive g situation. A further caveat is that the foam used is useful from a buckling stiffness perspective, in other words, no white foam. My very first vacuum bagged wing had white foam and folded on about its fourth launch. One wing folded over the other, and it fluttered to the ground. When I got to it and picked it up, it looked entirely undamaged, as the wing flopped back into place and the skin was unbroken![37] It’s interesting that the original opening of this thread had to do with the reinforcement of spruce spars with carbon f
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