ORIGINAL: AllTheGoodNamesAreTaken
I looked at your photos and one thing struck me as odd. The wing joiner on your model does not look like it's made of plywood like mine is. You mention that it's a hardwood, but it looks more like balsa to me - I'm not there to see it or touch it, but regardless I believe the wing joiner material is the root cause of your wing failure.
Have you contacted GP and asked them about the wing joiner? The factory may have accidently substituted the wrong materail for your plane.
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Holy smokes Jeff! You're right! That is not plywood!
Whether balsa is a hardwood or not technically is somewhat irrelevant. Undoubtedly that piece of wood would crack sooner or later. JR, you need to forget about what it's called and use 5-ply - I believe 1/4" (can't recall). Another extremely stiff and easy to build option would be two 1/8" 5-ply sandwiching a layer of CF all laid up and put under pressure. In some classic pattern designs, light dihedral braces (what this is) used to be built from cross-grained sheet balsa laminated with CF. In fact, CF laminated balsa can be structurally stiffer than ply but no need to use balsa here on a 27%.
David.