Steff,

Actually, it is probably best to glue the shear webbing to the back of the two spar members, as your plans suggested. The job of the shear webbing is to carry the shear load ( the force that makes the two spar members move relative to each other when the wing is loaded ). The glue joint that you get when you lap the shear webbing to the back of the spar members is much better than the joint you get when you butt it between them. Steel I-beams are different in several important ways, which is why their construction is different. Your wing will probably be fine, but sometimes the easy way is also the best way.

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