I have seen where the foam core had a [painstaking] honey comb pattern of holes cut into it prior to sheeting. The retract bays, and other mechanical systems in the wing were mounted on wood that you might think was too light to last long. Some carbon fiber was laid in across the inner 50% of the span prior to sheeting. The wing was sheeted with 3/32" medium balsa, and turned out very light, compared with other .60 sized pattern planes. The pilot never thrashed this plane, he only flew it like he was doing a pattern routine, so it didn't really ever get tested in a "real world", sport flyer application