wrote:
[...]
Once the early designers appreciated the advantage of the one over the
other they moved immediately to true monocoque structures of molded
plywood, welded steel tubing and so forth, but the structural integrity
of the 'box' structures combined with their simplicity of fabrication
makes the method ideal for homebuilders even today.

-R.S.Hoover

Which reminds me. A little googling will bring up an Avions Mudry Cap
10B (also known as Apex), the left spar of which failed in Texas a few
years ago (c. 2001), killing the pilot. The problem was compression
fractures in the upper box spar cap and some other damage which could
not be (or was not) inspected and repaired, even after compliance with
several service bulletins requiring inspection and repair near the wing
attach fittings at the wing roots. According to the NTSB report, none
of the glue bonds failed; the failures were compression cracks in the
Sitka spruce and other woods in the main spar. Having read the reports
and seen the extensive photos of the failures, I think if I had a Cap
10, I would rebuild the wings or ground it.