I'm on several other aircraft forums, and just now read a post from a guy on an "airchair" group. Look up red goat glider on utube if you want to see an airchair in action. Anyway here is what an engineer had to say about holes in the center section of an aluminum spar. Take it for what it's worth, I can't say he's right or wrong, but figured I would pass along what he said. Jim Chuk

I'm Richard Lamb - engineer and draftsman for the Texas Parasol project.
The TP is basically a "fat" ultralight. A few have been built under 254 pounds but most run 320 or more.*
*We sorta pioneered using extruded aluminum angle for fuselage construction, but my remarks today are about the wing we finally settled on.
*We operated at higher weights and speeds than the air chairs will EVER see.
And we have had NO failures due to rivet holes in the spar tubes with over 100 airplanes built.
*Our wing was actually designed by (the late) Paul Hammond of San Antonio, Texas,
It uses tube spars with bent sheet-metal compression ribs at each airfoil rib location to form an all-metal "ladder". The drawings specify .058 wall tube spars, but they have been built as light as .035.* The ,049 wall tubes cost more than the .058!
*The thing is to locate the holes on the spar tube neutral axis.
The front and back centerline of the tube.
Anything in an arc +/- 45 degrees from the neutral axis had negligible* impact on spar tube strength.*