Let's discuss wing spar design, and as a spinoff, the modes of spar failure. I'm
specifically looking at built-up wings, strong enough to withstand heavy-pedal
winching.
I'll chime in more as the discussion heats up, but my initial spar design in built
around spruce main spars with CF laminations, shear webs of vertical-grain balsa or
ply, and possibly a ply box structure to contain the wing joiner rod. I like the idea
of a secondary spar at 2/3 chord with a light joiner rod to keep the wing halves from
twisting. The center of the wing panels(s) should be sheeted and 'glased, and a
D-tube LE gives torsional rigidity.
I'll fine-tune this the more I build, but from an intuitive view, it seems correct.
I've not seen any (sailplane) failed wings, but it seems to be the stresses involved
in winching would put the lower surface in tension and the upper surface in
compression.
--Bill
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