Anyone out there know if there have been any sailplane
wings made that use a glass spar and carbon skins?

What that other guy said.

As much as you'd wish otherwise, stress follows the path of greatest
stiffness, and not necessarily the path of greatest strength.

So if you independently sized the carbon skin for panel stiffness,
ground handling, and torsion loads, and sized the fiberglass spar for
bending loads, you'd almost certainly find that the spar is so limber
that the skins would kink and buckle at loads well below a reasonable
design limit.

Of course, almost everything is possible. If for whatever reason you
really really had to combine carbon skin and fiberglass spar, you
could always just size the spar caps to the stiffness of the skin.
However, in order to achieve the necessary stiffness, the spar would
have something like six times as strong as it otherwise needs to be,
and commensurately heavy.

I ran up against a similar issue with an early design sketch of what
became the HP-24. I proposed a wing spar based on Graphlite pultruded
carbon ribbon, with PVC foam ribs and a bonded aluminum skin ala
HP-18. However, Jim Marske set me straight on that idea, calculating
that in order to be stiff enough so as to not kink the aluminum wing
skin within the design load limits, the spar would need four times the
material dictated by strength alone. And even then the skin/spar bond
and the fatigue life on the skin would be marginal.

Thanks, Bob K.
http://www.hpaircraft.com