On Sep 6, 8:39 pm, Eric Greenwell wrote:
Bob Kuykendall wrote:
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.

If the wing was a fairly thick, say 20% or so, could the spar be stiff
enough without much excess strength? I realize that a thick wing is an
unlikely choice for a glider, of course.

--
Eric Greenwell - Washington State, USA
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Thanks for replies on this guys. It was kind of an academic question,
as well as a practical one. We have a source of fiberglass laminate
pultrusions that are suitable for spars, and the price is quite
attractive, but the geometry of the wing and hence the spar requires
using different methods. The graphlite rods make the most sense and
the spar made using these is limber enough to allow a fore-aft bend to
hit the high points of the wing. Plus, carbon fiber is now getting
more available and prices are becoming more reasonable. I like the
idea of carbon skins and carbon spars, my Apis is built that way and I
rather enjoy how easily it is to rig.

Brad