Paul -

You're missing out on a key point: Changing materials to a lighter
structure does not change the aerodynamics of the wing.

It changes the wing-loading - but that's no different from adding or
removing water ballast. As long as the wing SHAPE remains the same, a
different weight/wing-loading simply shifts the polar. It doesn't
create an improvement.

Wing efficiency is affected by the shape of the airfoil, the wing
planform (and how the two add together to become a total 3-d package),
the smoothness of the skin/surface, the surface-area/skin drag, and
numerous other smaller factors.

Existing composite materials can be made to follow complex curves and
result in an extremely smooth surface, so changing to a fancier
material does nothing to improve the efficiency of the wing.

The reason that Pre-Preg and other fancy/costly exotic materials
aren't commonly in use is because they aren't NEEDED to achieve
aerodynamic optimums. Fancy materials are mostly used for
manufacturing reasons, to carry higher loads, to have more specific
stiffness in a particular load direction, etc. Aerodynamics doesn't
enter into the equation.

Small, incremental advances in overall performance have been made over
the last 20 years; but they are pretty tiny in comparison to the
performance jumps that were seen from the 50's to the late 70's (when
glide ratios doubled or tripled). We're down to the point where we're
fighting physics every step of the way in order to see any
improvements. And in order for theoretical gains to be realized in
actual flight, the tolerances are getting so tight that normal
manufacturing techniques cannot be applied. And with tolerances that
tight you also end up fighting the fact that the air gliders move
through is not sitting still. At some point the turbulence and
natural air movement disrupts the pristine theoretical predictions of
how the air will behave as the sailplane passes through it, rendering
supposed performance gains null (and in some cases causing an actual
performance penalty - see the part of the Diana 2 paper where they
talk about lift minimums in certain airfoils at high Cl).

Take care,

--Noel