Earlier, Eric Greenwell wrote:

Are there engineering or manufacturing
issues that make spoilers a more
desirable choice these days? For
example, a fiberglass wing might be
more flexible than a metal one, which
would make a 90 degree flap harder
to implement. The early ASW 20 had
problems this way with it's 60 degree
flap setting.

Boy, you picked up on that one quick!

I'm not qualified to address the actual engineering
aspects of this issue. But speaking from the perspective
of a sailplane development pundit:

I think that, absolutely, implementing 90-degree flaps
on a composite wing has complications that you wouldn't
find on a more rigid metal wing. However, the lessons
of the PIK-20B and the Zuni suggest that it is doable.

As you point out, the big problem is bending of the
wing with fully deployed flaps, which tries to bend
the flaps in the plane in which they are most rigid.
I suspect that overcoming this issue requires the right
layup type and fiber orientation. I'd have to do test
sections to be sure, but I think that either aramids
or possibly newer polyethelyne fibers on some sort
of bias orientation would be required. That might give
reasonable torsional stiffness without undue bending
stiffness. It seems to work for the LS-6, which uses
Kevlar (tm) laminates in the flaperons.

Of course, a stiffer wing than the old ASW-20 would
help, too. That, and more hinge points and more drive
points.

Before I tried it for sure, what I'd want to do would
be to test a candidate flap section, and see how close
I can get it to the predicted wing curvature at the
ultimate loading limit. It might turn out to be necessary
to either make the wing stiffer, or to limit loading
to a lower G value under landing flap deployment. Or
perhaps something else entirely. That's what testing
is for.

Thanks, and best regards to all

Bob K.