Earlier, Wallace Berry wrote:

...Wing skins form the hinge for the wing
control surfaces. No mylar needed...

A minor nitpick:

That is the way it looks from the outside. However, structurally, the
upper wing skin does not actually support the aileron. Inside the wing
there are several support linkage gadgets that transfer lift loading
between the aileron and the wing. I can't remember if they're four-bar
links or track-and-roller arrangements. However, these gadgets are
carefully arranged so that the effective hinge axis coincides with the
surface of the upper wing skin. There are other sailplanes that use
this sort of system; the flaps of the Slingsby T65 Vega come to mind.

It is a compelling system, since as Wallace points out it needs no
mylar and no internal curtain sealing. However, what I've seen of one
or two Speed Astirs in service is that the control loads tend to be
greater than in more conventional systems, especially as they age.
Also, the flexible skin portion tends not to hold its surface finish
well; I've seen cracking and bubbling in this area on both Vegas and
Speed Astirs. It's also hard to achieve the kind of surface travels
that it takes to make high-aspect ailerons do their job effectively.

Getting off-topic, there _is_ at least one Chris Heintz powered
airplane that does actually support the aileron on a flexible portion
of wing skin. For more information, see:

http://www.zenithair.com/kit-data/ht-aileron.html

Thanks, and best regards to all

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