The point I had hoped to make with the rib reference is that, I don't
quite see how the ribs in the extension will slide past the ribs in the

main wing.

Just do away with conventional ribs. A double skinned cored curve
might be made with only a slight weight penalty, and construction would
be very simple. After all they would be used for just slightly more
than a 1"G" climb, or flare, and are at the tips of the existing wing
where bending loads are small. While your at it might incorporate
leading edge slats in the extendable section.
=====================
Leon McAtee
thinking exoskeleton