The X-29 did some fiber orientation to get favorable bending-torsional coupling from the wing skins. When I talked with two different professors knowledgeable on the topic (while in grad school) they said that you can not do enough with skins alone - mostly the X-29 avoided excess divergence with a lot of bending and torsional stiffness aided by favorable bending-torsional coupling.
Could you do this on the spar? The same problem as the with the X-29: The spar set by its mission is much stronger and stiffer than the skin set in shear and spanwise bending, but not usually much for torsion. Wing skins are way capable in torsion, to the point where spar torsional stiffness in most airplanes can be just about forgotten about. The light way is to react torsion near the leading and trailing edges.
To make the spar work, you would need to put on a lot of plies oriented for bending-torsion coupling. You could probably make the caps this way, but by the time they are stout enough to carry all the bending with what is an unfavorable orientation for that purpose, the weight would be way high. And you would still need to do all of this with the skins too.
Anyone really interested needs to get a copy of Tsai and Hahn or Jones, learn the math, and start playing with wing skins and spars in it. They might discover a part of the design space that let's you get good coupling and light weight.
Billski
The X-29 did some fiber orientation to get favorable bending-torsional coupling from the wing skins. When I talked with two different professors knowledgeable on the topic (while in grad school) they said that you can not do enough with skins alone - mostly the X-29 avoided excess divergence with a lot of bending and torsional stiffness aided by favorable bending-torsional coupling.
I was under the impression that the wash-in angle vs load factor for the X-29 was negative. Was that not the case? I cannot find a direct statement or graphic that answers that question, and it's bothered me since the 80's in high school. Some papers claimed the torsional stiffness simply pushed divergence up sufficiently high, and other papers vaguely alluded to the bending torsion coupling voodoo that was responsible for the entire advancement.
I'd love to read a better source than I've been able to dredge up on the issue over the decades.
It seems to me that a sufficiently anisotropic material with a sufficiently weird Poisson ratio could be the basis for a design that would make the bending torsion coupling trick work completely. And although the idea is inelegant, an active system could modulate lift for low frequency instability.
Advantages? Carry through location, visibility, CLmax, and potentially reduced induced drag.
The estimates for the last one are difficult to get from the literature. The estimates range from a few percent (similar to winglets) to 20% (from a planning doc for the X-29, and only plausible for the glue sniffers). The X-29 saw only comparable L/D performance compared to other "comparable" operational aircraft except at high speed and high load factors it seems.
