This is an example of what I mean by going for zero torque ripple or at least getting control of it and put it to work. The point here is counter rotating two curved spars with a simple gear pass between. You can see that the ends are always the same short spacing in the Z direction, while they breathe fore and aft along an imaginary airfoil chord line.
I liken the fast meshing of the two rods as a set of one tooth gears.
So this one is flapping up and down, but room needs to be made for it to breathe fore and aft, or constrain them in order to store, for instance, some of the energy of the upward part of the flap cycle.