Could someone help me with my understanding. Here is a quote from the conclusion in a paper by A Bowers.
" Neither Prandtl nor Horten followed through to the logical and complete conclusion of their work. Prandtl did not extend the upwash outboard of the wingtip, which would have answered the question of formation flight in birds, and he did not find the induced thrust at the outboard ends of the wings, which leads to proverse yaw. In turn, with his approximation and objectives Horten did not understand the origin of the induced thrust at the outboard ends of the wings for proverse yaw, and he did not prove that proverse yaw exists.
It remained for the current authors to prove conclusively that proverse yaw is achievable through an efficient bell-shaped spanload, that an optimal solution integrating minimum structure and minimum drag can solve the problem of yaw control and stability of a flying wing, and that the bell spanload solution answers some of the great enduring mysteries of the flight of birds.
In the case of the flight of birds, the bell spanload is the only viable solution."
End of quote.
Would I be right in thinking that twist in the bell spanload distribution is the cause ( as well as wing shape) of proverse yaw and in which case, why are not wingtips deformed downwards at speed as has been the case with Horten gliders and other aircraft.
Birds only have lightweight structures with which to contain these forces and yes, I appreciate they can manipulate there planform, but if this bell spanload distribution is correct and it does seem to make sense, the author ( A. Bowers) mentions the wingtip feathers are not deformed, so can someone please put this in a simple context that I can grasp.
Many thanks