How does wing flex reduce wave drag? If I'm reading this post right: What is the effect of airfoil thickness on aerodynamic lift?


Airfoil thickness increases the lift coefficient.


And from here: What are the effects of the Boeing 787&#39;s very flexible wings?


Wing flex reduces wave drag.


I'm wondering about the how. My theory is that the air sees a thicker airfoil in a flexed wing, without actually building one. Is that assumption correct? Or is it related to the end-plate effect?

Front view of a flexed wing. Orange indicates that perceived increase in airfoil thickness.
No, it is torsion which reduces wave drag.

The sweptback wing will twist as well as bend, and this twist will reduce the local angle of attack, and with it the suction peak on the top side of the wing. The same mechanism twists the outer part of a forward swept wing to higher angles of attack, necessitating a stiffer structure to avoid aeroelastic instability.

Air will not "see" any difference in wing thickness, regardless of the dihedral or bending. For air, there is no difference between a wing and a vertical surface (which would have almost infinite thickness following your assumption, but obviously doesn't).