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FYI
Looking at this bird one wonders if the wing on a sail could be a cloth hinged unit that flipped over to the other side on tack as the bird and plane both exit the shape to the "lift" side.


In the picture attached, I suspect the feathers on the tip of the eagle's wings are there for stability in slow flight - not efficiency. They may even have very little affect on the airflow but may just be positioned that way to give the bird additional feedback about the airflow conditions on each end of it's wing so it can avoid a stall condition.

[Linked Image]

Jake Kohl