Q: Why don’t birds have propellers instead of wings? (to me via Quora)
A: On a macro biological scale (birds, humans, etc.), true, 360 degree, detached, continuous rotation around an axis or axle would be difficult to grow, and basically impossible to maintain/heal: How would nerves, blood vessels, and other necessary attachments span across a rotating gap?
I suppose you could postulate a rigid, initially non-rotating, bony propeller that grows like antlers/horns, static at first and perhaps covered in “velvet” or something similar to allow for the initial growth of the propeller; but there’s still the problem of managing wear on the rotating joint itself, and in “healing” any damage done to the propeller structure: Once the nutrient/blood supply is removed and the propeller is free to move, it’s just a piece of dead bone/antler; no further growth or repair work can occur until the next growing season. This does not sound like an evolutionary plus, assuming that the bird actually needs a propeller for locomotion. (And if not, why would evolution select towards growing a propeller in the first place?)
In real life nature, the only known examples of true, freely-rotating, natural biological structures occur only on the tiny, tiny, microscopic scale, where nutrients and repair proteins can easily be carried in the surrounding liquid medium, rather than having to be supplied by blood vessels and pumps.
On that minute scale, it actually can work:
“The only known example of a biological system capable of providing continuous propulsive torque about a fixed body—is the flagellum, a corkscrew-like tail used by single-celled prokaryotes for propulsion.” — Rotating locomotion in living systems – Wikipedia
Here’s a physical model of a bacterial flagellum :
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