Like a rifled bullet, or a well thrown football; it would be in the fletching, but I can’t find anything.
IIRC, “Ötzi the Iceman” had arrows with fletching that would impart a spin to them; but the idea was apparently lost in later eras.
Otzi’s arrows had canted flights. Not sure if it was enough to make them spin. As I understand it, an arrow upon launch is bent, then flexes black, and bends again causing it to undulate mildly at the start of its flight. That already gives it some kind of spin.
No. The flexing of the shaft, by itself, would not cause the arrow to spin.
Air pressure against the windward side of the fletching would cause the arrow to spin in flight, assuming the feather fletching wasn’t glued on totally inline with the shaft, in which case there would be neither a windward or leeward side.
Yes, ancient arrows could spin depending on what was used for fletching and how the fletching was positioned on the shaft. The larger feathers that would be chosen for fletching have a natural left or right twist but could be attached inline with the shaft if arrowmaker so desired. Leather, fur, and parchment could also be used.
As doorhinge notes, the wing feathers of birds are naturally curved. Arrows fletched with them will have ‘canted’ fletchings even without the fletcher shooting for it (heh). I learned this as a young boy making bows and arrows all day long. The fletchings can be canted in addition to their natural curve, to increase the spin, but this is not necessary as it is with modern, plastic vanes that are straight. Wing feathers with their curve can be forced straight onto an arrow shaft, but without a modern clamp-style fletching jig it is quite difficult, and it’s safe to say was rarely done historically, whereas fletchings made of parchment, paper, leaf etc. would need a cant to make the arrow spin. Very little curvature or cant is needed: an easy way to tell if an arrow spins in flight is to shoot it with a very short, light draw: as the slow arrow leaves the bow, one can readily tell that most any hand-fletched arrow will spin in flight. It is bullshit that Otzi’s arrows spun but later prehistoric arrows didn’t.