And third is one that I've never heard discussed is any slight bend that may be in the arrow.
this is called eccentric compression. definitely a factor in engineering calcs. of course all archers shoot straight arrows, no?
I think that the arrow should want to bend away from the bow on release
if you flip the nock end away from the bow with a Mediterranean release, the arrow center is bent towards the bow at the moment the string applies the load? I think the trick is to have the arrow be flexing away from the handle a half cycle later as it clears the bow. shooting off the thumb changes things of course.
spine oriented so that the arrow wants to bend away from the bow.
my guess is with a straight arrow and a mechanical release that imparts little lateral force on the nock at release, spine orientation would become more of a factor. Wouldn't a stiffer arrow vibrate faster, but with less deflection (amplitude)? Maybe Tuomo has some slo-mo of flight arrows somewhere?
A good test for arrows would be to have a jig that compresses them lengthwise. See which way they bend from that.
this is actually a preferred way to spine atlatl darts by many. I have duplicated a native dart of elliptical cross-section. (it was designed to be thrown flatter sides up and down). the dart, like many others, has a somewhat heavy head that makes the dart droop when held in throwing position. my conclusion is that the dart was tuned to the proper spine with respect to a single side, and just about had to flex in the desired direction with any reasonable throw.