Dave Jackson,
The quote from Leishman is right at the heart of my question, thanks.
But I still don't accept the explanation just yet.He uses the phrase"induced angle of attack", not sure there is such a thing. The "induced drag" is low at high speed. What he appears to be saying, I think, is that the tip region is going above terminal velocity,defined as: that speed at which profile drag exceeds the forward lift force of the airfoil.That could be the answer Dave!
I am unfamiliar with the dynamics of gliding wings at 450 mph, so that may be what I need to study. That may explain why autorotation performance is improved at the lower tip velocity.(550ft/sec according to Prouty)
The Leishman explanation is helpful but a little confusing.
I'll beat you up some other time, Thanks Dave