Autorotation and windmilling are the same thing, in fact, designers call autorotation "windmill brake state" in that a brake is needed to draw off the energy the rotor is producing as it autorotates, otherwise the rotor will overspeed.
Matthew is right, the rotor on a windmill has a different design point, but that is because that rotor is designed to autorotate only, so it has no need for a low drag powered cruise mode. My guess is that most autogyros and windmills have similar designs.
BTW, the designs of the several Dutch windmills that I have studied are simply awesome. In the 1600's they had variable blade area and radius, variable angle of attack and slotted leading and trailing edge high-lift devices. A windmill engineer back then probably had all the sophistication of a rotor designer today, but without any Microsoft programs to help him!
One can draw lift diagrams to show the torque produced by the blade section that drives the rotor, but frankly I have never got much out of lift diagrams for rotors, and generally use the pinwheel/windmill analogy to satisfy myself as to how the rotor is eating a stream of air that is forced into it, and in the process it produces both lift and torque. I am also an anti-bernoulli guy, since I never explain that lift is produced by curves. It is far easier to see how lift is produced by a wing that "planes" on the air and makes a bunch of air go down so the momentum of that air makes the wing go up. A waterski and a wing have much in common, and are much more intuitive to budding aviators.
Last edited by NickLappos; 5th October 2006 at 16:31.