Most of the books I mentioned do not go into the regions of the rotor in autorotations. Newman mentions it, as I recall and you know what Prouty says. One point as to why most texts do not go into 'regions of the rotor', is that it suggests a steady-state situation. Most helicopters autorotate within Vortex ring/Turbulent wake states, not Windmill brake as I've heard mentioned. As such, the flow through the rotor is not as straight forward as to have certain regions doing the driving and others not. As a simple model, to offer an explanation, what I've said above broadly approximates what is happening. With that model, which is a blade element approach, drawing the diagrams shows that with an increasing rotational speed, the inflow angle reduces, the airflow becomes more horizontal and with lift perpendicular to it, it will tilt back. It is crude approximation bearing in mind the real changing flow state which is why it doesn't appear in most texts, in my humble opinion.
Lets just be glad autorotation works!! Most plank drivers think helis fly on black magic anyway, which isn't far off..
Last edited by SEL; 9th Oct 2003 at 02:55.