There is no need for any part of the aerofoil to be in -ve AoA. It is
not blown around like a toy fan. Have a look at this page
http://www.copters.com/aero/autorotation.html

Hi Phil,

Great link! Turns out I understood this stuff better than I thought I did.
I've read plenty about the physics, just never seen it laid out this well in
pictures. Thanks for posting it.

Fly Safe,
Steve R.