I'm trying to tabulate the differences of (vertical) autorotating behavior for rotors of equal chord and span, but with different airfoils. Of course, they can be studied case by case, but there may be a simpler, less laborious way... Hence my question...
This NACA report may help you further.
Basically, determine $v_i$ = the induced velocity in the hover, then vertical autorotation speed is around:
- 2 * $v_i$ for a rotor with 12 degree blade twist.
- 1.8 * $v_i$ for an untwisted rotor.
More detailed information in Prouty chapter 2, although the annoying non-metric constants pop up all the time.