Crab I'm not sure what YOU mean by a discontinuity , and yes it would normally be steeper approaching the y axis , the gentle arrival at the y axis is typical for a power curve that includes ground effect (which probably should be considered as a 'special case').
LRP yup I have always thought that was a reasonable estimation. (Although probably unimportantly slightly different, curious as to how different, if anyone actually has a real graph of RoD against airspeed in autorotation). The reason I asked was that I was disconcerted that Crab would think that the rate of descent at zero airspeed would only be slightly greater than at 'bucket speed' (Vy) whereas the power required at zero would be significantly greater at zero airspeed (surely?). Since the power required at zero airspeed is so high compared to at Vy one would surely expect the Rate of Descent to be accordingly greater, since Rate of Descent is equivalent to power input for autorotation.
The rate of descent is of course directly equivalent to Power (supplied).
For each 330lbs that an aircraft weighs the VSI reading gives Horse Power directly.
ie "10" on a VSI (with units of 100fpm) IS 10HP per 330lbs.
so a RoD of 2500fpm is 25Hp per 330lbs, so for an aircraft weighing 3300lbs that would be 250Hp