Thrust and power analysis at autorotation and windmill states.

[Tophinater]

Im trying to analyze tandem rotors going from the typical shaft powered state(+ power) to the autorotation state(0 shaft power) and then into the windmill brake state(- power). So far I am able to analyze such a design based on information from "Principles of Helicopter Aerodynamics" up to the point of autorotation however the book doesn't cover much in the windmill state. It does cover the basic function of windmills and analysis but not in autorotative state with a Vc/vh ~= -1.85 to -1.9. Does anyone know how or know of a place where I can find information on this kind of analysis. Really the only missing piece of the puzzle I require is how to model the induced velocity of the air after it passes through the TTP.