For a rotor in vertical autorotation, what is the relation between pitch, vertical speed and rpms?
Unless I am misinterpreting your question, if you add pitch [pulling collective in a helicopter] your RRPM [rotor RPM] will decrease, as it is being used to accelerate air down decent, if in a vertical decent the aerodynamics are screwy. Now if you have some forward speed increase Pitch trades RRPM for lowering vertical speed. Decreasing Pitch, allows you to trade altitude for RRPM giving you higher VS and higher RRPM.