Autorotation: Positive Pitch during descent phase possible?
During autorotation there are two main angles which are important, the inflow angle created by the rate of descent flow and rotational airflow plus the pitch angle (normally minimal on initial entry to autorotation). It is these angles which produces the lift. The second angle produces the auto rotative force, or the force which maintains the rotor blades turning in auto. This is created by the forward tilt of the main lift vector (or the horizontal component of the lift vector) which is shown on your diagram. During autorotation it is important to maintain control over rotor rpm. Density altitude has a marked effect in auto, which is your consideration albeit to a greater extent. As density altitude increases rotor rpm for the same pitch angle will increase, this can’t be allowed to continue to increase and is contained by increasing blade pitch, hence the greater the DA, the more pitch is required including positive pitch if required. Another consideration which may help is, autorotation appears to be an aerodynamic effect of rotorcraft that is considered in isolation when a helicopter loses power, rather like an on/off switch. This isn’t the case, helicopters will experience a degree of autorotation in a normal descent due to the rate of descent flow but the rotor rpm will be maintained within normal limits due to the rotor governing system. It is a progressive process from the start of a descent to fully established auto when the torque to the rotors becomes zero.