During autorotation, what forces primarily drive the rotor blades?

During autorotation, the rotor blades are primarily driven by the upward flow of air through the rotor system and the forces generated by the blades themselves. In normal powered flight, the engine drives the rotor blades, forcing air downwards to create lift. However, in autorotation, the engine is disengaged from the rotor system, usually due to an engine failure. As the helicopter descends, the relative wind changes direction, flowing upwards through the rotor disk. This upward airflow strikes the rotor blades at an angle, causing them to rotate. The rotor blades are designed with specific aerodynamic properties to take advantage of this upward airflow. The blades are divided into three regions during autorotation: the driving region (usually the outer portion of the blade), the driven region (usually the inner portion of the blade), and the stall region (the area closest to the rotor hub). The driving region is angled so that the upward airflow creates lift, which helps to rotate the blades. The driven region experiences drag and slows the blades down. The pilot controls the overall rotor RPM by adjusting the collective pitch, balancing the forces of the driving and driven regions. By maintaining the correct rotor RPM, the pilot can control the rate of descent and prepare for a safe landing.