In autorotation, the helicopter's rotor blades generate lift and thrust without engine power, relying on the upward airflow during descent. Two key forces act on the rotor blades:

1. Rotor Drag: This resistance opposes blade rotation and includes aerodynamic drag and mechanical drag from components like the gearbox and tail rotor.
2. Autorotative Force: This aerodynamic force is generated by the upward airflow as the helicopter descends, keeping the blades spinning.

To maintain constant rotor RPM, the autorotative force must balance the rotor drag and additional mechanical drag. Pilots achieve this by:

• Managing Blade Pitch:
  • Adjusting the collective pitch to change the angle of attack and influence autorotative force.
• Controlling Descent Rate:
  • Adjusting the descent rate to affect the upward airflow through the rotor disk.
• Balancing Forces:
  • Continuously adjusting collective pitch and descent rate to maintain equilibrium between autorotative force and drag.
• Considering Mechanical Drag:
  • Accounting for constant mechanical resistance from the gearbox, tail rotor, and other driven equipment.

When the collective is lowered further during autorotation, rotor RPM usually increases; when the collective is raised, rotor RPM decreases. The collective can be fully lowered momentarily when increasing forward speed during autorotation to counteract the temporary reduction in rotor RPM.

After touchdown during an autorotation with forward run-on speed, immediately lowering the collective can cause the helicopter to tip forward due to the sudden shift of weight onto the skids. Therefore, after autorotation and once the helicopter has come to a complete stop, the collective should be lowered.