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rather than engine power. It is the means by which a helicopter can be landed safely in the event of an engine failure. In this case, you are using altitude as potential energy and converting it to kinetic energy dur- ing the descent and touchdown. All helicopters must have this capability in order to be certified. Autorotation is permitted mechanically because of a freewheeling unit, which allows the main rotor to con- 

tinue turning even if the engine is not running. In nor- mal powered flight, air is drawn into the main rotor sys- tem from above and exhausted downward. During autorotation, airflow enters the rotor disc from below as the helicopter descends. [Figure 3-20] 

## AUTOROTATION (VERTICAL FLIGHT) 

Most autorotations are performed with forward speed. For simplicity, the following aerodynamic explanation is based on a vertical autorotative descent (no forward speed) in still air. Under these conditions, the forces that cause the blades to turn are similar for all blades regardless of their position in the plane of rotation. Therefore, dissymmetry of lift resulting from helicop- ter airspeed is not a factor. 

During vertical autorotation, the rotor disc is divided into three regions as illustrated in figure 3-21—the 

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