Direction� of FlightRetreating�
Advancing� Side Side
 

 
 
 Blade Tip� 

Blade Tip� Speed Minus�
Speed Plus� Helicopter�
Helicopter� Speed�
Speed� (200 KTS)
(400 KTS)
Blade� Rotation
Forward Flight�100 KTS
Figure 3-14. The blade tip speed of this helicopter is approximately 300 knots. If the helicopter is moving forward at 100 knots, the relative wind speed on the advancing side is 400 knots. On the retreating side, it is only 200 knots. This difference in speed causes a dissymmetry of lift.
As shown in figure 3-15, as the rotor blade reaches the advancing side of the rotor disc (A), it reaches its maximum upflap velocity. When the blade flaps upward, the angle between the chord line and the resultant relative wind decreases. This decreases the angle of attack, which reduces the amount of lift produced by the blade. At position (C) the rotor blade is now at its maximum downflapping velocity. Due to downflapping, the angle between the chord line and the resultant relative wind increases. This increases the angle of attack and thus the amount of lift produced by the blade.
The combination of blade flapping and slow relative wind acting on the retreating blade normally limits the maximum forward speed of a helicopter. At a high forward speed, the retreating blade stalls because of a high angle of attack and slow relative wind speed. This situation is called retreating blade stall and is evidenced by a nose pitch up, vibration, and a rolling tendency—usually to the left in helicopters with counterclockwise blade rotation.
You can avoid retreating blade stall by not exceeding the never-exceed speed. This speed is designated VNE and is usually indicated on a placard and marked on the airspeed indicator by a red line.
During aerodynamic flapping of the rotor blades as they compensate for dissymmetry of lift, the advancing blade
 

VNE —The speed beyond which an aircraft should never be operated. VNE can change with altitude, density altitude, and weight.
 

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 during 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 continue turning even if the engine is not running. In normal powered flight, air is drawn into the main rotor system from above and exhausted downward. During autorotation, airflow enters the rotor disc from below as the helicopter descends. [Figure 3-20]