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 maxi- mum 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  V$_{NE}$ 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 As shown in figure 3-15, as the rotor blade reaches the advancing side of the rotor disc (A), it reaches its max- imum upflap velocity. When the blade flaps upward, the angle between the chord line and the resultant rela- tive wind decreases. This decreases the angle of attack, V$_{NE}$—The speed beyond which an aircraft should never be operated. V$_{NE}$ can change with altitude, density altitude, and weight.  3-7 Figure 3-15. The combined upward flapping (reduced lift) of the advancing blade and downward flapping (increased lift) of the retreating blade equalizes lift across the main rotor disc counteracting dissymmetry of lift. Figure 3-14. The blade tip speed of this helicopter is approxi- mately 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 differ- ence in speed causes a dissymmetry of lift. Relative Wind Relative Wind DirectionGLYPH<.notdef> of Flight AdvancingGLYPH<.notdef> Side Blade TipGLYPH<.notdef> Speed PlusGLYPH<.notdef> HelicopterGLYPH<.notdef> SpeedGLYPH<.notdef> (400 KTS) Blade TipGLYPH<.notdef> Speed MinusGLYPH<.notdef> HelicopterGLYPH<.notdef> SpeedGLYPH<.notdef> (200 KTS) RetreatingGLYPH<.notdef> Side Forward FlightGLYPH<.notdef> 100 KTS BladeGLYPH<.notdef> Rotation Direction of Rotation C h o r d L i n e Resultant RW C h o r d L i n e Resultant RW C h o r d L i n e Downflap Velocity R e s u lt a n t R W C h o r d L i n e R e s u lt a n t R W Upflap Velocity Angle of Attack atGLYPH<.notdef> 9 O'Clock Position Angle of Attack atGLYPH<.notdef> 3 O'Clock Position Angle of Attack overGLYPH<.notdef> Tail Angle of Attack overGLYPH<.notdef> Nose A B C D D A B C RW = Relative WindGLYPH<.notdef> = Angle of Attack