B l a d e r o $_{t}$a t $_{i}$o n B l a d e r o t a t i o n Relative wind Forward flight at 100 knots Relative wind Direction of Flight Advancing Side Retreating Side Blade tip  speed plus helicopter  speed (500 knots) Blade tip speed  minus helicopter speed (300 knots) Figure 2-36.  The blade tip speed of this helicopter is approximately  400 knots. If the helicopter is moving forward at 100 knots, the  relative windspeed on the advancing side is 500 knots. On the  retreating side, it is only 300 knots. This difference in speed causes  a dissymmetry of lift.  Dissymmetry of Lift wind velocity is the least. This downflap creates an upward  flow of air and has the same effect as decreasing the induced  flow velocity by imposing an upward velocity vertical vector  to the relative wind which increases the AOA. Dissymmetry of lift is the differential (unequal) lift between  advancing and retreating halves of the rotor disk caused by the  different wind flow velocity across each half. This difference  in lift would cause the helicopter to be uncontrollable in any  situation other than hovering in a calm wind. There must  be a means of compensating, correcting, or eliminating this  unequal lift to attain symmetry of lift. When the helicopter moves through the air, the relative  airflow through the main rotor disk is different on the  advancing side from the retreating side. The relative wind  encountered by the advancing blade is increased by the  forward speed of the helicopter, while the relative wind speed  acting on the retreating blade is reduced by the helicopter’s  forward airspeed. Therefore, as a result of the relative wind  speed, the advancing blade side of the rotor disk can produce  more lift than the retreating blade side.  [Figure 2-36] Pilots can avoid retreating blade stall by not exceeding the  never-exceed speed. This speed is designated V$_{NE}$ and is  indicated on a placard and marked on the airspeed indicator  by a red line. If this condition were allowed to exist, a helicopter with a  counterclockwise main rotor blade rotation would roll to the  left because of the difference in lift. In reality, the main rotor  blades flap and feather automatically to equalize lift across  the rotor disk. Articulated rotor disks, usually with three or  more blades, incorporate a horizontal hinge (flapping hinge)  to allow the individual rotor blades to move, or flap up and  down as they rotate. A semi-rigid rotor disk (two blades)  utilizes a teetering hinge, which allows the blades to flap as  a unit. When one blade flaps up, the other blade flaps down. Blade flapping compensates for dissymmetry of lift in the  following way. At a hover, equal lift is produced around the  rotor disk with equal pitch (AOI) on all the blades and at  all points in the rotor disk (disregarding compensation for  translating tendency). The rotor disk is parallel to the horizon.  To develop a thrust force, the rotor disk must be tilted in the  desired direction of movement. Cyclic feathering changes  the angle of incidence differentially around the rotor disk.  For a counterclockwise rotation, forward cyclic movement  decreases the angle of incidence on the right of the rotor disk  and increases it on the left. As shown in  Figure 2-37,  as the rotor blade reaches the  advancing side of the rotor disk (A), it reaches its maximum  up flap velocity. When the blade flaps upward, the angle  between the chord line and the resultant relative wind  decreases. This decreases the AOA, which reduces the  amount of lift produced by the blade. At position (C), the  rotor blade is now at its maximum down flapping velocity.  Due to down flapping, the angle between the chord line and  the resultant relative wind increases. This increases the AOA  and thus the amount of lift produced by the blade. When transitioning to forward flight either from a hover or  taking off from the ground, pilots must be aware that as the  helicopter speed increases, translational lift becomes more  effective and causes the nose to rise or pitch up (sometimes  referred to as blowback). This tendency is caused by the  combined effects of dissymmetry of lift and transverse flow.  Pilots must correct for this tendency by maintaining a constant  rotor disk attitude that will move the helicopter through the  speed range in which blowback occurs. If the nose is permitted  to pitch up while passing through this speed range, the aircraft  may also tend to roll to the right. To correct for this tendency,  the pilot must continuously move the cyclic forward as velocity  of the helicopter increases until the takeoff is complete, and  the helicopter has transitioned into forward flight.  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 AOA 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. 2-20