the rotor system while increasing the angle at another part.  Maximum downward flapping of the blade over the nose  and maximum upward flapping over the tail tilt both rotor  disk and thrust vector forward. To prevent blowback from  occurring, the pilot must continually move the cyclic forward  as the velocity of the helicopter increases.  Figure 2-42 illustrates the changes in pitch angle as the cyclic is moved  forward at increased airspeeds. At a hover, the cyclic is  centered and the pitch angle on the advancing and retreating  blades is the same. At low forward speeds, moving the cyclic  forward reduces pitch angle on the advancing blade and  increases pitch angle on the retreating blade. This causes  a slight rotor tilt. At higher forward speeds, the pilot must  continue to move the cyclic forward. This further reduces  pitch angle on the advancing blade and further increases  pitch angle on the retreating blade. As a result, there is even  more tilt to the rotor than at lower speeds. This horizontal lift component (thrust) generates higher  helicopter airspeed. The higher airspeed induces blade  flapping to maintain symmetry of lift. The combination of  flapping and cyclic feathering maintains symmetry of lift and  desired attitude on the rotor system and helicopter. disk is originally oriented with the front down following  the initial cyclic input. As airspeed is gained and flapping  eliminates dissymmetry of lift, the front of the disk comes  up, and the back of the disk goes down. This reorientation  of the rotor disk changes the direction in which total rotor  thrust acts; the helicopter’s forward speed slows, but can be  corrected with cyclic input. The pilot uses cyclic feathering  to compensate for dissymmetry of lift allowing him or her  to control the attitude of the rotor disk. Cyclic feathering compensates for dissymmetry of lift  (changes the AOA) in the following way. At a hover, equal  lift is produced around the rotor system with equal pitch and  AOA on all the blades and at all points in the rotor system  (disregarding compensation for translating tendency). The  rotor disk is parallel to the horizon. To develop a thrust force,  the rotor system must be tilted in the desired direction of  movement. Cyclic feathering changes the angle of incidence  differentially around the rotor system. Forward cyclic  movements decrease the angle of incidence at one part on  Autorotation is the state of flight in which the main rotor system  of a helicopter is being turned by the action of air moving up  through the rotor rather than engine power driving the rotor.  [Figure 2-43]  In normal, powered flight, air is drawn into the  main rotor system from above and exhausted downward, but  during autorotation, air moves up into the rotor system from  below as the helicopter descends. Autorotation is permitted  mechanically by a freewheeling unit, which is a special clutch  mechanism that allows the main rotor to continue turning even  if the engine is not running. If the engine fails, the freewheeling  Figure 2-43.  During an autorotation, the upward flow of relative wind permits the main rotor blades to rotate at their normal speed. In  effect, the blades are “gliding” in their rotational plane. Figure 2-42.  To compensate for blowback, move the cyclic forward.  Blowback is more pronounced with higher airspeeds.  Autorotation Normal Powered Flight Autorotation Direction of flight Direction of flight 2-27