ates through approximately 20 knots or if the headwind is approximately 20 knots. Translational lift is also present in a stationary hover if the wind speed is approximately 16 to 24 knots. In nor- mal operations, always utilize the benefit of translational lift, especially if maximum performance is needed. You can recognize transverse flow effect because of increased vibrations of the helicopter at airspeeds just below effective translational lift on takeoff and after passing through effective translational lift during land- ing. To counteract transverse flow effect, a cyclic input needs to be made.  As the rotor blades rotate they generate what is called rotational relative wind. This airflow is characterized as flowing parallel and opposite the rotor’s plane of rotation and striking perpendicular to the rotor blade’s leading edge. This rotational relative wind is used to generate lift. As rotor blades produce lift, air is acceler- ated over the foil and projected downward. Anytime a helicopter is producing lift, it moves large masses of air vertically and down through the rotor system. This downwash or induced flow can significantly change the efficiency of the rotor system. Rotational relative wind combines with induced flow to form the resultant relative wind. As induced flow increases, resultant rel- ative wind becomes less horizontal. Since angle of attack is determined by measuring the difference between the chord line and the resultant relative wind, as the resultant relative wind becomes less horizontal, angle of attack decreases. [Figure 3-13] When the helicopter moves through the air, the relative airflow through the main rotor disc is different on the advancing side than on the retreating side. The relative wind encountered by the advancing blade is increased by the forward speed of the helicopter, while the rela- tive 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 disc produces more lift than the retreating blade side. This situation is defined as dissymmetry of lift. [Figure 3-14] If this condition was 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 disc. Articulated rotor sys- tems, usually with three or more blades, incorporate a horizontal hinge (flapping hinge) to allow the individ- ual rotor blades to move, or flap up and down as they rotate. A semirigid rotor system (two blades) utilizes a teetering hinge, which allows the blades to flap as a unit. When one blade flaps up, the other flaps down. As the helicopter accelerates in forward flight, induced flow drops to near zero at the forward disc area and increases at the aft disc area. This increases the angle of attack at the front disc area causing the rotor blade to flap up, and reduces angle of attack at the aft disc area causing the rotor blade to flap down. Because the rotor acts like a gyro, maximum displacement occurs 90° in the direction of rotation. The result is a tendency for the helicopter to roll slightly to the right as it acceler- 3-6 Figure 3-13. A helicopter in forward flight, or hovering with a headwind or crosswind, has more molecules of air entering the aft portion of the rotor blade. Therefore, the angle of attack is less and the induced flow is greater at the rear of the rotor disc. INDUCED FLOW DISSYMMETRY OF LIFT TRANSVERSE FLOW EFFECT R e s u l t a n t R e l a t i v e W i n d R e s u l t a n t R e l a t i v e W i n d 10 to 20GLYPH<.notdef> Knots A A B B InducedGLYPH<.notdef> Flow InducedGLYPH<.notdef> Flow Angle ofGLYPH<.notdef> Attac k Angle ofGLYPH<.notdef> Attack Rotational Relative Wind Rotational Relative Wind