16–24 knots No recirculation of air More horizontal flow of air Reduced induced flow increases angle of attack Tail rotor operates in relatively clean air Helicopter movement Thrust Drag Weight Lift Resultant Resultant Figure 2-42.  Forces acting on the helicopter during sideward flight.  Figure 2-41.  Effective translational lift is easily recognized in actual  flight by a transient induced aerodynamic vibration and increased  performance of the helicopter. Sideward Flight Induced Flow Transverse Flow Effect 2-23 Transverse flow effect is recognized by increased vibrations  of the helicopter at airspeeds around 12 to 15 knots and can  be produced by forward flight or from the wind while in a  hover. This vibration happens at an airspeed just below ETL  on takeoff and after passing through ETL during landing. The  vibration happens close to the same airspeed as ETL because  that is when the greatest lift differential exists between the  front and rear portions of the rotor system. As such, some  pilots confuse the vibration felt by transverse flow effect with  passing through ETL. To counteract transverse flow effect,  a cyclic input to the left may be needed. In sideward flight, the tip-path plane is tilted in the direction  that flight is desired. This tilts the total lift-thrust vector  sideward. In this case, the vertical or lift component is still  straight up and weight straight down, but the horizontal or  thrust component now acts sideward with drag acting to the  opposite side.  [Figure 2-42] (with a main rotor turning counterclockwise) and forces the  pilot to apply right pedal (decreasing the AOA in the tail  rotor blades) in response. In addition, during this period, the  airflow affects the horizontal components of the stabilizer  found on most helicopters which tends to bring the nose of  the helicopter to a more level attitude.  Sideward flight can be a very unstable condition due to the  parasitic drag of the fuselage combined with the lack of  horizontal stabilizer for that direction of flight. Increased  altitudes help with control and the pilot must always scan in  the direction of flight. Movement of the cyclic in the intended  direction of flight causes the helicopter to move, controls the  rate of speed, and ground track, but the collective and pedals  are key to successful sideward flight. Just as in forward flight,  the collective keeps the helicopter from contacting the ground  and the pedals help maintain the correct heading; even in  sideward flight, the tail of the helicopter should remain behind  you. Inputs to the cyclic should be smooth and controlled,  and the pilot should always be aware of the tip-path plane in  relation to the ground.  [Figure 2-43] 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 accelerated over the foil and  projected downward. Anytime a helicopter is producing lift,  it moves large masses of air vertically and down through the  rotor disk. This downwash or induced flow can significantly  change the efficiency of the rotor disk. Rotational relative  wind combines with induced flow to form the resultant  relative wind. As induced flow increases, resultant relative  wind becomes less horizontal. Since AOA is determined by measuring the difference between the chord line and the  resultant relative wind, as the resultant relative wind becomes  less horizontal, AOA decreases.  [See Figure 2-21] As the helicopter accelerates in forward flight, induced flow  drops to near zero at the forward disk area and increases at the  aft disk area. These differences in lift between the fore and  aft portions of the rotor disk are called transverse flow effect.  [ Figure 2-41]  This increases the AOA at the front disk area  causing the rotor blade to flap up and reduces AOA at the aft  disk 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 accelerates through approximately  20 knots or if the headwind is approximately 20 knots.