Center of pressure Chord line Angle of attack Rotational relative wind Induced flow Resultant relative wind Center of pressure Rotational relative wind Flightpath of airfoil Chord line Angle of incidence 10–20 knots More horizontal flow of air Downward velocity of air molecules used by aft section of rotor Fore Fore Aft Aft Rotational relative wind Less induced flow Angle of attack Resultant relative wind Chord line  Rotational relative wind Greater induced flow Angle of attack Resultant relative wind Chord line  Figure 2-21.  A helicopter in forward flight, or hovering with a headwind or crosswind, has more molecules of air entering the aft portion  of the rotor disk. Therefore, at the rear of the rotor disk, the angle of attack is less and the induced flow is greater.  2-11 Figure 2-20.  Resultant relative wind.  Figure 2-19.  Rotational relative wind. Resultant Relative Wind Induced Flow (Downwash)  forward velocity results in decreased induced flow velocity.  This change results in an improved efficiency (additional lift)  being produced from a given blade pitch setting. The resultant relative wind at a hover is rotational relative  wind modified by induced flow. This is inclined downward  at some angle and opposite the effective flightpath of the  airfoil, rather than the physical flightpath (rotational relative  wind). The resultant relative wind also serves as the reference  plane for development of lift, drag, and total aerodynamic  force (TAF) vectors on the airfoil.  [Figure 2-20]  When the  helicopter has horizontal motion, airspeed further modifies  the resultant relative wind. The airspeed component of  relative wind results from the helicopter moving through  the air. This airspeed component is added to, or subtracted  from, the rotational relative wind depending on whether  the blade is advancing or retreating in relation to helicopter  movement. Introduction of airspeed relative wind also  modifies induced flow. Generally, the downward velocity  of induced flow is reduced. The pattern of air circulation  through the disk changes when the aircraft has horizontal  motion. As the helicopter gains airspeed, the addition of  At flat pitch, air leaves the trailing edge of the rotor blade  in the same direction it moved across the leading edge; no  lift or induced flow is being produced. As blade pitch angle  is increased, the rotor disk induces a downward flow of air  through the rotor blades creating a downward component of  air that is added to the rotational relative wind. Because the  blades are moving horizontally, some of the air is displaced  downward. The blades travel along the same path and pass a  given point in rapid succession. Rotor blade action changes the  still air to a column of descending air. Therefore, each blade has  a decreased AOA due to the downwash. This downward flow  of air is called induced flow (downwash). It is most pronounced  at a hover under no-wind conditions.  [Figure 2-21]