any changes in power or in cyclic movement, the helicopter  begins either to climb or to descend. Once straight-and-level  flight is obtained, the pilot should make note of the power  (torque setting) required and not make major adjustments to  the flight controls.  [Figure 2-34]  Airflow across the rotor disk in forward flight varies from  airflow at a hover. In forward flight, air flows opposite the  aircraft’s flightpath. The velocity of this air flow equals the  helicopter’s forward speed. Because the rotor blades turn  in a circular pattern, the velocity of airflow across a blade  depends on the position of the blade in the plane of rotation  at a given instant, its rotational velocity, and airspeed of the  helicopter. Therefore, the airflow meeting each blade varies  continuously as the blade rotates. The highest velocity of  airflow occurs over the right side (3 o’clock position) of  the helicopter (advancing blade in a rotor disk that turns  counterclockwise) and decreases to rotational velocity over  the nose. It continues to decrease until the lowest velocity of  airflow occurs over the left side (9 o’clock position) of the  helicopter (retreating blade). As the blade continues to rotate,  velocity of the airflow then increases to rotational velocity  over the tail. It continues to increase until the blade is back  at the 3 o’clock position.  The advancing blade in  Figure 2-35,  position A, moves in  the same direction as the helicopter. The velocity of the air  meeting this blade equals rotational velocity of the blade  plus wind velocity resulting from forward airspeed. The  retreating blade (position C) moves in a flow of air moving  in the opposite direction of the helicopter. The velocity of  airflow meeting this blade equals rotational velocity of the  blade minus wind velocity resulting from forward airspeed.  The blades (positions B and D) over the nose and tail move  essentially at right angles to the airflow created by forward  airspeed; the velocity of airflow meeting these blades equals  the rotational velocity. This results in a change to velocity  of airflow all across the rotor disk and a change to the lift  pattern of the rotor disk. As the relative wind speed of the advancing blade increases,  the blade gains lift and begins to flap up. It reaches its  maximum upflap velocity at the 3 o’clock position, where the  wind velocity is the greatest. This upflap creates a downward  flow of air and has the same effect as increasing the induced  flow velocity by imposing a downward vertical velocity  vector to the relative wind which decreases the AOA. As relative wind speed of the retreating blade decreases,  the blade loses lift and begins to flap down. It reaches its  maximum downflap velocity at the 9 o’clock position, where  Airspeed = 120 Knots Relative wind as a result of aircraft movement at 120 knots Relative wind as a result of aircraft movement at 120 knots 480  Knots  120 Knots  360 Knots  Rotational Velocity Aircraft Airspeed Wind Velocity =  =  =  – 480  Knots  120 Knots  600 Knots  Rotational Velocity Aircraft Airspeed Wind Velocity =  =  =  + A B C D D i r e c t i o n o f r o t a t i o n R o t a i o n a l v e l o c i t y = 4 8 0 k n o t s Power required (horsepower) Indicated airspeed (KIAS) 800 600 400 200 0  0  40  60  80  100  120 Minimum pow er for level flight (V$_{Y}$ ) Maximum continuo us level (horizon tal) flight airspeed (V$_{H}$ ) Maximum continuous power available Increasing power for decreasing airspeed Increasing power for increasing airspeed $_{ }$P o w e r e q u i r e d t o h o v e r O G E Figure 2-35.  Airflow in forward flight. Figure 2-34.  Power versus airspeed chart. Airflow in Forward Flight  Advancing Blade  Retreating Blade  2-19