It has been shown that the chordwise velocity of a blade element is given by:
U = )r + V sin ^
On the retreating side the components of rotational velocity and horizontal velocity are subtractive and a region of reversed flow will exist. It can be shown that this region is circular in nature with a diameter equal to pR. At low forward speed this region is usually of little consequence as the aerodynamic section of the blade does not begin until some distance from the hub (0.25R is typical). Since the flow in the reversed flow region is from trailing edge to leading edge it has the effect of pushing the rotor around the mast thereby extracting power from the airflow. Although the amount of negative power is usually quite small and often ignored, it may, however, be approximated by an 8% reduction in the forward speed dependent component of rotor power
[1.4] , thus, for example, reducing k from 4.65 to 4.30. Hence the power required in forward flight may be estimated using:
Cp=2 (a ) + ^Ct +1 ^ + 4.3^2)
Figure 2.14 shows the variation of angle of attack across a rotor disk at a high advance ratio (p = 0.34). Note that the reversed flow region is clearly delineated.