Fig. 1.1. The lift in (a) is the result of faster air sliding over the top of the wind foil. In (b),
the combination of actual wind and the relative wind due to blade motion creates a resultant
that creates the blade lift.
Figure 1.1a) shows an airfoil, where the air moving the top has a greater distance to pass before it can rejoin the air that takes the short cut under the foil.
So the air pressure on the top is lower than the air pressure under the airfoil. The air pressure creates the lifting force which can hold the airplane up.
In terms of the wind turbine blade, it is more complicated than the aircraft wing.
From Fig. 1.1b) we can find that a rotating turbine blade sees air moving only from the wind itself, but also from the relative motion of the blade.
So the combination of the wind and blade motion is the resultant wind which moves toward the blade at a certain angle.
The angle between the airfoil and the wind is called the angle of attack as shown in Fig. 1.2. Increasing the angle of attack can improve the lift at the expense of increased drag. However, if we increase the angle of attack too much the wing will stall and the airflow will have turbulence and damage the turbine blades.