Airflow past an aerofoil surface exerts a force on the surface. The component of this force perpendicular to the air flow is called the lift, and the component parallel to the airflow is called drag. Figure 1 below shows the flow of air over a typical low-speed aerofoil section and illustrates the way in which lift is generated;

Figure 1: flow of air over a typical low-speed aerofoil section

The airflow over the wing increases its speed causing a reduction in pressure; this generates a force (lift) perpendicular to the chord of the aerofoil. The airflow below the wing moves much more slowly generating greater pressure and less or negative lift.

The component of total drag resulting from the generation of lift is termed induced drag.

Lift is a function of aerofoil design (shape), airspeed and angle of attack:

  • Lift increases with airspeed and decreases with falling airspeed;
  • Lift increases with angle of attack up to a maximum angle;
  • Stall occurs if the airspeed falls too low or if the angle of attack increases too much.

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