An airfoil is any surface, such as a wing or rotor blade, designed to produce lift when air passes over
it. Air passing over the upper surface of a foil produces two-thirds of a foil's lift by creating a lower
pressure. One third of the foil's lift is produced by the higher pressure of air on the foil's under surface.
Relative wind is the air flowing opposite and parallel to the direction of airfoil motion. When an
aircraft is at rest, relative wind does not exist, only wind created by nature. Relative wind, shown in
figure 2.4, is created by the motion of the aircraft traveling through the air using its own power to reach
its desired speed.
Figure 2.4. Relative Wind.
A symmetrical airfoil is designed to have equal cambers on both sides. This kind of airfoil has the
characteristics of limiting center-of-pressure travel.
An asymmetrical airfoil is designed to have unequal cambers. This type of airfoil has the
characteristic of a rapid movement of center-of-pressure travel. Figure 2.5 shows the contrast between a
symmetrical and an asymmetrical airfoil.