Game Development Reference
In-Depth Information
Lift and Drag
When an airfoil moves through a fluid such as air, lift is produced. The mechanisms by
which this occurs are similar to those in the case of the Magnus lift force, discussed
earlier in Chapter 6 , in that Bernoulli's law is still in effect. However, this time, instead
of rotation it's the airfoil's shape and angle of attack that affect the flow of air so as to
create lift.
Figure 15-5 shows an airfoil section moving through air at a speed V . V is the relative
velocity between the foil and the undisturbed air ahead of the foil. As the air hits and
moves around the foil, it splits at the forward stagnation point located near the foil
leading edge such that air flows both over and under the foil. The air that flows under
the foil gets deflected downward, while the air that flows over the foil speeds up as it
goes around the leading edge and over the surface of the foil. The air then flows smoothly
off the trailing edge; this is the so-called Kutta condition. Ideally, the boundary layer
remains “attached” to the foil without separating as in the case of the sphere discussed
in Chapter 6 .
Figure 15-5. Airfoil moving through air
The relatively fast-moving air above the foil results in a region of low pressure above
the foil (remember Bernoulli's equation that shows pressure is inversely proportional
to velocity in fluid flow). The air hitting and moving along the underside of the foil
creates a region of relatively high pressure. The combined effect of this flow pattern is
to create regions of relatively low and high pressure above and below the airfoil. It's this
pressure differential that gives rise to the lift force. By definition, the lift force is per‐
pendicular to the line of flight—that is, the velocity vector.