Game Development Reference

In-Depth Information

Note that the airfoil does not have to be cambered in order to generate lift; a flat plate

oriented at an angle of attack relative to the airflow will also generate lift. Likewise, an

airfoil does not have to have an angle of attack either. Cambered airfoils can generate

lift at 0, or even negative, angles of attack. Thus, in general, the total lift force on an

airfoil is composed of two components: the lift due to camber and the lift due to attack

angle.

Theoretically, the thickness of an airfoil does not contribute to lift. You can, after all,

have a thin curved wing as in the case of wings made from fabric (such as those used

for hang gliders). In practice, thickness is utilized for structural reasons. Further, thick‐

ness at the leading edge can help delay stall (more on this in a moment).

The pressure differential between the upper and lower surfaces of the airfoil also gives

rise to a drag force that acts in line with, but opposing, the velocity vector. The lift and

drag forces are perpendicular to each other and lie in the plane defined by the velocity

vector and the vector normal (perpendicular) to the airfoil chord line. When combined,

these two force components, lift and drag, yield the resultant force acting on the airfoil

in flight. This is illustrated in
Figure 15-5
.

Both lift and drag are functions of air density, speed, viscosity, surface area, aspect ratio,

and angle of attack. Traditionally, the lift and drag properties of a given foil design are

expressed in terms of nondimensional coefficients C
L
and C
D
, respectively:

C
L
= L / [(1/2) ρ V
2
S]

C
D
= D / [(1/2) ρ V
2
S]

where
S
is the wing planform area (span times chord for rectangular wings),
L
is the lift

force,
D
is the drag force,
V
is the speed through the air, and ρ (rho) is air density. These

coefficients are experimentally determined from wind tunnel tests of model airfoil de‐

signs at various angles of attack. The results of these tests are usually presented as graphs

of lift and drag coefficient versus attack angle.
Figure 15-6
through
Figure 15-8
illustrate

some typical lift and drag charts for a wing section.