Game Development Reference
In-Depth Information
Figure 10-8.
Lift coefficient data for the NACA 0012 and 2412 airfoils
One thing that is obvious from looking at Figure 10-8 is that changing the angle of attack of
an airfoil changes its lift. This phenomenon makes sense because tilting the airfoil up or down
changes the way that air flows around it, which changes the surface pressure distribution over
the wing. If the airfoils are tilted downward (given a negative angle of attack), they generate a
negative, or downward, lift force.
Stall
If you look at Figure 10-8, the lift coefficient of the NACA 0012 and 2412 airfoils increases with
increasing angle of attack until about
a
= 16 degrees, at which point the lift coefficient begins
to decrease. This effect is known as a
stall
condition. What has happened is that when the angle
of attack reaches a certain value, the air is no longer able to follow the shape of the airfoil on the
upper surface. Instead, as shown in Figure 10-9, the air flow
separates
from the airfoil. Inside
the separation region, the flow is turbulent and chaotic and the surface pressure increases. The
pressure increase on the upper surface reduces (or eliminates) the pressure differential between
the top and bottom wing surfaces, reducing the lift generated by the airfoil.
Gaining speed and reducing the angle of attack so the air flow can
reattach
itself to the top
surface of the airfoil can correct a stall condition. Stall is most dangerous at low velocities where
it can be difficult to make the necessary corrections.
The
stall speed
of an airplane is the minimum speed at which the wings can generate
enough lift to sustain level flight. It is the point where
F
L
=
mg
, with
m
being the mass of the
