Game Development Reference
In-Depth Information
An interesting characteristic about the parasitic drag coefficient is that it is not a function
of angle of attack but is only a function of the airplane geometry. For example, the parasitic
drag coefficient won't change if the airplane changes its pitch angle, but the coefficient will
change if the wing flaps are deflected.
Induced Drag
A third type of drag experienced by airplanes, known as
induced drag
, is caused by the gener-
ation of lift. Recall from earlier in this chapter that lift occurs because the pressure is higher on
the bottom of the wing than it is on the top of the wing. In the region of the wing tip, the higher-
pressure air on the bottom of the wing flows in a swirling vortex structure towards the upper
surface of the wing. This effect is shown schematically in Figure 10-18. The vortices are “shed”
from the aft end of the wing tip and continue for some distance behind the plane. The generation
of the wing tip vortex structure creates drag.
Figure 10-18.
Wing tip vortices are due to air circulation from the bottom side of the wing to
the top.
Induced drag is characterized by a coefficient of induced drag,
C
Di
. The coefficient is a
function of the square of the lift coefficient,
C
L
; the wing aspect ratio,
A
R
; and a constant,
e
,
known as the
airplane efficiency factor
.
2
L
C
C
=
(10.22)
Di
p
Ae
R
The airplane efficiency factor has a value between 0 and 1. Its value depends on the shape
of the pressure distribution over the wing. If the shape of the pressure distribution is elliptic, as
it was for the RAF Spitfire, then the efficiency factor is equal to 1. Nonelliptic pressure distribu-
tions have an efficiency factor less than 1. The Cessna 172 Skyhawk, for example, has an airplane
efficiency factor of 0.77.
