Page 120 - Aeronautical Engineer Data Book

Section 6

Basic aerodynamics

6.1 General airfoil theory
When an airfoil is located in an airstream, the
flow divides at the leading edge, the stagna­
tion point. The camber of the airfoil section
means that the air passing over the top
surface has further to travel to reach the trail­
ing edge than that travelling along the lower
surface. In accordance with Bernoulli’s
equation the higher velocity along the upper
airfoil surface results in a lower pressure,
producing a lift force. The net result of the
velocity differences produces an effect equiv­
alent to that of a parallel air stream and a
rotational velocity (‘vortex’) see Figures 6.1
and 6.2.
For the case of a theoretical finite airfoil
section, the pressure on the upper and lower
surface tries to equalize by flowing round the
tips. This rotation persists downstream of the
wing resulting in a long U-shaped vortex (see
Figure 6.1). The generation of these vortices
needs the input of a continuous supply of
energy; the net result being to increase the drag
of the wing, i.e. by the addition of so-called
induced drag.

6.2 Airfoil coefficients
Lift, drag and moment (L, D, M) acting on an
aircraft wing are expressed by the equations:
U 2
2
l
Lift (L) per unit width = C L
2