An airfoil consists of two nearly parallel
can be produced in two ways--by increasing
the forward movement of the airfoil through
surfaces, with one surface being more rounded
the air (fig. 8-2), or by changing the angle
than the other. As air passes over these two
of attack. Angle of attack is the acute angle
surfaces, the air passing over the rounded
surface has farther to travel than the air passing
between the chord line of an airfoil and its
over the flat surface. However, two particles of
direction of motion relative to the air. T h e
air leaving the airfoil's leading edge at the same
chord of an airfoil is an imaginary straight
line drawn from the leading edge to the trailing
instant, one going over the rounded surface and
edge of the airfoil (fig. 8-3). As the angle
one over the flat, arrive at the trailing edge at the
of attack increases, the air strikes the leading
same time. Therefore, you can infer that air
edge closer to the flat portion of the airfoil.
passing over the rounded surface travels at a
The distance air must flow over the rounded
higher velocity than air passing over the flat
portion becomes even greater in relation to that
surface.
flowing over the flat portion. This action causes
a larger pressure difference, and develops more
Bernoulli's theory concerning the behavior of
lift.
fluids, E = V x P, explains how pressure is
changed and lift is produced. Here, E is the total
If the angle of attack increases too much,
energy produced by the airfoil passing through
airflow over the airfoil's rounded portion
the air, V is velocity energy, and P is pressure
separates from the surface and becomes turbulent.
energy. An airfoil that passes through air at a
velocity of 50 feet per second and exerts a pressure
Turbulence causes the pressure on both surfaces
of 10 pounds per square inch on the flat surface
to become nearly equal, and the airfoil is said to
produces a total energy of 500 foot-pounds per
stall.
square inch per second.
When producing lift, a secondary effect called
drag is also produced. Drag produced by a lifting
surface or airfoil is called induced drag. Induced
drag develops in direct proportion to lift--when
If the airflow velocity over the rounded surface
lift increases, induced drag also increases. At a
is increased to 60 feet per second, and the total
given speed and angle of attack, a thick airfoil
energy is unchanged, it exerts a pressure of 8.33
produces more lift and drag than does a thin
foot-pounds per square inch per second on the
rounded surface.
The difference in the pressure between the
rounded surface and the flat surface of the airfoil
is called lift.
In actual practice, the flat surface is not
perfectly flat and causes some decreased pressure.
The decreased pressure is negative lift. Negative
lift is compensated for by the creation of a high
pressure on the flat surface. Air packed beneath
the airfoil (dynamic lift) causes the high pressure.
The true measure of lift remains the difference
in pressure between the rounded and flat portions
of the airfoil.
Increased lift is the result of a larger pressure
Figure 8-2.-Lift increases as velocity increases.
difference between the surfaces. The difference
8-2
