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58 CHAPTER THREE

that lift is primarily a function of angle of attack, with little dependence
on the airfoil shape. So, why will not almost any wing section do? Here
we will discuss some of the specific airfoil design characteristics that
are used and how they affect performance. Characteristics that must be
considered in selecting a wing include lift at cruise angle of attack,
drag, stall characteristics, laminar flow, and room for internal struc-
tures.

Wing Incidence and Camber
The lowest drag for the fuselage will be achieved when the fuselage is
aligned with the relative wind. A symmetric wing would have to be
attached at an “incident angle” so that the fuselage is at a zero angle
to the wind while the wing is at some positive angle of attack. When
a child builds a simple hobby-shop glider with flat balsa wings, the
wings are attached at an angle to the fuselage. Alternatively, a wing
with sufficient camber can be selected such that when the leading and
trailing edges are aligned at cruise the lift balances the weight. This is
when the geometric angle of attack is zero (Figure 2.10). This config-
uration gives lift with a low parasitic drag and parasitic power. Typi-
cally, cambered wings are used and mounted to the fuselage with a
small incident angle for cruise conditions.
It might seem that a highly cambered airfoil would give the best
results. The appropriate amount of camber will depend on the
application. An acrobatic airplane, which spends much of the time
upside down, will have a symmetric airfoil section. Modern aircraft
frequently have positive camber over most of the airfoil but a
reversed, or negative, camber toward the trailing edge. This reversed
camber can reduce drag at high speeds. So, selecting a correct
The projected lifetime of a
airfoil for a particular application involves complex choices and
commercial airplane design is
can become more of an art than a science.
70 years.
Wing Thickness
Wing thickness is another design consideration. A thick wing can
result in a large wake, resulting in high parasitic drag, even at zero
angle of attack. The airflow around a thick wing may separate, caus-
ing form drag, a type of parasitic drag.