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altitude airplanes. This exception will be discussed in Chapter 6,
“High-Speed Flight.”

Sweep
Most modern aircraft use swept wings, as shown in the picture of an
X-5 with variable-sweep wings (Figure 3.7). The primary motivation
behind swept wings is to reduce drag at higher cruise speeds. It was
discovered in Germany in the late 1930s that at high speeds the para-
sitic drag of the wing was related to the angle the air makes with the
wing’s leading edge. Thus, by sweeping the wing, the drag at high
speeds is reduced. This is covered in greater detail in Chapter 6. In
flight near or above the speed of sound, swept wings are mandatory
to reduce the power required to sustain cruise speeds. Most commer-
cial transports, military aircraft, and newer business jets fly at near the
speed of sound. Thus they require swept wings. A glance at the wing
of a jet tells you how fast it is designed to go.
There are other reasons to sweep a wing. Swept wings impact
stability. A wing swept back is generally more stable than a wing
without sweep. This is desirable for passenger airplanes, since the
airplane will have a tendency to stabilize after it is upset by a gust of air.
Conversely, a wing swept forward will be less stable. Experimentation
with forward-swept wings has resulted in less stable airplanes, which
increases maneuverability. Although not yet found in production
airplanes, the fighter of the future may employ forward-swept wings.

Fig. 3.7. The variable-sweep X-5 research aircraft. (Photo courtesy of NASA.)