Page 79 - Understanding Flight

CH03_Anderson 7/25/01 8:56 AM Page 66

66 CHAPTER THREE

Another reason to sweep a wing might be to move the center of lift
of the wing forward or aft from where the wing root attaches to the
fuselage. This might be necessary to accommodate a certain structural
feature at the wing root and a certain position for the center of gravity
of the plane.
Virtually all aircraft that have swept wings have wings
DC-6 publicity photos used a
swept back, though it is interesting to note that the high-speed
model named Norma Jean Baker
drag reduction due to sweep is equal for both forward- and
inside the cabin. She later
backward-swept wings. The primary advantage to forward
became known as Marilyn
sweep is to increase maneuverability. But forward sweep is
Monroe.
very difficult to build structurally. A problem known as
structural divergence can occur if the wing is not stiff enough.
What happens is that the forward tip twists due to the load. The
twisting increases the tip loading, thus twisting the wingtip even
more. Eventually, the tip load is so great that it literally twists off the
airplane. The advent of modern composites that can be tailored to
specific stiffness requirements has led to the possibility of forward-
swept wings. Composites can be manufactured in such a way that it
eliminates the structural divergence. These airplanes are still very
much in the experimental stage, however.

In the late 1970s NASA started a program to study
swept-forward wings.The program resulted in the X-29,
forward-swept wing demonstrator seen in Figure 3.8.The
purpose behind the forward-swept wing was to create a
naturally unstable platform.This is highly desirable for
maneuverability.The airplane is so unstable that the pilot
alone cannot control it.A sophisticated computer control
system, called fly-by-wire, is added into the control loop
to make the airplane controllable. If the computer were
to fail, the airplane would instantly become unstable and
crash.

Taper
The designer may consider a tapered wing for an airplane. A tapered
wing has a shorter chord at the tip than at the root, as shown on a