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CH04_Anderson 7/25/01 8:57 AM Page 98

98 CHAPTER FOUR

Stable Neutral Unstable

Fig. 4.1. Three types of static stability.

The global positioning system Static Stability
(GPS) included a built-in error
There are three types of static stability. These are statically
that could only be corrected by
stable, unstable, and neutral. All three are illustrated in Figure
the military. The built-in error
4.1. The ball in the bowl illustrates a statically stable system. If
has been officially terminated
it is displaced from the bottom, it will tend to return. An
to give everyone the ability to
increase in the steepness of the sides of the bowl corresponds to
know their position within
an increase in stability. If one turns the bowl over, we have an
about 5 feet.
illustration of static instability. If the ball is moved from the
exact center of the bowl, it will continue to move away. The ball
on the table illustrates neutral stability. If the ball is moved, it will tend
to stay in its new position.
For an airplane, static stability means that if a gust of air or some other
perturbation causes a change in its current state such as heading, it will
experience a restoring force. Small general aviation and commercial
aircraft, if properly trimmed, will return to straight-and-level flight after a
gust or an abrupt disturbance of the controls. There have been instances
where a pilot has fallen asleep only to have the airplane continue its
course without an autopilot. Even Lindbergh got a short, unplanned nap
on his solo trip across the Atlantic, and lived to tell about it.
Stability should not be confused with balance. An airplane is
balanced when there is no net torque on the airplane. Stability is the
tendency for the airplane to return to its previous attitude once
disturbed. In the next section we lead you through pitch stability,
which should make this distinction clear.

Longitudinal Stability

Longitudinal stability is the tendency for an airplane at a specific pitch
attitude to return to that attitude when perturbed. Ultimately, we are