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Stability and Control 109

Positive dynamic stability

Neutral dynamic stability

Negative dynamic stability

Fig. 4.7. Three types of dynamic stability.

with the oscillations decreasing with time. This is like a car with good
shock absorbers.
The second case is neutral dynamic stability. The airplane is
statically stable because there is a restoring force. But the amplitude
of the oscillations in this case does not decrease with time. This is
kind of like a car without shock absorbers that hits a bump.
The third case in the figure is negative dynamic stability. Again the
airplane is statically stable but the amplitude of the oscillations
increases with time. This is kind of like a car without shock absorbers
going down a “washboard” (bumpy) road.
As with static stability, dynamic stability must be considered in
the longitudinal, lateral, and directional (pitch, roll, and yaw) axes.
What can make dynamic stability even more interesting is that more
than one axis can couple, producing some very interesting motion.