Page 124 - Understanding Flight

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Stability and Control 111

wiggle of the tail. The airplane does this while maintaining its head-
ing. Dutch roll has a short period and presents no major stability prob-
lem. The biggest problem is upset stomachs of passengers in the rear
of the airplane. Because of passenger discomfort, commercial air-
planes use yaw dampers which move the rudder to automati-
cally damp out Dutch roll. A yaw damper is not a critical flight The first Academy Award for Best
control, so a flight will continue if the yaw damper is inopera- Picture was awarded to Wings.
ble. But there may be more sick passengers on the flight.
In smaller airplanes most pilots do not feel the motion of Dutch roll.
That is because all motion occurs around the center of gravity. In a
small airplane, the pilot and passengers are usually sitting very close
to the center of gravity. Only on a large commercial transport do
passengers find themselves far enough away from the center of gravity
to feel this motion.

Spiral Instability
The last instability that we will consider is in spiral motion, frequently
referred to as spiral divergence. This is instability in yaw and roll,
which leads to a downward spiral. An airplane that has a spiral insta-
bility will eventually spin into the ground if the pilot does not inter-
vene. However, although the results are dramatic, the FAA allows for
some spiral divergence in airplane designs. This is because, like
phugoid motion, spiral divergence takes a long time to develop.
Again, only a sleeping pilot could ignore this. Note that Lindbergh’s
unplanned nap might have been disastrous if he had not woken up on
time. The Spirit of St. Louis was fairly deep into a spiral divergence at
the time he woke up.

Stability Augmentation

The newest military aircraft are exploring statically unstable airplanes,
which could not be flown without computer control. What the com-
puter does is kind of like when you try to balance a pencil vertically
on the tip of your finger. It is very difficult. This is a statically unsta-
ble situation. But, if you could react quickly enough, you could keep