Example: a weight suspended by a cord.
INSTABILITY is a condition whereby an object disturbed
has a natural tendency to move as far as possible away from
its first position, with no tendency to return. Example:
a stick balanced vertically upon your finger.
NEUTRAL INSTABILITY is a condition whereby an object
disturbed has no tendency to move farther than displaced
by the force of the disturbance, and no tendency to return
to its first position.
In order that an aeroplane may be reasonably controllable,
it is necessary for it to possess some degree of stability
longitudinally, laterally, and directionally.
LONGITUDINAL STABILITY in an aeroplane is its stability
about an axis transverse to the direction of normal horizontal
flight, and without which it would pitch and toss.
LATERAL STABILITY is its stability about its longitudinal
axis, and without which it would roll sideways.
DIRECTIONAL STABILITY is its stability about its vertical
axis, and without which it would have no tendency to keep
its course.
For such directional stability to exist there must be,
in effect,[[16]] more ``keel-surface'' behind the vertical axis
than there is in front of it. By keel-surface I mean every-
thing to be seen when looking at an aeroplane from the side
of it--the sides of the body, undercarriage, struts, wires, etc.
The same thing applies to a weathercock. You know what
would happen if there was insufficient keel-surface behind
the vertical axis upon which it is pivoted.
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