city and Climb.

CHAPTER II. STABILITY AND CONTROL

STABILITY is a condition whereby an object disturbed has a natural
tendency to return to its first and normal position. 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. It would turn off its proper
course, which is opposite to the direction of the wind. It is very much
the same in the case of an aeroplane.

The above illustration represents an aeroplane (directionally stable)
flying along the course B. A gust striking it as indicated acts upon the
greater proportion of keel-surface behind the turning axis and throws it
into the new course. It does not, however, travel along the new course,
owing to its momentum in the direction B. It travels, as long as such
momentum lasts, in a direction which is the resultant of the two forces
Thrust and Momentum. But the centre line of the aeroplane is pointing in
the direction of the new course. Therefore its attitude, relative to
the direction of motion, is more or less sideways, and it consequently
receives