IRECTIONAL 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 everything 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.

[Illustration]

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 an air pressure in the direction C. Such pressure, acting upon
the keel-surface, presses the tail back towards its first position in
which the aeroplane is upon its course B.

What I have described is continually going on during flight, but in a
well-designed aeroplane such stabilizing movements are, most of the
time, so slight as to be imperceptible to the pilot.

If an aeroplane was not stabilized in this way, it would not only be
continually trying to leave its course, but it would also possess a
dangerous tendency to "nose away" from the direction of the side gusts.
In such case the gust shown in the above illustration would turn the
aeroplane round the opposite way a very considerable distance; and the
right wing, being on the outside of the turn, would travel with greater
velocity than the left wing. Increased velocity means increased lift;
and so, the right wing lifting, the aeroplane would turn over sideways
very quickly.

LONGITUDINAL STABILITY.--Flat surfaces are longitudinally stable owing
to the fact that with decreasing angles of incide