"Spinning"). In the old days of crudely designed
and under-powered "pusher" aeroplanes this gyroscopic action was very
marked, and led the majority of pilots to dislike turning an aeroplane
to the right, since, in doing so, there was some danger of "stalling."

LATERAL STABILITY is far more difficult for the designer to secure
than is longitudinal or directional stability. Some degree of lateral
stability may be secured by means of the "lateral dihedral," _i.e._,
the upward inclination of the surface towards its wing-tips thus:

[Illustration]

Imagine the top =V=, illustrated opposite, to be the front view of a
surface flying towards you. The horizontal equivalent (H.E.) of the left
wing is the same as that of the right wing. Therefore, the lift of one
wing is equal to the lift of the other, and the weight, being situated
always in the centre, is balanced.

If some movement of the air causes the surface to tilt sideways, as in
the lower illustration, then you will note that the H.E. of the left
wing increases, and the H.E. of the right wing decreases. The left wing
then, having the greatest lift, rises; and the surface assumes its first
and normal position.

Unfortunately, however, the righting effect is not proportional to the
difference between the right and left H.E.'s.

[Illustration:
R, Direction of reaction of wing indicated.
R R, Resultant direction of reaction of both wings.
M, Horizontal (sideway) component of reaction.
L, Vertical component of reaction (lift).]

In the case of A, the resultant direction of the reaction of both wings
is opposed to the direction of gravity or weight. The two forces R R
and gravity are then evenly balanced, and the surface is in a state of
equilibrium.

In the case of B, you will note that the R R is not directly opposed
to gravity. This results in the appearance of M, and so the resultant
direction of motion of the aeroplane is no longer directly forward,
but is along a line the resultant of the Thrust and M. In other words,
it is, while flying forward, at the same time moving sideways in the
direction M.

In moving sideways, the keel-surface receives, of course, a pressure
from the air equal and opposite to M. Since such surface is greatest
in effect towards the tail, then the latter must be pushed sideways.
That causes the aeroplane to turn; and, the highest wing being on the
outside of the turn, it has a greater velocity than the lower wing. That