thrust and the momentum. It then moves in a more or
less sideways attitude, which results in an air pressure upon one
side of the V, and which tends to turn the aeroplane back to its first
course. This arrangement of the surface results in a bad drift. Vertical
surfaces at the wing-tips may also be set at an angle producing the same
stabilizing effect, but they also increase the drift.

The gyroscopic action of a rotary engine will affect the longitudinal
stability when an aeroplane is turned to right or left. In the case of
a Gnome engine, such gyroscopic action will tend to depress the nose of
the aeroplane when it is turned to the left, and to elevate it when
it is turned to the right. In modern aeroplanes this tendency is not
sufficiently important to bother about. In the old days of crudely
designed and under-powered 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:

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.

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 d