sponding camber. The wing-tips then act as
longitudinal stabilizers.

[Illustration]

This design of aeroplane, while very interesting, has not proved very
practicable, owing to the following disadvantages: (1) The plan design
is not, from a mechanical point of view, so sound as that of the
ordinary aeroplane surface, which is, in plan, a parallelogram. It is,
then, necessary to make the strength of construction greater than would
otherwise be the case. That means extra weight. (2) The plan of the
surface area is such that the aspect ratio is not so high as if the
surface was arranged with its leading edges at right angles to the
direction of motion. The lower the aspect ratio, then, the less the
lift. This design, then, produces less lift for weight of surface than
would the same surface if arranged as a parallelogram. (3) In order to
secure the longitudinal dihedral, the angle of incidence has to be very
much decreased towards the wing-tips. Then, in order that the lift-drift
ratio may be preserved, there must be a corresponding decrease in the
camber. That calls for surface ribs of varying cambers, and results in
an expensive and lengthy job for the builder. (4) In order to secure
directional stability, the surface is, in the centre, arranged to dip
down in the form of a V, pointing towards the direction of motion.
Should the aeroplane turn off its course, then its momentum in the
direction of its first course causes it to move in a direction the
resultant of the 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, fitted to a "pusher" aeroplane, 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. When fitted to a
"tractor" aeroplane, the engine is reversed so that a reverse condition
results. In modern aeroplanes this tendency is not sufficiently
important to bother about, except in the matter of spiral descents
(see section headed