irectly 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
produces greater lift, and tends to tilt the aeroplane over still more.
Such tilting tendency is, however, opposed by the difference in the
H.E.'s of the two wings.

It then follows that, for the lateral dihedral angle to be effective,
such angle must be large enough to produce, when the aeroplane tilts,
a difference in the H.E.'s of the two wings, which difference must be
sufficient to not only oppose the tilting tendency due to the aeroplane
turning, but sufficient to also force the aeroplane back to its original
position of equilibrium.

It is now, I hope, clear to the reader that the lateral dihedral is
not quite so effective as would appear at first sight. Some designers,
indeed, prefer not to use it, since its effect is not very great, and
since it must be paid for in loss of H.E. and consequently loss of lift,
thus decreasing the lift-drift ratio, i.e., the efficiency. Also, it is
sometimes advanced that the lateral dihedral increases the "spill" of
air from the wing-tips and that this adversely affects the lift-drift
ratio.

The disposition of the keel-surface affects the lateral stability. It
should be, in effect, equally divided by the longitudinal turning axis
of the aeroplane. If there is an excess of keel-surface above or below
such axis, then a side gust striking it will tend to turn the aeroplane
over sideways.

The position of the centre of gravity affects lateral stability. If
too low, it produces a pendulum effect and causes the aeroplane to roll
sideways.

If too high, it acts as a stick balanced vertically would act. If
disturbed, it tends to travel to a position as far as possible from its
original position. It would then tend, when moved, to turn the aeroplane
over sideways and into an upside-down position.

From the point of view of lateral stability, the best position for the
centre of gravity is one a little below the centre of drift.

Propeller torque affects lateral stabi