ce at which the lift-drift ratio is
highest.

_Wash-in_ is the term applied to the increased angle.

_Wash-out_ is the term applied to the decreased angle.

Both lateral and directional stability may be improved by washing out
the angle of incidence on both sides of the surface, thus:

[Illustration]

The decreased angle decreases the drift and therefore the effect of
gusts upon the wing-tips, which is just where they have the most effect
upon the aeroplane, owing to the distance from the turning axis.

The wash-out also renders the ailerons (lateral controlling services)
more effective, as, in order to operate them, it is not then necessary
to give them such a large angle of incidence as would otherwise be
required.

[Illustration: Note: Observe that the inclination of the ailerons to
the surface is the same in each case.]

The less the angle of incidence of the ailerons, the better their
lift-drift ratio, i.e., their efficiency. You will note that, while the
aileron attached to the surface with washed-out angle is operated to the
same extent as the aileron illustrated above it, its angle of incidence
is considerably less. Its efficiency is therefore greater.

The advantages of the wash-in must, of course, be paid for in some loss
of lift, as the lift decreases with the decreased angle.

In order to secure all the above described advantages, a combination is
sometimes effected, thus:

[Illustration: "Wash Out" on both sides relative to the Centre.]

BANKING.--An aeroplane turned off its course to right or left does not
at once proceed along its new course. Its momentum in the direction of
its first course causes it to travel along a line the resultant of such
momentum and the thrust. In other words, it more or less skids sideways
and away from the centre of the turn. Its lifting surfaces do not then
meet the air in their correct attitude, and the lift may fall to such an
extent as to become less than the weight, in which case the aeroplane
must fall. This bad effect is minimized by "banking," _i.e._, tilting
the aeroplane sideways. The bottom of the lifting surface is in that way
opposed to the air through which it is moving in the direction of the
momentum and receives an opposite air pressure. The rarefied area over
the top of the surface is rendered still more rare, and this, of course,
assists the air pressure in opposing the momentum.

The velocity of the "skid," or sideways movement, is then only