such
as is necessary to secure an air pressure equal and opposite to the
centrifugal force of the turn.

The sharper the turn, the greater the effect of the centrifugal force,
and therefore the steeper should be the "bank." _Experientia docet_.

_The position of the centre of gravity_ affects banking. A low C.G. will
tend to swing outward from the centre of the turn, and will cause the
aeroplane to bank--perhaps too much, in which case the pilot must remedy
matters by operating the ailerons.

A high C.G. also tends to swing outward from the centre of the turn. It
will tend to make the aeroplane bank the wrong way, and such effect must
be remedied by means of the ailerons.

The pleasantest machine from a banking point of view is one in which
the C.G. is a little below the centre of drift. It tends to bank the
aeroplane the right way for the turn, and the pilot can, if necessary,
perfect the bank by means of the ailerons.

_The disposition of the keel-surface_ affects banking. It should be,
in effect, evenly divided by the longitudinal axis. An excess of
keel-surface above the longitudinal axis will, when banking, receive an
air pressure causing the aeroplane to bank, perhaps too much. An excess
of keel-surface below the axis has the reverse effect.

SIDE-SLIPPING.--This usually occurs as a result of over-banking. It is
always the result of the aeroplane tilting sideways and thus decreasing
the horizontal equivalent, and therefore the lift, of the surface. An
excessive "bank," or sideways tilt, results in the H.E., and therefore
the lift, becoming less than the weight, when, of course, the aeroplane
must fall, _i.e._, side-slip.

[Illustration]

When making a very sharp turn it is necessary to bank very steeply
indeed. If, at the same time, the longitudinal axis of the aeroplane
remains approximately horizontal, then there must be a fall, and the
direction of motion will be the resultant of the thrust and the fall as
illustrated above in sketch A. The lifting surfaces and the controlling
surfaces are not then meeting the air in the correct attitude, with the
result that, in addition to falling, the aeroplane will probably become
quite unmanageable.

The pilot, however, prevents such a state of affairs from happening by
"nosing-down," _i.e._, by operating the rudder to turn the nose of the
aeroplane downward and towards the direction of motion as illustrated in
sketch B. This results in the higher wing, which is