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 stability. An aeroplane
tends to turn over sideways in the opposite direction to which
the propeller revolves.
This tendency is offset by increasing the angle of incidence
(and consequently the lift) of the side tending to fall; and it
is always advisable, if practical considerations allow it, to
also decrease the angle upon the other side.
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