(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, 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. In modern aeroplanes this tendency is not sufficiently
important to bother about.
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