''
``But,'' objected Efficiency, ``the Lift varies with the
Angle of Incidence, and surely such a condition will result in
one side of the Aeroplane lifting more than the other side?'
``That's all right,'' said the Propeller, ``it's meant to
off-set the tendency of the Aeroplane to turn over sideways
in the opposite direction to which I revolve.''
``That's quite clear, though rather unexpected; but how
do you counteract the effect of the gusts when they try to
overturn the Aeroplane sideways?'' said she, turning to
Lateral Stability again.
``Well,'' he replied, rather miserably, ``I'm not nearly
so perfect as the Longitudinal and Directional Stabilities.
The Dihedral Angle--that is, the upward inclination of the
Surfaces towards their wing-tips--does what it can for me,
but, in my opinion, it's a more or less futile effort. The
Blackboard will show you the argument.'' And he at once
showed them two Surfaces, each set at a Dihedral Angle
like this:
``Please imagine,'' said the Blackboard, ``that the top
V is the front view of a Surface flying towards you. Now
if a gust blows it into the position of the lower V you see
that the horizontal equivalent of the Surface on one side
becomes larger, and on the other side it becomes smaller.
That results in more Lift on the lower side and less on the
higher side, and if the V is large enough it should produce
such a difference in the Lift of one side to the other as to
quickly turn the Aeroplane back to its former and normal
position.
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