''
``I'm afraid I'm very stupid,'' said Efficiency, ``but
please tell me why you lay stress upon the words `IN
EFFECT.' ''
``Ah! I was wondering if you would spot that,'' he
replied. ``And there is a very good reason for it. You see,
in some Aeroplanes the Tail Surface may be actually set
at the same Angle on the machine as the Main Surface, but
owing to the air being deflected downwards by the front
Main Surface it meets the Tail Surface at a lesser angle,
and indeed in some cases at no angle at all. The Tail is then
for its surface getting less Lift than the Main Surface, although
set at the same angle on the machine. It may then be
said to have IN EFFECT a less Angle of Incidence. I'll just
show you on the Blackboard.''
``And now,'' said Efficiency, ``I have only to meet the
Ailerons and the Rudder, haven't I?''
``Here we are,'' replied the Ailerons, or little wings.
``Please hinge us on to the back of the Main Surfaces, one
of us at each Wing-tip, and join us up to the Pilot's joystick
by means of the control cables. When the Pilot wishes to
tilt the Aeroplane sideways, he will move the stick and depress
us upon one side, thus giving us a larger Angle of Incidence
and so creating more Lift on that side of the Aeroplane;
and, by means of a cable connecting us with the Ailerons on
the other side of the Aeroplane, we shall, as we are depressed,
pull them up and give them a reverse or negative Angle of
Incidence, and that side will then get a reverse Lift or downward
thrust, and so we are able to tilt the Aeroplane sideways.
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