But Efficiency, growing rather critical with her newly
gained knowledge, cried out: ``But where's the horizontal
Tail Surface? It doesn't look right like that!''
``This is when I have the pleasure of meeting you, my
dear,'' said Longitudinal Stability. ``Here's the Tail Surface,''
he said, ``and in order to help me it must be set IN
EFFECT at a much less Angle of Incidence than the Main Surface.
To explain we must trouble the Blackboard again,'' and
this was his effort:
``I have tried to make that as clear as possible,'' he
said. ``It may appear a bit complicated at first, but if
you will take the trouble to look at it for a minute you will find
it quite simple. A is the normal and proper direction of
motion of the Aeroplane, but, owing to a gust of air, it takes
up the new nose-down position. Owing to Momentum,
however, it does not fly straight along in that direction, but
moves more or less in the direction B, which is the resultant
of the two forces, Momentum and Thrust. And so you will
note that the Angle of Incidence, which is the inclination
of the Surfaces to the Direction of Motion, has decreased,
and of course the Lift decreases with it. You will also see,
and this is the point, that the Tail Surface has lost a higher
proportion of its Angle, and consequently its Lift, than has
the Main Surface. Then, such being the case, the Tail must
fall and the Aeroplane assume its normal position again,
though probably at a slightly lower altitude.
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