It would turn
off its proper course, which is opposite to the direction of
the wind. It is very much the same in the case of an aeroplane.
[[16]] ``In effect'' because, although there may be actually the greatest proportion
of keel-surface In front of the vertical axis, such surface may be much nearer to
the axis than is the keel-surface towards the tail. The latter may then be actually
less than the surface in front, but, being farther from the axis, it has a greater
leverage, and consequently is greater in effect than the surface in front.
The above illustration represents an aeroplane (directionally
stable) flying along the course B. A gust striking it
as indicated acts upon the greater proportion of keel-surface
behind the turning axis and throws it into the new course.
It does not, however, travel along the new course, owing to
its momentum in the direction B. It travels, as long as
such momentum lasts, in a direction which is the resultant
of the two forces Thrust and Momentum. But the centre
line of the aeroplane is pointing in the direction of the new
course. Therefore its attitude, relative to the direction of
motion, is more or less sideways, and it consequently receives
an air pressure in the direction C. Such pressure, acting
upon the keel-surface, presses the tail back towards its first
position in which the aeroplane is upon its course B.
What I have described is continually going on during
flight, but in a well-designed aeroplane such stabilizing
movements are, most of the time, so slight as to be imperceptible
to the pilot.
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