SPINNING.--This is the worst of all predicaments the
pilot can find himself in. Fortunately it rarely happens.
It is due to the combination of (1) a very steep spiral
descent of small radius, and (2) insufficiency of keel-surface
behind the vertical axis, or the jamming of the rudder
end or elevator into a position by which the aeroplane is forced
into an increasingly steep and small spiral.
Owing to the small radius of such a spiral, the mass of
the aeroplane may gain a rotary momentum greater, in effect,
than the air pressure of the keel-surface or controlling surfaces
opposed to it; and, when once such a condition occurs,
it is difficult to see what can be done by the pilot to remedy
it. The sensible pilot will not go beyond reasonable limits
of steepness and radius when executing spiral descents.
GLIDING DESCENT WITHOUT PROPELLER THRUST.--All
aeroplanes are, or should be, designed to assume their gliding
angle when the power and thrust is cut off. This relieves
the pilot of work, worry, and danger should he find himself
in a fog or cloud. The Pilot, although he may not realize
it, maintains the correct attitude of the aeroplane by observing
its position relative to the horizon. Flying into a
fog or cloud the horizon is lost to view, and he must then rely
upon his instruments--(1) the compass for direction; (2) an
inclinometer (arched spirit-level) mounted transversely to
the longitudinal axis, for lateral stability; and (3) an inclinometer
mounted parallel to the longitudinal axis, or the airspeed
indicator, which will indicate a nose-down position
by increase in air speed, and a tail-down position by decrease
in air speed.
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