Surface, Controlling--A surface the operation of which turns an aeroplane
about one of its axes.
Skin-Friction--The friction of the air with roughness of surface. A
form of drift.
Span---The distance from wing-tip to wing-tip.
Stagger--The distance the upper surface is forward of the lower surface
when the axis of the propeller is horizontal.
Stability--The inherent tendency of a body, when disturbed, to return
to its normal position.
Stability, Directional--The stability about an axis which is vertical
during normal horizontal flight, and without which an aeroplane
has no natural tendency to remain upon its course.
Stability, Longitudinal--The stability of an aeroplane about an axis
transverse to the direction of normal horizontal flight, and without
which it has no tendency to oppose pitching and tossing.
Stability, Lateral--The stability of an aeroplane about its longitudinal
axis, and without which it has no tendency to oppose sideways
rolling.
Stabilizer--A surface, such as fin or tail-plane, designed to give an
aeroplane inherent stability.
Stall, to--To give or allow an aeroplane an angle of incidence greater
than the ``maximum'' angle, the result being a fall in the lift-
drift ratio, the lift consequently becoming less than the weight of
the aeroplane, which must then fall, i.e., ``stall'' or ``pancake.''
Stress--Burden or load.
Strain--Deformation produced by stress.
Side-Slip, to--To fall as a result of an excessive ``bank'' or ``roll.
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