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.