SKIDDING: Sliding sidewise away from the center of the turn in flight. It is usually caused by insufficient banking in a turn and is the opposite of side slipping. SKIDS: Long wooden or metal runners designed to prevent nosing of a land machine when landing or to prevent dropping into holes or ditches in rough ground. Generally designed to function should the landing gear collapse or fail to act. SLIP STREAM (or _propeller race_): The stream of air driven aft by the propeller and with a velocity relative to the airplane greater than that of the surrounding body of still air. SOARING MACHINE: See Glider. SPAN (or _spread_): The maximum distance laterally from tip to tip of an airplane or the lateral dimension of an aerofoil. SPEED: _Air._—The speed of an aircraft relative to the air. _Ground._—The horizontal component of the velocity of an aircraft relative to the earth. SPIN: An aerial maneuver consisting of a combination of roll and yaw, with the longitudinal axis of the airplane inclined steeply downward. The machine descends in a helix of large pitch and very small radius, the upper side of the machine being on the inside of the helix, and the angle of attack being maintained at a large value. STABILITY: A body in any attitude has stability about an axis if, after a slight displacement about that axis, it tends to regain its initial attitude. _Directional._—Stability with reference to the normal axis. _Dynamical._—The quality of an aircraft in flight which causes it to return to a condition of equilibrium after its attitude has been changed by meeting some disturbance—e. g., a gust. This return to equilibrium is due to two factors: First, the inherent righting moments of the structure; second, the damping of the oscillations by the tail, etc. _Inherent._—Stability of an aircraft due to the disposition and arrangement of its fixed parts, i. e., that property which causes it to return to its normal attitude of flight without the use of the controls. _Lateral._—Stability with reference to displacements involving rolling or yawing, i. e., displacements in which the plane of symmetry of the airplane is rotated. _Longitudinal._—Stability with reference to displacements involving pitching, i. e., displacements in which the plane of symmetry of the airplane is not rotated. _Statical._—In wind-tunnel experiments it is found that there is a definite angle of attack, such that, for a greater angle or a less one, the righting moments are in such a sense as to tend to make the attitude return to this angle. This holds true for a certain range of angles on each side of this definite angle; and the machine is said to possess “statical stability” through this range.