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.
A machine possesses statical stability if, when its attitude is disturbed, moments tending to restore it to this attitude are set up by the action of the air on the machine; e. g., if an aircraft, after an initial disturbance, oscillates with swings of constantly increasing amplitude, it is statically stable but not dynamically stable.
Stabilizer: A fixed horizontal, or nearly horizontal, tail surface, used to steady the longitudinal motion and to damp oscillations in pitch.
Mechanical.—A mechanical device to steady the motion of an aircraft.
Stagger: The amount of advance of the entering edge of the upper plane of a biplane over that of the lower, expressed as percentage of gap; it is considered positive when the upper surface is forward and is measured from the entering edge of the upper plane along its chord to the point of intersection of this chord with a line drawn perpendicular to the chord of the lower plane at its entering edge, all lines being drawn in a plane parallel to the plane of symmetry.
(In directions for rigging).—The horizontal distance between the entering edge of the upper plane and that of the lower when the airplane is in the standard position; i. e., when the arbitrary line of reference in the airplane is horizontal. (This line is usually the axis of the propeller shaft.)