Stalling: A term describing the condition of an airplane which from any cause has lost the relative speed necessary for control.
Statoscope: An instrument to detect the existence of a small rate of ascent or descent, principally used in ballooning.
Stay: A wire, rope, or the like, used as a tie piece to hold parts together, or to contribute stiffness. For example, the stays of the wing and body trussing.
Step: A break in the form of the bottom of a float.
Stream-line flow: The condition of continuous flow of a fluid, as distinguished from eddying flow.
Stream-line shape: A shape intended to avoid eddying and to preserve stream-line flow.
Strut: A compression member of a truss frame. For instance, the vertical members of the wing truss of a biplane.
Suspension band: The band around a balloon to which are attached the basket and the main bridle suspensions.
Suspension bar: The bar used for the concentration of basket suspension ropes in captive balloons.
Sweep back: The horizontal angle between the lateral axis of an airplane and the entering edge of the main planes.