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Screw—Another term for propeller; properly, screw-propeller.
Single-surfaced—A term used to designate wings or planes whose frames are covered with fabric only on the upper side. See [Double-Surfaced].
Skid—Another name for runner.
Skin Friction—The retarding effect of the adherence of the air to surfaces moving rapidly through it. It is very slight with polished surfaces, and in case of slow speeds is entirely negligible.
Slip—The difference between the actual progress of a moving propeller, and the theoretical progress expressed by its pitch. It is much greater in some propellers than in others, due to the “churning” of the air by blades of faulty design and construction.
Soaring Flight—The sailing motion in the air achieved by some of the larger birds without the flapping of their wings. It is to be distinguished from gliding in that it is in an upward direction. Soaring has never been satisfactorily explained, and is considered to be the secret whose discovery will bring about the largest advance in the navigation of the air.
Spar—A stick of considerable length used in the framing of the body of aeroplanes, or as the long members in wing structures.
Stabilize—To maintain balance by the automatic action of adjunct surfaces, as distinguished from the intentional manipulation of controlling devices.
Stabilizer—Any surface whose automatic action tends to the maintaining of balance in the air.
Stable Equilibrium—That equilibrium which is inherent in the construction of the machine, and does not depend upon automatic or controlling balancing devices.
Starboard—The right-hand side of an aircraft as one faces forward. See [Port.]
Starting Area—An area of ground specially prepared to facilitate the starting of aeroplanes into flight.
Starting Device—Any contrivance for giving an aeroplane a powerful impulse or thrust into the air. See [Derrick].
Starting Impulse—The thrust with which an aeroplane is started into the air for a flight. Most machines depend upon the thrust of their own propellers, the machine being held back by force until the engines have worked up to flying speed, when it is suddenly released.
Starting Rail—The rail upon which the starting truck runs before the aeroplane rises into the air.
Starting Truck—A small vehicle upon which the aeroplane rests while it is gaining sufficient impulse to take flight.
Stay—A construction member of an aeroplane sustaining a pulling strain. It is usually of wire.
Straight Pitch—That type of [pitch] (which see) in a propeller blade in which every cross-section of the blade makes the same angle with its axis of revolution.
Strainer—Another term for Turnbuckle—which see.
Strut—An upright, or vertical, construction member of an aeroplane sustaining a compression strain; as distinguished from a brace which sustains a diagonal compression strain.
Supplementary Surface—A comparatively small surface used as an adjunct to the large surfaces for some special purpose; as, for instance, the preserving of balance, or for steering.
Sustaining Surface—The large surfaces of the aeroplane whose rapid movement through the air at a slight angle to the horizontal sustains the weight of the machine.