_Rarefaction Side_--A correct term for the incorrect “vacuum side,” so-called. The side opposite the compression side: the forward side of a revolving propeller blade, or the upper side of a flying surface, or the side of a rudder-surface turned away from the wind. _Reactive Stratum_--The layer of compressed air beneath a moving aeroplane surface, or behind a moving propeller blade. _Rib_--The smaller construction members used in building up surfaces. Generally they run fore-and-aft, crossing the spars or wing-bars at right angles, and they are bent to form the curve of the wings or planes. _Rising Angle_--Technically, the steepest angle at which any given aeroplane will rise into the air. _Rudder_--A movable surface by which the aeronaut is enabled to steer his craft in a desired direction. See Horizontal Rudder and Vertical Rudder. _Runner_--A construction similar to the runners of a sleigh, used for alighting on some machines, instead of the wheel alighting gear; a skid. S _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.