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rudder angle-See ANGLE, RUDDER
rudder bar-The foot bar by means of which the control cables leading to the rudder are operated. (See fig. 5.)
rudder pedals-The foot pedals by means of which the controls leading to the rudder are operated.
runway-An artificial landing strip permitting the landing and take-off of airplanes under all weather conditions.
safety, factor of-See FACTOR OF SAFETY.
sailplane-A performance-type glider.
scale effect-The change in any force coefficient, such as the drag coefficient, due to a change in the value of Reynolds Number.
school ground-A school in which instruction is given to a student undergoing flight training.
scoop, air-See AIR SCOOP.
seaplane-An airplane designed to rise from and alight on the water.
semirigid airship-See AIRSHIP, SEMIRIGID
separation-The phenomenon in which the flow past a body placed in a moving stream of fluid separates from the surface of the body.
separation point-The point at which the separation of the boundary layer begins.
service ceiling-See CEILING, SERVICE.
servo control-See CONTROL, SERVO.
sesquiplane-A form of biplane in which the area of one wing is less than half the area of the other.
shaft:
    climbing shaft-A shaft, fitted with a ladder, which
    leads from the bottom to the top of an airship hull to 
    provide access to the upper portion. (See fig. 4.)
    gas shaft-A passageway between the gas cells of an
    airship to permit the escape of gas which has been 
    discharged from the cells.
shed, airship-See DOCK.
shipboard plane-A landplane designed to rise from and alight on the deck of a ship.
side direction (stress analysis)-The direction perpendicular to the plane of symmetry.
side force or component (stress analysis)-A force, or component, perpendicular to the plane of symmetry.
sideslipping-Motion of an aircraft relative to the air,in which the lateral axis is inclined and the airplane has a velocity component along the lateral axis. When it occurs in connection with a turn, it is the opposite of skidding. (See fig. 11.)
signal flare-See FLARE, SIGNAL.
sinking speed-The vertical downward component of velocity that an aircraft would have while descending in still air under given conditions of equilibrium.
skid, tail-See TAIL SKID.
skid, wing-See WING SKID.
skidding-Sliding sidewise away from the center of curvature when turning. It is caused by banking insufficiently, and is the opposite of sideslipping.
skid fin-A longitudinal vertical surface, usually placed above the upper wing to increase the lateral stability.
skin friction-The tangential component of the fluid force at a point on a surface.
sky writing-The act of emitting from an aircraft a trail of smoke or other visible substance, the flight of the aircraft being so directed as to cause the trail to assume the form of letters or symbols.
slat-A movable auxiliary airfoil, attached to the leading edge of a wing, which when closed falls within the original contour of the main wing and which when opened forms a slot. (See fig. 1.)
sleeve:
    deflation sleeve-(1) A sleever or appendix made of 
    fabric provided for the special purpose of
    facilitating the deflation of an aerostat. (2) The 
    sleeve or appendix fitted in the lower lobe of a kite
    balloon and used to permit the rapid escape of the air
    in the lobes when the balloon is hauled down.
    inflation sleeve-A tubular fabric attachment to an 
    envelope or gas bag, serving as a lead for the 
    inflation tube. (See figs. 3 and 4.)
    towing sleeve-A tubular fabric envelope towed by an
    aircraft and used as a target.
slip-The difference between the geometrical pitch and the effective pitch of a propeller. Slip may be expressed as a percentage of the mean geometrical pitch, or as a linear dimension.
slip function-The ratio of the speed of advance through the undisturbed air to the product of the propeller diameter and the number of revolutions per unit time; i.e., V/nD.
slipstream-The current of air driven astern by a propeller.
slot-The nozzle-shaped passage through a wing whose primary object is to improve the flow conditions at high angles of attack. It is usually near the leading edge and formed by a main and an auxiliary airfoil, or slat. (See fig. 1.) (Cf. slat.)
slotted aileron-See AILERON, SLOTTED.
soar-To fly without engine power and without loss of altitude, as does a glider in ascending air currents.
solidity-The ratio of the total blade area of a rotor to the area of the disk swept by the blades.
sounding balloon-See BALLOON, SOUNDING.
span-The maximum distance, measured parallel to the lateral axis, from tip to tip of an airfoil, of an airplane wing inclusive of ailerons, or of a stabilizer inclusive of elevator. (See fig. 5.)
    effective span-The true span of a wing less 
    corrections for tip loss.