=Pitot Tube=--A form of air-speed indicator consisting of a tube with open end facing the wind, which, combined with a static pressure or suction tube, is used in conjunction with a gauge for measuring air pressures or velocities. (_No. 1 in diagram._)

=Pitch, Propeller=--The distance a propeller advances during one revolution supposing the air to be solid.

=Pitch, to=--To plunge nose-down.

=Reaction=--A force, equal and opposite to the force of the action producing it.

=Rudder=--A controlling surface, usually hinged to the tail, the operation of which turns an aeroplane about an axis which is vertical in normal horizontal flight; causes an aeroplane to turn to left or right of the pilot. [22]

=Roll, to=--To turn about the longitudinal axis.

=Rib, Ordinary=--A light curved wooden part mounted in a fore and aft direction within a surface. The ordinary ribs give the surface its camber, carry the fabric, and transfer the lift from the fabric to the spars. [23]

=Rib, Compression=--Acts as an ordinary rib, besides bearing the stress of compression produced by the tension of the internal bracing wires. [24]

=Rib, False=--A subsidiary rib, usually used to improve the camber of the front part of the surface. [25]

=Right and Left Hand=--Always used relative to the position of the pilot.

When observing an aeroplane from the front of it, the right hand side of it is then on the left hand of the observer.

=Remou=--A local movement or condition of the air which may cause displacement of an aeroplane.

=Rudder-Bar=--A control lever moved by the pilot's feet, and operating the rudder. [26]

=Surface=--See "Aerofoil."

=Surface, Detrimental=--All exterior parts of an aeroplane including the propeller, but excluding the (aeroplane) lifting and (propeller) thrusting surfaces.

=Surface, Controlling=--A surface the operation of which turns an aeroplane about one of its axes.

=Skin-Friction=--The friction of the air with roughness of surface. A form of drift.

=Span=--The distance from wing-tip to wing-tip.

=Stagger=--The distance the upper surface is forward of the lower surface when the axis of the propeller is horizontal.

=Stability=--The inherent tendency of a body, when disturbed, to return to its normal position.

=Stability, Directional=--The stability about an axis which is vertical during normal horizontal flight, and without which an aeroplane has no natural tendency to remain upon its course.

=Stability, Longitudinal=--The stability of an aeroplane about an axis transverse to the direction of normal horizontal flight, and without which it has no tendency to oppose pitching and tossing.

=Stability, Lateral=--The stability of an aeroplane about its longitudinal axis, and without which it has no tendency to oppose sideways rolling.

=Stabilizer=--A surface, such as fin or tail-plane, designed to give an aeroplane inherent stability.

=Stall, to=--To give or allow an aeroplane an angle of incidence greater than the "maximum" angle, the result being a fall in the lift-drift ratio, the lift consequently becoming less than the weight of the aeroplane, which must then fall, _i.e._, "stall" or "pancake."

=Stress=--Burden or load.

=Strain=--Deformation produced by stress.

=Side-Slip, to=--To fall as a result of an excessive "bank" or "roll."

=Skid, to=--To be carried sideways by centrifugal force when turning to left or right.

=Skid, Undercarriage=--A spar, mounted in a fore and aft direction, and to which the wheels of the undercarriage are sometimes attached. Should a wheel give way the skid is then supposed to act like the runner of a sleigh and to support the aeroplane. [28]

=Skid, Tail=--A piece of wood or other material, orientable, and fitted with shock absorbers, situated under the tail of an aeroplane in order to support it upon the ground and to absorb the shock of alighting.

[28_a_]

=Section=--Any separate part of the top surface, that part of the bottom surface immediately underneath it, with their struts and wires.

=Spar=--Any long piece of wood or other material.

=Spar, Main=--A spar within a surface and to which all the ribs are attached, such spar being the one situated nearest to the centre of pressure. It transfers more than half the lift from the ribs to the bracing. [29]

=Spar, Rear=--A spar within a surface, and to which all the ribs are attached, such spar being situated at the rear of the centre of pressure and at a greater distance from it than is the main spar. It transfers less than half of the lift from the ribs to the bracing. [30]

=Strut=--Any wooden member intended to take merely the stress of direct compression.

=Strut, Interplane=--A strut holding the top and bottom surfaces apart. [31]

=Strut, Fuselage=--A strut holding the _fuselage longerons_ apart. It should be stated whether top, bottom, or side. If side, then it should be stated whether right or left hand. _Montant_. [32]

=Strut, Extension=--A strut supporting an "extension" when not in flight. It may also prevent the extension from collapsing upwards during flight. [33]

=Strut, undercarriage=-- [33_a_]

=Strut, Dope=--A strut within a surface, so placed as to prevent the tension of the doped fabric from distorting the framework. [34]

=Serving=--To bind round with wire, cord, or similar material. Usually used in connection with wood joints and wire cable splices.

=Slip, Propeller=--The pitch less the distance the propeller advances during one revolution.

=Stream-Line=--A form or shape of detrimental surface designed to produce minimum drift.

=Toss, to=--To plunge tail-down.

=Torque, Propeller=--The tendency of a propeller to turn an aeroplane about its longitudinal axis in a direction opposite to that in which the propeller revolves.

=Tail-Slide=--A fall whereby the tail of an aeroplane leads.

=Tractor=--An aeroplane of which the propeller is mounted in front of the main lifting surface.

=Triplane=--An aeroplane of which the main lifting surface consists of three surfaces or pairs of wings mounted one above the other.

=Tail-Plane=--A horizontal stabilizing surface mounted at some distance behind the main lifting surface. _Empennage_. [36]