=Foot-pound=--A measure of work representing the weight of 1 lb. raised 1 foot.

=Fairing=--Usually made of thin sheet aluminium, wood, or a light construction of wood and fabric; and bent round detrimental surface in order to give it a "fair" or "stream-like" shape. [15]

=Gravity=--Is the force of the Earth's attraction upon a body. It decreases with increase of distance from the Earth. See "Weight."

=Gravity, Specific=--

Density of substance -------------------- Density of water.

Thus, if the density of water is 10 lb. per unit volume, the same unit volume of petrol, if weighing 7 lb., would be said to have a specific gravity of 7/10, _i.e._, 0.7.

=Gap= (_of an aeroplane_)--The distance between the upper and lower surfaces of a biplane. In a triplane or multiplane, the distance between any two of its surfaces. [16]

=Gap, Propeller=--The distance, measured in the direction of the thrust, between the spiral courses of the blades.

=Girder=--A structure designed to resist bending, and to combine lightness and strength.

=Gyroscope=--A heavy circular wheel revolving at high speed, the effect of which is a tendency to maintain its plane of rotation against disturbing forces.

=Hangar=--An aeroplane shed.

=Head-resistance=--Drift. The resistance of the air to the pa.s.sage of a body.

=Helicopter=--An air-screw revolving about a vertical axis, the direction of its thrust being opposed to gravity.

=Horizontal Equivalent=--The plan view of a body whatever its att.i.tude may be.

=Impulse=--A force causing a body to gain or lose momentum.

=Inclinometer=--A curved form of spirit-level used for indicating the att.i.tude of a body relative to the horizontal.

=Instability=--An inherent tendency of a body, which, if the body is disturbed, causes it to move into a position as far as possible away from its first position.

=Instability, Neutral=--An inherent tendency of a body to remain in the position given it by the force of a disturbance, with no tendency to move farther or to return to its first position.

=Inertia=--The inherent resistance to displacement of a body as distinct from resistance the result of an external force.

=Joy-Stick=--See "Control Lever."

=Keel-Surface=--Everything to be seen when viewing an aeroplane from the side of it.

=King-Post=--A bracing strut; in an aeroplane, usually pa.s.sing through a surface and attached to the main spar, and from the end or ends of which wires are taken to spar, surface, or other part of the construction in order to prevent distortion. When used in connection with a controlling surface, it usually performs the additional function of a lever, control cables connecting its ends with the pilot's control lever. [17]

=Lift=--The vertical component of the reaction produced by the action of driving through the air a surface inclined upwards and towards its direction of motion.

=Lift, Margin of=--The height an aeroplane can gain in a given time and starting from a given alt.i.tude.

=Lift-Drift Ratio=--The proportion of lift to drift.

=Loading=--The weight carried by an aerofoil. Usually expressed in pounds per square foot of superficial area.

=Longeron=--The term usually applied to any long spar running length-ways of a fuselage. [18]

=Ma.s.s=--The ma.s.s of a body is a measure of the quant.i.ty of material in it.

=Momentum=--The product of the ma.s.s and velocity of a body is known as "momentum."

=Monoplane=--An aeroplane of which the main lifting surface consists of one surface or one pair of wings.

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

=Montant=--Fuselage strut.

=Nacelle=--That part of an aeroplane containing the engine and/or pilot and pa.s.senger, and to which the tail-plane is not fixed. [19]

=Neutral Lift Line=--A line taken through a surface in a forward direction relative to its direction of motion, and starting from its trailing edge. If the att.i.tude of the surface is such as to make the said line coincident with the direction of motion, it results in no lift, the reaction then consisting solely of drift. The position of the neutral lift line, _i.e._, the angle it makes with the chord, varies with differences of camber, and it is found by means of wind-tunnel research.

=Newton's Laws of Motion=--1. If a body be at rest, it will remain at rest; or, if in motion, it will move uniformly in a straight line until acted upon by some force.

2. The rate of change of the quant.i.ty of motion (momentum) is proportional to the force which causes it, and takes place in the direction of the straight line in which the force acts. If a body be acted upon by several forces, it will obey each as though the others did not exist, and this whether the body be at rest or in motion.

3. To every action there is opposed an equal and opposite reaction.

=Ornithopter (or Orthopter)=--A flapping wing design of aircraft intended to imitate the flight of a bird.

=Outrigger=--This term is usually applied to the framework connecting the main surface with an elevator placed in advance of it. Sometimes applied to the "tail-boom" framework connecting the tail-plane with the main lifting surface. [20]

=Pancake, to=--To "stall."

=Plane=--This term is often applied to a lifting surface. Such application is not quite correct, since "plane" indicates a flat surface, and the lifting surfaces are always cambered.

=Propeller=--See "Air-Screw."

=Propeller, Tractor=--An air-screw mounted in front of the main lifting surface.

=Propeller, Pusher=--An air-screw mounted behind the main lifting surface.

=Pusher=--An aeroplane of which the propeller is mounted behind the main lifting surface.

=Pylon=--Any V-shaped construction from the point of which wires are taken.

=Power=--Rate of working. [21]

=Power, Horse=--One horse-power represents a force sufficient to raise 33,000 lb. 1 foot in a minute.

=Power, Indicated Horse=--The I.H.P. of an engine is a measure of the rate at which work is done by the pressure upon the piston or pistons, as distinct from the rate at which the engine does work. The latter is usually termed "brake horse-power," since it may be measured by an absorption brake.

=Power, Margin of=--The available quant.i.ty of power above that necessary to maintain horizontal flight at the optimum angle.