=Biplane=--An aeroplane of which the main lifting surface consists of a surface or pair of wings mounted above another surface or pair of wings.

=Bay=--The s.p.a.ce enclosed by two struts and whatever they are fixed to.

=Boom=--A term usually applied to the long spars joining the tail of a "pusher" aeroplane to its main lifting surface. [4]

=Bracing=--A system of struts and tie wires to transfer a force from one point to another.

=Canard=--Literally "duck." The name which was given to a type of aeroplane of which the longitudinal stabilizing surface (_empennage_) was mounted in front of the main lifting surface. Sometimes termed "tail-first" aeroplanes, but such term is erroneous, as in such a design the main lifting surface acts as, and is, the _empennage_.

=Cabre=--To fly or glide at an excessive angle of incidence; tail down.

=Camber=--Curvature.

=Chord=--Usually taken to be a straight line between the trailing and leading edges of a surface.

=Cell=--The whole of the lower surface, that part of the upper surface directly over it, together with the struts and wires holding them together.

=Centre (Line) of Pressure=--A line running from wing-tip to wing-tip, and through which all the air forces acting upon the surface may be said to act, or about which they may be said to balance.

=Centre (Line) of Pressure, Resultant=--A line transverse to the longitudinal axis, and the position of which is the resultant of the centres of pressure of two or more surfaces.

=Centre of Gravity=--The centre of weight.

=Cabane=--A combination of two pylons, situated over the fuselage, and from which the anti-lift wires are suspended. [5]

=Cloche=--Literally "bell." Is applied to the bell-shaped construction which forms the lower part of the pilot's control lever in a Bleriot monoplane, and to which the control cables are attached.

=Centrifugal Force=--Every body which moves in a curved path is urged outwards from the centre of the curve by a force termed "centrifugal."

=Control Lever=--A lever by means of which the controlling surfaces are operated. It usually operates the ailerons and elevator. The "joy-stick." [6]

=Cavitation, Propeller=--The tendency to produce a cavity in the air.

=Distance Piece=--A long, thin piece of wood (sometimes tape) pa.s.sing through and attached to all the ribs in order to prevent them from rolling over sideways. [7]

=Displacement=--Change of position.

=Drift= (_of an aeroplane as distinct from the propeller_)--The horizontal component of the reaction produced by the action of driving through the air a surface inclined upwards and towards its direction of motion _plus_ the horizontal component of the reaction produced by the "detrimental" surface _plus_ resistance due to "skin-friction."

Sometimes termed "head-resistance."

=Drift, Active=--Drift produced by the lifting surface.

=Drift, Pa.s.sive=--Drift produced by the detrimental surface.

=Drift= (_of a propeller_)--a.n.a.logous to the drift of an aeroplane.

It is convenient to include "eddies" and "cavitation" within this term.

=Drift, to=--To be carried by a current of air; to make leeway.

=Dive, to=--To descend so steeply as to produce a speed greater than the normal flying speed.

=Dope, to=--To paint a fabric with a special fluid for the purpose of tightening and protecting it.

=Density=--Ma.s.s of unit volume; for instance, pounds per cubic foot.

=Efficiency=--

Output ------ Input.

=Efficiency= (_of an aeroplane as distinct from engine and propeller_)--

Lift and Velocity --------------------------- Thrust (= aeroplane drift).

=Efficiency, Engine=--

Brake horse-power ---------------------- Indicated horse-power.

=Efficiency, Propeller=--

Thrust horse-power -------------------------------- Horse-power received from engine (= propeller drift).

NOTE.--The above terms can, of course, be expressed in foot-pounds. It is then only necessary to divide the upper term by the lower one to find the measure of efficiency.

=Elevator=--A controlling surface, usually hinged to the rear of the tail-plane, the operation of which turns an aeroplane about an axis which is transverse to the direction of normal horizontal flight. [8]

=Empennage=--See "Tail-plane."

=Energy=--Stored work. For instance, a given weight of coal or petroleum stores a given quant.i.ty of energy which may be expressed in foot-pounds.

=Extension=--That part of the upper surface extending beyond the span of the lower surface. [9]

=Edge, Leading=--The front edge of a surface relative to its normal direction of motion. [10]

=Edge, Trailing=--The rear edge of a surface relative to its normal direction of motion. [11]

=Factor of Safety=--Usually taken to mean the result found by dividing the stress at which a body will collapse by the maximum stress it will be called upon to bear.

=Fineness= (_of stream-line_)--The proportion of length to maximum width.

=Flying Position=--A special position in which an aeroplane must be placed when rigging it or making adjustments. It varies with different types of aeroplanes. Would be more correctly described as "rigging position."

=Fuselage=--That part of an aeroplane containing the pilot, and to which is fixed the tail-plane. [12]

=Fin=--Additional keel-surface, usually mounted at the rear of an aeroplane. [13]

=f.l.a.n.g.e= (_of a rib_)--That horizontal part of a rib which prevents it from bending sideways. [14]

=Flight=--The sustenance of a body heavier than air by means of its action upon the air.