=Air Pocket=--A local movement or condition of the air causing an aeroplane to drop or lose its correct attitude.
=Aspect-Ratio=--The proportion of span to chord of a surface.
=Air-Screw (Propeller)=--A surface so shaped that its rotation about an axis produces a force (thrust) in the direction of its axis. [2]
=Aileron=--A controlling surface, usually situated at the wing-tip, the operation of which turns an aeroplane about its longitudinal axis; causes an aeroplane to tilt sideways. [3]
=Aviation=--The art of driving an aeroplane.
=Barograph=--A recording barometer, the charts of which can be calibrated for showing air density or height.
=Barometer=--An instrument used for indicating the density of air.
=Bank, to=--To turn an aeroplane about its longitudinal axis (to tilt sideways) when turning to left or right.
=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 space 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.
=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.