Centre Of Lift

See Centre of Pressure.

Centre Of Pressure

An imaginary centre in which the air pressure on a supporting surface is theoretically concentrated.

Centre Of Thrust

See Centre of Effort.


The line connecting the ends of the segment of a circle.

Compound Control

A system of hand-levers and ropes or hand-wheels and ropes by which two controlling operations are simultaneously carried out with but a single operating device, as, for example, the single lever in a Wright machine, which serves not only to warp the main planes, but also to swing the vertical rudder at the same time.

Compression Side

The side of a surface, such as an aeroplane or air-propeller, which faces the flow of air current.


A vertical plane, as in the Voisin cellular biplane, between the main planes, serving to insure a certain amount of lateral stability.


A diagonal brace in a framework.


A pyramidal structure from the top of which a weight can be mechanically dropped in order to start a flying-machine in motion on a rail. Sometimes called a " pylon".

Dihedral Angle

The angle formed by two planes placed at opposite sides of a median line, so as to form a very wide " V".


A synonym of biplane (q. v.).

Double Monoplane

A machine having two sets of supporting surfaces arranged in a single tier. Such a machine is also called a " following-surface" machine.

Double Rudder

A rudder having two surfaces of more or less similar surface and outline, which surfaces may or may not act simultaneously.


Covering both sides of the framework of a supporting surface.


The resistance offered to forward motion of a plane or curved surface in the air by the horizontal component of the air pressure against the plane. It is to be carefully distinguished from mere head resistance (q. v.).


The horizontal rudder of a flying-machine, used for steering in a vertical plane.

Entering Edge

The front or leading edge of an aeroplane.


The tail of a flying-machine.


In flying-machine parlance the term is used in the same sense as " stability." Properly speaking, an aeroplane is in equilibrium when travelling at a uniform rate in a straight line, or, again, when it is steered around a horizontal arc or circle. It is necessary for stability that if the aeroplane be not in equilibrium and moving uniformly it shall tend toward a condition of equilibrium.

Equivalent Head Area

The area which would offer head resistance equal to that of the supporting surfaces of a flying-machine plus the struts, stays, wires, chassis, etc.


Said of surfaces which are manoeuvred in a manner to pass edgewise and flatwise in alternate directions while in motion.


A rigid vertical surface which acts somewhat like the keel of a sailing yacht.

Fish Section

A section resembling in shape the body of a fish. Such sections are commonly found in flying-machine struts.

Fixed Wheel

In contradistinction to a caster-wheel (q. v.), a wheel that always preserves its relative position in the alighting-gear.

Flapping-Wing Flight

Flight by means of beating wings as distinguished from flight obtained by means of rigid aeroplanes. See Ornithopter.

Flexible Propeller

A fabric propeller, capable of adjusting itself in flight.

Flying Angle

Flying attitude. See Angle of Incidence.

Following Edge

The rear edge of an aeroplane surface.

Following Surface

The rear surfaces of two similar surfaces arranged in tandem.

Fore And Aft


Front Control. Front Rudder

The framework and planes situated at the extreme front of the aeroplane, in advance of the operator.




The framework or body of an aeroplane.


The distance between two planes in a multiplane machine.


To travel without power.


An aeroplane without a motor.

Gliding Angle

The angle at which a machine glides down without power.

Ground Attitude Or Incidence

The difference in the angle formed by the aeroplane surface when on the ground and when in flight.


A wire connecting two members of an aeroplane, usually parts of the controlling system.


A flying-machine with rotating planes. See Heliocopter.


A freely-hung, rapidly-rotating fly-wheel, which resists forces that tend to throw it from its plane of rotation.


A term said to be of Hungarian origin, now also used in English, to designate a shed for housing aeroplanes or airships.

Head Area

The total head resistance offered by the entire framework of an aeroplane.

Head Resistance

The resistance a surface offers to movement through the air.


A term applied to all air-craft not sustained by a buoyant gas.

Helicopter Or Helicoptere

A heavier-than-air machine in which flight is secured by lifting screw propellers revolving in more or less horizontal planes.


The path of a point moving uniformly around a cylinder and uniformly along the cylinder.

Horizontal Rudder

See Elevator.


The under framing of an aeroplane to stiffen it both laterally and vertically. Sometimes used as a synonym of fin (q. v.).

Land Speed

The rate of travel of an aeroplane on the ground before ascension.

Landing Area

A special allotment of ground on which a machine can land safely.

Landing Skid

See Skid.