### 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.

### Chord

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.

### Curtain

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

### Diagonal

A diagonal brace in a framework.

### Derrick

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".

### Double-Decker

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.

### Doubled-Surfaced

Covering both sides of the framework of a supporting surface.

### Drift

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.).

### Elevator

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

### Entering Edge

The front or leading edge of an aeroplane.

### Equilibrator

The tail of a flying-machine.

### Equilibrium

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.

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.

### Feathering

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

### Fin

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.

Longitudinally.

### Front Control. Front Rudder

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

Spindle-shaped.

### Fuselage

The framework or body of an aeroplane.

### Gap

The distance between two planes in a multiplane machine.

### Glide

To travel without power.

### Glider

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.

### Guy-Wire

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

### Gyroplane

A flying-machine with rotating planes. See Heliocopter.

### Gyroscope

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

### Hangar

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

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

The resistance a surface offers to movement through the air.

### Heavier-Than-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.

### Helix

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

See Elevator.

### Keel

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.

See Skid.