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This glossary of historic aviation terms is reproduced from 'The Spotters Glossary' which appeared in 'The Aeroplane Spotter' from January 1940 onwards. 'The Aeroplane Spotter' was the first journal devoted exclusively to the study and practice of aircraft recognition.

We are pleased to have been able to reproduce the entire glossary from A-Z, offering a unique insight into aviation terminology (and therefore aviation history and technology) prior to and during the 1940s.

NB: In the interests of clarity we have substituted current equivalents where words and phrases used in the original publication have fallen from general use (eg, 'aero motor' has been replaced by 'aero engine').

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Valve: A mechanism which may be opened or closed to permit or restrict the passage of a fluid or gas through an orifice.

Variable Datum Boost Control: An automatic boost control which varies progressively with the opening or closing of the throttle of an aero-engine.

Variable-Pitch Propeller: A propeller the angle of whose blades may be varied while it is in rotation.

Vee Engine: An aero-engine with two banks of cylinders arranged in the form of a "V," either upright or inverted.

Veering: A clockwise change of wind.

Velocity: Rate of motion along a defined path.

Vent: The central hole in the canopy of a parachute which promotes stability through the escape of superfluous air pressure.

Venturi Tube: A tube made with a gradually reduced cross-section about its centre which speeds up fluid flow through the tube at that point and so produces suction.

Vertical Axis: Is the normal upright axis when the longitudinal and lateral axes of an aeroplane are horizontal.

Viscosity: The stiffness of a fluid, or the converse of fluidity.

Visibility: The distance at which objects may be clearly seen. Fog and haze produce poor visibility.

Volume (lighter-than-air craft): The capacity or displacement of the envelope of an airship or balloon.

Vortex: A fluid in rotational motion, such as a whirlpool.