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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equivalents where words and phrases used in the original publication have fallen from general use (eg, 'aero motor' has been replaced by 'aero engine').

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Tab: A small hinged flap usually set in the trailing edge of a control surface. Its purpose is so to adjust the control that the aeroplane is properly trimmed when the controls are centralised.

Tachometer: A revolution indicator (or "rev counter").

Tailbooms: Elongated girder or tubular structures used on some aeroplanes instead of the normal fuselage to carry the tail.

Tail-first Aeroplane: Often called a "canard", a form of aeroplane design in which the tail precedes the main supporting surfaces. Notable examples were the early Santos-Dumont types, the Aeronautical Syndicate Valkyrie of 1910/12, and the German Focke-Wulf Ente of 1927.

Tail Heaviness: The faulty characteristic of an aeroplane caused by the Centre of Gravity (c.g.) being too far back or by badly trimmed elevators.

Tailplane: The fixed horizontal tail surface of an aeroplane.

Tailskid: A lever and shoe which, mounted on a fixed bracket and fulcrum, takes the weight of the rear end of the fuselage on the ground. Nowadays it is usually replaced by a tailwheel.

Tailslide: The backward and downward movement of an aeroplane which has been stalled after too steep a climb.

Tail Unit: The complete assembly of tail surfaces, comprising fin, rudder, tailplane and elevators.

Tailwheel: The small wheel which takes the weight of an aeroplane fuselage on the ground at the tail.

Tailless Aeroplanes: A basic aeroplane design, first evolved by Lt J.W. Dunne in 1907. It dispenses with tail surfaces carried on a fuselage or tailbooms. The wings are swept back in plan and the control surfaces are usually carried at the wing tips. The Westland-Hill Pterodactyl of 1926-30 was a notable example.

Tail-setting Angle: The acute angle formed by the chord line of the mainplane and that of the tailplane.

Take-off: The movement of aircraft from a position of rest to the moment they are airborne.

Tanks: Fuel containers on aircraft.

Taxying: Manoeuvring aircraft on the ground or on water.

Temperature: Degrees of heat. There is a temperature variation of 129 degrees Fahrenheit (71·5 degrees Centigrade) between sea level (59 degrees F International Standard Atmosphere) and a height of 40,000ft (-69·7 degrees F International Standard Atmosphere).

Tempering: The final stage in the treatment of high-grade steels which relieves strains in the material and produces any required combination of hardness and toughness.

Tensile Strength: The property of a material to resist a stress which tends to pull molecules apart.

Terminal Velocity: The maximum speed attainable by an aeroplane at its most acute diving angle, the engine and propeller operating under specified conditions.

Thermometer: An instrument for measuring temperatures. Three different systems are employed. The Fahrenheit scale records freezing point of water at 32 degrees and boiling point at 212 degrees; the Centigrade scale registers freezing point at 0 degrees and boiling point at 100 degrees; the Reaumur scale has a freezing point of 0 degrees and a boiling point of 80 degrees. Absolute minimum temperatures are - 273·16 degrees Centigrade or -459·7 degrees Fahrenheit.

Thickness Ratio: In an aerofoil its chord divided by its maximum depth. In modern cantilever wings the average thickness ratio is about 6:1.

Three-point Landing: When the three wheels (or two wheels and tailskid) of an aeroplane touch down simultaneously on landing.

Thrust: The force exerted by a propeller along its thrustline.

Thunderstorm: Strong convectional movements in the atmosphere which give rise to intense vertical currents of air. Thunderstorms are usually heralded by towering cumulo-nimbus clouds followed by heavy rain or hail, thunder and lightning.

Tornado: A violent whirling windstorm of small radius which advances over land while circling round its centre. Strong ascending currents of air form in the region of a tornado. In West Africa a tornado is defined as a squall accompanying a thunderstorm.

Torque: The moment of the aerodynamic forces about the thrust line of a propeller which tends to turn the aeroplane in the opposite direction to that in which the propeller is rotating.

Townend Ring: A narrow-chord cowling ring round the cylinders of a radial engine to reduce drag and improve cooling. The Townend ring was the invention of Dr H.C.H. Townend in 1929. It caused a great reduction in the drag of radial engines and was widely used before the long-chord NACA cowling came into general use. Aeroplanes with Townend rings were the Vickers Wellesley, the Westland Wallace and the Gloster Gauntlet.

Track: A course followed by projection of the path of the centre of gravity of an aeroplane on to the surface of the Earth.

Track Angle: The angle, at any moment, between the track of an aeroplane and the Meridian, measured clockwise from 0 degrees to 360 degrees.

Tractor Aeroplane: An aeroplane which has a tractor propeller or propellers, i.e. mounted in front of the engines so as to pull the aircraft through the air. The opposite of a pusher aeroplane, which has its propeller(s) mounted aft of the engine.

Tractor Propeller: A propeller designed to pull on its propshaft. A tractor propeller is usually fitted in the nose or in the front of engine nacelles in the wings and is distinct from a pusher propeller which propels the aeroplane and is thus situated in the rear.

Trade Winds: Persistent winds which blow in latitudes 30 degrees North and South, towards the equitorial doldrums, north-east in the northern hemisphere and south-east in the southern hemisphere. The reverse winds above the trade winds are called the anti-trades.

Trailing Aerial: A weighted radio aerial which trails in flight, mounted in an aeroplane and usually capable of being wound in or out.

Trailing Edge: The rear edge of a streamlined body, aerofoil, or control surface.

Trainers: A generic term for aircraft used for flying training.

Transport: A civil or military aeroplane used for freight or troop carrying.

Trim: The set or angle at which an aeroplane flies under given conditions.

Trimming Tabs: Small auxiliary movable tabs on the trailing edges of control surfaces. Originally designed to provide trim as an improvement on incidence gears, they were found to have a potent effect on the aerodynamic balance of control surfaces.

Tricycle Undercarriage: A landing gear which incorporates a nosewheel and two mainwheels which are located behind the Centre of Gravity so that the aeroplane remains in approximately flying attitude when on the ground.

Triplane: An aeroplane with three mainplanes superimposed one above the other.

Troop Carrier: A transport aeroplane expressly designed or converted for carrying airborne troops.

Tropical Revolving Storms: The generic term for the type of cyclonic depression known in various parts of the world as hurricane, cyclone or typhoon.

Tropopause: The region of the atmosphere between the stratosphere and the troposphere.

Troposphere: The lower layer of the atmosphere in which the change of temperature with height is relatively large and decreases with an increase in height. The troposphere extends up to approximately 33,000ft.

Trough: A valley of low atmospheric pressure, as opposed to a ridge of high pressure.

True Air Speed: The speed of an aeroplane through the air in which it is flying, corrected for errors in Indicated Air Speed and contrasted with ground speed.

True Course: The angle between the longitudinal axis of an aeroplane and a true meridian.

Turbo-supercharger: An exhaust-driven supercharger, the advantage of which is that it can maintain sea-level engine performance at great heights. Disadvantages are high working temperature and excessive turbine speed.

Turbulent Flow: Irregular periodic motion of a fluid, such as air. Air flow is said to be turbulent instead of laminar (or smooth) when it breaks away from the surface of an aerofoil.

Turnbuckle: A double-ended eyebolt with a left and right-handed thread in the central nut. A turn of the latter draws both bolts together, thus increasing the tension in the rod or wire to which the turnbuckle is attached. Turnbuckles were used profusely on the older wire-braced aeroplane structures.

Turn Indicator: A gyroscopic instrument which registers the deviation of the course of an aeroplane to right or left.

Turret: The transparent enclosed position of the gunner of a military aeroplane. Most turrets are power-operated, hydraulically, electrically, and both.

Two-stroke: The cycle of operations of an internal-combustion engine which has a power stroke in each cylinder for every revolution of the crankshaft.

Type: Every new design of aeroplane has a distinctive type name (e.g. Tiger Moth) or number (e.g. D.H.82). Many different systems of naming and numbering new types exist.

Typhoon: A cyclonic depression in the China Seas, similar to a hurricane or cyclone.