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where words and phrases used in the original publication have fallen from general use (eg, 'aero motor' has been replaced by 'aero engine').

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Radial Engine: An aero-engine with its cylinders arranged radially around the crankshaft, the cylinders being stationary and the crankshaft revolving.

Radiator: In an aeroplane, a device which disposes of the surplus heat of a liquid-cooled aero-engine by employing a liquid, known as the coolant, to convey the heat from the cylinders to the radiator. The latter transfers the heat from the coolant to the atmosphere.

Radio Telephony: Usually abbreviated to R/T to distinguish from W/T - wireless telegraphy. The transmission by radio of the human voice. This method of communication is in general use for short range work between aeroplanes in flight and between aeroplanes and ground stations.

Radius of Action: Half the range of an aeroplane in still air.

Rain: A precipitation from clouds taking the form of drops of water.

Rain Gauge: An instrument to measure rainfall.

Range: The maximum distance an aeroplane can travel in level flight under Standard Atmosphere Conditions without refuelling.

Rate of Climb Indicator: An aeroplane instrument which shows the rate at which a change of height is taking place.

Recalescence Point: The point on the cooling curve of a steel where carbides are precipitated.

Receiver: That portion of a wireless apparatus which receives the signals through the ether waves and modulates them.

Reconnaissance: Exploration from aircraft of the whole area in the occupation of the enemy, from the front line outposts to the back areas; in other words, all the centres where his war effort operates.

Records: All records achieved in aviation must be certified by the Fédération Aéronautique Internationale, the controlling authority of air sport as recognised by the principal countries of the World.

The World air records held in 1943 were:-

Speed: 481·4 m.p.h., Messerschmitt Bf 109R monoplane
(1,660 h.p. Daimler-Benz DB601 engine).

Height: 56,100ft, Caproni 161bis biplane
(1,000 h.p. Piaggio OXI-RC engine); 72,394ft, free balloon "Explorer" (USA).

Distance: 7,158·65 miles, Vickers-Armstrongs Wellesley monoplanes
(1,010 h.p. Bristol Pegasus XXII engines).

Recovery: The process of returning to horizontal straight flight after a manœuvre, voluntary or involuntary, such as a dive.

Registering Balloon: A small free balloon released into the upper air carrying a meteorograph which registers graphically a reading of the pressure, temperature and humidity at different heights.

Registration Marks. All civil aircraft engaged in international navigation must bear their nationality and registration marks. The first letter or the first two letters indicate the nationality. Immediately following is a hyphen, after which the registration letters are placed. Registration of aircraft is under the control of the Air Ministry in Great Britain. Before the Second World War the national letter of Great Britain was G; of the United States, NA to NZ; of France, F; of Germany, D.

Research: Research into the problems of aeronautical science is officially under the control of the Advisory Committee of Aeronautics, which was appointed in 1909. Scientific investigations are carried out by the Royal Aircraft Establishment, the National Physical Laboratory, and privately by the leading aeroplane companies.

Reshabar: A strong dry squally NE wind which blows down certain mountain ranges in Southern Kurdistan and which is a great danger to flying.

Resistance Derivatives: Quantities expressing the variation of the forces and movements on aircraft through the disturbance of steady motion. They represent the experimental basis of the theory of stability, and from them the damping factors of aircraft motions can be calculated.

Restoring (or Righting) Moment: That moment tending to restore an aeroplane to its original position after any disturbing movement.

Reversal: A change of more than 90° in direction between the surface wind and the wind in the upper area.

Revolution Indicator: An instrument which records the rate of revolution of the crankshaft of an engine in turns per minute. Sometimes known as a tachometer, and colloquially as a "rev counter" - although this latter term is, strictly speaking, inaccurate.

Reynolds Number: A non-dimensional ratio expressing conditions for similar motions in viscous fluids such as air. Comparisons of Reynolds Numbers permits comparison of wind tunnel results for small models with full-scale practice. The Reynolds Number is obtained by multiplying the density of the air by the speed and length of the model and dividing by the viscosity of the air.

Rhumb Line: A term used in navigation for a line cutting the meridians on a map at equal angles. An aeroplane flying on a constant course follows a rhumb line.

Rib: A member which gives the desired shape to the external covering of planes and control surfaces and maintains that shape under load.

Ridge: A tongue of high pressure on a weather map. The opposite of a trough of low pressure.

Rigger: A qualified person who is employed in assembling and aligning aircraft.

Rigging: The relative adjustment or alignment of the different components of an aeroplane - particularly the wings.

Rigging Lines: Those cords joining the canopy of a parachute to the harness.

Rigid Airship: An airship which has a rigid framework to maintain the shape of the hull.

Rip Cord: A manually operated device which withdraws a parachute from its pack.

Roak: A blow-hole of carbon monoxide gas in steel which becomes oxidised and causes a permanent flaw after working the material.

Roaring Forties: A nautical term for the prevailing westerly winds of the temperate zones in the Southern Hemisphere.

Rockets: Rockets carried on the interplane struts were used in the First World War for attacking airships and balloons.

Rocket Bombs: Rocket-projected bombs are being used from aeroplanes by the Russians as an alternative method of attacking ground targets.

Rocket Propulsion: Rockets have been experimentally tried out on aeroplanes as motive power, particularly in Germany by Opel and Espenlaub. Rocket-assisted take-off is used by the Luftwaffe for overloaded bombers.

Roll: An aerobatic manœuvre by which an aeroplane makes a full corkscrew revolution about its longitudinal axis.

Rotary Engine: A radial aero-engine in which the crankshaft is stationary and the cylinders revolve, carrying the propeller on an extension.
The type is now out of date, but did extremely valuable work in making flying possible with a very low weight-power ratio for its time. The chief advantages were limitation of size and excessive oil consumption. The 50 h.p. Gnome - a French design - was the outstanding early example.

Rotor: A narrow aerofoil rotating in approximately a horizontal plane about a common axis (like the spokes of a wheel) which produces lift corresponding to the normal fixed wings of an aeroplane. The system can be either freely revolving or power-driven. Rotor is also the name given to the driven vane mechanism of an aero-engine supercharger.

Royal Aero Club: Has for its objects the encouragement and control of all competitions, sporting events, trials and record attempts in connection with aeronautics in the United Kingdom. Founded as the "Aero Club" in 1901, the Royal Aero Club is by agreement the sole authority in matters of sporting flying. Its offices are at 119 Piccadilly, London W1.

Royal Aeronautical Society: The oldest aeronautical society in the World with an unbroken record, was founded in 1866 as the Aeronautical Society of Great Britain, and has acknowledged authority in matters of aeronautical engineering and science. Papers read at its meetings by authorities in one branch or another of these subjects are published in the Journal of the Royal Aeronautical Society. The Society possesses one of the most comprehensive libraries of aeronautical books in existence and is always willing to extend its help to bona-fide students and others interested in aeronautical science in addition to providing library service for its members. The Gold Medal of the Society is a rare award for outstanding service to aeronautics. There are various grades of membership and application should be made to The Secretary, Royal Aeronautical society, 4, Hamilton Place, London W1.

Royal Aircraft Establishment: The Government aeronautical research station founded at South Farnborough, Hampshire, to succeed the Royal Aircraft Factory in 1918. In addition to windtunnel work on aeroplane models and full-scale components of aircraft, much research is carried out in flight on specific types. This latter branch has always been one of the principal concerns of the Establishment.

Royal Aircraft Factory: The predecessor of the Royal Aircraft Establishment. In the same way that the Admiralty is responsible for the design of warships, it was thought at one time that military aircraft should be Government designed. The Royal Aircraft Factory produced a number of types between 1910 (the prefix Royal was substituted for Army in 1912) and 1917. The best known which took part in the First World War were the B.E.2c, the F.E.2b, the R.E.8 (Harry Tate), and the famous S.E.5a, which, with the Sopwith Camel, shared the distinction of being the greatest British fighter in the First World War.

Royal Air Force: The Royal Air Force was created on April 1, 1918, by the amalgamation of the Royal Flying Corps and the Royal Naval Air Service. Hitherto they had been separate air arms of the Army and the Royal Navy respectively, although some liaison was provided for when they had been originally formed in 1912. A Fleet Air Arm was provided by the RAF from 1923 for service in aircraft carriers and other Naval units until May 1939 when the Admiralty assumed all administrative control. Many cogent reasons could be given for the existence of one air force, but perhaps the strongest are the growing superiority of air power over all arms and the avoidance of competition for personnel and material. This was one of the main reasons for the agitation in the First World War which resulted in the formation of the Royal Air Force and the Air Ministry.

Royal Flying Corps: Formed on May 13, 1912, with a Naval wing, a Military wing, a Central Flying School at Upavon, and an Aircraft Factory and "Park" at Farnborough. The Naval Wing separated in 1914. The Military Wing consisted of three squadrons, No 1 (Airship) and No 2 (Aeroplane) being based at Farnborough, and No 3 (Aeroplane) at Larkhill, Salisbury Plain. At the outbreak of the First World War the RFC had 179 aeroplanes. The RFC was amalgamated with the RNAS to form the RAF on April 1, 1918.

Royal Naval Air Service: Never happy with the RFC, the Naval wing was detached in July, 1914, just before the outbreak of the First World War. The two services were together again as the RAF in 1918 and the Fleet Air Arm was formed in 1923 to look after Naval flying under the RAF. History repeated itself just before the outbreak of the Second World War, when the Royal Navy once more took over the Fleet Air Arm as an air service of its own. At the outbreak of the First World War in 1914 the RNAS had 151 aeroplanes of which 52 were able to take off from water, and six airships, mostly from RFC Squadron No 1, which had become an aeroplane squadron.

Rudder: A vertical moving surface usually hinged to the sternpost of the fin in the tail unit of an aeroplane. Its functions are to provide directional control and to assist directional stability.

Rudder Bar: The foot control by which the pilot of an aeroplane controls the rudder. It is hinged about its centre and when pushed forward with the left foot yaws the nose to the left and vice versa.

Runway: A prepared track on an aerodrome to facilitate the landing and taking-off of heavily loaded aeroplanes. Runways may be either concrete or metal lattice-work. The former type is more permanent.