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where words and phrases used in the original publication have fallen from general use (eg, 'aero motor' has been replaced by 'aero engine').

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Damping Factor. With regard to the stability of an aeroplane, the rate of change in the violence of any movement from the level path.

Damping Moment. The moment of the force which tends to resist movement when damping is positive.

Dead Reckoning. In air navigation the estimation of true direction and speed of an aeroplane during flight, and hence of its distance from any known point at any moment.

Decalage. Decalage is the angle between the chord line of the upper plane and that of the lower plane and is positive when the top plane is at a greater angle of incidence than the bottom. The reason for giving a different angle of incidence to the upper and lower wings of a biplane is to shorten the aeroplane's take-off run and adjust its landing characteristics.

Decorations. In the Royal air Force, the Air Force Cross, the Air Force Medal, the Distinguished Flying Cross and the Distinguished Flying Medal.

De-Icer. A device for dispersing ice which has formed on any part of an aeroplane, particularly the wings and control surfaces. The best-known type is the Goodrich de-icer. In this system pulsating rubber overshoes are fitted along the leading edges of the surfaces. The rubber tubes can be inflated and deflated alternately to crack off the ice.

Delayed Drop. A parachute descent begun by a free fall of greater duration than that normally allowed for the opening parachute to clear the aeroplane. Falling freely in the air, the average rate of descent of a man is constant at 119 m.p.h. The rate after the parachute has opened is 14 m.p.h. at sea level - approximately equal to a jump from a 10ft wall.

Depression. A region of relatively low barometric pressure. The winds circulate in an anti-clockwise direction around its centre in the Northern Hemisphere and in a clockwise direction in the Southern Hemisphere. A Depression is sometimes called a "Low."

Detachable Pack. A parachute pack which can be quickly attached to and detached from the parachute harness.

Dew Point. The lowest temperature to which air can be cooled at constant pressure without causing condensation and hence the formation of cloud.

DF. Direction-Finding Wireless Telegraphy.

DFC. Distinguished Flying Cross.

DFM. Distinguished Flying Medal.

DGCA. Director-General of Civil Aviation.

D.H. de Havilland.

Diesel. The name given to a compression-ignition engine after its inventor, Rudolf Diesel. In the Diesel cycle the heavy oil fuel is ignited by the heat of compression of the air in the cylinder. The chief advantages of compression-ignition engines are their lower specific fuel consumption, the use of cheaper fuel, reduced risk of fire, and the absence of electrical ignition systems which need shielding to prevent interference with radio. The disadvantage is the increased weight per horsepower of the power plant.

Differential Ailerons. Ailerons interconnected so that the aileron which moves upwards moves through a larger angle than the other which moves downwards. The reason for this is to increase the drag as well as the lift of the wing with the upgoing aileron and at the same time to keep down the drag of the downgoing aileron because of its smaller movement.

Diffusion. Function of a carburettor. The diffusion tube is perforated to assist in atomising the fuel before it is introduced into the choke tube.

Dihedral Angle. The angle at which the port and starboard mainplanes of an aeroplane or glider are inclined upwards to the lateral axis. When they are inclined downwards the angle is termed Anhedral or Negative Dihedral; an older term was Kathedral.

The purpose of dihedral is to improve lateral stability. It tends to restore the aeroplane to a level keel if one wing is pushed downwards by a gust. When a wing drops the resultant force between the lift and the weight is inclined towards the tip of the downgoing wing. This causes the aeroplane to sideslip slightly so that the relative airflow is directed along the lateral axis. As a result, the downgoing wing receives a greater righting force than the wing which is tilted up. In general, less dihedral is needed for a high-wing monoplane than a low-wing monoplane because of the pendulum effect of the underslung fuselage. In fact, the Do 217 has no dihedral, the Flamingo slight anhedral, whereas the He 111K has a marked dihedral. Dihedral angle and fin area are to some extent complementary. Thus the Hurricane which has a big fin has little dihedral, whereas the Spitfire, which has a small fin, has more dihedral. Most aeroplanes have a form of longitudinal dihedral (to give fore and aft stability) in the fact that the angle of incidence of the main planes is greater than that of the tailplane.

Direct-drive Aero-engine. An engine in which the propeller is driven at crankshaft speed without reduction gear. In most modern aero-engines the speed of rotation of the propeller shaft is geared down so that it revolves at only about one-half the crankshaft speed. In this manner a bigger propeller can be used and the great drag caused by the high speed of the tips of the blades is reduced.

Directional gyro. More properly called Direction Indicator. An instrument to indicate any change in the direction of an aeroplane. Deviation from a straight course is shown by marks along a graduated card kept at a fixed datum by a gyroscope.

Directional Instability. A flying defect in an aeroplane which causes it to tend to deviate from a straight course by a combination of side-slipping and yawing movements whenever the pilot attempts to keep it on a level keel by moving the ailerons.

Direction Finder. (DF) Radio apparatus the function of which is to determine the bearing of a distant radio transmitter.

Dirigible. A steerable airship (the term does not denote a rigid airship.)

Disc Area. In a gyroplane or helicopter the area of the circle described by the tips of the blades.

Disc Loading. The product of the loaded (gross) weight of the gyroplane or helicopter divided by the disc area of its rotor.

Disposable Load. Same as Useful Load. The difference between the loaded weight (gross) and the empty weight of an aeroplane. It includes Payload or Military Load.

Distance Records. The last world absolute distance record to be ratified by the Federation Aeronautique Internationale (FAI) was a distance of 24,986·664 miles/40,212·139km achieved by Dick Rutan and Jeana Yeager of the USA in their Voyager aircraft on their round the world non-stop flight between December 14 and 23, 1986.

Distinguished Flying Cross (DFC). An award granted to Commissioned Officers of the Royal air Force and of the Fleet Air Arm while serving under an Air Officer Commanding-in-Chief in the RAF for exceptional valour, courage or devotion to duty whilst flying in active operations against the enemy. The DFC was instituted by Royal Warrant on June 3, 1918. The colours of the ribbons are violet and white in alternate diagonal stripes one-eighth of an inch wide.

The DFC is distinct from the AFC because the DFC is awarded for flying in the face of the enemy, the AFC for meritorious flying in peacetime.

Distinguished Flying Medal (DFM). An award granted to Non-commissioned Officers and Airmen of the Royal Air Force and of the Fleet Air Arm while serving under an Air Officer Commanding-in-Chief in the RAF for exceptional valour, courage or devotion to duty whilst flying in active operations against the enemy. The DFM was instituted by Royal Warrant on June 3, 1918. It bears the same relation to the Air Force Medal as does DFC to AFC.

Distress Signal. Civil aircraft in danger and in need of immediate assistance have international signals laid down for use.

They are:

Radio-telegraphy: S.O.S.

Radio Telephony: Mayday (corruption of French "M'aidez").

By Flags (balloon or airships or aeroplanes on the water): N.C.

Dive. A steep descent with the nose down. A "Terminal Velocity Dive" is the steepest dive in which the aeroplane accelerates until its drag becomes equal to the thrust of gravity pulling it downwards so that the speed remains constant. The terminal velocity of a World War Two single-seat fighter was around 650 m.p.h.

D Licence. A Ground Engineer's licence permitting the holder to inspect and pass as airworthy aero-engines after overhaul.

Doldrums. The region of calms and variable winds in equatorial seas, often accompanied by heavy rain, thunderstorms and squalls.

Doping. A system of chemical treatment of the fabric used for the covering of structural parts of aeroplanes to protect, strengthen, tauten and render it airtight.

Dorsal. A term used to denote the longitudinal member which runs from bow to stern along the top centre line of the hull of a flying-boat. Applied to a gun turret means that on top of the fuselage. Derived from the back fin of fish.

Dover Control. A device linking the throttles of multi-engined aeroplanes so that the throttles can be opened simultaneously or differentially.

Downwash. A term applied to airstream deflected downwards behind a lifting aerofoil. The angle of downwash is the angle of the inclined airstream relative to the longitudinal axis of the aeroplane. The downwash behind the mainplane may have an important influence on the tailplane.

Drag. The total resistance of an aeroplane along its line of flight. The total drag is made up of a number of components.

(i) Cooling Drag. The part of the Parasite Drag covered by the need to cool the aero-engine. It includes the resistance of the radiator of a liquid-cooled engine.

(ii) Induced Drag. The part of the wing drag associated with lift.

(iii) Parasite Drag. The drag of the wings excluding the Induced Drag. Profile Drag is the sum of the drag caused by surface friction and the Pressure Drag.

(iv) Profile Drag. The drag of the wings excluding the Induced Drag. Profile Drag is the sum of the drsag caused by surface friction and the Pressure Drag.

(v) Pressure Drag or Form Drag. That part of the drag caused by the pressure on a wing at right angles to the surface.

(vi) Skin-Friction Drag. The frictional portion of the Parasite and Profile Drags caused by the surface roughness of the fuselage or wing.

Draught. The vertical distance from the loaded waterline to the lowest point of the hull of a flying-boat or the floats of a float seaplane.

Drift. The movement of an aeroplane in a horizontal plane through the influence of a cross-wind. Drift makes necessary the deflection of the longitudinal axis of the aeroplane away from the line of track to be followed. That is to say an aeroplane must be headed slightly towards a beam wind to avoid being drifted off its course.

Drift-angle. The angle between the longitudinal axis of an aeroplane and the track along which it is flying. A drift-angle sight or drift sight is an instrument for determining the angle of drift. It may employ direct vision or may be periscopic.

Drift Indicator. An instrument by which the speed and the direction of the wind can be determined from an aeroplane in flight.

Drizzle. Rain in which the drops are very small.

Drogue. A conical canvas sleeve open at both ends with a metal hoop at the larger end, used as a sea anchor by seaplanes. In lighter form a drogue is towed behind an aeroplane to serve as a target for aircraft or anti-aircraft guns.

Dry Weights. A term applied to the weight of an aero-engine. The gross dry weight includes the whole of the engine, including propeller hub and stub exhausts, but excluding radiator, tanks, fuel, oil or instruments. The net dry weight of the engine is the gross dry weight not including exhaust stubs, starting units or accessories.

DSR. Director of Scientific Research.

DTD. Director of Technical Development.

Dual Control. Flying controls which are duplicated in an aeroplane either for instruction or so that one pilot may conveniently relieve another.

Ducted Cooling. A system of low velocity cooling in which the cooling air is constrained to flow in ducts to the radiators or the cylinder fins of an aero-engine. By means of ducted cooling the cooling drag is greatly reduced, and can, in theory, be made to add thrust.

Duralumin. The name for a wrought light alloy material extensively used for aircraft construction. Duralumin, commonly abbreviated to "Dural", is an aluminium alloy containing approximately 4 per cent copper, 0·6 per cent magnesium, 0·6 per cent manganese ore, 0·3 per cent iron. Its specification numbers are L1, L3, L37, L39, T4, DTD 147 and 150. As Duralumin is the name for a particular material it should not be used loosely for light alloy products in general. Duralumin was first used in 1915 and for some years was the only light ally suitable for aircraft in the form of tubes, sections and sheets.

Dust Devil. A small whirlwind formed by strong convection over hot desert regions. It raises dust or sand in a vertical column which progresses across country.

Dust Interference. Interference with radio reception in an aeroplane flying through a dust storm, and is caused by the accumulation of static charges produced by the impact of particles on the airframe.

Dust Storm. A squall in desert regions which carries dust or fine sand to a height of several thousand feet.

Dynamic Lift. The lift from the envelope of a balloon or airship or the fuselage of an aeroplane caused by its passage through the air.

Dynamotor (Usually Dynamo). A battery-driven generator which supplies power for a radio installation. It consists of low- and high-tension armature windings running in a common field system.