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where words and phrases used in the original publication have fallen from general use (eg, 'aero motor' has been replaced by 'aero engine').

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Earth. Used, in connection with the radio apparatus carried by an aeroplane, to describe the metallic portions of an airframe which are connected by bonding wires to give electrical continuity and so to serve the purpose of an "earth".

Economical Cruising Power. The percentage of the maximum level horse-power of an aero-engine at which the specific consumption is the lowest. Usually about 62·5 per cent of the maximum power.

Economical Cruising Speed. The speed at which an aeroplane achieves its greatest range in still air. This speed is usually about 85 per cent of the maximum speed. Maximum range is attained at a speed about 40 per cent greater than the stalling speed. Maximum duration is obtained at a speed about 20 per cent greater than the stalling speed.

Eddy. The irregular motion of a fluid - in aeronautics, air - when flowing above a certain speed or around an object. In eddy motion the flow moves in irregular swirls.

Eddy Drag. The portion of the drag of a body which is caused by the eddies formed in flowing around the surface.

Edge. (a) Leading Edge - The forward edge of an aerofoil (including a propeller blade, strut, or any streamlined body). (b) Trailing Edge - The rear edge of an aerofoil, strut, or any streamlined body).

Elastic Limit. Applied to materials for aircraft construction, the limit of stress or force which can be exerted on a body without causing permanent distortion of the stressed body.

Elasticity. The material of a body is said to be elastic up to the point at which the whole of the strain, or distortion of the body, disappears when the stress, or force, which has produced it is removed.

Electrical Starter. An electric motor turned by batteries in an aeroplane or externally, used to rotate an aero-engine to start it.

Electrically Heated Clothing. Flying suits which are heated by electricity through the medium of thin, high-resistance wires enclosed in glass insulators and inserted at points vulnerable to cold. In modern flying clothing only the gloves and boots are heated to give protection against the severe cold at great heights.

Elektron. A magnesium alloy increasingly used in the form of castings for aircraft components. Elektron was developed in Germany during the 1914-18 War as a substitute for aluminium alloy. Elektron is unusually light and has a specific gravity of about 1·8 compared with the 2·8 of aluminium alloy.

Elevator. Control surface hinged to the trailing edge of the tailplane to provide longitudinal control. By raising the elevators the tail is depressed and the nose raised, and vice versa.

Empennage. A general term, now little used, to refer to the tail unit of an aeroplane. Empennage includes tailplane, elevators, fins and rudders.

Endurance. The maximum time which an aeroplane can continue to fly without refuelling. The speed for maximum endurance is normally about 20 per cent above the stalling speed, changing as the stalling speed varies with the constant diminution in weight caused by the burning of fuel.

Engine. A loose term used for the aero-engine which provides the motive power for any flying machine or airship. Aero-engines are both liquid-cooled and air-cooled. The former have less drag, the latter need less maintenance because of the absence of "plumbing."

The most common types of air-cooled aero-engines are the radial and the in-line, although there are also some Vee-types and H-types. The most common types of liquid-cooled aero-engines are the Upright and Inverted Vees, although there are some X-types and H-types. Examples of common types are:
Category Air-cooled Liquid-cooled
Radial Bristol Pegasus -
In-line Gipsy Major -
Upright-Vee - Rolls-Royce Merlin
Inverted-Vee Gipsy Twelve Daimler-Benz DB.601
H-type Napier Dagger Napier Sabre
X-type - Rolls-Royce Vulture

Most aero-engines run on the petrol four-stroke cycle, but there are some modified Diesel-type engines, notably in German seaplanes.

Engine Instruments. The most common instruments concerning the aero-engine used in the cockpits of modern aeroplanes are:

(i) The Engine Speed Indicator or Tachometer which measures the revolutions of the crankshaft in rpm.
(ii) The Fuel Pressure Gauge.
(iii) The Fuel Flow Indicator.
(iv) The Fuel-Contents Gauge which indicates the quantity of fuel in the tanks.
(v) The Boost Gauge which shows the pressure in the induction manifold relative to atmospheric pressure at sea-level (14·7lb per sq in). In principle it is similar to the aneroid barometer.
(vi) The Oil Pressure Gauge.
(vii) The Oil Temperature Thermometer.
(viii) The Radiator Temperature Thermometer.
(ix) The Exhaust Gas Analyser which indicates the quantity of carbon monoxide in the exhaust gas from which the extent of combustion of the fuel/air mixture can be judged.
(x) Fuel Flow Meter which indicates the rate of flow of fuel.
(xi) Synchroscope which indicates whether the two engines are running at the same speed.

English Channel. The first people to fly across the English Channel were Blanchard and Jeffries in a gas balloon - Jan 7, 1785. The first flight across the English Channel in an aeroplane was made by Louis Blériot in a Blériot XI monoplane (25 h.p. Anzani engine) in 37 minutes - July 25, 1909.

Envelope. The gas container of a balloon or airship or the outer cover surrounding the gas bags.

Equivalent Full-throttle Power. The theoretical brake-horsepower which a supercharged aero-engine would develop if run at full throttle and maximum rpm at sea level. In practice the power cannot be attained because the loads on the engine would be too great. For instance, the equivalent full-throttle power of the Rolls-Royce Merlin II would be about 1,900 h.p., although only 1,030 h.p. is normally permissible for take-off.

Eta Patch. A fan-shaped patch of fabric and webbing secured to the envelope of a balloon to hold rigging.

Evaporative Cooling. A cooling system for aero-engines which makes use of the "latent heat of evaporation" by allowing the cooling fluid to boil, then condensing the steam from it and returning it to the cylinder jackets. The Rolls-Royce Goshawk engine of 1932 was designed on this principle, but was not really successful.

Everest. A Westland Wallace and a Houston-Westland P.V.3 biplane flew over Mount Everest (29,028ft) for the first time on April 3, 1933. The expedition which made this flight, and another on April 19, 1933, was named the Houston Mount Everest Expedition and was led by Air Commodore P.F.M. Fellowes. The pilots were Lord Clydesdale and Flt Lt D.F. McIntyre. Its achievements were reported in The Times by Mr E. Colston Shepherd, who accompanied the expedition.

Exhaust. The burnt gases discharged from an aero-engine. About 47 per cent of the latent energy in the fuel supplied to an aero-engine is normally wasted in the exhaust.

Exhaust Branch Pipe. A short pipe which conveys exhaust gases from the cylinder of an aero-engine to the exhaust manifold.

Exhaust Driven Turbo-supercharger. A type of supercharger specially suitable for high flying in which the rotor is turned by the exhaust gases. The chief difficulty in this form of supercharger is to find metals to withstand the intense heat.

Exhaust Flame-damper. An exhaust pipe designed to prevent exhaust gases from passing into the atmosphere while incandescent and thus visible at night.

Exhaust Gas Analyser. An instrument for indicating the quantity of carbon monoxide in the exhaust gases and so showing the efficiency with which the fuel is being consumed.

Exhaust Manifold or Collector Ring. A pipe or chamber into which exhaust gases are led from a number of cylinders through branch pipes. In Bristol radial engines the collector ring forms the nose of the cowling ring. This serves to cool the gases, but has the disadvantage of glowing red-hot at night. Most American radials have an exhaust collector ring inside the cowling behind the cylinders. The disadvantage of this is that it might increase the risk of fire in an accident.

Exhaust Stub Pipe. Stub exhausts were fitted to most fighters before ejector exhausts were developed. The stub exhausts discharge the gases straight into the atmosphere at right angles to the direction of flight. The ejector exhausts turn the gases and discharge them backwards, thus aiding propulsion.

Exhaust Tail Pipe. The pipe which leads the exhaust gases away from a manifold or collector ring.

Experimental Mean Pitch. The theoretical distance which a propeller advances along its own axis during one revolution when there is no slip.

Eye of Storm. The central calm area of a tropical cyclone.