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where words and phrases used in the original publication have fallen from general use (eg, 'aero motor' has been replaced by 'aero engine').

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Safety-belt: A strong belt of webbing about 4 inches wide with a quick release device which keeps the wearer in his seat in an aeroplane. For violent aerobatics a more elaborate harness is worn.

Sailplane: A high performance glider capable of soaring flight.

Sandstorm: A wind laden with sand or dust which may cover a large area and reduce visibility. Sandstorms frequently held up air operations in the Second World War in North Africa.

Sausage: A colloquial name for the windsock which gives wind direction on an aerodrome. Also applied to the captive observation balloon during the First World War.

SBAC: The Society of British Aircraft Constructors Ltd (nowadays the Society of British Aerospace Companies). A body incorporated in 1916 to foster the collective interests of a rapidly growing aircraft industry. The Society represents every established British manufacturer of aircraft and aero-engines and most of the manufacturers of aeronautical materials, equipment and accessories.

Scale Effect: In technical language, the effect upon the absolute coefficients of a change in the Reynolds number. In more simple words, the corrections that have to be applied to the results of model tests in a wind tunnel or similar apparatus in order to gain accurate forecasts of the characteristics of the full scale aeroplane which the model represents.

Scarff Ring: A circular rail mounted on the perimeter of an open cockpit of an aeroplane. It carries a frame on runners. The adjustable tube frame mounts one or more light machine-guns.

Scavenge Pump: In a dry sump oil system as used on piston aero-engines, a separate mechanically driven pump drains the sump and returns the lubricant to the oil tank, usually through an oil cooler.

Schneider Trophy: The International Seaplane contest which was won outright by Great Britain in 1931. It originated as a result of the Trophy presented in 1913 by M. Jacques Schneider to the Aero Club of France. The first race was won in 1913 at an average speed of 45·75 m.p.h. and the last at 340·08 m.p.h. The contest had many vicissitudes, including its rescue from outright wins on two occasions. The last win was, in fact, a walk-over, but probably this was a good thing as it had become a rival race between national Air Forces backed by their respective Governments. This was far beyond the original intentions of the donor, although the science of high-speed flying was much advanced as a result of the race. The low-wing fighter was developed as one result and the famous Spitfire descended directly from the Supermarine S.4, S.5 and S.6 racing seaplanes.

Screening: The earthed sheath which surrounds the entire ignition system on an aeroplane to prevent interference with the wireless equipment.

Scud: Ragged pieces of cloud drifting rapidly in a strong wind. Scud is seen during stormy weather.

Sea Breeze: Because land heats more rapidly than water, air moves in from the sea, generally during the day from the heat of the sun.

Sea Dromes: Projected floating airports anchored at sea to assist in the operation of transoceanic airlines.

Sea Marker: A device dropped in the sea to give navigation readings for aircraft.

Seams: Flaws produced in the surface of tubes during drawing operations. They are usually the result of dirt in the dies.

Seaplane: A generic term covering all aeroplanes which can take off from and alight on the sea. It includes both flying-boats and floatplanes.

Secondary Depression: The small area of low pressure which accompanies a large depression.

Secretary of State for Air: The President of the Air Council, sometimes referred to as the Air Minister, was the political chief of the RAF.

Section: A technical term for the transverse projections of the drawings of flying-boats or floats from which moulds and frames are made. Also used generally to describe a cross-section of an engineering detail, such as an aerofoil.

Segrave Trophy: A trophy awarded annually for the most outstanding British achievement in air, sea or land transport. Sir Henry Segrave, whose memory it commemorates, was a noted figure in British aviation, racing motoring and record motor boating.

Seistan: A strong North wind which blows for about four months in the province of Seistan, Persia (Iran).

Seizing: When an aero-engine suddenly stops as a result of mechanical or lubrication breakdown so that the pistons fuse with the cylinder walls. Before the days of feathering propellers this trouble was usually disastrous, for the revolving blades would either tear the engine to pieces or out of the airframe.

Semi-Rigid Airship: An airship in which the envelope, in addition to maintaining its shape by gas pressure also has a rigid keel. From this keel are suspended the passenger car and driving mechanism.

Separation Point: The point at which the air flow separates from the surface of a body.

Series: The same as the mark of an aeroplane or aero-engine which identifies a particular type.

Service Ceiling: The height at which the rate of climb of an aeroplane falls below 100ft per minute.

Service Load: The total weight of the crew, removable armament, bombs and equipment normally carried in a military aeroplane.

Servo Control: A mechanical or aerodynamic means of supplementing the pilot's physical effort in controlling aircraft.

Sesquiplane: A biplane with a much smaller bottom plane, sometimes called a one-and-a-half plane.

Sextant: An instrument which gives the angular elevation or depression of an object, usually celestial. A bubble sextant uses a bubble to maintain an artificial horizon.

Shamal: A North-Westerly wind in Iraq which causes severe dust storms.

Sheathing: Metal leading-edges to wooden propellers, usually made of brass sheet. They protect the fast-moving edges from damage by rain, gravel, dust and other foreign bodies.

Sheer Lines: The side elevations of a flying-boat hull or float.

Ship Plane: Any aeroplane which is shipborne and can take off from and alight on the deck. "Take-off" includes catapulting.

Sideslip: The deliberate or involuntary movement of an aeroplane downwards and outwards from its true course. A stable type with adequate keel surface will right itself.

Signal Rocket: A pyrotechnic flare fired from the ground to indicate a landing ground or code signal to aircraft

Signalling Lamp: a portable lamp used for visual signalling either to or from aircraft. The usual type is the Aldis, which has a continuous beam and dipping reflector.

Silencer: A device fitted to the exhaust pipe to reduce engine noise.

Simoon: A hot, dry, southerly wind which blows in North Africa, Syria and Arabia.

Sirocco: A troublesome Mediterranean wind which varies in character as it traverses different terrain. It is usually accompanied by bad visibility.

Skidding: A form of sideslip experienced in flat turns in an aeroplane.

Skin Friction: That portion of drag which is covered by the movement of a fluid (e.g. air) over the surface of a body (e.g. aerofoil). The rougher the surface the greater the skin friction.

Sky-writing and Sky-shouting: The projection of sounds or visual marks into or on the sky for advertising or propaganda purposes. The most familiar form was smoke writing. This was done by specially adapted S.E.5as in clear weather at about 10,000ft by Major Savage, Captain Tait-Cox and others. Sky-shouting was used in Russia from loudspeakers fitted into aircraft.

Slat: An auxiliary aerofoil which runs along the front edge of an aerofoil fitted with Handley Page slots. The hinged slat moves outwards to form the slot.

Sleet: Rain and snow being precipitated together.

Sleeve: A drogue or a windsock.

Slipstream: The stream of air behind a propeller which usually has about 20 per cent greater speed than that of the aeroplane, at every speed.

Slipway: The inclined place used for hauling boat and float seaplanes into and out of the water.

Slot: A narrow air passage running spanwise on an aerofoil, flap, aileron, elevator or rudder. Slots give greater control at critical angles of an aeroplane and act on the principle of smoothing the turbulent wake and so delaying the stall of a surface.

Slow Roll: An aerobatic manoeuvre consisting of a slow revolution about the longitudinal axis.

Slow Running Jet: On a normal carburettor, the by-pass jet which enables an aero-engine to fire steadily at a slow rate of revolutions.

Small End: The small end of a connecting rod which carries the gudgeon-pin and piston in an aero-engine.

Smoke Generators: A pyrotechnic device either for dropping from aircraft to indicate wind direction, or for laying smoke screens from aircraft. A smoke-float is dropped at sea.

Snow: Precipitation in the form of feathery ice crystals.

Soaring: Maintaining free flight without power, usually in a thermal current of ascending air.

Solidity: The ratio of the blade area of a propeller or rotor to the total disc area.

Southerly Burster: A line-squall in East or South-East Australia.

Span: The overall distance from wingtip to wingtip.

Spar: The main longitudinal beam or beams of a wing or control surface. Secondary structures are built around the spar to transfer the load to it, and in the case of wing ribs, also to give correct aerofoil form.

Sparking Plug: The firing device of an internal-combustion engine, consisting of a central electrode, metal points and body, and an insulator of mica or porcelain. In a piston aero-engine there are normally two sparking plugs per cylinder, to ensure even firing and reliability.

Special Areas: Areas over which there are special regulations for flying, such as air bases or military establishments.

Specific Consumption: The quantity of fuel or oil consumed stated in pounds (or pints) per brake horsepower per hour.

Speed: The rate at which a body moves through space. Airspeed is measured on the ASI (Air Speed Indicator) and differs both from the true airspeed and the groundspeed. The true airspeed is the indicated airspeed corrected for variations caused by changes in temperature and pressure. The groundspeed is the true airspeed from which the component caused by the wind has been added or subtracted. Thus an aeroplane flying at 100 m.p.h. against a 60 m.p.h. wind has a ground speed of 40 m.p.h. If it is flying with a 60 m.p.h. wind it will have a ground speed of 160 m.p.h.

An indicated airspeed of 100 m.p.h. at 5,000ft equals (approx) a true airspeed (TAS) of 104 m.p.h.; 100 m.p.h. IAS at 10,000ft is 112 m.p.h. TAS, at 20,000ft 130 m.p.h. TAS, at 30,000ft 155 m.p.h. TAS.

Spill: The escape of air at the outer edge of a parachute either from irregularity of descent or intentionally by a pull at one side of the suspension cords. This gives a measure of control over the descent by side-slipping the parachute in the direction of the spill.

Spin: The rotationary movement of an aeroplane around its lateral axis, either in descent or, with extreme power, for a short period in a climb. A spin is usually entered purposely by putting on full upward elevator and full rudder at the same time, or, involuntarily, after a stall when one wing drops. Some aeroplanes will fall into a flat spin from which recovery is difficult or impossible. The normal method of coming out of a spin is to centralise the controls.
The first observed spin was performed involuntarily by Lieutenant Wilfred Parke RN at the British Military Trials on Salisbury Plain in 1912. Lieutenant Parke was flying the Avro cabin biplane and recovered automatically to his great surprise.

Dr Lindemann and Major Goodden undertook the first research into spinning problems at the RAE in 1916 and the work was subsequently carried forward in the vertical spinning tunnel.

Spinner: A streamline fairing fitting co-axially over the hub or variable pitch mechanism of a propeller, and rotating with it.

Spiral: A continuous banked turn made during a glide with the engine stopped or "ticking-over".

Spiral Instability: A particular case of lateral instability, in which the banking angle is always too great, causing side-slipping.

Split Flap: A flat plate, hinged along its leading edge, and mounted on the underside of the trailing portion of a wing. Split flaps are used to increase the lift of a wing at comparatively low forward speeds and to steepen the glide in approaching to land.

Spoiler: A device to reduce lift of a wing surface in order to increase sinking rate of a glider or aeroplane.

Sponsons: Short wing-like projections on each side of the lower portion of a flying-boat hull to maintain its lateral balance on the water. They are usually of aerofoil section to give some return for their parasitic influence on aerodynamic performance. They serve as useful landing platforms as well as their main purpose of giving stability on the water, and are sometimes used to stow fuel.

Sporting Events: The range of events of a racing and competitive character allied with social intercourse which are exemplified in the Flying Club organisations of pre-Second World War days. Some association of the interest in the spotting of aircraft with sporting flying may be envisaged for the future.

Spotter: Any person who practises aircraft recognition. Industrial raid spotters were employed by vital factories and organisations during the Second World War.

Squadron: A Royal Air Force formation of two or more flights.

Squall: A strong wind of short duration usually associated with a change of wind direction. This distinguishes a squall from a gust, which is a sudden increase of strength of any wind. A line squall advances over a large area and is caused by the displacement of colder air.

Stabilisers: Originally the pressure-inflated fabric fins at the rear of a dirigible airship or kite balloon. Now loosely applied to all fixed stabilising fins. In America the term for tailplane.

Stability: Stability is the property possessed by aircraft (aerostat or aerodyne) in more or less degree which enables it to return to normal conditions of flight automatically after a disturbance of its course. Lateral stability is the maintaining of the horizontal attitude of the wings or envelope to the horizon. Longitudinal stability is the maintaining of the body on an even keel fore and aft. Directional stability is the effective control of the fixed keel surfaces or fins in keeping the aircraft on a straight course. There are other more obscure but nevertheless important factors such as spiral or spinning stability.

Before the invention of mechanical control devices such as the British robot pilot known colloquially as "George" and the American Sperry automatic control ("Iron Mike"), inherent stability in aircraft was a most important quality. Much research was done before the First World War at the Royal Aircraft Factory (subsequently RAE) under the direction of Mervyn O'Gorman into stability problems. The names usually associated with this work are Dr Lindemann and E.T. Busk, the latter actually carrying his theories into the air and trying them out in practice. As a result, the R.E.1 was evolved and was claimed as the first inherently stable aeroplane. This was closely followed by the B.E.2c, another stable type, which proved eventually to be too steady for handling quickly in evolutions of war. The great value of this work was that stability became a calculable quantity in aeroplane design.

Stagger: The "set" of the wings of a biplane in relation to one another in side elevation. In positive stagger the upper wing is in front of the lower, and in negative stagger the reverse arrangement applies. Wings directly above one another are said to have no stagger.

Stall: The wing of an aeroplane is said to stall when the smooth flow over the top surface breaks down and degenerates into turbulence. Reverse flow may even be present. The lift of the wing then drops suddenly and with it controllability. A dive is necessary to pick up speed and restore the flow. Most wings stall normally at 15°, or at 25° with slots.

Starboard: A nautical term borrowed for flying terminology indicating the right-hand side of an aeroplane when looking forward from the pilot's seat.

Stars: The heavenly bodies are of great utility in the navigation of aircraft and a study of the stellar system is included in all initial training courses for air crews.

Starter: A device for starting aero-engines. Several different types exist, the more modern of these being operated by induced gas, compressed air, electricity or an explosive charge. Hand or auxiliary engine starters, and priming the induction in conjunction with a starting magneto, are other means used for starting.

Static Balance: When a propeller, mounted in a steel spindle and resting on knife edges, remains at rest it is said to be statically balanced.

Station: A term applied to Royal Air Force aerodromes and depots.

Steel: A high-grade iron alloy with a content of carbon and sometimes with other elements added, such as nickel. Stainless steel has a high chromium content and is non-corrosive.

Stem: The principal frame member in the bows of a flying-boat hull. It runs downwards from the deck to the keel.

Step: A break in the planing bottom of a flying-boat hull which assists "unsticking" from the water.

Sternpost: The rearmost vertical member of a fuselage or hull.

Storm: A term given to any violent atmospheric disturbance.

Strain: The distortion of any body caused by forces acting on it.

Stratosphere: The outer layer of the atmosphere beginning at about 35,000ft defined as the strata of air in which the temperature gradient remains constant - at about -70°F.

Stratus: A fog-like layer of low cloud.

Streamline: A form designed to reduce resistance to motion through a fluid and to eliminate eddies. A good streamline form appears to have a longitudinal section rather like an elongated peardrop with the wider end foremost. First used on airships.

Streamline Wires: First introduced after research at Farnborough, streamline or Raf-wires are rolled to an oval section and are used for external bracing, particularly of biplanes.

Stress: The product of the load on a body divided by the area subjected to load, originally expressed in foot-pounds per square inch. A body is in tension when the force tends to pull it apart. It is in compression when the force tends to squeeze the body together. The change in shape caused by tension or compression is called strain. Stress divided by strain equals a constant called Young's Modulus which varies for different materials. Young's Modulus (E) is about 30million lb per square inch for steel. Shear stress is present when the force acts in such a way that the molecules or sections of the body tend to slide over one another.

Stressed Skin: The form of aircraft construction in which the external skin carries part or all of the main loads.

Stringer: A secondary member of an aeroplane structure, usually running parallel to the mainspars or longerons, which assist in maintaining the external form.

Structure: The basis framework or skeleton of aircraft. or other engineering construction.

Strut: A member of a structure which takes compression stress along the direction of its length. The interplane struts of a biplane are the most graphical example in aeroplane structures.

Stub: Another name for a sponson or similar short projection from an aeroplane.

Stunts: a colloquial name for aerobatics.

Submarine: A small ship specifically designed to submerge and proceed under water to avoid detection and so take advantage of the element of surprise, one of the most telling tactics of warfare.

Suction: A lowering of pressure in a confined space causing a fluid to enter, or one body to adhere to another, under atmospheric pressure.

Supercharger: A device which forces a greater weight of charge into the combustion chamber of an aero-engine than would be taken in by normal induction, particularly in reduced atmospheric pressure at great heights.

Super-Compression Engine: A high-compression engine designed for full-power operation only above a predetermined height. At lower levels the throttle opening is controlled by a throttle gate.

Superheat: A term used in airship piloting to denote the difference in temperature between the gas in the airship's envelope and the surrounding atmosphere.

Supersonic Speed: A speed greater than the speed of sound through the medium concerned. The speed of sound in air is about 770 m.p.h. at sea level, falling to about 660 m.p.h. at 36,000ft. At supersonic speeds bodies have high drag and cause a loud noise, as with the propeller tips of the Harvard.

Supporting Surfaces: The lifting surfaces of an aeroplane or glider.

Surface Friction Drag: The drag caused by the actual contact of the air flowing over a body with the surface of that body.

Surface Radiator: An aero-engine's radiator so mounted as to form an integral part of one or more of the aeroplane's external surfaces. These radiators were used on some of the later Schneider Trophy floatplanes, but fell from favour following the introduction of the ducted radiator.

Sutton Harness: A system of four straps used to hold a pilot in his seat during aerobatics. It consists of two straps passing over the pilot's shoulders and two others over the thighs. The four meet in a quick-release pin in front of the pilot's chest.

Sweep Back: The mainplanes of many aeroplanes are so designed as to appear bent backwards in plan, the tips being aft of the centre section. The angular set back of the planes is known as "sweepback."

Swinging Base: Another term for a compass base which is a circular level piece of ground on which aeroplanes are stood while their compasses are calibrated. Lines marking the points of the compass are usually marked out on the base so that the aeroplane can be correctly aligned in each direction.

"Switch Off": Term used in starting an aero-engine to ensure that the ignition switches are off.

Synchronising Gear: A mechanism designed to permit the firing of a machine-gun or guns through the disc of a revolving propeller. The best known synchronising gear was that developed by George Constantinesco in 1917, which remained in use until the introduction of wing-mounted guns shortly before the Second World War.

Synchroscope: An instrument for indicating whether two engines are running at the same speed.

Synoptic Weather Chart: A chart showing the weather by means of pressure variations at a given time over a particular area.