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where words and phrases used in the original publication have fallen from general use (eg, 'aero motor' has been replaced by 'aero engine').

A | B | C | D | E | F | G | H | I/J | K | L | M | N | O | P/Q | R | S | T | U | V | W | X/Y/Z

A

Absolute ceiling. The greatest height which can be reached by an aerodyne or aerostat in Standard Atmosphere. The theoretical height at which the rate of climb is zero.

Accelerometer. An instrument for measuring acceleration in a definite direction.

Accessory Gearbox. A gearbox, usually attached to the back of an aero engine, to transmit the drive of the engine to accessories such as instruments, electric generators, de-icing apparatus, undercarriage retracting mechanism, etc.

Acorn. A streamlined fitting placed at the intersection of bracing wires to prevent abrasion.

Acrobatics. A term used erroneously to describe voluntary evolutions in the air other than those requied for normal flying. The proper description is 'Aerobatics'.

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

Adjustable Pitch Propeller (airscrew). A propeller the angle of which can be adjusted on the ground.

Aerial. The wire, fixed or trailing, used for radio reception and transmission from aircraft. Sometimes erroneously used as an adjective such as 'Aerial Reconnaissance' which should be Air Reconnaissance'.

Aerobatics. Voluntary evolutions performed in the air other than those required for normal flying.

Aerodrome. A prepared area of land or water, including all building and fixtures, intended to be used for the operation of aircraft.

Aerodynamics. The science of the laws relating to forces acting on bodies moving in the air.

Aerodyne. A generic term for heavier-than-air aircraft which derives its lift when flying chiefly from aerodynamic forces - aeroplanes, gyroplanes, helicopters, ornithopters, kites or gliders.

Aero-engine. The power-plant for an aerodyne or aerostat.

Aerofoil. A surface designed to produce an aeropdynamic force at approximately right angles to its direction of motion. Usually a wind, tailplane or fin.

Aeronautics. The study of all branches and activities of flying.

Aeroplane. A mechanically driven heavier-than-air flying machine which has fixed wings. An aerodyne with fixed wings) the terms includes landplanes, seaplanes and amphibians. The word should be used in preference to the generic term 'aircraft' when an aeroplane is specifically meant. The abbreviation 'plane', commonly used is not correct and should be avoided. 'Plane properly means a wing.

Aerostat. A generic terms for aircraft which derive their lift chiefly from buoyancy in the air: lighter than air craft, balloons or airships.

Aileron. Movable aerofoil fitted near the wing-tip of an aeroplane and designed to make possible a rolling movement about the longitudinal axis. Ailerons are invariably connected differentially to the control column so that when one is raised to depress a wing the other is lowered to raise its wing.

Air Brake. A device, usually in the form of a flap, designed to increase the drag of an aeroplane at will. Used to steepen the glide, shorten the approach and reduce the landing run.

Aircraft. A generic term for all types of flying machines, both heavier-than-air and lighter-than-air. The term 'aircraft' includes aeroplanes, gyroplanes, helicopters, ornithopters, kites, gliders, airships an balloons.

Aircraft Carrier. A ship specially designed for the housing and servicing of aircraft and providing means for them to take off and alight (land).

Airframe. The structure of an aeroplane without the engine(s).

Air Intake. The scoop through which air is fed to the induction system of an aero-engine. Forward facing air-intakes give a degree of supercharge from the forward speed of the aeroplane.

Airline. A concern operating transport aircraft over an air route.

Airliner. A slang term for a transport aeroplane.

Airplane. Americans derivative of 'aeroplane'.

Air Pocket. An erroneous term describing the effect of vertical currents which cause 'bumps' to a flying machine passing through them.

Airport. An aerodrome provided with Customs amenities and used as a station on a commercial airway.

Air Route. A definite route between two or more aerodromes, towns or countries.

Air Scoop. A scoop through which air is taken into the ballonnets of a balloon or an airship by wind pressure.

Airscrew. All. types of screws with helical blades designed to rotate in air and more particularly power-driven screws designed to produce thrust by rotation. Synonymous with the term 'propeller'. Airscrew slip is the ratio of the actual advance per revolution of an airscrew to the theoretical r advance per revolution. Airscrew efficiency is an expression of this ratio, normally about 82 per cent.

Airship. A mechanically driven aerostat (lighter-than-air craft) .Includes rigid airships, semi-rigid airships and non-rigid airships. A dirigible (i.e. steerable) balloon.

Airspeed. The speed of a flying machine or airship relative to the air as distinct from its speed relative to the ground. Thus an aeroplane flying at 200 m.p.h. airspeed against a 50 m.p.h. headwind will have a ground speed of only 150 m.p.h.

Airspeed Indicator. An instrument which registers the speed of aircraft through the air, as distinct from relative groundspeed .

Airway. An air route with ground organisation.

Airworthy. A term used to denote that an aeroplane has been examined and passed as safe for flying.

Alighting. The act of making contact with water.

Altimeter. An instrument, usually a converted type of aneroid barometer, graduated to indicate height above a given datum, usually sea level.

Altitude. Height of aircraft above sea level. The word should be used with discretion. Thus 'high altitude' means merely 'high height' and should be avoided.

'Maximum Power Altitude' is the lowest height at which full throttle should be used at maximum permissible r .p.m. of an aero engine. For a supercharged engines it is the greatest height at which the rated boost pressure can be maintained at max. permissible r .p.m.

'Rated Altitude' - preferably 'rated height' - is the lowest height at which full throttle should be used at normal r.p.m. For a supercharged engine it is the greatest height at which 'the rated boost pressure can be maintained at normal r.p.m.

Alto-cumulus. Clouds of average height (between 7,000 and 20,000 ft.), usually in the form of a layer made up of small masses arranged regularly in groups, lines or rolls.

Alto-stratus. Clouds of average height! (between 7,000 and 20,000ft.), usually in the form of a bluish-grey fibrous veil.

Amphibian. An aeroplane designed for taking off from and alighting on both land and water. Examples: Supermarine Walrus, Grumman Goose, Consolidated Canso.

Anemometer. An instrument for measuring the speed of the wind.

Angle of Attack. The angle between the chord line of an aerofoil and the relative airstream, no matter what the angle of the aeroplane. This is also the True Angle of Downwash - the angle through which the airstream relative to the main axis of the aeroplane is deflected by the aerofoil.

Angle of Incidence. The angle between the chord line of a wing and the centre-line of the aeroplane.

Angle of Incidence (Rigging). The angle between the chord line of the mainplane of an aeroplane and the horizontal when the aeroplane is in the specified 'rigging position' on the ground. It should not be confused with the 'True Angle of Incidence' See Angle of Attack.

Angle of Pitch. The angle between the relative airstream and the longitudinal axis of the areoplane.

Angle of Roll. The angle between a line joining the wing-tips and the horizontal.

Angle of Sideslip. The angle between the longitudinal axis of the areoplane and its direction of motion.

Anodic Treatment. A treatment for metal parts of aeroplanes whereby a protective coating of oxide is imposed by electrolytic action.

Antenna. Aerial wire for radio communication.

Anticer. A device for preventing the formation of ice on aircraft, usually in the form of liquid paste. Should not be confused with 'de-icers', which are devices for removing ice when it has formed.

Anticyclone. Refers to weather. A region of relatively high barometric pressure in which the pressure is highest at the middle. The winds circulate clockwise around the centre in the Northern Hemisphere and anti-clockwise in the Southern Hemisphere. Anticyclone is the opposite of Depression.

Anti-drag Wires. Wires or cables, usually in the inside of non-stressed-skin wings, which resists forces set up by the drag of the aeroplane as it flies.

Anti-lift wires. Wires or cables, usually running from the top of the top centre-section plane to the outer wing struts of a biplane, designed to resist forces opposite to the normal direction of lift. They are sometimes called 'landing wires' because they help support the weight of the wings on the ground. They are units of the bracing system of a biplane.

Aperiodic. Non-oscillating. Applied for example, to the need of an instrument, usually a compass, which is so designed that, after a slight lag, it indicates the correct reading with no over-swing.

Approach Beacon. Radio equipment on the ground which, in conjunction with a radio receiver in an aeroplane, indicates its displacement to one side or another of a predetermined radio beam. The approach beacon is used for the final approach to landing, usually in conjunction with two marker beacons, when an aeroplane is coming in to land in bad visibility or in darkness.

Apron. A prepared area near the hangars on an aerodrome, usually made with a hard surface, designed to facilitate the manoeuvering of aeroplanes on the ground.

Arrester Gear. Cables laid across the deck of and aircraft carrier to engage with the deck arrester hook of a deck-landing aeroplane to return it's run on touch-down.

Artificial Horizon. An instrument, operated by a gyroscope, devised to keep an indicator permanently parallel with the true horizon and thus show the pilot the attitude of his aeroplane when flying in cloud or bad visibility.

ASI. Common abbreviation for for an 'Air Speed Indicator'.

Aspect Ratio. The ration of the span to the mean chord of an aerofoil; the ration of the square of the span to the total area of an aerofoil. Determined by the formula: 'span multiplied by span divided by total wing area' for all aerofoils - tapered, elliptical, circular, rectangular or square. Aspect Ration is also important to the structural engineer in building the wing and to the performance estimator in calculating the rate of climb and ceiling of an aeroplane. Aspect Ratio has an important bearing on the drag of a wing at low speeds high up. Aspect ratio vries between 12:1 (high) and about 4:1 (low)

Atmosphere. The mass of air which surrounds the earth and rotates with it. International Standard Atmosphere is an imaginary condition of the atmosphere to which the performance of all aeroplane is referred for exact comparison. It assumes, at mean sea level, Temperature = 15º C; Pressure = 1013.2 millibars. The temperature is calculated to fall by 6.5º C. for every additional 1,000m (3,281ft) of height up to 11,000m (36,089ft) above sea level. A that height it is assumed to be constant at minus 56.5º C.

Attitude (Flying). The inclination of the axes of an aeroplane to the relative airstream.

Attitude (Ground). The inclination of the axes of an aeroplane relative to the ground.

Autogyro. A trade name copyrighted by the Cierva Autogiro Company Limited for the gyroplanes made by that Company.

Automatic Boost Control. A device which automatically regulates the boos pressure from the supercharger of an aero engine so that a predetermined value is not exceeded.

Automatic Mixture Control. A device which automatically adjust the fuel mixture fed to an aero engine so that the quantities of air and fuel are varied by a predetermined amount with increase in height.

Automatic Parachute. A parachute which is opened by a static line attached to and aerostat or aerodyne. Also a parachute the pack of which is attached to an aerostat or aerodyne and opened by the freely falling body.

Automatic Pilot. An apparatus, operated by gyroscopes, which automatically moves the flying controls of an aeroplane to keep it on an even keel and on a fixed course.

Auxiliary Fuel Tank. A tank in which a reserve a reserve supply of fuel is carried.

Aviation. The operation of all types of aircraft.

Axes. Imaginary lines about which an aeroplane may change its attitude of flight. An aeroplane has three axes. They are:

(i) Lateral Axis. The straight line through the Centre of Gravity parallel to a line which would join the wing tips.

(ii) Londitudinal Axis. The straight line through the Centre of Gravity parallel to a line which would join nose to tail.

(iii) Normal Axis.The straight line through the Centre of Gravity at right angles to the lateral axis and the longitudinal axis. When an aeroplane is flying so that the longitudinal axis and the lateral axis are horizontal the normal axis is vertical.

There are also the Lift Axis, the straight line through the Centre of Gravity perpendicular to the relative air-stream, and the Drag Axis, the straight line though the Centre of Gravity parallel to the airstream.

Azimuth. Used in aeronautics usually with respect to instruments to denote angular displacement in a horizontal plane. In astronomical navigation, azimuth is the true bearing of the star under observation from the North point on the horizon as seen by the observer.

B

Backing. A change of wind in an anticlockwise direction. This is the opposite of Veering, which is a change of wind in a clockwise direction.

Baffle. A metal plate used between the cylinders of an air-cooled aero engine to guide the cooling air. Properly: "inter-cylinder baffle".

Balanced Surfaces. Control surfaces of an aeroplane (ailerons, elevators, rudder and flaps) in which the aerodynamic movements about the hinge are wholly or partly self-balanced.

Horn Balance. A small balanced area at the tip of a control surface, usually projecting forward of the main surface.

Mass Balance. A weight set forward of a control surface designed especially to prevent flutter.

Balance Tab. A hinged area at the rear portion of a control surface designed to move in the opposite direction to the control surface and so balance it.

Bale Out. An expression which has become common parlance for the abandoning of an aeroplane and descending by parachute. It was derived from the act of baling out a boat.

Ballast. A weight carried in an aeroplane, balloon or airship to trim it with respect to its centre of gravity or to alter its buoyancy. In aeroplanes, ballast usually takes the form of lead disks. In balloons and airships sand or water is usually carried as ballast, as they can be easily discharged.

Ballonet. A compartment within the envelope of a balloon or airship into which air can be blown to counteract changes of volume of the gas in the envelope. This is necessary to maintain pressure and alter trim, and in non-rigid types to preserve envelope shape.

Balloon. An aerostat which consists of an envelope containing a gas which is lighter than air and which is not equipped for mechanical propulsion. Types of balloons are: Barrage Balloons, Captive Balloons, Free Balloons and Kite Balloons.

Bank. The angle between the lateral axis of an aeroplane and the horizontal plane. Banking is the angular motion about the longitudinal axis of an aeroplane when turning.

Barograph. A recording barometer used in aeronautics to record the varying heights reached by any aircraft during flight. The barograph is carried in the aeroplane and traces out a record on a revolving drum.

Bay. The lateral space between the interplane struts on one side of a biplane. Thus a Gauntlet is a two-bay biplane, a Gladiator is a single-bay biplane.

Beacon. An apparatus, usually on an airway, which emits light signals to indicate a particular geographical position to aircrews.

Beaufort Scale. A numerical system, ranging from 0 (Calm) to 12 (Hurricane) to classify the speed of the wind and covering a range of 0 to 75 m.p.h. or over. The system was originated by Rear-Admiral Sir Francis Beaufort and is based on observing the effect of winds on common objects.

Biased Fabric. Multi-ply fabric used for covering aircraft with one or more plies cut diagonally so that the warp is at an angle to the length.

Big End. In an aero engine, the crankpin end of the connecting rod.

Biplane. An aeroplane or glider which has two mainplanes (main wings) one above the other.

Bise. A cold, dry wind which blows in the winter from a northerly direction in Southern France.

Blade Angle. Of an propeller, the acute angle between the flat undersurface of one blade and the right angle from the propeller shaft. - the plane of rotation. The blade angle is in effect the angle at which a propeller blade bites into the air. It is variable in most propellers by turning the blade in its socket to alter the 'pitch'.

Blade Loading. Used with reference to a gyroplane or helicopter. It is the loaded weight of the machine divided by the total area of the rotor blades, not to be confused with disc loading, which, which is the equivalent of wing loading.

Bleeding. A term used with reference to the hydraulic system of an aircraft which concerns the expulsion of air from a system.

Bonding. The joining together of all metal parts of an aeroplane to form a continuous electrical conductor so that no sparks are caused between separate parts of the structure when they are charged with static electricity.

Bonding Noise. Interference in radio reception caused by the relative movement of the metal parts of an aeroplane in contact with each other.

Boom Well (or Trough). A depression in the plating of a float of a seaplane to take the end fittings of struts or booms.

Booms. Usually Tail Booms, a longitudinal structure which is used on some aeroplanes in place of the fuselage to carry the tail unit.

Boost. The pressure, which may be above that of atmospheric pressure, in the induction manifold of an aero engine. It is measured in lb/in2 above or below standard sea-level atmospheric pressure or in inches of mercury absolute. In the USA, boost pressure is usually termed Manifold Pressure.

Bora. A cold, squally wind which blows in the winter from the North-East on the northern shore of the Adriatic and Aegean Seas.

Boss. Used only with a fixed-pitch propeller, is a collar with flanges which fits on to the propeller shaft and on to which the hub of the propeller fits. Should not be confused with a propeller hub which, in a fixed-pitch and in a controllable pitch propeller, is the central portion on to which the roots of the blades are attached.

Boundary Layer. The thin layer of air immediately surrounding the surfaces of an aeroplane or airship. This layer is slowed down in flight by the friction exerted on the surface as it moves through the air and represents an important element of drag.

Boundary Light. A light, of which there are usually a number, arranged round the limits of an aerodrome. It marks the extent of the landing area.

Bracing. A system, usually consisting of struts and/or wires, by which a compound structure is made rigid.

Brake Horsepower. (Abbreviated b.h.p.) The Horsepower available at the propeller shaft of an aero engine. It differs from the Indicated Horsepower by the power needed to overcome the internal friction of the engine. The term is derived from the convenient method of measuring the horsepower developed at the propeller shaft by running the engine against a calibrated brake.

Breeze. A wind of moderate strength. In the Beaufort Scale graduated from 'Light Breeze' to 'Fresh Breeze' for a wind speed from between 4 to 24 m.p.h.

Bubble Sextant. A sextant used in air navigation in which a bubble level is provided to afford an artificial horizon for determining the altitude of a celestial body.

Buffeting. An irregular oscillation of any portion of an aeroplane distinct from 'flutter', because it is produced and maintained by the eddying wake behind some other part of the aeroplane, whereas 'flutter' is an unstable oscillation in a normal airflow. Buffeting can occur in the tailplane of a low-wing monoplane when it is literally buffeted by the wake of the mainplanes at high angles of incidence or when the flaps are down.

Bulkhead. A solid partition which is used to isolate one part of a structure from another. Used particularly with regard to the Fireproof Bulkhead behind an aero engine.

Bump. A colloquial term used to denote and sudden vertical or lateral movement of an aeroplane caused by thermal or eddy currents. Often erroneously referred to as 'air pockets'.

Bunt. The first half of an inverted or outside loop in which an aeroplane dives past the vertical into an inverted position and then returns to a normal attitude by a half roll or through half a normal loop. In a bunt the forces tend to throw the pilot from his seat and to break the wings downwards. Because of the abnormal stresses put upon an aeroplane in a bunt it is not encouraged as a manoeuvre.

Buran. (pronounced Boorarn). A strong wind from the north-east in Russia and Central Asia

C

Cabane. The term for a tripod or similar arrangement of struts, usually at the centre-section of a biplane. Sometimes called a pylon.

Cabin. An enclosed compartment in an aeroplane or airship for housing the crew and passengers (if any).

Calm. The absence of any appreciable wind.

Camber. The curvature of the surface and/or centre line of an aerofoil section which causes lift. A heavily cambered aerofoil is usually a high-lift section. A slightly cambered aerofoil is usually a high-speed section.

Camera Obscura. A darkened enclosure which has a lens in the roof designed to throw the image of an aeroplane or cloud on a horizontal screen. Sometimes used for determining the speed of an aeroplane.

Canopy. The large envelope of a parachute which, when opened, offers resistance to motion through the air and so slows up the fall of the parachutist.

Cant. To tilt or to incline at an angle.

Cantilever. A term for a structural member held at one end by a rigid support, and free from any external bracing members. The term is often used with reference to wings or undercarriage legs which have no external bracing.

Captive Balloon. A balloon secured or towed by a cable.

Carburetter. A device which, with the aid of a draught of air, converts the fuel supply of an aero engine from a fluid into a vapour which is mixed with an appropriate amount of air and introduced into the combustion chamber as an explosive mixture.

Cartridge Starter. A means of starting an aero engine in which a cartridge is electrically fired in a cylinder and forces down one piston, thus rotating the crankshaft.

CAS. Chief of the Air Staff.

Cascades. Blades of aerofoil section set vertically, after the style of baffles, in a return-flow wind tunnel, to turn the airstream smoothly round a bend in the closed section.

Castoring Nosewheel. A nosewheel of a tricycle undercarriage which is not directly steerable but is free to swivel.

Castoring Tailwheel. A small wheel at the tail of an aeroplane which is free to swivel to assist manoeuvring on the ground.

Catapult. A mechanism for assisting the take-off of an aeroplane by giving an initial acceleration in addition to the normal thrust from the aero engine.

Ceiling. The maximum height to which an aeroplane can climb. The Absolute Ceiling is the height at which the rate of climb is zero and at which the aeroplane has only one possible flying speed. Absolute ceiling is sometimes called the theoretical ceiling. The Service Ceiling is the height at which the rate of climb of any aeroplane has dropped to 100ft. per minute.

Centre-Line (of an aerofoil). A line drawn through an aerofoil from leading to trailing edges. Each point of this line is equidistant from the upper and lower surfaces of the aerofoil. In aerofoil design the curvature of the centre line is used for designation of the type of wing section.

Centre-line Camber. The ratio between the chord of an aerofoil and the maximum height of the centre line above the chord line. A measure of the amount of curvature between the leading and trailing edges.

Centre of Buoyancy. The point through which the entire upward thrust of an aeroplane, glider, gyroplane, balloon or airship, or the hull or floats of a seaplane may be assumed to act.

Centre of Gravity. The point in a body through which the sum of the weights of the parts which make up that body may be assumed to pass whatever the attitude of the body.

Centre of Pressure. An imaginary line along the span of an aerofoil along which all the aerodynamic forces on the aerofoil affecting lift may be assumed to act. The centre of pressure of an aerofoil is normally about one-third of the way back from the leading edge, but in certain flying attitudes and with flaps extended it may vary greatly and may even move right off the aerofoil altogether. The movement in the centre of pressure is of importance structurally as well as aerodynamically.

Centrifugal Supercharger. A supercharger in which air or the ready mixed charge is compressed by centrifugal action.

Centripetal Force. The equal and opposite of centrifugal force. It is the force acting on the body and maintaining it in the curved path.

Chart Board. A table carried in most long-range aeroplanes on which the navigator can work. There is usually provision for attaching a map and parallel motion arms carrying a protractor.

Chassis. An archaic term often applied to the undercarriage of an aeroplane.

Chili. A dry southerly wind which blows in Tunis. Similar to the Sirocco.

Chine. The line which runs along the side of the hull of a flying-boat or a float, parallel to the keel and marks the change in angle between the side plating and the planing bottom.

Chinook. A warm dry wind which blows from the West on the eastern side of the Rocky Mountains. Similar to the 'Foehn' in Europe.

Chock. A wedge placed in front of the wheels of an aeroplane on the ground to prevent it from moving forward when the engine is run up.

Chord. The width of an aerofoil section, usually the wing, measured in a straight line from leading to trailing edge.

Cirro-cumulus (mackerel sky). High clouds, usually found above 20,000 ft. and composed of ice crystals. Appears like ripples in the sky resembling sand on the sea shore or in groups and lines. Cirro-cumulus is invariably white and flaky.

Cirro-stratus. High clouds usually seen in a thin whitish veil which does not blur the outline of the sun or moon but sets a halo around them. Like Cirro-cumulus, is composed of ice crystals and found around 20,000 ft.

Cirrus (mares' tails). High cloud around 20,000 ft. which appears in detached and fibrous wisps. Usually dead white and silky looking.

"C" Licence (Ground Engineer). A licence issued to Ground Engineers for the inspection of aero engines before flight.

Clinometer. An instrument for indicating the lateral attitude of an aeroplane.

Closed-jet Wind Tunnel. A wind tunnel in which the air stream passing over the body under test is enclosed by rigid walls.

Cloud Amount. The proportion of sky obscured by cloud. Usually expressed in tenths of the sky covered. Thus 10/10ths indicates complete overcast. Also expressed in eighths or 'oktas'.

Cloud Base. The height at the bottom of a towering cloud.

Cloud Height. The height of the cloud base above the ground at the point indicated.

Clouds. Clouds, like fog, are caused by condensation of water vapour in the air when it is cooled.

Cloud formation is usually caused by the expansion of rising air. There are three main conditions in which clouds can arise:-

(i) GEOGRAPHICALLY. Wind blowing against mountains and thus causing the air to rise over them, with consequent fall in temperature. A long bank of cloud results.

(ii) CONVECTION. By the sun heating the ground and causing, large vertical movements of air wit a mushroom of cumulus at the top of each rising column. Clouds of this type are formed by day and disappear at night and are usually bumpy to fly through. Clouds also form in this way behind' a depression when a current of cold air meets a mass of warm air and undercuts it, forming a cold front.

(iii) BY THE RISING OF AIR SLOWLY OVER A BIG AREA. In front of a depression clouds are formed by the gradual ascent of air over a large area. A warm front is thus formed by the advance of warm air over a wedge of colder air. Warm air rises in a gentle slope over the colder and denser air and gives rise to clouds usually at great heights. The warm front is distinguished from the cold by the fact that it is the body of warm air which moves forward and by the relative slowness of its descent. Clouds are also formed by eddy motion when a current of cold air blows over a warmer surface and by the mixing of two air currents of different temperatures.

Flying in Cumulo-nimbus, Cumulus, Nimbo-stratus and Strato-cumulus clouds is liable to be very bumpy.

Alto-stratus clouds are usually smooth and smooth flying is encountered above Strato-cumulus.

Clouds can be divided into four general categories, according to height:

(a) HEAP CLOUDS. Clouds with a vertical structure: Cumulus, Cumulo-nimbus and Fracto-cumulus. Height from 1,500ft. to 20,000ft.

(b) HIGH CLOUDS. Clouds which have a base normally higher than 20,000ft. Usually composed of ice crystals: Cirro-cumulus, Cirro-stratus, Cirrus.

(c) LOW CLOUDS. Clouds which have a base normally lower than 8,000ft: Nimbo-stratus, Strato-cumulus, Stratus, Fracto-stratus.

(d) MEDIUM CLOUDS. Clouds which have a base normally between 8,000ft. and 20,000ft: Alto-cumulus, Alto-stratus.

Cockpit. The portion of a fuselage, hull or nacelle designed to accommodate the pilot and/or crew.

C of A. Certificate of Airworthiness.

Col. With reference to a weather map, a region of weather between two areas of relatively high pressure and two areas of relatively low pressure. A col is usually associated with light winds, with thunderstorms in Summer and fog in Winter.

Cold Front. The boundary line between a mass of advancing cold air and a mass of warmer air under which it pushes.

Combustion Starter. A device to start an aero engine by the firing of a charge within a cylinder.

Commercial Load. That part of the disposable load of, an aeroplane from which revenue is derived. (Passengers, mails or freight.) Usually termed 'Payload'.

Compass. An instrument which consists essentially of a magnetic needle free to swing and which, subject to correction for conflicting magnetic fields, always points to the magnetic North. It thus indicates the angle in the horizontal plane between the magnetic North and the longitudinal axis of the aeroplane.

Compass Base. A circular area, usually on the edge of an aerodrome, marked with magnetic bearings upon which aircraft may be orientated for the compensation of errors in their compasses.

Compass Course. The angle between the: longitudinal axis of an aeroplane and the compass needle, measured clockwise from the compass needle between 0 degrees and 360 degrees.

Composite Cooling. (Usually termed Evaporative or Steam Cooling.) A cooling system for aero engines which makes use of the latent heat of evaporation by allowing the cooling fluid to boil, then passing the vapour to a condenser and returning the resultant liquid to the cylinder jackets. An example of the steam-cooled engine was the Rolls-Royce Goshawk of 1932.

Compressed Air Starter. A device for starting an aero engine by making use of the expansive energy of compressed air to push down a piston in a cylinder.

Compressed Air Wind Tunnel. (Sometimes called the Variable Density wind tunnel.) A wind tunnel in which the pressure of the circulated air can be increased so that the small scale of the models tested can be offset by the compensating factor of higher pressure.

Compressibility Drag. The great increase in drag, which arises when a body is moving at speeds around the speed of sound (770 m.p.h. at sea level decreasing steadily to 660 m.p.h. at 36,000ft., after which it is constant).

Compressibility Stall. The sudden loss of lift of a wing which is moved through the air at such a speed that the air flow over its upper surface reaches the speed of sound.

Compression Ignition Engine. An engine in which the ignition of the explosive charge is produced by the heat of compression alone without any electric spark. CI engines usually run on oil fuel and work on the Diesel cycle.

Compression Ratio. The ratio of compression, in the cylinder of an aero engine. Determined by the formula (r + R)/r, where 'r' is the compression space in the cylinder when the piston is at Top Dead Centre and 'R' is the volume swept by the piston in the cylinder.

Condensation. The conversion of a fluid from vapour into a liquid state. This happens when air is cooled below its dew point. Near the ground or sea this produces fog. When rising air is cooled, cloud forms.

Coning. The upward angular displacement of the rotor blades of a gyroplane or helicopter under the influence of lift. The Coning Angle is the angle between the rotor blades and the plane normal to the axis of the hub.

Connecting-rod Assembly. The complete assembly of two or more connecting rods of an aero engine working on one crankpin.

Constant-speed Propeller. Propeller in which the pitch of the blades is varied automatically by a governor so that the aero engine preserves a set, constant speed of rotation, whatever the load.

Consumption. The quantity of fuel or oil consumed by an aero engine or engines, defined in gallons per hour.

"Contact". A colloquial term used to announce that the switch of an aero engine is on and the pilot is ready for the engine to be started. Largely a relic now from the days when propellers swung by hand.

Contours. In maps, the imaginary line joining points of equal height above sea level.

Contra-rotating Propeller. Two propellers mounted one in front of the other on concentric shafts and designed to rotate in opposite directions. By this method twice the horsepower is absorbed by a given diameter propeller and propeller torque is cancelled out. This method was used by the Italians on the 2,500hp Fiat A.S.6 motor of the Macchi Castoldi 72 racing seaplane of 1931, which set a world's Speed Record of 440.67 m.p.h. in 1934.

Control Column. The lever, sometimes surmounted by a wheel, by which the elevators and ailerons are operated. Sometimes called the Control Stick or Joystick, the control column is pulled back to raise the elevators and depress the tail to put an aeroplane into a climb. It is pressed forward to lower the elevators, raise the tail and so promote a dive. Movement of the column to one side raises the ailerons on that side and lowers the ailerons on the other side, so that the aeroplane is tilted towards the side on which the ailerons are raised. When a wheel is set on top of the column this wheel is used to operate the ailerons instead of moving the stick sideways.

This diagram illustrates the means by which a conventional aeroplane is controlled. Elevators control pitching movement, ailerons control rolling movement. Directional movement is effected by the rudder. The lower inset diagrams show how control surfaces are operated. The small upper diagram shows the forces operating on an aeroplane in straight and level flight.

Control Surface. A surface, movable in flight, to control the motion of an aeroplane about its axes. The three principal control surfaces are the elevators, the rudder and the ailerons.

Controllable Pitch Propeller. (Abb. "c.p. prop"). Propeller, the pitch of the blades of which can be changed, within previously determined limits during rotation.

Convection. In meteorology the transference of heat by the movement of vertical currents in the atmosphere.

Coolant. The liquid which is made to flow around the walls and head of the cylinders of a liquid-cooled engine and then passed through a radiator to disperse the excess heat from the engine. Coolant is usually water or glycol or a mixture of both.

Cooling Drag. The part of the drag of an aeroplane which is caused by the cooling system of the engine. For a liquid-cooled engine this is the drag of the radiator. For an air-cooled engine it is the drag caused by the air passing over the finned cylinders of the engine. The cooling drag needs on an average about 3 per cent of the total horsepower to overcome it.

Corrosion. The destruction of the surface of a metal by the chemical action of the fluid in which it is immersed. This takes the form of rust in steels and pitting in light alloys. Corrosion is caused particularly through the action of sea water. Various chemical treatments such as anodising are used to combat corrosion.

Course. In air navigation, the direction of the horizontal longitudinal axis of an aeroplane with reference to the angle it makes with a specified datum. The course is set on the verge ring of the compass and includes both the bearing of one's destination and an allowance for the effect of wind and the consequent drift.

Course and Distance Calculator. A navigational device used for the solution of the triangle of velocities. Sometimes called the Course and Speed Computor. With this instrument the air navigator, knowing wind speed and direction, the bearing of his destination and his own cruising speed, can read off the true compass course which must be flown and the distance in "air miles." Any of these five quantities can be read off, provided four of them are known.

Course and Drift Indicator. A device for measuring the angle between the longitudinal axis of an aeroplane and any other object and between that axis and the line of travel. Sometimes called a 'Bearing Plate'.

Cowling. A metal cover enclosing the whole or part of the power unit of an aero engine.

Cracked Spirit. The fuel obtained by high temperature distillation of crude oil. Petrol of high octane value is prepared by this process.

Crew. Persons carried in an aeroplane who are actively engaged in its pilotage, navigation, maintenance and in other duties connected with its commercial or military purposes. The crew varies from one in a fighter up to about ten in a large bomber or flying-boat.

Cross-bracing. Wires, cables or girders used in certain forms of construction to keep the structure of wings or fuselage rigid.

Cross-level. An instrument used to indicate the direction of the resultant force, or apparent direction of gravity, in a transverse plane. Sometimes called the Lateral Climometer.

Cross-section. A section of the fuselage, hull, float or ring of an aeroplane perpendicular to the longitudinal axis.

Crosswind Axis. The straight line through the centre of gravity of an aeroplane perpendicular to the axes of lift, drag and thrust.

Crosswind Force. The component of the resultant force on an aeroplane along the cross-wind axis, caused by the relative air stream. This force is zero except when the aeroplane is yawed or side-slipped.

Cruising Speed. The normal operating speed of an aeroplane. This may vary widely according to circumstances. An aeroplane normally cruises at about 85 percent of its top speed at any particular height.

Cumulo-nimbus. Heap clouds formed of heavy masses, which may rise to a great height. The tops of these clouds pile up like mountains and appear to have a fibrous texture.

Cumulus. Thick clouds of a woolly type, usually with flat base low down and rising up to cauliflower tops.

Cyclogyro. A flying machine which is supported in the air by power-driven rotors, which rotate about a horizontal axis, like the paddle-wheels of a steamboat. These paddle-wheels replace the normal wings. Each paddle-plane consists of long thin aerofoils, which are feathered at the turn. Machines of this type have been designed by Rohrbach in Germany and Platt in the U.S.A., but there is no record of any successful flights.

Cyclone. A tropical revolving storm with winds of hurricane force, circulating anti-clockwise in the northern Hemisphere and clockwise in the Southern hemisphere. In temperate latitudes a cyclone is properly a Depression. In the West Indies a cyclone is called a hurricane. In the China Seas it is known as a typhoon. In Australia it is called a 'Willy-Willy'. The name cylone originated in the Indian Ocean.

D

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.

E

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.

F

FAA. Fleet Air Arm (in USA, Federal Aviation Administration).

Fabric. A material, usually of linen used for the outer covering of wings, fuselages and tail units of some aeroplanes. Fabric is usually of multi-ply weave.

Factor of Safety. A strength factor to which all aeroplanes are built. Thus a factor of safety of five means that any part of the aeroplane is designed to withstand five times its normal load before failing. A factor of safety of one would mean that the machine would be on the point of breaking at its normal load. Aeroplanes intended for high-speed aerobatics have a higher factor of safety than those intended for normal level flying only.

FAI. Fédération Aéronautique Internationale.

Fairing. A secondary structure added to reduce drag. Common examples of fairings are wing-root fillets and streamlining of struts.

Falling Leaf. An aerobatic manœuvre of a purely spectacular variety. The aeroplane is first stalled and is then forced into a spin. As soon as the spin develops the controls are reversed so that a spin begins in the opposite direction.

Fédération Aéronautique Internationale. Founded in June 1905 by Major Moedebeck, Fernand Jacobs and the Count de la Vaulx in Brussels to regulate aeronautical sport internationally throughout the World. All world records in flying are homologated and officially recognised by the FAI.

Feed Pipes. The oil pipes which convey lubricating oil from the oil tank to the aero-engine.

Figure of Eight. A series of reverse banking turns in which the aeroplane describes a horizontal figure of eight in the air. This was one of the required manœuvres for the gaining of a civil pilot's "A" Licence.

Fillet. A fairing at the junction of two surfaces to improve the airflow and so cut down drag. A fillet is often seen at the trailing-edge wing root.

Fin. A fixed vertical surface, usually at the tail, designed to contribute to both directional and lateral stability. Often called the "tail fin" and usually used to carry the rudder. On lighter-than-air craft the term fin is also used to define the horizontal stabilising surfaces termed the tailplane in an aeroplane.

Fishtailing. Swinging the tail of an aeroplane from one side to another when coming in to land in order to shorten the glide and landing run. It is an alternative manœuvre to sideslipping.

Fix. A navigational term. A "fix" is the point of intersection of two position lines drawn on a map to determine the location of an aeroplane.

Fixed Aerial. The aerial of an aeroplane permanently attached at two points. Fixed aerials usually run from an aerial mast to the fin or from a wing-tip to the tail.

Fixed Loop-aerial. An aerial permanently fixed on an aeroplane and used as a homing receiver.

Flame-float. A "firework" device which can be dropped from an aeroplane at night to burn on the water and so make possible the determination of drift.

Flap. A hinged surface, usually at the trailing edge of a wing, used to increase the lift of a wing at slow speeds, to steepen the glide and to act as an air brake during the approach and landing.

Flap Angle. The angle between the chord of the wing and the chord of the flap. Flaps are often lowered about 5° to assist take-off and usually about 30° to 45° for landing.

Flare. (i) A pyrotechnic flare attached to the underside of the wing of an aeroplane to illuminate the ground for a night landing. Parachute flares are used to illuminate targets for night bombing.

(ii) The outward slope of the hull of some designs of flying-boats from the chine to the gunwale.

(iii) The action of "holding the aircraft off", i.e. reducing the descent rate, during landing.

Flarepath. A line of lights laid out on an aerodrome or surface of a harbour to act as a guide for aeroplanes taking off or alighting.

Flat Spin. A spin in which the tail of the aeroplane drops and the machine rotates in an almost horizontal plane. Recovery from a flat spin is difficult and sometimes impossible. Low-wing monoplanes are more prone to the flat spin than any other type of aeroplane so that an anti-spin parachute is usually carried on trials. This small parachute is attached to the tail and can be released if necessary. It pulls up the tail of the aeroplane and makes possible recovery from the spin. It can then be jettisoned.

Flattening Out. In approaching to land an aeroplane, the transition from the gliding approach to the position to alight is termed "flattening out."

Fleet Air Arm. Descended from the Royal Naval Air service - which, with the Royal Flying Corps, became part of the Royal Air Force on April 1, 1918 - the Fleet Air Arm was formed in April 1924. It was then a part of the RAF, under the operational control of the Admiralty only when afloat. The Fleet Air Arm comprised all aircraft carried on ships of the Royal Navy. For 14 years the FAA was under the administrative control of the Air Ministry and remained part of the RAF.

In 1937 the Government decided to transfer the whole control of the FAA to the Royal Navy. The personnel of the FAA was then about 3,000. The transfer did not take place at once, but on May 24, 1939, the administrative as well as the operational control was assumed by the Admiralty.

The Fleet Air Arm distinguished itself in action in World War Two - particularly in the spotting of the Graf Spee (Fairey Seafox), in the action at Taranto (Fairey Swordfish), in action off Norway and against German and Italian dive bombers in the Mediterranean (Blackburn Skuas, Gloster Sea Gladiators and Fairey Fulmars), in the battle of Cape Matapan (Fairey Albacores and Fairey Swordfish) and in the destruction of the Bismarck (Fairey Albacores and Fairey Swordfish).

When war began the Royal Navy had seven aircraft carriers with six more being constructed. Since the outbreak a programme of converting merchant ships into auxiliary carriers has been started and many are now in service. The carriers completed at the outbreak of the war were, in order of age, Argus (14,000 tons, about 20 aircraft), Hermes (12,900 tons, about 20 aircraft), Eagle (20,400 tons, about 21 aircraft), Courageous (22,500 tons, about 48 aircraft), Glorious (22,500 tons, about 48 aircraft), Furious (22,450 tons, about 33 aircraft) and Ark Royal (22,000 tons, about 60 aircraft).

Six aircraft carriers had been officially named in the course of construction when the war began. They were the Illustrious (23,000 tons, about 60 aircraft), Victorious (23,000 tons, about 60 aircraft), Formidable (23,000 tons, about 60 aircraft), Indomitable (23,000 tons, about 60 aircraft), Implacable (23,000 tons, about 60 aircraft) and Indefatigable (23,000 tons, about 60 aircraft).

The Courageous was sunk by torpedoes in September, 1939, Glorious was sunk off Norway in June, 1940, Hermes was sunk off Trincomalee in April 1942, Ark Royal torpedoed off Gibraltar in November 1941 and Eagle was sunk in the Mediterranean in August 1942.

Flettner Rotor. A cylinder rotated with forward motion to provide lift at right angles to its path. Flettner rotors have been used to provide lateral control in place of ailerons, without outstanding results.

Flick Roll. A rapidly executed roll in a horizontal plane in which the aeroplane is turned completely around its longitudinal axis by use of both rudder and elevators and continues in level flight. A flick roll is in reality a horizontal spin and the aeroplane is temporarily out of control during its course. The flick roll was first demonstrated on a Bristol Scout in 1914 and was popular as a spectacular aerobatic during the age of small manœuvrable single-seat fighter biplanes. Powerful elevators are needed to force a modern aeroplane into a flick roll.

Flight. (a) The act of flying.

(b) A unit of a formation of aeroplanes of the Royal Air Force. A flight usually consists of three aeroplanes, but may also be four, five or six.

(c) The aeronautical weekly journal founded by Stanley Spooner in January 1909.

Flight Engineer. A member of the crew of a large aeroplane who controls the engines in the air, thus relieving the pilot of an onerous duty.

Flight Lieutenant. A commissioned rank in the Royal Air Force equivalent to Lieutenant in the Royal Navy and Captain in the British Army. The rank may be distinguished by two sleeve stripes and two stripes on the coat shoulder straps.

Flightpath. The path of the centre of gravity of an aeroplane relative to the air. If the aeroplane is climbing steeply or gliding flatly the flightpath may be at an angle less acute than the attitude of the longitudinal axis of the machine.

Flight Sergeant. A non-commissioned rank in the Royal Air Force which corresponds with Chief Petty Officer in the Royal Navy and Colour Sergeant, Chief Quarter Master Sergeant, etc in the British Army. The pay of a Flight Sergeant in 1942 varied from 7s 9d per day to 12s per day on ground duties up to 15s 6d per day for a Flight Sergeant Pilot or Flight Sergeant Observer.

Flightway. A specified air space beyond the end of a runway of an aerodrome. Obstructions must be kept below the specified heights in the flightways.

Float. (a) When approaching to alight, the distance travelled horizontally in the air after flattening out and before touching down. The better streamlined the aeroplane the greater its float, unless fitted with flaps. Flaps act as air brakes and shorten the float and they make possible landing in a smaller space.

(b) A watertight body of streamlined form which gives buoyancy and stability on the water to a seaplane or flying-boat and enables it to take off and alight. There are four types of float (in America called "pontoons").

(i) Main float - usually two, sometimes one.

(ii) Wingtip floats - to provide stability for flying-boats or single-float seaplanes.

(iii) Tail float - used with the short main floats of old-type seaplanes.

(iv) Inboard floats - perform the same function as wing-tip floats but are mounted close to the hull.

Float Seaplane. A seaplane provided with floats (American pontoons) as its means of buoyancy on the water.

Floating Ailerons. Ailerons so connected that they remain free, while the control column is centralised, to preserve a negative angle of attack in relation to the air flow whatever the angle of the mainplane.

Floodlight. Wide angle lights used on an aerodrome to illuminate all or part of the landing area at night.

Flotation Gear. Emergency flotation equipment installed in landplanes liable to fly over the sea to give them buoyancy should they be forced down on the water. Flotation gear usually consists of watertight bags or compartments in the fuselage.

Flutter. Usually applied to wings or ailerons - the unstable oscillation caused by interaction between aerodynamic forces, elastic reactions in the structure and the force of inertia. Unless damped quickly flutter will break a wing. The cure is usually to dispose the weights so that the centre of gravity of the wing is as far forward as possible. Ailerons, elevators and rudders are mass-balanced to prevent flutter.

Flutter Speed. The lowest airspeed at which flutter occurs in a given aeroplane.

Flying Blind. Flying solely by instruments in thick cloud or darkness.

Flying-boat (Boat Seaplane). A seaplane in which the main body or hull provides the means of support on the water.

Flying Clubs. The Flying Club movement began with the advent of the light aeroplane in 1924. The Clubs received a Government subsidy up to the outbreak of World War Two which made possible the training of pupils - particularly the Civil Air Guard - at reasonable rates.

Flying Controls. The mechanism whereby the pilot operates the control surfaces from the control column.

Flying Instruments. (i) The Air Speed Indicator (ASI) linked to the Pitot Tube and the Static Tube to measure the air pressure and hence the forward airspeed.

(ii) Altimeter, which is in reality a barometer measuring pressures and recording them as heights.

(iii) Artificial Horizon, which indicates whether the aeroplane is banking, climbing or diving (worked by a gyroscope).

(iv) Turn and Bank Indicator, partly worked by a gyroscope and partly consisting of a simple pendulum.

(v) Rate of Climb Indicator. Shows the rate of change of height by providing a visual indication of the air entering or leaving a "Thermos" flask.

(vi) Compass - An instrument which consists essentially of a magnetic needle free to swing and which, subject to correction for conflicting magnetic fields, always points to the magnetic North. It thus indicates the angle in the horizontal plane between the magnetic North and the longitudinal axis of the aeroplane.

(vii) Directional Gyros used to indicate any change in course, without continual reference to the compass, which must be mounted horizontally.

Flying Machine. A power-driven heavier-than-air craft. Includes all aeroplanes, helicopters and gyroplanes, but not gliders, sailplanes or airships.

Flying Officer. A junior commissioned rank in the RAF corresponding to a Sub-Lieutenant in the Royal Navy and a Lieutenant in the British Army.

Flying Position. The attitude of an aeroplane when it is flying straight and level.

Flying Speed. Maximum: The maximum airspeed of an aeroplane when flying level in Standard Atmosphere. Mimimum: The minimum airspeed at which an aeroplane can fly level in Standard Atmosphere.

Flying Wires. Those bracing wires in a biplane which transfer the lift of the wings to the main structure. Also known as "lift wires." They extend diagonally outwards and upwards. Anti-lift wires which take the weight of the wings on the ground extend outwards and downwards.

Fog. A condition of bad visibility in the lower atmosphere caused by the presence of particles of condensed moisture, of smoke, or of a mixture of the two held in suspension in the air.

Föhn (Pronounced "Furn"). A warm dry wind which blows down the northern slopes of the Alps when the general wind current comes over the mountains. Can be applied to any wind experienced under similar conditions.

Folding Wings. The mainplanes of an aeroplane designed to fold so that it may be stored in a confined space.

Force. That which produces or tends to produce a change of motion or shape of a body. Measured in pounds or dynes.

Forecast. A statement of weather to be expected over a given area or route during a definite period.

Forced Landing. The unpremeditated landing of an aeroplane usually caused by mechanical breakdown or bad weather.

Four-stroke engine. An internal-combustion engine in which explosion of the mixture in each cylinder occurs at every alternate revolution of the crankshaft. Nearly all aero-engines work on the four-stroke cycle at present.

Fracto-cumulus. Small cumulus clouds with ragged tops which continually change their form.

Fracto-nimbus. Low, dark, ragged clouds associated with bad weather.

Fracto-stratus. Layer cloud broken into irregular, ragged fragments.

FRAeS. Fellow of the Royal Aeronautical Society.

Free Balloon. A balloon intended for cross-country flying carried by the wind.

Freeboard. In a flying-boat hull, the vertical distance from the loaded waterline to the lowest opening through which water could enter the hull or to the level of the top decking if there is no opening below its level.

"Frise" Ailerons. Ailerons with their hinges a little back from their leading edges and so shaped that when the aileron is raised a horn projects into the airflow below the wing, thus creating turbulence and destroying lift. Their function is to equalise aileron drag in a banked turn. Named after Mr Frise, Chief Designer to the Bristol Aeroplane Co Ltd who invented them.

FRMetSoc. Fellow of the Royal Meteorological Society.

Front. A line of discontinuity between cold and warm masses of air.

Frost. A condition in which the temperature of the air is below the freezing point of water.

Froude's Law. A law used in Hydrodynamic calculations to convert the full size data obtained with scale models in testing tanks. It states that to get similar flow on model and full scale, the model should be towed at a speed equal to the full-scale speed multiplied by the square root of the scale of the model.

Fuels. Aero-engine fuels are either refined from crude petroleum or obtained from coal, the latter source being that on which Germany has had largely to depend.
The great majority of aero-engines run on what is so commonly termed "high grade aviation spirit," but a few compression-ignition engines running on heavier fuels are in service, notably in Germany.

Fuel-contents Gauge. A gauge for indicating the contents of a fuel tank ie full or empty etc.

Fuel-flow Meter. An instrument for indicating the rate of flow of fuel.

Fuel-jettison Gear. Gear to enable part of an aeroplane's fuel load to be discharged in case of emergency. This is particularly important with modern aeroplanes because of the high wing-loadings at take-off in general use - loadings too great for safe landing if one should become necessary soon after take-off.

Fuel Tank. A tank containing the fuel supply for an aero-engine.

Fuselage. The main body of a landplane, float seaplane, or float amphibian, providing accommodation for the crew and useful load.

G

Gale. A wind of "Force 8" or above on the Beaufort Scale of Wind Force defined as strong enough to "break twigs off trees and generally impede progress".

Gap. The distance separating the mainplanes of a biplane, triplane or quadruplane.

GAPAN. The Guild of Air Pilots and Air Navigators of the British Empire.

Gas Bag.A gas container in a rigid airship. Rigid airships are provided with a large number of gas bags (usually about 16) which hold the supporting gas and are anchored within the rigid framework.

Geared Engine. An engine in which the power developed on the crankshaft is transmitted to the propeller shaft through a reduction gear. The vast majority of large aero-engines are of this type, although in the smaller classes direct-drive engines are still widely used. A geared engine makes possible a slower-moving bigger propeller of higher efficiency.

General Inference. A general description of the meteorological situation at a given time deducted from the pressure distribution shown on a synoptic weather chart, together with the changes in progress and a statement of the kind of weather likely to be experienced over a particular area as a result of these changes.

Geodetic Structure. A structure built up of geodetic members, that is to say, of members which have the shape of the shortest (surface) distance between two points on a curved surface.

Geometric Pitch. The distance which an element of a propeller would advance in one revolution under conditions of no slip.

Gibli. A hot dry southerly wind in Tripoli, similar to the Sirocco.

Glacier Breeze. A cold breeze, blowing down the course of a glacier, owing its origin to the cooling of the air in contact with the ice.

Glazed Frost. A layer of smooth ice formed by rain falling on exposed objects when air temperature is below freezing point.

Glide. The gradual descent of an aeroplane with gravity alone providing the motive power.

Glider. A fixed-wing aerodyne intended for motorless flight. Gliders are generally divided into four classes:

(1) Primary Glider. Used for training and designed for simple glides with continuous loss of height.

(2) Intermediary Sailplanes. A more advanced form of glider capable of continuous flight without loss of height.

(3) High Performance Sailplanes. A highly developed glider of great aerodynamic efficiency designed for competition and other high performance flying.

(4) Towed Glider. This is towed behind an aeroplane singly or in a "train" carrying troops or freight.

Gliding Angle. The angle between the path of flight of an aeroplane and the horizontal while gliding.

Best Gliding Angle. That gliding angle at which an aerodyne will cover a maximum horizontal distance for a given loss of height.

Gliding Certificates. There are five classes of Gliding Certificate: "A", "B", "C", "Silver C" and "Gold C." Some 1,639 people had been awarded certificates in England up to the time when gliding was stopped by World War Two.

Gnomonic Projection. A method of projecting a map used for charts of the Polar regions and for harbour surveys. The Gnomonic projection produces a map on which direct measurements are accurate only over short distances. It assumes that each subdivision of the surface of the earth is flat. The Great Circle then appears as a straight line. Its advantages are that for small areas it is nearly accurate in every way.

Gordon Bennett Balloon Race. The first classic air race, held between 1906 and 1938. Free balloons were wafted from a joint starting place by the wind and the one to go farthest won. The record distance was 1,368 miles, set up by France in 1912 - one of the few air records standing for so long.

Gore. A fabric segment of a parachute, airship or balloon.

Gosport Tube. A system of voice pipes and headphones widely used in training aircraft for communication between instructor and pupil. The name comes from the fact that it was first used at the Special School of Flying, Gosport, under Smith-Barry in 1916. Sometimes called "de Havilland 'phones."

Göttingen. The German aeronautical research centre. A family of related aerofoils are called "Göttingen sections" after their place of origin.

Gradient Wind. A wind induced in a certain direction by flow of air balancing the pressure gradient caused by the earth's rotation and centrifugal force.

Gravity. The force exerted on a body by the mass of the earth. It is equal to the "weight" of the body in pounds. The speed of a falling body under the action of gravity depends in practice on its density; that is, on the relation between its weight and the area it presents to air resistance. In a vacuum, the rate of descent of any body increases at the rate of 32ft per second every second.

Gravity Tank. A fuel tank so situated that the contents flow to the engine by gravity alone.

Great Circle. The line over the surface of the earth in the shortest curved path. It is, in fact, the shortest line joining two points on the earth's surface.

Gregale (pronounced "gregahley"). A strong wind from the North-East usually in Winter, in the Ionian Sea and neighbouring parts of the Central Mediterranean.

Grid Deviation. The angle between a grid line and True North.

"Grids," that is to say patterns of squares, are super-imposed on some maps to enable a point to be quickly defined by co-ordinates.

Many flying - and other - maps, because of the projection employed, have slightly converging meridians. The grid lines are parallel, however, and are therefore inclined to all the meridians (except that through the origin) which converge at True North. These angles of inclination are termed "grid deviations".

Gross Weight. The maximum weight at which an aeroplane is permitted to fly. This is stated on the certificate of airworthiness for a civil aeroplane.

Ground Engineer. A person licensed to certify the safety for flight of an aeroplane or its engine or to inspect and improve repairs, replacements and overhauls to aeroplanes or aero-engines.

Groundloop. An involuntary, uncontrolled and usually sudden circular turn when moving on the ground, caused by directional instability. Groundlooping can afflict tailwheel aircraft (as distinct from tricycle-undercarriage aircraft) on take-off or landing, and if it happens at high speed the undercarriage may collapse.

Ground Speed. The speed of an aeroplane relative to the earth's surface, as distinct from its speed through the air. Thus if an aeroplane is flying at 200 m.p.h. through the air and is flying against a steady wind of 20 m.p.h. its ground speed will be 180 m.p.h.

"Ground Strafing". A term given to low-flying gun attacks on ground targets.

Group. The RAF formation into which the Commands are divided. The Group, in turn, is subdivided into Wings and the Wings into Squadrons.

Group Captain. A commissioned rank in the RAF equivalent to that of Captain in the Royal Navy and Colonel in the British Army.

A Group Captain may be distinguished by the four rings on his sleeve or the four bars on the shoulders of his greatcoat.

Guild of Air Pilots and Air Navigators of the British Empire. Founded on October 1, 1929, the Guild has as its object the promotion, consideration and discussion of all questions affecting air pilots and air navigation, and the protection by all proper legal methods of the interests of its Members and the procession of Commercial Aviation.

Gust. A sudden local fluctuation in velocity of the wind.

Gyroplane. A mechanically-driven aeroplane deriving its lift from the reaction of the air on one or more rotors freely rotating in a horizontal plane. This type of aircraft, of which the Autogiro is one example, should be clearly distinguished from the helicopter, in which the rotors are power-driven in normal flight.

Sometimes termed "Rotaplane."

H

H-Engine. An aero-engine with its cylinders arranged in two banks, which, with crankshafts between them, represent the form of an "H" in end view.
Notable examples of this type of engine are the Napier series, the 16-cylinder Rapier, the 24-cylinder Dagger and the 24-cylinder Sabre. H-type engines were produced also by Hispano-Suiza and Jalbert-Loire.

Haar. A local name in Eastern Scotland for a sea fog which spreads inland.

Haboob. A disturbance of the line squall type which occurs in the Sudan during the rainy season and is usually accompanied by a dense sandstorm.

Hail. Precipitation in the form of hard pellets of ice which fall from cumulo-nimbus clouds and are often associated with thunderstorms.

Hand Starter. A mechanism for the rotation by hand of an aero-engine's crankshaft for starting.

Hangar. A building to house aircraft. This was originally a French term which came into use in this country before the Second World War.

Harmattan. A very dry North-East wind prevalent in West Africa during the dry season (November to March) carrying with it great quantities of dust from the Sahara.

Haze. Slight fog caused by the presence of solid matter such as dust, smoke, or hydroscopic particles carrying a deposit of water in air not saturated with water vapour.

Head Resistance. More correctly "drag." The total resistance of an aeroplane to forward motion along its line of flight.

Header Tank. A tank in the cooling circuit of liquid-cooled aero-engines in which coolant is received from the engine and distributed to the cooling elements of the radiator.

Heap Clouds. Clouds with vertical structure and usually found between 1,500ft and 20,000ft. They are:

(a) Cumulus. High flat-based clouds with rounded billowy summits.

(b) Fracto-cumulus. Broken masses of cumulus.

(c) Cumulo-nimbus. Heavy masses of cloud with solid bases and fibrous tops.

Heavier-than-air Craft. More correctly "aerodyne." Includes all aeroplanes, gliders, gyroplanes and helicopters.

Height Computer. A calculating device for converting indicated to true height by taking the density factor into consideration.

Helicopters. Aircraft deriving their lift from one or more mechanically driven rotors.

Helium. An inert colourless gas, one-sevcnth of the weight of air. It is preferable to hydrogen for use in airships because it is non-inflammable. Unfortunately it is very costly and is only found in workable quantities in North America, and it is twice as heavy as hydrogen.

High Clouds. Mean height of more than 20,000ft. Usually composed of ice crystals. They are:

(a) Cirro-cumulus (mackerel sky). Layer cloud of small white flakes or of small globular masses. Often resembles the ripples in the sand on the seashore.

(b) Cirro-stratus. A thin whitish veil which does not blur the outlines of the sun or moon but gives rise to haloes.

(c) Cirrus (Mare's tails). Detached clouds of delicate and fibrous appearance.

Hucks Starter. A device consisting of a lorry chassis fitted with an overhead shaft geared to the engine and provided with a dog which can be engaged with the boss of the propeller for starting purposes.

Hull. (i) The main structural and flotation body of a flying-boat or boat amphibian.
(ii) The structural framework of a rigid airship.

Humidity. The amount of water vapour per unit volume of air.

Absolute Humidity. The mass of water vapour per unit volume of air.

Relative Humidity. The ratio of the humidity of a given volume of air to its maximum possible humidity at the same temperature.

Hump. That point in the take-off run at which a seaplane passes from displacement lift to hydrodynamic lift. It represents the process of climbing, by means of the "step" in the floats or the hull and with the help of increasing speed, out of the water on to its surface.

Hump Speed. The speed of a seaplane at the hump point at which its resistance is a maximum.

Hurricane. (1) A wind of "Force 12" (e.g. maximum) on the Beaufort Scale of wind force. Rarely experienced except in tropical revolving storms or tornadoes. Velocity of 75+ m.p.h.

(2) The name given in the West Indies to tropical revolving storms.

(3) the famous Hawker single-seat fighter in use in many theatres of the Second World War.

Hydrodynamics. That part of the science of mechanics which deals with the dynamics of water and of the effects of the motion of water past bodies on its surface or immersed in it.

Hydrofoil. A surface immersed in water underneath the planing bottom of a hull or float to assist take-off with its hydrodynamic lift.

Hydrogen. The lightest known gas which has been, and still is, extensively used in airships and balloons. Its one great drawback is that it is highly inflammable and this undesirable characteristic has been the cause of many disasters. Because of this the heavier gas, helium, is more suitable for airship use.

Hydrogen has one-fourteenth the weight of air and is one-half the weight of helium. One thousand cubic feet of hydrogen will lift 70.746lb. One thousand cubic feet of helium will lift 65.57lb.

Hydromatic. The trade name for the Hamilton hydraulic constant-speed full-feathering propeller. It was built under license in the UK by the de Havilland Aircraft Co Ltd.

Hydroplane. A flat-bottomed craft designed to skim over the surface of the water.
Seaplanes are said to hydroplane as soon as they have passed the hump speed during take-off and are supported by the hydrodynamic lift of the hull or floats.

Hydrostatics. That part of the science of mechanics which deals with the equilibrium of stationary bodies immersed in water or on its surface.

Hydrovane. A planing surface sometimes fitted to landplanes to make possible an emergency alighting on water.

Hyetograph. A recording rain gauge.

Hygrograph. A recording instrument for measuring the humidity of the air.

Hygrometer. An instrument giving the humidity of the air at any instant.

I/J

IATA: International Air Traffic Association.

ICAN: International Commission for Air Navigation.

Ice Accretion: The formation of ice on the exposed surfaces of an aeroplane in flight.
There are three types of ice formation: (1) A white crystalline deposit (hoarfrost) formed by direct change from water vapour in the air. The formation is slow and the ice does not adhere firmly. (2) Feathery ice crystals (rime) which form when an aeroplane is flying in clouds containing super-cooled water drops. (3) Clear ice (glazed frost) formed when rain is falling through air with a temperature below freezing point. The ice forms when the rain strikes the exposed surfaces of the aeroplane.

Various de-icing systems, both chemical and mechanical, are in use to combat the menace of ice formation.

Identification Light: A light situated on or near a ground beacon and serving to identify it when seen from the air.

IHP: Indicated Horse Power.

Immelmann Turn: An obsolete term for a half roll off the top of a loop - a useful manoeuvre, particularly in air fighting, for reversing direction and gaining height. It took its name from the famous German pilot of World War One, Max Immelmann, who is credited with having introduced the manoeuvre and who certainly used it to good effect in air combat in the Fokker Monoplane. It was subsequently used with equal effect by pilots of D.H.2 single seat pusher fighters.

Impellor: The rotor of a centrifugal supercharger.

Incidence Wires: The diagonal wires or cables on a biplane, triplane or quadruplane which brace each pair of front and rear interplane or centre section struts. They get their name from the fact that they hold the planes at their correct angle of incidence.

Independent Air Force: A force formed on June 5, 1918 for the strategic bombing of Germany to be performed quite apart from the Army Cooperation work, then the chief occupation of the British Air Forces. By the end of World War One it comprised nine bomber squadrons and one night fighter squadron and had dropped 543 tons of bombs. "Independent Air Force" was really a misnomer. The force remained part of the RAF and was in effect the predecessor of the Bomber Command of today.

Indian Air Force: This Service was formed in October 1932.

Indicated Airspeed: The airspeed as shown by an airspeed indicator. The reading only agrees with the true airspeed if atmospheric conditions are "standard". As height increases the indicated airspeed falls below the true airspeed.

Indicated Horse Power: The power of an engine calculated from the pressure in the cylinders during each working stroke. In effect it is the total power developed by the engine without allowing for the power lost in mechanical friction. The IHP is obtained from the following formula:

IHP per cylinder = p.L.A.N horse power
= 33,000

where p = the mean effective pressure in the cylinder in lb/in2
L = the stroke of the piston in ft.
A = the area of the piston in in2.
N = the number of power strokes per minute of the engine.

Induced Drag: That part of the drag which is caused by vortices at the wing tips. It is the drag which results from the lift. Pushing a wing through the air, to generate lift in it, causes violent disturbance of the air at the tips. That expresses itself in turbulence or vortices which tend to withhold the wing from moving forward although in fact they are mainly beyond the limits of the wing. Therefore, in a wing of infinite span, there would be no induced drag.

Induction Manifold: A branched pipe which distributes the fuel mixture evenly to the cylinders of an internal combustion engine.

Inertia Starter: a device for starting aero engines consisting of a large flywheel which is set in motion by hand or mechanical means. When the flywheel is turning rapidly it is suddenly connected to the engine and its momentum rotates the crankshaft.

Inflation: The process of introducing a gas into the envelope or gas bag of an aerostat.

Inflow: The increase in air velocity in front of a propeller produced by its rotation.

In-Line engine: An engine with its cylinders arranged in one or more vertical rows running parallel to the longitudinal axis of the aircraft.

Instability: The tendency of an aeroplane to depart involuntarily from the set line of flight. Instability may express itself in yawing (swinging to right or left), rolling or porpoising (following an undulating path). Such faults may arise from a variety of causes, eg insufficient fin area, too short a fuselage, inefficient tailplane, failure to compensate in wing or aileron adjustment for the torque (twisting tendency) of the engine.

Instructors' Certificates: In Great Britain these take the form of an endorsement by the Air Ministry on either an "A" or a "B" Pilot's Licence. The endorsement may be made on the recommendation of the Guild of Air Pilots and Air Navigators.

Instrument Flying: The art of flying an aeroplane solely by instruments without reference to any external datum or horizon.

Intake air heater: A device for raising the temperature of the air entering the carburetter to prevent icing.

Interceptor: A small plate on the upper surface of a slotted wing. It is situated immediately behind the slot and is used to counteract or "spoil" the extra lift when the aileron behind it goes up. The interceptor remains down when the aileron moves downwards and is automatically raised when the aileron moves upwards.

Interceptor Fighter: A specialised single-seat fighter of short duration with a minimum of equipment and relatively light armament. A high rate of climb and high maximum speed are characteristics of this class of fighter, the chief function of which is to make contact with enemy aircraft at short notice.

Intercooler: A device installed on the delivery side of a supercharger to cool either the compressed air or the fuel mixture as it passes to the cylinders.

Inter-cylinder Baffle: A plate fixed between the cylinders of an air cooled engine to direct the cooling air on to the cylinders.

Interference: The increase in drag and, in certain cases, the reduction in lift caused by the interference of the airflow round one body with that round an adjoining body.

Internal combustion engine: An engine in which the combustion of the fuel takes place within the cylinder.

All petrol engines are of the internal combustion type, whereas steam engines employ external combustion. The energy released in burning the fuel in the furnace of a steam plant is transmitted to the piston by a "working agent" - air - receives the energy from the fuel when already inside the cylinder and the transmission of power to the piston is accordingly more direct. This results in greater efficiency in the petrol engine because of smaller heat loss. The overall effective thermal efficiency of an aero engine is about 20 per cent. The same for a steam locomotive is about 8 per cent. The internal combustion and steam turbines have effective efficiency of the order of 65 per cent.

International Power: The b.h.p. an engine is rated to develop at full throttle and at the highest crankshaft rpm which may be maintained in climbing flight for periods exceeding 5min. This power is developed in Standard Atmosphere at the lowest height at which full throttle is permissible at that particular engine speed. In effect the International Power works out to be the maximum power available for continuous climb.

International Standard Atmosphere: An imaginary atmosphere used in comparing the performance of aircraft. It assumes at mean sea level a temperature of 15° C and a pressure of 1,013·2 millibars, and a fall in temperature of 6·5°C per 1,000m of increased height from sea level up to 11,000m, above which height the temperature is assumed constant at 56·5°C.

Interplane Interference: The interference between the airflows round the planes of a biplane. This is one of the serious drawbacks to the biplane arrangement. Interplane interference (sometimes termed "biplane effect") may be reduced by having the gap between the planes at least equal to the maximum chord and by the use of positive stagger, that is to say, by having the top main plane slightly in advance of the lower.

Interplane Strut: A structural member in a biplane, triplane or quadruplane which braces the mainplanes to each other. Interplane struts are usually of streamline sections and are attached to the planes at the spars.

Interrupter Gear: An obsolete gear designed to make possible the firing of a machine-gun or guns through the disc of a revolving propeller. The interrupter gear should not be confused with the synchronising gear; the former interrupts the firing of the gun when the blades of the propeller are in the way, the latter fires the gun when the blades are clear. The latter type of gear is still in use today, although it is going out of fashion as it prevents high rates of fire from being used. The guns of single engine fighters are today usually mounted in the wings to fire outside the propeller disc.

Anthony Fokker, the famous Dutch designer who worked for Germany during World War One, fitted the first interrupter gear in his E I monoplane fighter of 1915.

Invar: A nickel iron containing 36 per cent of nickel. It has an exceedingly low rate of expansion and has a high resistance to water corrosion.

Inversion: An increase of temperature with height through a given layer of air in place of the decrease which normally occurs.

Inverted Loop: An outside loop. One of the less common evolutions which come under the general heading of aerobatics. In an inverted loop the upper surfaces of the aeroplane are on the outside of the loop and the pilot must accordingly be firmly strapped in, as the tendency of centrifugal force is to catapult him out of his seat.

Inverted Engine: An engine with its cylinders below the crankshaft. In single engine aeroplanes the use of an inverted engine usually improves the pilot's view in a forward direction.

Inwales: The longitudinal members in a seaplane hull at the junction of the topsides and decking.

Iraqi Air Force: A service formed in 1931 in Iraq. Its equipment was of British and Italian manufacture and some of its aircrews were trained in this country.

Irreversible Control: A flying control so designed that, although it is easily moved by the pilot, it cannot be moved by the air forces acting on it. Controls are made irreversible as a precaution against flutter.

Isallobar: A line on a weather map drawn through points where similar changes in barometric pressure have occurred in a given period of time. Isallobars show regions of rising and falling pressure.

Isobar: A line on a weather map drawn through points of equal pressure reduced to mean sea level. Isobars show the pressure distribution at any given time.

Isotherm: A line on a weather map drawn through points of equal temperature. Isotherms are usually drawn to show average monthly or annual distribution of temperature over a given area.

Isothermal Layer: An obsolete term for the Stratosphere. Generally, any layer of the atmosphere in which there is no appreciable variation of temperature with height.

Izod Test: A test employed to determine particular characteristics of structural materials. The testing machine consists of a weighted pendulum which is arranged so as to strike and fracture a test piece of fixed dimensions after it has swung through a specified arc. The height swung by the pendulum after impact with the test piece provides a measure of the properties of the material under impact.

Japanning: The application of an enamel coat by any process for protective or decorative purposes.

Johnston Memorial Air Navigation Trophy: A silver plaque presented in memory of the late Squadron Leader E.L. Johnston, who lost his life in the disaster to the R101 airship in October 1930, and before the Second World War, awarded annually by the Guild of Air Pilots and Air Navigators for what is considered by the Guild to be the best feat of air navigation during the preceding year. The trophy was first awarded, in 1931, to Mr Francis Chichester.

Joystick: A colloquial term for Control Column, the primary flying control of an aeroplane or glider which is linked to the ailerons and elevators. It controls the aeroplane in the rolling and pitching planes.

Junk Ring: A ring for maintaining a gas-tight seal between the cylinder head and the bore of a sleeve-valve on an aero engine.

Jury Strut: A strut inserted to provide temporary support for a structure. A common example is the strut used to support the wing structure of an aeroplane when folded.

K

Katabatic Wind: A local wind produced by the downward motion of cold air off high ground. On clear nights in hilly country, when the air over the higher ground cools because of the radiation into space, it becomes heavier than the air farther from the surface and thus flows downhill on to lower ground. This flow is the Katabatic wind.

Kaus: South-easterly or easterly winds in the Persian Gulf which herald the approach of a winter depression. They are usually associated with wet and cloudy weather.

Keel: (1) A longitudinal centre member running the length of a hull or float. (2) An internal or external framework integral with the underside of a rigid airship and serving to distribute the concentrated loads along the hull. (3) A rigid or articulated framedwork in a semi-rigid airship serving the same purpose as (2), but in this instance transmitting the loads direct to the envelope.

False Keel: A member fitted under the main keel to deepen it or to protect the hull (or float) plating from damage when the seaplane is aground or drawn up on a slipway.

Keel Angle: The angle formed by the straight lines joining the central keel and the chines at any particular section of a hull or float.

Khamsin (pronounced Khumseen): A hot dry Southerly wind blowing over Egypt in front of depressions passing Eastward along the Mediterranean. This wind is similar to the Sirocco.

Kharif: An intermittent but severe dust storm that blows from May to September in Somaliland.

King-post: A compression member which, together with suitable bracing wires, limits the deflection of a major member (such as a spar) which has to resist bending loads. King-posts raised above the wings of early flying boats were used to reinforce the interplane bracing.

King's Cup Air Race : A handicap air race open only to British and Empire pilots flying British or Dominion aeroplanes, competed for annually up to the outbreak of World War Two. The Cup was originally presented by King George V and the race was first held in 1922. On that occasion it was won by the late Captain F.L. Barnard in a D.H.4A (RR Eagle VIII) at an average speed of 120 m.p.h.

Kinetic Pressure: The increase in pressure which arises when a stream of fluid meets an obstruction. Air has many of the properties of a fluid and the pedestrian has to overcome kinetic pressure in meeting a high wind. In the reverse sense, kinetic pressure is built up in front of an aeroplane by reason of its movement through relatively still air.

Kite: An aerodyne not mechanically driven and normally moored to the ground, or towed by a line. Kites are the oldest form of aircraft and have been made and flown by the Chinese from ancient times. Their aerodynamic principle is analogous to that of an aeroplane. At the beginning of the 20th Century experiments were made with man-lifting kites for military purposes. Since then kites have been used for meteorological purposes and, in special circumstances, in place of barrage balloons.

Kite Balloon: A captive balloon so shaped as to derive stablility and aerodynamic lift from the relative wind. There are two types of kite balloon:

(1) The Observation Balloon: This is provided with a basket for one or more observers who are connected by telephone with the ground and can thus signal the results of artillery fire to the battery concerned and perform other types of observation. The observation balloon was much used during World War One. Captive balloons are used for training parachute troops.

(2) The Barrage Balloon: Balloon barrages were first introduced during World War One and were employed on a large scale in World War Two as a deterrent to dive bombers. They were used to protect important land areas and merchant ships in convoy.

Knot: A nautical unit of speed being equal to one nautical mile (6,080ft) per hour. To convert knots to miles per hour multiply by 1·15.

L

LAC: Leading Aircraftman - an RAF rank between Aircraftman 1st Class and Corporal, indicated by an "airscrew" badge on the arms of the tunic.

Lagging: The extent to which a rotor of a gyroplane in flight is displaced rearwards by air resistance from its normal position relative to the rotor head. The amount of lag is limited by the action of the drag pivot, a damping device which gives the rotor flexibility in the plane of rotation.

Laminar Flow: Airflow free of turbulence, that is to say, free of any eddying motion - a smooth stream.

Laminar Flow Wing: An aeroplane wing of thin section designed so that turbulence is reduced to a minimum. The Davies wing is an example.

Land and Sea Breezes: Offshore and onshore winds which occur up to about 1,000ft at night and in the daytime respectively. They are caused by the more rapid heating and cooling of the air over land than over water. Thus during the day when the land heated by the sun becomes hotter than the sea, the air above is warmed and expands. When air expands, its pressure falls and thus a region of lower pressure is created over land. The air over the sea has not heated to such an extent and has therefore a higher pressure. Because of these conditions there is a wind from sea to land - this is known as a "sea breeze." At night the above conditions are reversed, the land tending to become cooler than the sea. This creates a "land breeze."

Landing: The act of bringing an aeroplane under full control into contact with the ground.

Landing Area: An area of smooth and level land or of sheltered water providing sufficient space for the take-off and alighting of an aeroplane.

Landing Flare: A pyrotechnic flare attached to the underside of an aeroplane which, when lighted by the pilot, provides a bright illumination of the ground beneath, thus facilitating landing at night.

Landing Ground: A selected piece of ground suitable for the landing of aeroplanes, but without the facilities of an aerodrome.

Landing Light: A lamp (or lamps) usually installed in the leading edge of the mainplane of modern aeroplanes for use in illuminating the surface on which the aeroplane is alighting.

Landing Run: The distance travelled over the ground by an aerodyne after landing. Thus it is the distance between the first point of contact with the ground and the point at which the aeroplane comes to rest.

Landing Speed: That speed at which an aeroplane with a normal type two-wheel undercarriage stalls when making contact with the ground in the course of a "three-point landing." Aeroplanes with tricycle undercarriages do not have an exact landing speed, but are flown on to the ground at some speed within a specified range and are "pulled up" by wheel brakes.

Landing "T". A large symbol shaped like a letter T which is the standard indicator used on landing areas to show the direction of the wind. The cross stroke of the T is up wind. Landing T indicators may be illuminated for use at night.

Landplane: An aeroplane with a wheel undercarriage designed to operate from an aerodrome or aircraft carrier.

Laps: Defects in steel caused by splashing of the molten metal when pouring it into the ingot mould. They appear as a crack on the surface of the metal after rolling.

Lapse Rate: The rate at which temperature decreases with height. The average lapse rate in the troposphere is 3°F per 1,000ft, but the actual rate varies a good deal between the two extreme conditions of dry and saturated air.

Lateral Axis: An imaginary line passing through the centre of gravity and parallel to a line joining the wing tips. Movement about the lateral axis is termed "pitching."

Lateral Clinometer: An instrument used to indicate the direction of the resultant force, or apparent direction of gravity, in a transverse plane. Sometimes called the Cross-level.

Lateral Divergence: A combination of rolling, yawing and side-slipping of an aeroplane which precedes a spin or spiral descent with increasing rate of turn.

Lateral Stability: That quality which tends to correct any disturbance out of the plane of symmetry (that is in side-slipping or rolling). An aeroplane is laterally stable when its wings are not easily tilted sideways by external forces.

Lay Off: A term used in hull and float construction. It means to transfer the design of the hull or float lines from scale drawings to full size on the mould loft floor.

Leading: The extent to which a rotor blade of a gyroplane is displaced forward from its normal position relative to the rotor head.

Leading Edge: (1) The forward edge of a streamline body or aerofoil. (2) The structural member at the leading edge of such a body or aerofoil.

Leading-edge Radiator: A radiator built into the leading edge of a mainplane.

Leading Sweep: As applied to a propeller blade, a general curvature of the blade towards the leading edge.

Leakage: The loss of gas from the envelope or gas bags of an airship or balloon.

Leakage Drag: The drag caused by the leaks in the structure of an aeroplane. This only becomes important at high speeds. In modern fast aeroplanes leakage drag is an important component of the total drag. Experiments, chiefly carried out by Wing Commander Sidney Cotton, showed that a worthwhile increase in maximum speeds of existing aeroplanes can be achieved by carefully sealing all leaks in the structure.

Leak Detector: An instrument, used in airships, which detects the presence of hydrogen and other gases in the air. It is used to find leaks in an envelope or gas bag.

Leste: (pronounced Lestey): A hot, dry, southerly wind in Madeira and Northern Africa in front of an advancing depression.

Levanter: An easterly wind experienced in the Straits of Gibraltar. This wind is stronger in winter than in summer. When of moderate force it causes a cloud to appear over the Rock. This phenomenon is known as the "Levant."

Leveche (pronounced Levetchey): A hot, dry south-west wind in Spain which occurs in front of an advancing depression.

Life Line: A feature of certain designs of parachute. The flexible connection between the rigging lines and the harness when these are not directly connected.

Lift: (1) The component in a vertical, upward direction in straight and level flight of the resultant force created by the relative wind acting on the lifting surfaces of an aeroplane. (2) The total buoyancy of an airship.

Lift Axis: An arbitrary line through the centre of gravity of an aeroplane, which is vertical in straight and level flight.

Lift Wires: Wires or cables the principal function of which is to transfer the lift of non-cantilever wings to the main structure.

Light Aeroplane: An aeroplane having a total all-up weight of less than 1,200lb.

Limit of Proportionality: In effect this is the elastic limit of a material. The load applied is proportional to the extension it produces up to this limit. Beyond this point the material stretches far more rapidly for a given increase of load, and when the load is removed the material remains partly stretched.

Line Squall: A squall advancing on a wide front caused by the replacement of a warmer by a cooler body of air. The passage of a well-developed line squall is usually marked by a sudden or very rapid change of wind direction; heavy rain, hail or snow, thunder and lightning; rapid rise in barometric pressure; a sudden fall of temperature; and violent vertical disturbances. A frequent characteristic is a long arch or line of low black cloud.

Link Trainer: A device which was widely used for ground instruction in instrument flying. It resembles a small aeroplane built round an exact replica of an aeroplane's cockpit. The cockpit is provided with a hood so that "blind" conditions can be reproduced and is fully equipped with all the normal controls and instruments of an aeroplane. These are ingeniously linked so that actual flying conditions are reproduced when the student pilot sits inside the trainer and "flies" it under the hood.

Load Factor: The ratio of the load at which a member or structure fails to the assumed working load under specified conditions of flight. The load factor of an aeroplane, when no conditions are specified, is that existing in flight with the centre of pressure in its most forward position. Load factor should not be confused with Factor of Safety, which is the ratio of the failing load to the assumed maximum load.

Load Ring: A ring on which the basket suspensions and the net of a free balloon are secured.

Load Water Line: A horizontal line on a seaplane hull or float, marking the level of the water when in the fully loaded condition.

Loadings. (1) Power loading. The loaded weight of an aeroplane divided by the total power of the engine(s). (2) Span loading. The loaded weight of an aeroplane or glider divided by the square of the wing span. (3) Surface loading. The mean force on unit area carried by an aerofoil under specified aerodynamic conditions. (4) Wing loading. The loaded weight of an aeroplane or glider divided by the net wing area.

Longeron: A main longitudinal strength member of a fuselage or nacelle.

Longitudinal Axis: A line fore and aft through the centre of gravity of an aeroplane and parallel to the line of flight.

Longitudinal Divergence: The pitching motion of an aeroplane which precedes a dive or a stall.

Longitudinal Stability: That quality which tends to correct any disturbance in the plane of symmetry (that is, in pitching). An aeroplane is longitudinally stable when it resists any external force tending to raise or lower the nose.

Loop: A manoeuvre consisting of a complete revolution in flight in a vertical plane. The upper surface of the aeroplane or glider performing the manoeuvre is on the inside of the circle.

Loop Aerial: A radio aerial consisting of one or more turns of a conductor, either self-supporting or in a protective covering. Loop aerials are mounted on the fuselage and may be of three types. (1) The fixed-loop aerial. A loop aerial permanently fixed with respect to the centre line of an aeroplane and used with a homing receiver. (2) Rotatable-loop aerial. A loop aerial used for direction finding. It can be rotated in a horizontal plane. (3) Streamline-loop aerial. A rotatable loop aerial enclosed in a streamline fairing of insulating material.

Louvre: A movable vane for controlling an opening for the flow of cooling air.

Low: A meteorological term used to denote a region of low pressure, or depression.

Low Clouds: Clouds which have an average height below 7,000ft. They are:
(a) Nimbo-stratus. A low uniform layer of dark grey cloud.
(b) Fracto-nimbus. Low, dark, ragged clouds associated with bad weather.
(c) Strato-cumulus. A layer or bank of cloud composed of large masses showing some vertical structure and following one or two general directions.
(d) Stratus. A uniform layer of cloud resembling fog, but not resting on the ground.
(e) Fracto-stratus. A layer of cloud broken up into irregular, ragged fragments.

Lubber Line: A line fixed to the forward end of a compass bowl. It corresponds to the nose of an aeroplane and enables the pilot to determine the direction in which the aeroplane is heading at any time.

M

Maestro: A North-Westerly wind experienced in the Adriatic. It occurs most frequently in Winter and Spring; when it blows in the Summer it is followed by fine weather.

Magnetic Course: The angle (measured in a clockwise direction) between the course of an aeroplane - that is, the direction steered - and Magnetic North. It is measured on a map from the nearest magnetic meridian.

Magnetic Track-angle: The angle (measured in a clockwise direction) between the track of an aeroplane - that is, its path over the ground - and Magnetic North. It is measured on a map from the nearest magnetic meridian.

Main Float: The principal watertight body of a floatplane which provides the necessary buoyancy on the water.

Mainplane: A complete main lifting surface of an aeroplane or glider.

Main Step: A discontinuity in the under surface of a float or hull designed to help the seaplane "unstick" from the water.

Malleability: The property which allows a material to be permanently deformed, without fracture, when beaten or rolled.

Maneton: The detachable short end of the crankshaft in a rotary or radial engine.

Manoeuvrability: The ability of an aeroplane to change its attitude under the influence of its controls. Manoeuvrability and control are two conflicting requirements. The former is particularly desirable in fighters in which it can be developed at the expense of the stability. In bombers and transport stability is more important, although in some such machines a high degree of manoeuvrability is also obtained.

Maps: Four main types of air map were recognised internationally before the Second World War: (1) Basic Maps. These cover the whole World in 24 sheets and are intended for the planning of routes and to give general aeronautical information. (2) Route Maps. Drawn to a small scale, these are intended for navigational use on long journeys. (3) Local Maps: The normal one to one-million scale. International Maps of the World modified to meet the needs of air navigation. These are used for ordinary navigation work. (4) General Maps: Maps drawn to Mercator's projection to a scale in which one degree of longitude equals three centimetres. This type of map has special applications.

Marine Distress Signal: A pyrotechnic device for use by aircraft in distress after landing on water.

Marshal of the Royal Air Force: The highest rank in the RAF, equivalent to an Admiral of the Fleet in the Navy and a Field Marshal in the Army.

Martensite: A hard, brittle, constituent of carbon steel. It is formed when the steel is cooled rapidly by quenching.

Mask Microphone: A microphone mounted in a face mask used for radio and intercommunication transmission in flight.

Master Connecting Rod: In a radial aero engine the large connecting rod linking one of the pistons to the crankshaft. Articulated rods are connected to the big end of the master rod and transmit the power from the other pistons.

Master Pilot's Certificate: The highest grade of commercial pilot's certificate. This certificate is awarded to "B" license pilots who have at least five years' experience and have satisfied other stringent requirements.

Maximum Permissible r.p.m.: The number of revolutions per minute of the crankshaft of an engine which must not be exceeded or maintained beyond a specified time - usually five minutes.

Maximum Power: The power developed by an aero engine at maximum permissible r.p.m. (see above)

Maximum-Power Altitude: The lowest height at which full throttle is permissible at maximum rpm for level flight. With a supercharged engine it is also the greatest height at which maximum boost pressure can be maintained in continuous climbing flight.

McKee Trans-Canada Trophy: A trophy awarded annually to the Canadian pilot who does most towards the sound advancement of Canadian Aviation. It was presented in 1926 by the late J Dalzell McKee, who made the first aeroplane flight across Canada.

Mean Chord: The total area of an aerofoil divided by the span.

Medium Clouds: Average height between 8,000ft and 20,000ft. They are: (a) Alto-cumulus. A layer of small masses of cloud arranged in groups, lines or rolls. (b) Alto-stratus. A grey fibrous cloud resembling a veil, through which the sun or moon shows vaguely without the halo effect which occurs with cirro-stratus cloud.

Mesh Wiring: A wire network which prevents the gas bags of a rigid airship chafing against the longitudinals of the hull structure.

Meteorograph: An instrument which records two or more of the basic meteorological conditions of pressure, temperature or humidity.

Mid-Wing Monoplane: A monoplane in which the mainplane is located approximately midway between the top and bottom of the fuselage.

Millibar: The thousandth part of a bar, a bar being a unit of barometric pressure equal to 750·1mm (29·531in) of mercury at 0°C in latitude 45°.

Maximum Flying Speed: The lowest speed at which an aeroplane or glider can be maintained in level flight.

Mist: a slight fog caused by condensation of moisture.

Mistral: A cold, dry wind from the North or North-West, common on the Mediterranean coast of France.

Mixture Control: A device embodied in the carburettor of an aero engine for reducing the quantity of fuel supplied to the cylinders as height increases so as to maintain a constant ratio of fuel to air.

Monocoque: A method of construction in which the skin carries the whole or the greater part of the main loads; applied particularly to fuselages.

Monoplane: An aeroplane or glider with a single mainplane.

Monsoon: By definition: winds which in certain regions blow in opposite directions at different seasons of the year. In Eastern and Southern Asia the term is commonly used to denote seasonal rains without reference to wind.

Mooring Area: An area of water reserved as a place for the mooring of seaplanes.

Mooring Mast: A relatively short mast to which airships are moored by a quick-release attachment in the nose. Great Britain originated the mooring mast and developed it into the mooring tower. The Americans use a mobile motor-driven form of mast for all types of airship. This has the advantage that it can be used for manoeuvring the airship while on the ground.

Mooring Tower: An elaborate high mast for mooring rigid airships. Mooring towers provide a means of entry to the airship from a platform near the top across a gangway let down from the airship. A lift runs from the foot of the tower to the platform, as do fuel and water ballast pipes.

Mortar Signal: A signal flare which after reaching a height of about 1,000ft burns steadily for part of its descent.

Mountain Breeze: A local wind caused by the downward flow of cold air off high ground at night or in Winter.

Multiplane: An aeroplane or glider with two or more mainplanes one above the other. There have been several quadruplanes and many triplanes.

N

Nacelle: A body accommodating crew, load or engines of an aeroplane, glider or airship.

Navigation (Avigation): The science of determining and plotting the position of an aircraft and of determining the courses to steer to reach any required destination.

Navigation Flame-Float: A pyrotechnic device dropped from an aeroplane on to water for determining the drift-angle by night.

Navigation Flare: A pyrotechnic device dropped from an aeroplane on to land for determining the drift-angle by night.

Navigation Light: A light on an aeroplane to reveal its presence and direction of motion at night. A complete set of navigation lights comprise a red light on the port (left) wingtip, a green light on the starboard (right) wingtip and a white light at the tail.

Neck: A tube at the base of a balloon through which the gas is passed during inflation and through which it can escape when it expands.

Nimbo-stratus: A low, rainy layer of cloud, grey in colour.

Nitralloy: A steel that can be surface-hardened by the Nitriding process.

No-Lift Direction: That direction of the airflow over an aerofoil in which no lift is generated.

Non-Induced Drag: The total drag of an aeroplane or glider less the drag which results from the lift of the wings. More often called the parasitic drag.

Non-Rigid Airship: An airship without a rigid keel and depending on the pressure of the gas within the envelope to retain its shape.

Normal Axis: A line passing through the centre of gravity of an aerodyne which is vertical in straight and level flight.

Normal Flight: All manœuvres needed in ordinary flying, such as level climbing and gliding flight, turns and side-slipping, but excluding aerobatics.

Normalising: The object of normalising is to relieve the stresses and refine the crystal structure after it has been coarsened by previous hot working.

Normally Aspirated Engine: An aero-engine the cylinder charge of which is normally taken in at the existing atmospheric pressure and temperature.

Norte (pronounced "nortey"): A cold, dry wind from the north over the Gulf of Mexico.

Nor' Wester: A disturbance of the line-squall type associated with thunderstorms and experienced in Bengal and Assam.

Nose Cap: A hub fairing fitted to the front of the boss of a fixed-pitch one-piece propeller.

Nose-Dive: A steep descent with the nose of the aeroplane down. More correctly, simply "dive".

Nose-heaviness: The tendency of an aeroplane to lower its nose of its own accord when flying.

Nose Radiator: An aero-engine radiator installed in the nose of a fuselage or nacelle.

Nose Rib: A rib between the front spar and leading edge of a main plane, tailplane or control surface.

Nose Slot Cowling: An aero-engine cowling in which the cooling air is discharged in the low-pressure area near the nose.

NPL: The National Physical Laboratory - an official centre of scientific research in Great Britain

Nurse Balloon: A fabric container used either as a gas reservoir or to maintain a constant pressure in an aerostat's envelope on the ground.

O

Observation Mirror: An engraved mirror used for bombing training in the same way as a camera obscura. The bomber flies over the mirror, which is suitably mounted on the ground. A light is flashed in the aeroplane at the release point of the bomb and this is plotted on the mirror. In this way the accuracy of the "bombing" can be determined.

Obstruction Angle: The angle between the horizontal and a line joining the highest obstruction in the approaches to an aerodrome to the nearest point of the appropriate runway.

Obstruction Light: A light indicating the presence of an object which is dangerous to an aircraft in flight.

Obturator Ring: An L-section ring fitted round a piston to maintain a gas-tight seal between the piston and the cylinder wall.

Occlusion: A meteorological term for the "front" (that is, junction between masses of warm and cold air) which remains after a cold front has met a warm front and the latter has been forced upwards.

Occulting Light: A flashing light in which the light period is distinctly longer than the dark period.

Octane Number: The quality of a fuel to produce power in a high-performance aero-engine without pre-ignition through high compression. The higher the octane number the greater the power that can be taken from the fuel, provided that the engine is designed to take advantage of it. The present highest octane number in use is 100. In peacetime ordinary motor fuel was about 67 octane. (NB: Contemporary reference circa 1940)

Omni-directional Beacon: A ground radio transmitter which is non-directional and is used for navigational purposes by aeroplanes with direction finders. The aeroplane's radio operator recognises the beacon from its code signal and then finds its bearing from the aeroplane with his directional receiver.

Open-jet Wind Tunnel: A wind tunnel in which the air stream passing the body under test is not enclosed, but is in the form of a free jet rather like water squirting from a hose.

Opposed-piston Engine: An engine in which the compression is effected by two pistons in the same cylinder acting in opposition. The Junkers Jumo 205 C.1. aero-engine works on this principle.

Optimum Angle: (1) The gliding angle at which the least height is lost in proportion to the ground covered. (2) The angle of attack of an aerofoil at which the ratio of lift to drag is greatest.

Ornithopter: A mechanically driven aerodyne whose lift in flight is obtained by the action of flapping wings.

Orographic Rain: Rain caused by rising ground in the path of moisture-laden wind.

Oswald Watt Gold Plaque: A gold medal presented by the late Colonel W. Oswald Watt for yearly award to the airman who performed the most outstanding performance during the year in the Commonwealth of Australia, or to the Australian-born airman who performs the most brilliant feat outside Australia. The plaque was first awarded in 1921 to F.S. Briggs.

Outer Area: A circular zone round an airport within which the obstructions must not be above a certain height.

Outer Cover: The external covering of the hull of a rigid airship.

Outer Marker-beacon: A radio beacon comprising part of a blind approach installation which defines a predetermined distance from the aerodrome boundary. As the aeroplane passes over the beacon in its approach its position is signalled to the pilot by the sound of a buzzer or the flash of a lamp on the instrument panel.

Outflow: The increase in air velocity behind an airscrew produced by its rotation.

Overhang: (1) The extent to which the wingtip of one or two superimposed planes projects beyond the tip of the other. (2) The distance from the outer point of support to the tip of an aerofoil.

Overstrain: The condition of a structural material which has been stressed past the yield point. Materials can be annealed - given a form of heat treatment - to remove the effects of overstrain.

Oxidation: The action of combining any substance with oxygen. Oxidation may be rapid, as in an explosion; of moderate speed, as in the burning of solid fuels; or slow, as in the rusting of metals.

Oxygen Equipment: Oxygen equipment is provided in all modern Service aircraft. The installation includes cylinders containing oxygen under pressure, regulators to control the supply pressure, flow meters and a breathing mask for each member of the crew.

Oxygen-mask Microphone: A microphone for radio transmission and intercommunication between aircrew members fitted to an oxygen breathing mask

P/Q

Pack cover: A canvas covering which accommodates a parachute when folded.

Packing: The operation of folding a parachute and inserting it within the pack cover ready for use. Parachute packing is done by qualified specialists.

Pampero (pronounced "pampearaw"): Line squalls experienced in Argentina and Uruguay which are accompanied by rain, thunder and lightning, and a sudden change of wind to the South-West.

Pancaking: Landing an aeroplane in a stalled condition so that it has an abnormally high rate of descent or an abnormally low forward speed.

Panel: A section of a parachute canopy or gasbag.

Parachute: A collapsible device used to retard the descent of a falling body. Parachutes, which comprise a pack, a canopy, shroud lines and harness, may be of two basic types:-

(a) The Free Parachute: This type is used by aircrews and is a purely emergency device, although it was used for exhibition purposes before the Second World War. Free parachutes are operated by the wearer after he has left the aircraft.

(b) The Automatic Parachute: This type, known as the "Statichute" in the RAF, is used by parachute troops, and for dropping supplies. It is operated by a "static line" attached to the aeroplane which pulls the canopy out of the pack as the body falls clear of the aeroplane. Drops from very low levels are possible with this type of parachute.

The first successful parachute descent was made by André-Jacques Garnerin, who descended from a balloon over Paris on October 22, 1797.

Parachute Flare: A pyrotechnic flare attached to a parachute, which can be dropped from an aeroplane to illuminate the ground beneath it.

Parachute Troops: The parachute soldier was much in the news during the Second World War and with other types of airborne troops - carried in both aeroplanes and towed gliders - was considered to be of growing importance in the future. The Russians started training parachute troops in 1934, and were later copied by the Germans. British parachute units were formed in 1940 and were incorporated in the Army Air Corps on its formation on February 27, 1942.

Parachutist: A person using a parachute.

Parasitic Drag: The total drag of an aeroplane or glider less the drag which results from the lift of the wings. Sometimes called "non-induced drag."

Parasol Monoplane: A monoplane with the mainplane mounted above the fuselage on struts.

Parkerising: A process whereby a protective coating is applied to iron and steel parts to prevent rusting and corrosion.

Partially Evaporative Cooling: A cooling system for aero-engines in which the coolant (cooling liquid) is allowed to boil during part of its passage round the cooling circuit.

Patch: A piece of fabric sewn to the envelope of a balloon or non-rigid airship, to which the rigging lines are attached.

Payload: That part of the useful load of a civil aeroplane which can be devoted to paying load; that is, the weight of passengers, mails and freight that may be carried.

Performance: The essential flying characteristics of aircraft under standard atmospheric conditions. The figures obtained under any existing conditions are corrected so as to apply to a standardised theoretical state of the atmosphere.

Petrol: A trade name for the spirit consisting of a mixture of volatile fractions of the paraffin, napthene and aromatic series of hydrocarbons used in internal-combustion engines.

Phillips Entry: The form of dipping leading edge to a wing section which was patented in 1884 by Horatio Phillips, one of the British pioneers of heavier-than-air flight.

Pilot: The individual who operates the flying controls of aircraft.

Pilotage: The art of piloting aircraft safely and accurately from one place to another by means of map-reading and the recognition of ground objects.

Pilot Balloon: A small free balloon used to determine the direction and speed of the wind at various heights. The position of the balloon after given intervals of time is observed with a special type of theodolite. From the figures obtained in this way the speed of the wind at successive heights can be calculated.

Pilot Officer: A junior commissioned rank in the RAF corresponding to a Midshipman in the Navy and a Second Lieutenant in the Army.

Pilot Parachute: A small parachute connected to the main canopy of many types of free parachute to ensure its rapid and correct withdrawal from the pack cover.

Piston: A cup-shaped member of circular cross-section which slides up and down in the cylinder of an engine. It absorbs the expansion force of the fuel in the cylinder and transmits it via the connecting rod to the crankshaft.

Piston Ring: A ring inset into the piston of an engine intended to preserve a gas-tight seal between the piston and the cylinder wall.

Pitch: (i) The geometric pitch of a propeller is the distance that propeller would advance in one revolution provided there was no slip.

(ii) The angle by which the nose of an aircraft is inclined up or down from the horizontal.

Pitch Indicator: An instrument for indicating angle of pitch of an aeroplane in flight. Sometimes called a fore-and-aft level.

Pitching: The rotary motion of an aircraft about its lateral axis. That is to say, a motion in which its nose tends to rise or fall.

Pithio: A high carbon steel used for valve ends and similar parts of an aero-engine.

Pitot Tube: Parts of an airspeed indicator consisting of a tube with an open end exposed to the airstream. The compressed air in the pitot tube is conveyed by a length of tubing to the pilot's cockpit. There the pressure is shown by the airspeed indicator, which gives a reading in terms of miles an hour airspeed.

Plane: The complete wing of an aeroplane or glider. The term "plane" should not be used for "aeroplane."

Planing Bottom: A smooth surface on the underside of the hull of a flying-boat or of a float lying forward of the main step.

Polar Front: A meteorological term for the line of discontinuity which occurs under certain conditions between a mass of air coming from polar regions and another mass from more temperate zones.

Porpoising: The undulating motion in a fore-and-aft direction of a seaplane when taxying. It is a sign of bad hull design and was a common characteristic of early seaplanes.

Port: The left side (looking forward) of aircraft and ships. The port navigation light is red in colour.

Port Drift-angle: A navigational term for the angle between the true course and the true track when the bearing of the former is greater than that of the latter.

Position Error: The reading of an airspeed indicator differs from the true airspeed because of a number of errors. The error arising from the mounting of the pitot tube in a position where the airflow is disturbed by its proximity to the aeroplane is termed the "position error."

Pour le Mérite: An obsolete German decoration for military bravery which at one time ranked highest and was approximately equivalent to our own Victoria Cross, although it was more freely awarded. Most of the outstanding German fighter pilots of the First World War gained the Pour le Mérite. Its French name was derived from the fact that it was instituted by Frederick the Great, the founder of the German Empire, who spoke only French. It is better known today as the Blue Max.

Power: The time-rate of expended energy when work is done.

Power Jet: (i) A fuel jet in the carburettor of an aero-engine which comes into operation when the throttle is opened beyond the setting for maximum cruising conditions. (ii) A device for propulsion of aircraft as used by the Caproni-Campini C.C.2 jet-propulsion aeroplane.

Power Loading: The ratio of maximum loaded weight of an aeroplane to its total maximum power. That is to say, Power loading = loaded weight total power.

The power loadings of modern aeroplanes vary from about 5lb per h.p. for fighters to about 12lb per h.p. for heavy bombers. Power loading chiefly affects the take-off distance, rate of climb and stalling-speed with engine on.

Precipitation: A general meteorological term for the forms in which water may fall after condensation from the atmosphere. Precipitation may take the form of rain, drizzle, snow or sleet.

Precipitation Interference: Disturbance in radio reception caused by the impact of rain, snow, hail or sleet on the aerials of an aircraft in flight.

Pre-ignition: The ignition of the charge in the cylinder of an internal-combustion engine before it is fired by the sparking plug(s). Pre-ignition is caused by some overheated part, such as the sparking plug or exhaust valve or by incandescent carbon.

Pressure Gradient: The rate of change of pressure in an imaginary horizontal plane running between areas of high and low pressure on a weather map.

Pressure Head: A combination of pitot and static pressure tubes forming part of airspeed indicator installation.

Primer: A device for spraying fuel into the induction system or combustion chambers of an aero-engine to facilitate starting.

Profile Drag: The drag (that is to say total head resistance) of the wings of an aeroplane or glider excluding that caused by the vortices at the wing tips. In other words, profile drag is the sum of the drag caused by surface friction and the pressure drag.

Profile Drag Power Loss: The power expended in overcoming the profile drag of the blades of a propeller.

Projection: Geographically, a method whereby the outlines of features on the surface of the world may be represented on a map. As the world is spherical whereas the map is flat, the outlines on maps are only approximations to the true outlines on the world itself. Well-known projections include Mercator's, Cassini's Gnomonic, and Gauss Conformal.

Proofing: The treatment of fabric to render it gas-tight or water-resisting.

Proof Stress: The load for a given cross sectional area under which a material must not stretch permanently beyond a specified percentage of its length.

Propaganda Balloon: A small free balloon, sent up without passengers but with a device by which propaganda leaflets may be dropped at intervals. Propaganda balloons were suggested in the mid-19th Century and were used is the First World War. They were also employed to a small extent by the French during the early months of the Second World War.

Propeller: A device commonly used to convert the power of an aircraft piston or turboprop engine into propulsive force; each blade is essentially a rotating wing, twisted and varying in section from root to tip to achieve maximum efficiency, which produces lift in a forward direction, i.e. thrust. In the 1940s propellers were often referred to as airscrews.

Prototype: The first trial aeroplane built to a new design.

psa: Graduate of the RAF Staff College.

Pterodactyl: The name of a series of tailless aeroplanes designed by Captain G.T.R. Hill and built by Westland Aircraft Ltd. Captain Hill was aided in the design work by Major J.W. Dunne, the famous pioneer of this type of aeroplane.

Purity: Of the gas in an aerostat, the ratio of the volume of hydrogen or helium (depending on which is used) in the envelope to the total volume of gas contained.

Pusher Aeroplane: An aeroplane with pusher engine(s).

Pusher Engine: An aero-engine which drives a pusher propeller, that is to say, which propels the aeroplane forward by means of a propeller situated aft of the wing.

Pylon: The support or mast for the rotors of a gyroplane or helicopter. Pylons are also occasionally used to brace, from above, the wings of a wire-braced monoplane or to protect the pilot from injury in a crash should the aeroplane overturn.

Q

Qibli (pronounced "kibly"): A hot, dry, southerly wind experienced in Tripoli.

Quadruplane: An aeroplane with four mainplanes mounted one above another

R

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.

S

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.

T

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.

U

Undercarriage: The main alighting gear of an aeroplane or floatplane. Modern undercarriages are usually retractable to reduce drag.

Under Way: A nautical term adapted to aviation to indicate that an aircraft is gathering or losing momentum immediately before or after flight.

Unstick: The separation of a seaplane from the water at the end of its take-off run. Sometimes applied to the take-off of a landplane.

Useful Load: The total weight of the crew, fuel, oil and payload of an aeroplane. A preferable term is "disposable load".

V

Valve: A mechanism which may be opened or closed to permit or restrict the passage of a fluid or gas through an orifice.

Variable Datum Boost Control: An automatic boost control which varies progressively with the opening or closing of the throttle of an aero-engine.

Variable-Pitch Propeller: A propeller the angle of whose blades may be varied while it is in rotation.

Vee Engine: An aero-engine with two banks of cylinders arranged in the form of a "V," either upright or inverted.

Veering: A clockwise change of wind.

Velocity: Rate of motion along a defined path.

Vent: The central hole in the canopy of a parachute which promotes stability through the escape of superfluous air pressure.

Venturi Tube: A tube made with a gradually reduced cross-section about its centre which speeds up fluid flow through the tube at that point and so produces suction.

Vertical Axis: Is the normal upright axis when the longitudinal and lateral axes of an aeroplane are horizontal.

Viscosity: The stiffness of a fluid, or the converse of fluidity.

Visibility: The distance at which objects may be clearly seen. Fog and haze produce poor visibility.

Volume (lighter-than-air craft): The capacity or displacement of the envelope of an airship or balloon.

Vortex: A fluid in rotational motion, such as a whirlpool.

W

Wake: That part of a fluid, such as air, in which the pressure head has been changed by the passage through it of a body, such as an aerofoil.

Warm Front: A body of warm air advancing over a mass of cold air. Being forced upwards, it expands. Its moisture tends to condense in the lower temperatures at height, clouds form and rain is likely to fall.

Warning: Advance notice of the approach of a rapid change in meteorological conditions.

Wash-in: An increase of the angle of incidence of a wing towards the tips.

Wash-out: A decrease of the angle of incidence of a wing towards the tips designed to delay tip stalling.

Water Lines: The lines of a flying-boat, such as keelson and chines, which determine its behaviour when water-borne.

Water Spout: The equivalent of a tornado over water. It is visible because of condensation produced by the lowering of pressure within the core.

Water Tanks: Special model testing tank for ascertaining in advance the probable behaviour of flying-boat hulls when water-borne.

Weather: The state of the atmosphere with reference to climatic conditions.

Wedge: An area of high pressure radiating from an anticyclone, which when plotted as isobars appears as a wedge.

Weight: The relative mass of a body or the relative reaction on a body caused by some attractive force such as gravity.

Wetted Area: In aerodynamics, the total area of an aeroplane or airship exposed to airflow.

Whirlwind: A small region of air revolving rapidly round a low-pressure core and acting for some hundreds of feet up from the ground.

Wind: Air in motion in the atmosphere.

Windmill: A term at one time used to describe the action of a propeller when revolving freely under the forward airspeed of an aeroplane but without engine power. Also a colloquial term for the rotor of a gyroplane.

Wind Cone: An indicator of wind strength and direction erected on an aerodrome. It consists of an open-ended fabric sleeve attached to the top of a mast. Also called windsock.

Wind "T": A horizontal T-shaped marker placed on the surface of an aerodrome to indicate wind direction to the pilots of incoming aeroplanes.

Wind Tunnel: A chamber of tunnel shape in which experiments are made to collect aerodynamic data. A steady stream of air is blown through the tunnel by a large fan at a predetermined speed. This airflow reacts on aerofoil sections or aeroplane models mounted on sensitive balances which measure the force of reaction. Corrections have to be made for scale effect to facilitate the application of the data to the design of aeroplanes.

Wing: The main lifting surface of an aeroplane.

Wing Section: The form of the cross-section of an aerofoil which determines its aerodynamic characteristics, in particular the lift and drag coefficients and the lift-drag ratio.

Wing Plan: The shape of the wing or wings of an aeroplane as seen from directly above or below.

Wing Tip: The outboard end of a wing. The shape of wing tips is important aerodynamically and in aircraft recognition.

Wing Tip Float: A buoyant body of streamline shape attached to the wing tips of a flying-boat or single float seaplane to give it lateral stability when waterborne.

Wireless: Nowadays called radio, wireless apparatus is installed in aircraft for sending or receiving signals (through the ether waves) to and from other aircraft or ground stations.

Wiring: A term applied to the electrical circuits of an aeroplane. Electrical apparatus on aircraft is now so complex that servicing is only possible by the use of intricate wiring diagrams and wires of diverse colours on different circuits.

Wood Construction: Wood was mostly used to construct early aeroplanes. Since the early 1920s, metal construction has largely replaced wood construction for aircraft.

W/T: An abbreviation for wireless telegraphy, or radio communication by dots and dashes, as contrasted with wirelessed speech or R/T (radio telephony).

X/Y/Z

"X" Engine: An aero-engine with four rows of cylinders forming in end view the letter "X."

Yawing: An unstable side-to-side motion, about the vertical axis of an aeroplane, which may be described as rotary or angular.

Yield Point: The point where material gives way to excessive strain (such as elongation) without any further stress being applied.

Zephyr: A light warm westerly wind in the Mediterranean.

Zoning: The delimitation of areas urrounding an aerodrome to permit freedom of flight for aircraft approaching or leaving.

Zooming: Utilising excessive forward or downward motion of an aeroplane to gain height suddenly by quickly raising the elevators