I/J

This glossary of historic aviation terms is reproduced from 'The Spotters Glossary' which appeared in 'The Aeroplane Spotter' from January 1940 onwards. 'The Aeroplane Spotter' was the first journal devoted exclusively to the study and practice of aircraft recognition.

We are pleased to have been able to reproduce the entire glossary from A-Z, offering a unique insight into aviation terminology (and therefore aviation history and technology) prior to and during the 1940s.

NB: In the interests of clarity we have substituted current equivalents where words and phrases used in the original publication have fallen from general use (eg, 'aero motor' has been replaced by 'aero engine').

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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.