The most significant technical development during the Second World War was the appearance of jet-propelled aircraft, powered by gas turbines. In a sense, all aircraft propulsion can be classified as ‘jet propulsion’, for a conventional airscrew produces a jet of air which has been accelerated, and the propulsive impulse is equal to the increment in the momentum of this air jet; but the term is commonly restricted to the case where the air jet is accelerated inside the engine unit. The incentive to develop this system arose from the reduced efficiency of conventional propellers at high speeds: when the tip speed approaches sonic velocity, the drag increases sharply owing to the formation of shock waves. This phenomenon effectively constrained propeller-driven aircraft to speeds substantially less than the speed of sound, even in a dive.
In practice, a propulsive jet can be produced in two ways—from a rocket engine or a gas turbine. Rocket engines inevitably imply high effective fuel consumption, because the oxygen for combustion has to be carried in some form as well as the fuel itself; consequently rocket-propelled aircraft have been limited to very specific and unorthodox roles. The most noteworthy example was the German Messerschmitt Me 163 interceptor fighter developed in 1944 to counter American bomber formations. Other applications for rocket power have been mainly to boost the take-off of conventionally-powered aircraft, or for supplementary short-duration boost at high altitudes.
The gas turbine engine, in which the air passing through the engine is continuously compressed, heated and expelled through a turbine which drives the compressor, was postulated as a desirable alternative to a reciprocating engine soon after the steam turbine had been invented, but its practical development awaited the availability of suitable materials able to withstand high stress at high temperatures, and the incentive of a perceived requirement for high-speed flight. Under this stimulus, development began almost simultaneously around 1936 in several countries. The names of Frank Whittle in Britain and Hans von Ohain in Germany will always be preeminent as the most successful pioneers in the field, but several others were close behind. Von Ohain was the first to achieve a flight engine, and this powered the Heinkel He 178 on its first flight in August 1939. Whittle followed in May 1941, and his engines developed in Britain were also built in the USA and laid the foundation of the gas turbine engine industry in those countries. But the first production jet aircraft was the German Messerschmitt Me 262 which was powered by the Jumo 004 gas turbine developed by a team at the Junkers company. The Jumo 004 had a multi-stage axial flow compressor rather than the centrifugal compressors used by Whittle and von Ohain; this type of compressor has dominated subsequent gas turbine development.
The Me 262 was capable of a top speed around 870kph (525mph) and could operate at higher altitudes (up to 12,000m (40,000ft)) than its piston-engined contemporaries; and the four-engined Arado Ar 234 bomber with similar performance was potentially almost impossible to intercept, but never came into large-scale service before the end of the war.
After 1945 the major air forces of the world rapidly adopted the gas turbine for new fighter and bomber designs. The problem of excessive drag caused by shock waves at near-sonic speeds which had previously affected propeller blades now recurred on the aircraft wings. In particular there were difficulties with control, because deflection of a control surface had unexpected effects on the airflow over the wing. The combination of increased drag and control problems gave rise to the popular misconception of a ‘sound barrier’, preventing flight at higher speeds. In this climate, the American rocket-powered Bell XS-1 experimental aircraft made the first flight at a speed greater than the speed of sound on 14 October 1947, but this was really something of a freak machine. The real solution was found in the adoption of swept wings, and in substantial programmes of theoretical and experimental research to understand the detailed phenomena of transonic airflow. The first successful swept-winged aircraft powered by a gas turbine was the North American F-86 Sabre, a single-seat fighter first flown in October 1947 and later built in large numbers and in several different versions. An F-86 prototype was flown at supersonic speed (in a dive) in April 1948; aircraft of similar configuration were subsequently developed in several countries.
The useful measure of an aircraft’s speed now became the Mach number — the flight velocity as a proportion of the speed of sound at the flight altitude. As engines of higher thrust were progressively developed, Mach 1 in level flight was achieved by the North American F-100 in 1953, Mach 2 by the Lockheed F-104 in 1958, and Mach 3 by the Lockheed A-11 in 1963.
A wide variety of wing shapes has been employed by these high-speed military aircraft. The drag of a wing at high speed depends primarily on its thickness/chord ratio and on the sweepback of the maximum thickness locus; the strength and stiffness of the wing depends on the same basic variables, and the aircraft designer also has to solve practical problems such as retracting the undercarriage and accommodating the fuel. Therefore the F-104 used a very thin unswept wing; the English Electric Lightning used a wing swept back by 60°; and the Dassault Mirage used a delta wing with leading edge similarly swept but trailing edge unswept, thus reducing thickness/chord ratio by increasing the wing chord. All have comparable performance.
These high-speed aircraft also share common features of powered control surfaces, actuated by hydraulic pressure in response to the pilot’s control movements, but monitored by electronic control systems to give the appropriate movement to produce the desired aircraft response over a wide range of flight speeds and altitudes. In the 1980s the increasing reliability of electronics made it feasible to dispense with a physical connection between the pilot’s control mechanism—the traditional ‘control column’—and the actuators, so that electric signalling or even signalling by modulated light signals in an optic fibre (colloquially ‘fly-by-wire’ or ‘fly-by-light’) became feasible. Experimental installations have been made in several countries, and the first production aircraft with a ‘fly-by-wire’ control system, the Airbus A.320 airliner, was put into service in 1988.
Increasing flight speed has also necessitated changes in materials for the airframe construction: air friction at high speeds produces kinetic heating of the structure. Aluminium alloys of various formulations have been developed with adequate strength for speeds up to about Mach 2, but stainless steel and titanium are needed above that speed.