PART THREE: TRANSPORT 632 other types of similar configuration, notably in the USA by the 14-seat Douglas DC-2 and 21-seat DC-3 (Figure 12.9). With these aircraft, the American airlines were able to provide regular, reliable and safe passenger services. Safety and reliability were enhanced by effective de-icing measures, blind-flying instruments, air-toground radio communications, meteorological forecasting services and the provision of hard-surfaced runways at airports. The pattern was followed in Europe, and eventually world-wide. THE SECOND WORLD WAR The new technologies were applied also to military aircraft, albeit somewhat hesitantly. New designs of fabric-covered biplane fighters with fixed undercarriages were still being introduced into Royal Air Force squadron service as late as February 1937, but by the outbreak of the Second World War in 1939 the front line fighter and bomber units of all the major air forces had been equipped with cantilever monoplanes with enclosed cockpits and retractable undercarriages. Engines of around 750kW (1000hp), supercharged to maintain power at altitudes around 6000m (20,000ft) became the norm. The Supermarine Spitfire may be taken as typical: in 1939 the original (‘Mark 1’) version with a Merlin engine of 738kW (990hp) driving a two-bladed wooden airscrew had a maximum speed of 588kph at 6150m (353mph at 20,000ft). Wartime development of the basic design produced the final operational version, Mark 22, with a 1530kW (2050hp) Griffon engine and five-bladed airscrew; this achieved 735kph at 8000m (457mph at 25,000ft) (see Figure 12.10). Figure 12.10: A Supermarine Spitfire VII of 1942, typical of the high-performance fighter of its time, with all-metal structure and thin cantilever wings. The engine is a 1120kW (1500hp) Rolls-Royce Merlin 61 with two-speed supercharger and the cabin was pressurized to allow operation up to 12,000m (40,000ft) altitude. AERONAUTICS 633 Progress in bomber design was rather less impressive, but the twin-engined Vickers Wellington 1 bomber of 1939—with two 750kW (1000hp) engines, it carried 2050kg (4500lb) of bombs at a speed of around 315kph at 3075m (195mph at 10,000ft) —can be compared with the Avro Lancaster 3 at the end of the war—with four 1200kW (1600hp) engines, it carried up to 6350kg (14,000lb) of bombs about 55kph (30mph) faster at 6150m (20,000 ft). The most efficient bomber of the war was the De Havilland Mosquito, which was quite outside the main conventional design trend. The De Havilland company had concentrated on civil aircraft production up to 1939, and their bomber was based on the pre- war DH 88 Comet built for long-distance air racing. The Mosquito structure was built entirely of wood, and the aircraft had a crew of only two, dispensing with the heavy defensive armament of conventional bombers. With two 1275kW (1700hp) Rolls-Royce Merlin engines it carried up to 2250kg (5000lb) of bombs at some 500kph (310mph). The same basic airframe was used in numerous other roles—as a photo-reconnaissance aircraft operating up to 13,000m (38,000ft) with a pressurized cabin, as a radar-equipped night fighter, as a low-altitude strike aircraft and even as a high-speed transport. Air transport played a substantial part in the military operations of the Second World War. Using variants of civil airliners and conversions of bomber designs, large numbers of passengers and significant quantities of freight were carried on regular services throughout the world, including for the first time regular passenger services across the North and South Atlantic. In addition, direct intervention in land battles was provided by troops dropped by parachute or carried in large towed gliders. Other technical developments during the war which had a significant and lasting effect on the operational use of aircraft included the introduction of various radar navigation systems and of reliable automatic pilots. An oxygen supply was routinely provided to aircrew flying above 4500m (15,000ft), and pressurized cabins were introduced on a limited scale for operations above 10,000m (35,000ft). Another very important influence on the future development of aircraft operations was the proliferation of hard-surfaced runways throughout the world, largely replacing grass airfields. This allowed landing and take-off speeds to rise, allowing an increase in wing-loading and thus in cruise performance. DEMISE OF THE FLYING-BOAT Improvements to runways also caused the virtual elimination during the 1950s of large flying-boats from both civil and military operations. Before and during the war, these aircraft were widely used for anti-submarine warfare and for long-range passenger transport: the first regular airline services across the Atlantic and Pacific Oceans were provided by four-engined flying-boats made PART THREE: TRANSPORT 634 by the Boeing and Sikorsky companies. A good deal of research work went into improved hull shapes for a post-war generation of large flying-boats of high performance, including such aircraft as the Saunders-Roe ten-engined boat intended to carry 85 passengers for 8000km (5000 miles), and the Martin P6M Seamaster four-jet military reconnaissance machine. None of these came into service because land-based machines could meet similar design targets more efficiently. The last four-engined passenger flying-boats were withdrawn from scheduled service on major routes in 1959. EXPANSION OF CIVIL AVIATION A new generation of passenger transport aircraft was initiated on 31 December 1938 with the first flight of the Boeing 307 Stratoliner, a four-engined airliner with a pressurized passenger cabin. Only a few Stratoliners were built, but it set the pattern for the Lockheed Constellation (January 1943) and Douglas DC-6 (February 1946). These two American manufacturers went into large- scale production at the end of the war, and variants of these types operated almost all the major long-distance air services until the 1960s. A typical version, the Constellation 749A, with four 1850kW (2500hp) engines carried fifty passengers at 525kph at 6100m (325mph at 20,000ft) for a range of 4000km (2500 miles). With this type of machine, the number of passengers carried increased exponentially: the major world airlines carried 3 million passengers in 1939, 27 million in 1949 and 98 million in 1959. By 1980 piston- engined airliners had virtually disappeared from service: the reliability of the gas turbine in service had caused the engine manufacturers to stop making piston engines of more than 1000hp, and all large and medium-sized transport aircraft were powered with turbojet or turboprop engines. INTRODUCTION OF JET PROPULSION 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 AERONAUTICS 635 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 (see Chapter 5), 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 pre- eminent 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’, PART THREE: TRANSPORT 636 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. AERONAUTICS 637 JET AIRLINERS It seemed at first that the gas turbine’s fuel consumption would be too high for jet-propelled aircraft to be used for civil airline services, so the turboprop engine was developed, mainly in Britain, for this purpose (see Chapter 5). The relative mechanical simplicity of the turbine engine promised greater reliability than the high-powered piston engine, while the application of a propeller improved the propulsive efficiency of the system and hence reduced the fuel consumption—at the cost of limiting flight speeds to around 600kph (400mph). The first proposals for a gas turbine driving a propeller—a turboprop engine— were made as early as 1929 by A.A.Griffith at the Royal Aircraft Establishment in Britain, but after a limited experiment to check his theories on the design of an axial compressor, the work was shelved until 1936 when a complete axial- flow compressor was built and tested. Full-scale tests of a turboprop in flight were made in September 1945 with modified Rolls-Royce Derwent engines in a Gloster Meteor. Rolls-Royce then developed their Dart engine, four of which powered the Vickers Viscount airliner on its first flight in July 1948. After some initial hesitation, the Viscount was taken up by several airlines, mainly for routes between major European cities, and it entered service in April 1953. Over 400 Viscounts were built, and its success encouraged other manufacturers to build turboprop airliners, but on most major routes they were replaced by faster turbojet aircraft. Turboprop airliners were still being built in the late 1980s, mainly for use on secondary routes where relatively small passenger loads are expected, and there is little economic advantage in high speeds. The introduction of the turbojet engine into airline service was spurred by the realization that the higher flight speeds which it made possible could increase the productivity of a jet airliner sufficiently to outweigh the high fuel consumption. The first jet-propelled airliner, the De Havilland Comet 1, first flew in July 1949, and went into service with British Overseas Airways in May 1952; unfortunately it had to be withdrawn from service after two years as a consequence of structural deficiencies, and Comet services were not resumed until October 1958 (Figure 12.11). By this time the first American jet airliner, the Boeing 707, was ready to enter service. With a 35° sweptback wing it was significantly faster than the Comet, cruising at about Mach 0.82, approximately 900kph at 10,000m (6oomph at 30,000ft), and therefore offered a more economic vehicle to the operator. The Boeing 707 configuration was followed by other manufacturers in America, Britain and the Soviet Union, but the 707 retained its initial commercial advantage and was built in far larger numbers —almost 1000—than its competitors. The first Comets and Boeing 707s had barely sufficient range for the main North Atlantic routes, but these routes provided the major application for the early jet transports. Extension of services to shorter-range routes led to the PART THREE: TRANSPORT 638 development of new designs specifically developed for these markets—the Boeing company produced consecutively the Boeing 720, the three-engined 727 and the twin-engined 737, all of essentially the same configuration but of decreasing size and passenger capacity. Continual increase in air traffic brought a potential market for much larger airliners. The later 707s carried about 180 passengers in tourist-style seating, so the target for the new designs was set around 400 passengers. Boeing announced its new design, the 747, in April 1966; it first flew in February 1969, and it entered service in January 1970. The basic configuration remained essentially similar, with a 38° sweptback wing. Four engines of around 22,000kg (50,000lb) thrust were fitted and these were of a new generation of ‘turbofans’. Essentially, the turbofan engine can be considered as a conventional gas turbine with an additional turbine driving a large multi-blade fan as the first stage of the compressor. Only part of the air from this first stage passes through the rest of the engine: the greater part is ‘bypassed’ in an external duct surrounding the core engine. The combined exhaust stream therefore has a larger mass flow but a lower velocity than that from a conventional turbojet engine, and therefore provides better propulsive efficiency and a lower fuel consumption. Conceptually, the turbofan engine is akin to the turboprop engine, but uses a ‘propeller’ with many more blades working inside a duct rather than in the open. The large passenger capacity of the Boeing 747 proved economically attractive, and the pattern was followed by other manufacturers with a generation of so-called jumbos during the next ten years. A somewhat fortuitous by-product of the high propulsive efficiency of the turbofan engine was a relatively lower noise level, associated with the lower jet velocity. This Figure 12.11: The prototype De Havilland Comet, the world’s first jet-propelled airliner, during a test flight in 1949. AERONAUTICS 639 provoked public pressure to reduce aircraft noise generally, so that turbofan engines largely replaced turbojets in civil use in the 1980s. SUPERSONIC COMMERCIAL AIRCRAFT Throughout the development of the airline industry there has always been a pressure to increase speed: apart from any influence on the productivity of the vehicle, it seems clear that passengers have preferred to travel as quickly as possible. By the late 1950s, preliminary design studies were set in hand in several countries to investigate the possibilities of a transport aircraft operating at supersonic speeds—several times the speed of any civil aircraft then in service. In the early 1960s the British and French governments agreed to provide funds for an Anglo-French airliner designed to be operated at Mach 2, the US government supported the design of a larger machine to fly at Mach 3, and the Soviet Union also started development of a Mach 2 design. The Anglo-French design eventually appeared as the prototype Concorde, which flew in March 1969; the Russian Tupolev Tu-144 flew three months earlier, but the American project never materialized. Concorde and Tu-144 were remarkably similar in general configuration, with highly swept delta wings of complex shape, a long slender fuselage and four under-wing engines. To obtain sufficient thrust for take-off and transonic acceleration, the engines utilized after-burning, a process hitherto restricted to military operations in which additional fuel is injected into the exhaust stream of the engine. The economics of operating a supersonic airliner are very marginal, and only two airlines, British Airways and Air France, put the Concorde into operation, in January 1976. The Russian Tu-144 was operated only experimentally and never entered full service. Concorde, flying regular transatlantic services at around 2150kph at 15,000m (1350mph at 50,000ft), is faster than almost all military aircraft now flying, but in many ways its technology can be considered conventional. It is built of aluminium alloys, with limited use of titanium and steel. The aerodynamic controls are hydraulically operated, with multiple redundancies to provide adequate reliability in service; in addition, fuel is moved during flight to adjust the aircraft’s centre of gravity to match the variation of the aerodynamic lift distribution as speed increases through Mach 1. JET-SUPPORTED FLIGHT The development of increasingly powerful jet engines brought in the possibility of building aircraft with a thrust/weight ratio greater than one, which would be potentially capable of vertical flight on jet thrust alone. A precursor was a PART THREE: TRANSPORT 640 rocket-propelled fighter, Bachem Ba 349 Natter, produced in Germany in 1945. This was launched vertically using rocket power, but it was envisaged as an expendable missile with the pilot returning by parachute from his mission. The first successful demonstration of jet-supported flight was provided in Britain by the Rolls-Royce Thrust Measuring Rig—better known by its nickname Flying Bedstead—in August 1954. The rig comprised two Rolls-Royce Nene engines mounted horizontally in a tubular framework on four wheels, with the jets deflected downwards through cascades of vanes in the nozzles. The pilot sat on top of the rig, and it was stabilized and controlled by four small air-jets on outriggers. Essentially, this rig was capable only of vertical flight; the next development was to mount four lightweight Rolls-Royce RB 108 engines vertically in a delta-winged aircraft, the Short SC.1, with a fifth RB 108 mounted horizontally for forward propulsion. Jet reaction controls were provided to stabilize and control the aircraft in the hover, and conventional aerodynamic controls for use in forward flight. The first of two Short SC.1 machines first flew in 1957, and achieved a transition from hovering to forward flight in 1960. A number of other aircraft were built or conceived to this formula, notably the German Dornier 31 experimental transport aircraft in 1967, but the need to carry separate jet engines for horizontal and vertical flight made them inherently inefficient. The alternative was to use the same engine for both regimes of flight. In its simplest form this was possible by sitting a more-or-less conventional aircraft on its tail for take-off, and flying a curved trajectory into forward flight. Several experimental machines of this type were flown in the USA and France—the Convair XFY-1 and Lockheed XFV-1 in 1954, Ryan X-13 in 1956 and SNECMA Coleoptere in 1959—but none were put into production. A third basic concept using progressive deflection of the exhaust jet from vertical to horizontal has proved to be technically the most successful. In 1960 the British Hawker Aircraft company and Bristol Aero Engines developed the Hawker P.1127 single-seat fighter with a Bristol Pegasus engine (Figure 12.12). The single engine, mounted in a conventional swept-winged aircraft, is provided with four nozzles containing cascades of vanes, which can be rotated through about 100 degrees. The forward nozzles take compressed air from the front fan stage of the engine, and the rear nozzles carry the exhaust. Control jets at the extremities of the airframe provide stability and control in hovering flight and conventional aerodynamic controls take over in horizontal wingborne flight. Developed from the original P.1127, the later Kestrel and Harrier aircraft were built in substantial quantities and entered service in several countries in the 1970s. The term VTOL (an acronym of Vertical Take-Off and Landing) is often applied to this class of aircraft. It is misleading: helicopters are excluded, although they are equally capable of vertical take-off and landing; and jet- AERONAUTICS 641 supported aircraft usually take-off from a running start, either horizontally or from a ‘ski-jump’ ramp, because this generates additional lift allowing the carriage of a greater load. The label STOVL (Short Take-off and Vertical Landing) is more appropriate. HELICOPTERS AND ROTARY WINGS The basic idea of generating lift from a rotating wing can be traced back certainly to the early fourteenth century: an illuminated manuscript of this period gives the earliest known illustration of a child’s toy consisting of a four- bladed ‘airscrew’ mounted on a vertical spindle. This is rotated by pulling a string wrapped round the spindle, whereupon it flies into the air. Essentially the same principle is involved in a toy with two contra-rotating screws operated by a spring-bow mechanism, which was described to the Paris Academy of Sciences by Launoy and Bienvenu in 1784 during that period of acute interest in all matters aeronautical. A similar device was described by Cayley (see p. Figure 12.12: The Hawker P.1127 supported by the thrust from the four deflected jets produced by its Bristol Pegasus engine. Hawker Aircraft photograph. . stage of the compressor. Only part of the air from this first stage passes through the rest of the engine: the greater part is ‘bypassed’ in an external duct surrounding the core engine. The combined. the pattern for the Lockheed Constellation (January 1943) and Douglas DC-6 (February 1946). These two American manufacturers went into large- scale production at the end of the war, and variants. high-speed transport. Air transport played a substantial part in the military operations of the Second World War. Using variants of civil airliners and conversions of bomber designs, large numbers of passengers