PART THREE: TRANSPORT 612 Throughout the nineteenth century, ballooning was largely an activity for showmen, with a limited number of ascents for scientific (mainly meteorological) purposes. Most of these flights were made using coal-gas, which was conveniently available in most towns from about 1820. Charles Green was the first to use coal-gas in London in 1821; although the Academy of Lyons had suggested it in 1784. Coal gas is heaver than hydrogen, so a somewhat larger balloon was needed, but the lower cost and the convenience of inflating from a permanent supply were great advantages. Hot-air balloons were used occasionally during the nineteenth and early twentieth centuries, but their very large size compared to gas balloons made them difficult to handle, and it was only when sheer size was a desirable feature required by an entrepreneur that they were noticed. However, it was often realized that if a continuous supply of heat could be contrived, hot-air balloons could be useful in remote places where coal gas or hydrogen was not conveniently available. Experiments on these lines were regularly made, using alcohol or petroleum fuels, from the latter part of the nineteenth century onwards. Another interesting but abortive proposal was to combine the constant lift of a hydrogen balloon with the variable lift of a hot-air balloon carrying a fuel supply. This concept was first tried by Pilâtre de Rozier, the pilot of the first Montgolfier balloon, in 1785. He made a combination balloon which had a conventional hydrogen balloon on top of a cylindrical bag heated by a brazier, fuelled with straw blocks. Its only flight, on 15 June 1785, was an attempt to cross the English Channel from Boulogne; this ended in disaster when the balloon caught fire after a few minutes’ flight. Subsequent attempts to produce combination balloons were equally unsuccessful, though not so spectacularly fatal. Although the use of balloons for military observation purposes was pioneered by the French in 1794, it was neglected thereafter. During the Franco-Prussian War of 1870 attention was focused on the successful use of free balloons to allow individuals and dispatches to leave Paris during the seige; tethered observation balloons were employed on both sides but were not significantly useful. However, in the aftermath of the war, experiments were resumed in Britain, France, Germany and elsewhere into the use of captive balloons for observation work. The major technical advances arising from this work were the development by the British army of steel cylinders to hold compressed gas and special wagons to carry them; and portable winches to raise and lower the balloon quickly. Electrolysis of water was also introduced as an alternative method of generating acid-free hydrogen on a large scale. It was soon realized that in strong winds, a spherical balloon was very unstable when tethered, especially near the ground. Two German officers, Major von Parseval and Captain von Sigsfeld, developed (about 1896) an elongated balloon with large inflated tail-fins which flew at an angle to the AERONAUTICS 613 wind, like a kite, and was much more stable. Such kite-balloons were then taken up by most other countries; a version invented in 1916 by the Frenchman Albert Caquot supplanted the Parseval design during the First World War. Apart from their use at the front for observing the results of artillery bombardment, Caquot balloons were used also as an anti-aircraft screen around London—the so-called ‘balloon barrage’—and this use was repeated on a large scale in the Second World War. After a number of high-altitude balloon flights in which the pilots were killed, a sealed pressurized cabin was first used by the Swiss scientist Auguste Piccard in 1931 for a flight to 15,780m (51,775ft) to investigate cosmic radiation. The spherical aluminium cabin or gondola was designed and made in Belgium, but closely resembled a published design of 1906 by Horace Short which was never built. Piccard’s balloon was made of rubberized cotton, and dispensed with the conventional net—the gondola was supported from a band sewn into the envelope. A number of similar flights were made in the 1930s in several countries. The last pre-war flight, by the Americans Stevens and Anderson, used helium as the lifting gas, and reached 22,066m (72,395ft). Helium was identified as a minor constituent of natural gas in some American oilfields in 1907, and its potentialities for inflating balloons and airships were realized on the outbreak of the First World War. Although heavier than hydrogen, it is completely non-flammable. Several plants were installed to separate helium from natural gas in the United States in 1917–18; the American military services prohibited its export and from 1923 it replaced hydrogen entirely in US military airships, but was rarely used in balloons because of the cost. The resumption of high-altitude balloon flights for cosmic ray research after the Second World War utilized the recently developed lightweight polythene films for the envelope. Initially these were not thought safe enough for manned flight, so from 1947 a series of unmanned flights was made using helium-filled balloons developed by the American Otto Winzen and funded by the US navy and air force. Eventually a number of manned flights were made in the USA with polythene balloons between 1956 and 1961. A by-product of these high-altitude flights was the revival of hot-air ballooning. In 1960, the American Paul E. Yost introduced the ‘Vulcoon’, with an envelope made of nylon fabric laminated with an internal mylar plastic film. Commercially available propane gas cylinders (normally used for fuelling portable cooking stoves) fed a burner system which heated the air in the balloon. The single pilot sat on a sling, although small wicker or aluminium baskets were soon introduced. Although nominally developed as a military project by Yost’s company Raven Industries, these new hot-air balloons were primarily produced for sport-flying. After a somewhat hesitant start, the revival of hot-air ballooning spread world-wide during the 1970s, and improved PART THREE: TRANSPORT 614 designs of envelopes and burners made it possible to build a large variety of sizes and shapes, including many novelties to advertise commercial products. The new fabric technology was also applied to gas balloons, and long- distance flights lasting several days became possible. The first successful balloon flight across the Atlantic was made by the Americans Ben L.Abruzzo, Max L. Anderson and Larry Newman in August 1978 in a helium-filled balloon. AIRSHIPS Almost as soon as the free balloon had been invented, attempts were made to control its direction of flight, by using hand-operated paddles (tried by Lunardi in 1784), airscrews (Blanchard, 1784) or even by letting out a jet of hot air from the side of the envelope (Joseph Montgolfier, 1784). It was soon realized that it would be an advantage to reduce the cross-section area of the balloon, so elongated and pointed shapes were proposed. The French General J.B.M.Meusnier produced a detailed design for such a dirigible balloon or airship in 1785; it had the form of an ellipsoid, 79m (260ft) long with a capacity of 60,000ft 3 (1700m 3 ), and was intended to be driven by manually-powered airscrews. Meusnier’s most significant invention in this design was the ballonet—an air-bag inside the envelope, into which air was pushed by a bellows to maintain internal pressure inside the airship. Varying the amount of air in the ballonet compensated for changes in the volume of hydrogen as the altitude of the ship changed, while maintaining the external shape of the envelope. Although Meusnier’s airship was never built, it embodied many of the features of later designs. As long as manual effort was the only available power source to propel an airship, little progress was possible. The idea of using a pair of horses working a treadmill was proposed in a design by Dr E.C.Génet in 1825, but the first satisfactory power source was a 2.25kW (3hp) steam engine employed by the French engineer Henri Giffard in 1852. This unit, with its coke-fired boiler, was slung some 12m (40ft) below the cigar-shaped envelope, which was 44m (144ft) long and contained 2500m 3 (88,000ft 3 ) of hydrogen. Giffard’s airship was calculated to have a maximum speed of 10kph (6mph) in still air, so in practice it was impossible to fly to a pre- determined destination except down-wind. An Austrian engineer, Paul Haenlein, was the first to build an airship with an internal combustion engine; in 1872 he constructed a four-cylinder gas engine of Lenoir pattern, but it was too heavy for his balloon which was never flown. He was followed by two French army engineers, Charles Renard and Arthur Krebs, who in 1884 flew their airship La France powered by an electric motor of 6.5kW (8.5hp) supplied with current from batteries. This is usually AERONAUTICS 615 regarded as the world’s first successful airship, since it was able to return to its starting point on five of its seven flights; however, it was capable of no more than 24kph (15mph) in still air so was only able to fly in the lightest breezes. With the development of petrol-engined road vehicles in 1886 (see Chapter 8), it seemed that a suitable power unit for airships had appeared. The German engineer Karl Woelfert, in conjunction with Gottlieb Daimler, produced an airship with a 1.5kW (2hp) single-cylinder Daimler engine which was test- flown in 1888. This was too small to be practicable; Woelfert’s next airship with a 4.5kW (6hp) Daimler engine probably never flew because it lacked sufficient lift; and a larger airship built for trials by the Prussian army in 1897 caught fire and killed the inventor on its only flight in Berlin. In the same year, at the same place, a unique all-metal airship designed by the Austrian David Schwarz and fitted with a Daimler engine was wrecked on its first trial flight. In France, the Brazilian amateur enthusiast Alberto Santos-Dumont fitted a 1.5kW (2hp) De Dion-Bouton engine in the first of a series of small airships which he flew fairly successfully. In 1901, in his No. 6 airship, he just succeeded in flying from St Cloud to the Eiffel Tower and back inside the time limit of 30 minutes and thus won a prize of 100,000 francs offered by Henri Deutsch de la Mearthe. The enormous publicity surrounding this flight gave Santos-Dumont’s airships a rather unwarranted reputation, for in truth they were hardly capable of outflying Renard and Krebs’ La France of 1884. Inspired by Santos-Dumont’s activities, the French Lebaudy brothers, owners of a large sugar refinery, commissioned their chief engineer Henri Julliot to build a much larger airship. This machine Lebaudy I was 57m (187ft) long with a capacity of 2250m 3 (80,000ft 3 ); a 30kW (40hp) Daimler engine driving two 3m (9ft) diameter airscrews gave it a speed of about 40kph (25mph) and it was the first really practical airship when it flew in November 1902. During the next ten years a considerable number of essentially similar airships were made in several European countries, and these were further developed during the First World War. Most were non-rigid airships, colloquially known as Blimps, which had envelopes made of rubberized cotton or linen fabric, whose shape was maintained by having the gas at slightly greater than atmospheric pressure: this pressure was generated by forcing air into internal ballonets by scoops behind the propellers. The car (often called a gondola) containing crew and engines was slung beneath the envelope, usually hanging from support patches sewn into the fabric (see Figure 12.1). The so-called semi-rigid airships (which included the Lebaudy types) had a rigid keel of wood or a metal framework, attached directly to the bottom of the envelope; the car was slung from this. Probably the largest of the non-rigid airships built during this period were the British North Sea class of 1917, with two 250hp (166kW) engines and a crew of ten; these were 80m (262ft) long with a capacity of 10,000m 3 (360,000ft 3 ). During and after the Second World War much larger airships of this general type, filled with helium, were developed by the US navy, PART THREE: TRANSPORT 616 eventually culminating in the Goodyear ZPG-3W of 1958 with a capacity of 42,500m 3 (1.5 million ft 3 ), powered by two 1120kW (1500hp) engines. These carried early-warning radar systems for fleet protection, with large aerials inside the envelope. Small non-rigid airships continue to be built in the 1980s, using modern synthetic materials for the gas-bag and cabins; the basic configuration, however, remains essentially similar to the successful Blimps of the First World War. The ‘semi-rigid’ design was particularly developed in Italy, the largest being the Roma of 34,000m 3 (1.2 million ft 3 ), launched in 1919. Its six 500hp engines gave a maximum speed of almost 70mph. This was sold to the United States and its destruction by fire in 1922 caused the US military authorities to use only helium for inflating their airships thereafter. The largest and most spectacular airships were the ‘rigids’, which had a wooden or metal framework structure, covered externally with a fabric skin, and containing a number of internal gas bags to provide lift. Accommodation for the crew and passengers was either in a cabin attached directly to the main hull, or in a separate gondola slung below it. Engines were slung from the hull, usually in discrete pods distributed along the length of the ship. Figure 12.1: A ‘Coastal’ class airship of the Royal Naval Air Setting setting off for an anti-submarine patrol during the First World War, 1914–18. This is a typical non-rigid airship, inflated by hydrogen, and powered by two 112kW (150hp) Sunbeam engines. AERONAUTICS 617 The ill-fated Schwarz metal airship was technically a rigid airship, for it had an internal structure of metal tubes supporting the external skin, but the accepted originator of the classical rigid airship was Count Ferdinand von Zeppelin. His first design, LZ.1, was developed with the assistance of Professor Müller-Breslau, a structural engineer. A framework of internally-based ring girders joined by longitudinal members was made in aluminium—the whole structure being a 128m (420ft) long cylinder of 11.75m (38.5ft) diameter with tapered ends. Two gondolas were slung beneath the hull, each containing a 10.5kW (14hp) Daimler engine. Seventeen individual gas bags made of cotton with a rubber lining were installed between the frames, and the outside of the hull was covered in a varnished cotton fabric. LZ.1 was launched in 1900, and flew only three times because it had inadequate controls. A second ship, LZ.2, built in 1905, introduced triangular section girders made of the new high- strength duraluminium alloys. This had two 63kW (85hp) motors and a more satisfactory control system, but crashed on its second flight. However, LZ.3 of 1906 proved sufficiently successful to spawn a long line of airships, and successful passenger-carrying services were operated from 1911 with LZ.10 and three other ships. The main uses of Zeppelins were to be for military purposes, and notably for the inauguration of night-bombing attacks on French and British targets, in which they were able to avoid the opposition of guns and aeroplanes by flying at altitudes above 6000m (20,000ft). There were other rigid airships, notably the wooden-framed German Schutte-Lanz designs of 1911–18. The British government sponsored a series of designs culminating in the passenger ships R100 and R101 (Figure 12.2) of 1929, and the US Navy purchased the Akron and Macon built by Goodyear (with considerable Zeppelin input) in 1931–3; but the Zeppelin company continued to dominate the field. Their LZ.127 Graf Zeppelin, built 1928, was 236m (775ft) long with a volume of 106,000m 3 (3.7 million ft 3 ), and operated passenger services on a regular schedule across the South Atlantic for several years. However, the spectacular losses of the British R.101 in 1931 and the LZ.129 Hindenburg in 1937, following a series of earlier accidents, brought about a cessation of work on rigid airships. Although new designs were proposed in the 1970s using modern materials and various novel design principles, it seems unlikely that the rigid airship will reappear. HEAVIER-THAN-AIR FLYING MACHINES: THE PIONEERS An entirely new approach to achieving dynamic flight with heavier-than-air apparatus was initiated by Sir George Cayley, a scholarly Yorkshire landowner with wide practical interests who remained fascinated by flying throughout his life. Although at various times he worked on model helicopters and clockwork PART THREE: TRANSPORT 618 Figure 12.3: Sir George Cayley scratched this sketch (a) on a small silver disc in 1799 to illustrate his concept that the aerodynamic force on a wing could be resolved into lift and drag. On the other side (b) he shows an aircraft with the wing, a boat-shaped nacelle for the pilot, a controlling tail unit and a pair of propulsive flappers. Figure 12.2: The British airship R101 leaving the passenger access tower at Cardington during test flying, October 1929. Water-ballast is being discharged near the nose, and the four ‘forward’ engines are stationary while the ship is maneuvered by the ‘reverse’ engine. AERONAUTICS 619 powered airships, his main contribution was to formulate clearly the basic principle of dynamic flight using a rigid wing surface to provide lift and a separate propulsion unit to provide forward motion (see Figure 12.3). In a classic paper published in three numbers of Nicholson’s Journal of Natural Philosophy (November 1809–March 1810), he summarized the basic principle in the words: The whole problem is confined within these limits, viz. To make a surface support a given weight by the application of power to the resistance of air. In 1804 he had measured the lift produced by a flat plate moving through the air at a small angle of incidence, using a whirling arm driven by a falling weight, similar to that used by John Smeaton in 1752 for comparing various designs of windmill (see Chapter 4). With the data thus obtained he was able to design and make model gliders incorporating a kite-like wing and a stabilizing tail unit. He also found that the stability of his gliders in the lateral plane was improved by bending the wing to incorporate a dihedral angle. There is some evidence that he built what would now be called a hang-glider in about 1810—a full-size winged machine supporting a man who launched it by running forward (probably downhill) and controlled its flight by moving his body. Towards the end of his life he made two gliders which certainly carried a live load—a ‘boy-carrier’ in 1849 and a ‘man-carrier’ in 1853. Unfortunately the evidence for their construction and operation is little better than anecdotal and although it is clear that a controlling tail unit was fitted at least to the later machine, it is not clear whether the occupant could really control the flight path to any significant extent. Although Cayley experimented with hot-air engines, clockwork springs and even a gunpowder motor as potential power units, he never solved the problem of propelling his aircraft and was consequently unable to exploit his vision that an uninterrupted navigable ocean, that comes to the threshold of every man’s door, ought not to be neglected as a source of human gratification and advantage (1816). STEAM POWER Most of Cayley’s work was never published, and although the significance of his paper of 1809–10 was later well recognized, it had less influence on his contemporaries than he had hoped. However, he did directly inspire William Samuel Henson, whose widely publicized patent of 1843 for an Aeriel Steam Carriage fixed the idea of Cayley’s classical aeroplane shape in the minds of later workers. Henson postulated a high-wing monoplane with cambered wing PART THREE: TRANSPORT 620 section, externally braced with streamline wires to reduce drag, with a separate fuselage containing accommodation for passengers and crew, and housing a steam engine which drove two airscrews behind the wing. A large tail unit comprising horizontal and vertical rudders was intended to steer the machine. The most impractical feature of Henson’s machine was the intention to launch it down an inclined rail. The idea was to use the force of gravity to accelerate the aircraft, so the steam engine needed only to be sufficiently powerful to overcome drag in forward flight. Clearly Henson recognized that the weight of the power plant—including boilers, fuel and water—would be critical, and must be kept as small as possible. Henson’s project was never built, though there is evidence that contracts were placed for construction of the airframe and engine. The company formed to exploit it suffered much ridicule as a result of its excessively optimistic publicity for passenger-carrying services to India and China, and the scheme foundered. Henson then combined with John Stringfellow to test a large model of about 6.6m (20ft) wingspan—something that might prudently have been done earlier. However, no real success was obtained, partly because the trials (on a hillside near Chard in Somerset) were conducted at night to maintain secrecy and avoid ridicule. These tests were made in 1847; in the next four years or so Stringfellow continued the work alone, building a number of models and light steam engines. Limited success was obtained, but any real demonstration of free flight was prevented by Stringfellow’s lack of a suitably large building in which to fly his models. In 1851, frustrated by lack of funds to procure such a building, he postulated ‘an Aerial tent of canvass or calico rendered impervious to air and to be filled and kept up by a blowing machine so that no timber would be required to support it’. This imaginative prevision of the inflated structures of the late twentieth century remained as no more than an idea. Stringfellow briefly resumed his aeronautical work around 1866, stimulated by the formation of the Aeronautical Society of Great Britain under the presidency of the Duke of Argyll; at the Society’s Exhibition at the Crystal Palace in 1868, he exhibited a steam powered triplane which seems to have been rather less successful than his model of 1848. He also exhibited one of his earlier small steam engines, and received a prize for it as the lightest practical power unit entered. Steam power being virtually all that was available in the nineteenth century, it is not surprising that it was employed by later inventors, who made little further progress towards successful mechanical flight. Clément Ader, a famous French electrical engineer, built his Eole between 1882 and 1890, with a 15m (49ft) wingspan patterned on the model of a bat. Driven by a steam engine of about 13.5kW (18hp) and piloted by Ader himself, it made a single straight- line ‘flight’ of about 50m (160ft) just clear of the ground on 9 October 1890. Inspired by this, Ader was funded by the French government in 1892 to make AERONAUTICS 621 a new machine which appeared in 1897. This Avion III (No. 2 having been abandoned before completion) had two steam engines driving curious feathered propellers, and huge bat-like wings of complex construction. It was twice tested in October 1897, but failed totally to fly and was blown off its track and damaged on the second attempt, after which the War Ministry refused further support. The machine still exists in a French museum and is the oldest full-size ‘aircraft’ to survive. Equally abortive was the enormous steam-powered test-rig built by Sir Hiram Maxim in Kent in 1893–4. After extensive tests of aerofoils and other components on a whirling arm and in a wind tunnel, Maxim built a carriage propelled by two very lightweight steam engines of 135kW (180hp) each, running on a level railway track some 550m (1800ft) long. On this carriage were mounted wing surfaces extending to some 400m 2 (4000ft 2 ), which were assembled in various configurations. The lift and drag developed were measured, and the carriage was restrained from rising more than a few centimetres by outrigged wheels running beneath a set of guard rails alongside the main track. Although testing continued for a couple of years, nothing came of it. Maxim did not attempt to build a real flying machine until 1910, and that was totally unsuccessful. He ascribed his failure variously to lack of money, inability to find a sufficiently large open space to continue trials, and to the need to develop a better power-plant. The last reason is certainly valid, but the real reason for his procrastination seems to have been a fear that he might be subject to ridicule if he failed. GLIDERS The first man to make repeated flights with a heavier-than-air machine was the German Otto Lilienthal. His original intention was to develop an ornithopter, but lacking a suitable power source he developed a series of fixed-wing gliders between 1891 and 1896. These machines were launched from a variety of eminences, including a special constructed earth mound some 15m (50ft) high. He supported himself from his forearms, placed through sleeves under the wing roots, with his lower body hanging below the wing, launched himself by running downhill into the wind, and controlled the flight by swinging his body to manipulate the centre of gravity. His gliders were fairly crudely made, with a single-surface fabric wing supported by wooden spars which could be folded up for easier portability. The machines themselves were not technically significant, but the publicity given to his numerous flights was a spur to other workers, particularly as for the first time photographic illustrations of a man flying successfully were given wide circulation. The publicity given to Lilienthal’s flying was enhanced when he was killed by a crash in August 1896. . changes in the volume of hydrogen as the altitude of the ship changed, while maintaining the external shape of the envelope. Although Meusnier’s airship was never built, it embodied many of the features. navy and air force. Eventually a number of manned flights were made in the USA with polythene balloons between 1956 and 1961. A by-product of these high-altitude flights was the revival of hot-air ballooning However, in the aftermath of the war, experiments were resumed in Britain, France, Germany and elsewhere into the use of captive balloons for observation work. The major technical advances arising