Up to this point an attempt has been made to give some idea of the progress that was made during the eleven years that had elapsed since the days of the Wrights' first flights. Much advance had been made and aeroplanes had settled down, superficially at any rate, into more or less standardised forms in three main types—tractor monoplanes, tractor biplanes, and pusher biplanes. Through the application of the results of experiments with models in wind tunnels to full-scale machines, considerable improvements had been made in the design of wing sections, which had greatly increased the efficiency of aeroplanes by raising the amount of 'lift' obtained from the wing compared with the 'drag' (or resistance to forward motion) which the same wing would cause. In the same way the shape of bodies, interplane struts, etc., had been improved to be of better stream-line shape, for the further reduction of resistance; while the problems of stability were beginning to be tolerably well understood. Records (for what they are worth) stood at 21,000 feet as far as height was concerned, 126 miles per hour for speed, and 24 hours duration. That there was considerable room for development is, however, evidenced by a statement made by the late B. C. Hucks (the famous pilot) in the course of an address delivered before the Royal Aeronautical Society in July, 1914. 'I consider,' he said, 'that the present day standard of flying is due far more to the improvement in piloting than to the improvement in machines.... I consider those (early 1914) machines are only slight improvements on the machines of three years ago, and yet they are put through evolutions which, at that time, were not even dreamed of. I can take a good example of the way improvement in piloting has outdistanced improvement in machines—in the case of myself, my 'looping' Bleriot. Most of you know that there is very little difference between that machine and the 50 horse-power Bleriot of three years ago.' This statement was, of course, to some extent an exaggeration and was by no means agreed with by designers, but there was at the same time a germ of truth in it. There is at any rate little doubt that the theory and practice of aeroplane design made far greater strides towards becoming an exact science during the four years of War than it had done during the six or seven years preceding it.
It is impossible in the space at disposal to treat of this development even with the meagre amount of detail that has been possible while covering the 'settling down' period from 1911 to 1914, and it is proposed, therefore, to indicate the improvements by sketching briefly the more noticeable difference in various respects between the average machine of 1914 and a similar machine of 1918.
In the first place, it was soon found that it was possible to obtain greater efficiency and, in particular, higher speeds, from tractor machines than from pusher machines with the air screw behind the main planes. This was for a variety of reasons connected with the efficiency of propellers and the possibility of reducing resistance to a greater extent in tractor machines by using a 'stream-line' fuselage (or body) to connect the main planes with the tail. Full advantage of this could not be taken, however, owing to the difficulty of fixing a machine-gun in a forward direction owing to the presence of the propeller. This was finally overcome by an ingenious device (known as an 'Interrupter gear') which allowed the gun to fire only when none of the propeller blades was passing in front of the muzzle. The monoplane gradually fell into desuetude, mainly owing to the difficulty of making that type adequately strong without it becoming prohibitively heavy, and also because of its high landing speed and general lack of manoeuvrability. The triplane was also little used except in one or two instances, and, practically speaking, every machine was of the biplane tractor type.
A careful consideration of the salient features leading to maximum efficiency in aeroplanes—particularly in regard to speed and climb, which were the two most important military requirements—showed that a vital feature was the reduction in the amount of weight lifted per horse-power employed; which in 1914 averaged from 20 to 25 lbs. This was effected both by gradual increase in the power and size of the engines used and by great improvement in their detailed design (by increasing compression ratio and saving weight whenever possible); with the result that the motive power of single-seater aeroplanes rose from 80 and 100 horse-power in 1914 to an average of 200 to 300 horse-power, while the actual weight of the engine fell from 3 1/2-4 lbs. per horse-power to an average of 2 1/2 lbs. per horse-power. This meant that while a pre-war engine of 100 horse-power would weigh some 400 lbs., the 1918 engine developing three times the power would have less than double the weight. The result of this improvement was that a scout aeroplane at the time of the Armistice would have 1 horse-power for every 8 lbs. of weight lifted, compared with the 20 or 25 lbs. of its 1914 predecessors. This produced a considerable increase in the rate of climb, a good postwar machine being able to reach 10,000 feet in about 5 minutes and 20,000 feet in under half an hour. The loading per square foot was also considerably increased; this being rendered possible both by improvement in the design of wing sections and by more scientific construction giving increased strength. It will be remembered that in the machine of the very early period each square foot of surface had only to lift a weight of some 1 1/2 to 2 lbs., which by 1914 had been increased to about 4 lbs. By 1918 aeroplanes habitually had a loading of 8 lbs. or more per square foot of area; which resulted in great increase in speed. Although a speed of 126 miles per hour had been attained by a specially designed racing machine over a short distance in 1914, the average at that period little exceeded, if at all, 100 miles per hour; whereas in 1918 speeds of 130 miles per hour had become a commonplace, and shortly afterwards a speed of over 166 miles an hour was achieved.
In another direction, also, that of size, great developments were made. Before the War a few machines fitted with more than one engine had been built (the first being a triple Gnome-engined biplane built by Messrs Short Bros. at Eastchurch in 1913), but none of large size had been successfully produced, the total weight probably in no case exceeding about 2 tons. In 1916, however, the twin engine Handley-Page biplane was produced, to be followed by others both in this country and abroad, which represented a very great increase in size and, consequently, load-carrying capacity. By the end of the War period several types were in existence weighing a total of 10 tons when fully loaded, of which some 4 tons or more represented 'useful load' available for crew, fuel, and bombs or passengers. This was attained through very careful attention to detailed design, which showed that the material could be employed more efficiently as size increased, and was also due to the fact that a large machine was not liable to be put through the same evolutions as a small machine, and therefore could safely be built with a lower factor of safety. Owing to the fact that a wing section which is adopted for carrying heavy loads usually has also a somewhat low lift to drag ratio, and is not therefore productive of high speed, these machines are not as fast as light scouts; but, nevertheless, they proved themselves capable of achieving speeds of 100 miles an hour or more in some cases; which was faster than the average small machine of 1914.
In one respect the development during the War may perhaps have proved to be somewhat disappointing, as it might have been expected that great improvements would be effected in metal construction, leading almost to the abolition of wooden structures. Although, however, a good deal of experimental work was done which resulted in overcoming at any rate the worst of the difficulties, metal-built machines were little used (except to a certain extent in Germany) chiefly on account of the need for rapid production and the danger of delay resulting from switching over from known and tried methods to experimental types of construction. The Germans constructed some large machines, such as the giant Siemens-Schukhert machine, entirely of metal except for the wing covering, while the Fokker and Junker firms about the time of the Armistice in 1918 both produced monoplanes with very deep all-metal wings (including the covering) which were entirely unstayed externally, depending for their strength on internal bracing. In Great Britain cable bracing gave place to a great extent to 'stream-line wires,' which are steel rods rolled to a more or less oval section, while tie-rods were also extensively used for the internal bracing of the wings. Great developments in the economical use of material were also made in the direction of using built-up main spars for the wings and interplane struts; spars composed of a series of layers (or 'laminations') of different pieces of wood also being used.
Apart from the metallic construction of aeroplanes an enormous amount of work was done in the testing of different steels and light alloys for use in engines, and by the end of the War period a number of aircraft engines were in use of which the pistons and other parts were of such alloys; the chief difficulty having been not so much in the design as in the successful heat-treatment and casting of the metal.
An important development in connection with the inspection and testing of aircraft parts, particularly in the case of metal, was the experimental application of X-ray photography, which showed up latent defects, both in the material and in manufacture, which would otherwise have passed unnoticed. This method was also used to test the penetration of glue into the wood on each side of joints, so giving a measure of the strength; and for the effect of 'doping' the wings, dope being a film (of cellulose acetate dissolved in acetone with other chemicals) applied to the covering of wings and bodies to render the linen taut and weatherproof, besides giving it a smooth surface for the lessening of 'skin friction' when passing rapidly through the air.
An important result of this experimental work was that it in many cases enabled designers to produce aeroplane parts from less costly material than had previously been considered necessary, without impairing the strength. It may be mentioned that it was found undesirable to use welded joints on aircraft in any part where the material is subjectto a tensile or bending load, owing to the danger resulting from bad workmanship causing the material to become brittle—an effect which cannot be discovered except by cutting through the weld, which, of course, involves a test to destruction. Written, as it has been, in August, 1920, it is impossible in this chapter to give any conception of how the developments of War will be applied to commercial aeroplanes, as few truly commercial machines have yet been designed, and even those still show distinct traces of the survival of war mentality. When, however, the inevitable recasting of ideas arrives, it will become evident, whatever the apparent modification in the relative importance of different aspects of design, that enormous advances were made under the impetus of War which have left an indelible mark on progress.
We have, during the seventeen years since aeroplanes first took the air, seen them grow from tentative experimental structures of unknown and unknowable performance to highly scientific products, of which not only the performances (in speed, load-carrying capacity, and climb) are known, but of which the precise strength and degree of stability can be forecast with some accuracy on the drawing board. For the rest, with the future lies—apart from some revolutionary change in fundamental design—the steady development of a now well-tried and well-found engineering structure.
Francesco Lana, with his 'aerial ship,' stands as one of the first great exponents of aerostatics; up to the time of the Montgolfier and Charles balloon experiments, aerostatic and aerodynamic research are so inextricably intermingled that it has been thought well to treat of them as one, and thus the work of Lana, Veranzio and his parachute, Guzman's frauds, and the like, have already been sketched. In connection with Guzman, Hildebrandt states in his Airships Past and Present, a fairly exhaustive treatise on the subject up to 1906, the year of its publication, that there were two inventors—or charlatans—Lorenzo de Guzman and a monk Bartolemeo Laurenzo, the former of whom constructed an unsuccessful airship out of a wooden basket covered with paper, while the latter made certain experiments with a machine of which no description remains. A third de Guzman, some twenty-five years later, announced that he had constructed a flying machine, with which he proposed to fly from a tower to prove his success to the public. The lack of record of any fatal accident overtaking him about that time seems to show that the experiment was not carried out.
Galien, a French monk, published a book L'art de naviguer dans l'air in 1757, in which it was conjectured that the air at high levels was lighter than that immediately over the surface of the earth. Galien proposed to bring down the upper layers of air and with them fill a vessel, which by Archimidean principle would rise through the heavier atmosphere. If one went high enough, said Galien, the air would be two thousand times as light as water, and it would be possible to construct an airship, with this light air as lifting factor, which should be as large as the town of Avignon, and carry four million passengers with their baggage. How this high air was to be obtained is matter for conjecture—Galien seems to have thought in a vicious circle, in which the vessel that must rise to obtain the light air must first be filled with it in order to rise.
Cavendish's discovery of hydrogen in 1776 set men thinking, and soon a certain Doctor Black was suggesting that vessels might be filled with hydrogen, in order that they might rise in the air. Black, however, did not get beyond suggestion; it was Leo Cavallo who first made experiments with hydrogen, beginning with filling soap bubbles, and passing on to bladders and special paper bags. In these latter the gas escaped, and Cavallo was about to try goldbeaters' skin at the time that the Montgolfiers came into the field with their hot air balloon.
Joseph and Stephen Montgolfier, sons of a wealthy French paper manufacturer, carried out many experiments in physics, and Joseph interested himself in the study of aeronautics some time before the first balloon was constructed by the brothers—he is said to have made a parachute descent from the roof of his house as early as 1771, but of this there is no proof. Galien's idea, together with study of the movement of clouds, gave Joseph some hope of achieving aerostation through Galien's schemes, and the first experiments were made by passing steam into a receiver, which, of course, tended to rise—but the rapid condensation of the steam prevented the receiver from more than threatening ascent. The experiments were continued with smoke, which produced only a slightly better effect, and, moreover, the paper bag into which the smoke was induced permitted of escape through its pores; finding this method a failure the brothers desisted until Priestley's work became known to them, and they conceived the use of hydrogen as a lifting factor. Trying this with paper bags, they found that the hydrogen escaped through the pores of the paper.
Their first balloon, made of paper, reverted to the hot-air principle; they lighted a fire of wool and wet straw under the balloon—and as a matter of course the balloon took fire after very little experiment; thereupon they constructed a second, having a capacity of 700 cubic feet, and this rose to a height of over 1,000 feet. Such a success gave them confidence, and they gave their first public exhibition on June 5th, 1783, with a balloon constructed of paper and of a circumference of 112 feet. A fire was lighted under this balloon, which, after rising to a height of 1,000 feet, descended through the cooling of the air inside a matter of ten minutes. At this the Academie des Sciences invited the brothers to conduct experiments in Paris.
The Montgolfiers were undoubtedly first to send up balloons, but other experimenters were not far behind them, and before they could get to Paris in response to their invitation, Charles, a prominent physicist of those days, had constructed a balloon of silk, which he proofed against escape of gas with rubber—the Roberts had just succeeded in dissolving this substance to permit of making a suitable coating for the silk. With a quarter of a ton of sulphuric acid, and half a ton of iron filings and turnings, sufficient hydrogen was generated in four days to fill Charles's balloon, which went up on August 28th, 1783. Although the day was wet, Paris turned out to the number of over 300,000 in the Champs de Mars, and cannon were fired to announce the ascent of the balloon. This, rising very rapidly, disappeared amid the rain clouds, but, probably bursting through no outlet being provided to compensate for the escape of gas, fell soon in the neighbourhood of Paris. Here peasants, ascribing evil supernatural influence to the fall of such a thing from nowhere, went at it with the implements of their craft—forks, hoes, and the like—and maltreated it severely, finally attaching it to a horse's tail and dragging it about until it was mere rag and scrap.
Meanwhile, Joseph Montgolfier, having come to Paris, set about the construction of a balloon out of linen; this was in three diverse sections, the top being a cone 30 feet in depth, the middle a cylinder 42 feet in diameter by 26 feet in depth, and the bottom another cone 20 feet in depth from junction with the cylindrical portion to its point. The balloon was both lined and covered with paper, decorated in blue and gold. Before ever an ascent could be attempted this ambitious balloon was caught in a heavy rainstorm which reduced its paper covering to pulp and tore the linen at its seams, so that a supervening strong wind tore the whole thing to shreds.
Montgolfier's next balloon was spherical, having a capacity of 52,000 cubic feet. It was made from waterproofed linen, and on September 19th, 1783, it made an ascent for the palace courtyard at Versailles, taking up as passengers a cock, a sheep, and a duck. A rent at the top of the balloon caused it to descend within eight minutes, and the duck and sheep were found none the worse for being the first living things to leave the earth in a balloon, but the cock, evidently suffering, was thought to have been affected by the rarefaction of the atmosphere at the tremendous height reached—for at that time the general opinion was that the atmosphere did not extend more than four or five miles above the earth's surface. It transpired later that the sheep had trampled on the cock, causing more solid injury than any that might be inflicted by rarefied air in an eight-minute ascent and descent of a balloon.
For achieving this flight Joseph Montgolfier received from the King of France a pension of of L40, while Stephen was given the order of St Michael, and a patent of nobility was granted to their father. They were made members of the Legion d'Honneur, and a scientific deputation, of which Faujas de Saint-Fond, who had raised the funds with which Charles's hydrogen balloon was constructed, presented to Stephen Montgolfier a gold medal struck in honour of his aerial conquest. Since Joseph appears to have had quite as much share in the success as Stephen, the presentation of the medal to one brother only was in questionable taste, unless it was intended to balance Joseph's pension.
Once aerostation had been proved possible, many people began the construction of small balloons—the wholehole thing was regarded as a matter of spectacles and a form of amusement by the great majority. A certain Baron de Beaumanoir made the first balloon of goldbeaters' skin, this being eighteen inches in diameter, and using hydrogen as a lifting factor. Few people saw any possibilities in aerostation, in spite of the adventures of the duck and sheep and cock; voyages to the moon were talked and written, and there was more of levity than seriousness over ballooning as a rule. The classic retort of Benjamin Franklin stands as an exception to the general rule: asked what was the use of ballooning—'What's the use of a baby?' he countered, and the spirit of that reply brought both the dirigible and the aeroplane to being, later.
The next noteworthy balloon was one by Stephen Montgolfier, designed to take up passengers, and therefore of rather large dimensions, as these things went then. The capacity was 100,000 cubic feet, the depth being 85 feet, and the exterior was very gaily decorated. A short, cylindrical opening was made at the lower extremity, and under this a fire-pan was suspended, above the passenger car of the balloon. On October 15th, 1783, Pilatre de Rozier made the first balloon ascent—but the balloon was held captive, and only allowed to rise to a height of 80 feet. But, a little later in 1783, Rozier secured the honour of making the first ascent in a free balloon, taking up with him the Marquis d'Arlandes. It had been originally intended that two criminals, condemned to death, should risk their lives in the perilous venture, with the prospect of a free pardon if they made a safe descent, but d'Arlandes got the royal consent to accompany Rozier, and the criminals lost their chance. Rozier and d'Arlandes made a voyage lasting for twenty-five minutes, and, on landing, the balloon collapsed with such rapidity as almost to suffocate Rozier, who, however, was dragged out to safety by d'Arlandes. This first aerostatic journey took place on November 21st, 1783.
Some seven months later, on June 4th, 1784, a Madame Thible ascended in a free balloon, reaching a height of 9,000 feet, and making a journey which lasted for forty-five minutes—the great King Gustavus of Sweden witnessed this ascent. France grew used to balloon ascents in the course of a few months, in spite of the brewing of such a storm as might have been calculated to wipe out all but purely political interests. Meanwhile, interest in the new discovery spread across the Channel, and on September 15th, 1784, one Vincent Lunardi made the first balloon voyage in England, starting from the Artillery Ground at Chelsea, with a cat and dog as passengers, and landing in a field in the parish of Standon, near Ware. There is a rather rare book which gives a very detailed account of this first ascent in England, one copy of which is in the library of the Royal Aeronautical Society; the venturesome Lunardi won a greater measure of fame through his exploit than did Cody for his infinitely more courageous and—from a scientific point of view—valuable first aeroplane ascent in this country.
The Montgolfier type of balloon, depending on hot air for its lifting power, was soon realised as having dangerous limitations. There was always a possibility of the balloon catching fire while it was being filled, and on landing there was further danger from the hot pan which kept up the supply of hot air on the voyage—the collapsing balloon fell on the pan, inevitably. The scientist Saussure, observing the filling of the balloons very carefully, ascertained that it was rarefaction of the air which was responsible for the lifting power, and not the heat in itself, and, owing to the rarefaction of the air at normal temperature at great heights above the earth, the limit of ascent for a balloon of the Montgolfier type was estimated by him at under 9,000 feet. Moreover, since the amount of fuel that could be carried for maintaining the heat of the balloon after inflation was subject to definite limits, prescribed by the carrying capacity of the balloon, the duration of the journey was necessarily limited just as strictly.
These considerations tended to turn the minds of those interested in aerostation to consideration of the hydrogen balloon evolved by Professor Charles. Certain improvements had been made by Charles since his first construction; he employed rubber-coated silk in the construction of a balloon of 30 feet diameter, and provided a net for distributing the pressure uniformly over the surface of the envelope; this net covered the top half of the balloon, and from its lower edge dependent ropes hung to join on a wooden ring, from which the car of the balloon was suspended—apart from the extension of the net so as to cover in the whole of the envelope, the spherical balloon of to-day is virtually identical with that of Charles in its method of construction. He introduced the valve at the top of the balloon, by which escape of gas could be controlled, operating his valve by means of ropes which depended to the car of the balloon, and he also inserted a tube, of about 7 inches diameter, at the bottom of the balloon, not only for purposes of inflation, but also to provide a means of escape for gas in case of expansion due to atmospheric conditions.
Sulphuric acid and iron filings were used by Charles for filling his balloon, which required three days and three nights for the generation of its 14,000 cubic feet of hydrogen gas. The inflation was completed on December 1st, 1783, and the fittings carried included a barometer and a grapnel form of anchor. In addition to this, Charles provided the first 'ballon sonde' in the form of a small pilot balloon which he handed to Montgolfier to launch before his own ascent, in order to determine the direction and velocity of the wind. It was a graceful compliment to his rival, and indicated that, although they were both working to the one end, their rivalry was not a matter of bitterness.
Ascending on December 1st, 1783, Charles took with him one of the brothers Robert, and with him made the record journey up to that date, covering a period of three and three-quarter hours, in which time they journeyed some forty miles. Robert then landed, and Charles ascended again alone, reaching such a height as to feel the effects of the rarefaction of the air, this very largely due to the rapidity of his ascent. Opening the valve at the top of the balloon, he descended thirty-five minutes after leaving Robert behind, and came to earth a few miles from the point of the first descent. His discomfort over the rapid ascent was mainly due to the fact that, when Robert landed, he forgot to compensate for the reduction of weight by taking in further ballast, but the ascent proved the value of the tube at the bottom of the balloon envelope, for the gas escaped very rapidly in that second ascent, and, but for the tube, the balloon must inevitably have burst in the air, with fatal results for Charles.
As in the case of aeroplane flight, as soon as the balloon was proved practicable the flight across the English Channel was talked of, and Rozier, who had the honour of the first flight, announced his intention of being first to cross. But Blanchard, who had an idea for a 'flying car,' anticipated him, and made a start from Dover on January 7th, 1785, taking with him an American doctor named Jeffries. Blanchard fitted out his craft for the journey very thoroughly, taking provisions, oars, and even wings, for propulsion in case of need. He took so much, in fact, that as soon as the balloon lifted clear of the ground the whole of the ballast had to be jettisoned, lest the balloon should drop into the sea. Half-way across the Channel the sinking of the balloon warned Blanchard that he had to part with more than ballast to accomplish the journey, and all the equipment went, together with certain books and papers that were on board the car. The balloon looked perilously like collapsing, and both Blanchard and Jeffries began to undress in order further to lighten their craft—Jeffries even proposed a heroic dive to save the situation, but suddenly the balloon rose sufficiently to clear the French coast, and the two voyagers landed at a point near Calais in the Forest of Gaines, where a marble column was subsequently erected to commemorate the great feat.
Rozier, although not first across, determined to be second, and for that purpose he constructed a balloon which was to owe its buoyancy to a combination of the hydrogen and hot air principles. There was a spherical hydrogen balloon above, and beneath it a cylindrical container which could be filled with hot air, thus compensating for the leakage of gas from the hydrogen portion of the balloon—regulating the heat of his fire, he thought, would give him perfect control in the matter of ascending and descending.
On July 6th, 1785, a favourable breeze gave Rozier his opportunity of starting from the French coast, and with a passenger aboard he cast off in his balloon, which he had named the 'Aero-Montgolfiere.' There was a rapid rise at first, and then for a time the balloon remained stationary over the land, after which a cloud suddenly appeared round the balloon, denoting that an explosion had taken place. Both Rozier and his companion were killed in the fall, so that he, first to leave the earth by balloon, was also first victim to the art of aerostation.
There followed, naturally, a lull in the enthusiasm with which ballooning had been taken up, so far as France was concerned. In Italy, however, Count Zambeccari took up hot-air ballooning, using a spirit lamp to give him buoyancy, and on the first occasion when the balloon car was set on fire Zambeccari let down his passenger by means of the anchor rope, and managed to extinguish the fire while in the air. This reduced the buoyancy of the balloon to such an extent that it fell into the Adriatic and was totally wrecked, Zambeccari being rescued by fishermen. He continued to experiment up to 1812, when he attempted to ascend at Bologna; the spirit in his lamp was upset by the collision of the car with a tree, and the car was again set on fire. Zambeccari jumped from the car when it was over fifty feet above level ground, and was killed. With him the Rozier type of balloon, combining the hydrogen and hot air principles, disappeared; the combination was obviously too dangerous to be practical.
The brothers Robert were first to note how the heat of the sun acted on the gases within a balloon envelope, and it has since been ascertained that sun rays will heat the gas in a balloon to as much as 80 degrees Fahrenheit greater temperature than the surrounding atmosphere; hydrogen, being less affected by change of temperature than coal gas, is the most suitable filling element, and coal gas comes next as the medium of buoyancy. This for the free and non-navigable balloon, though for the airship, carrying means of combustion, and in military work liable to ignition by explosives, the gas helium seems likely to replace hydrogen, being non-combustible.
In spite of the development of the dirigible airship, there remains work for the free, spherical type of balloon in the scientific field. Blanchard's companion on the first Channel crossing by balloon, Dr Jeffries, was the first balloonist to ascend for purely scientific purposes; as early as 1784 he made an ascent to a height of 9,000 feet, and observed a fall in temperature of from degrees—at the level of London, where he began his ascent—to 29 degrees at the maximum height reached. He took up an electrometer, a hydrometer, a compass, a thermometer, and a Toricelli barometer, together with bottles of water, in order to collect samples of the air at different heights. In 1785 he made a second ascent, when trigonometrical observations of the height of the balloon were made from the French coast, giving an altitude of 4,800 feet.
The matter was taken up on its scientific side very early in America, experiments in Philadelphia being almost simultaneous with those of the Montgolfiers in France. The flight of Rozier and d'Arlandes inspired two members of the Philadelphia Philosophical Academy to construct a balloon or series of balloons of their own design; they made a machine which consisted of no less than 47 small hydrogen balloons attached to a wicker car, and made certain preliminary trials, using animals as passengers. This was followed by a captive ascent with a man as passenger, and eventually by the first free ascent in America, which was undertaken by one James Wilcox, a carpenter, on December 28th, 1783. Wilcox, fearful of falling into a river, attempted to regulate his landing by cutting slits in some of the supporting balloons, which was the method adopted for regulating ascent or descent in this machine. He first cut three, and then, finding that the effect produced was not sufficient, cut three more, and then another five—eleven out of the forty-seven. The result was so swift a descent that he dislocated his wrist on landing.
Meusnier, toward the end of the eighteenth century, was first to conceive the idea of compensating for the loss of gas due to expansion by fitting to the interior of a free balloon a ballonet, or air bag, which could be pumped full of air so as to retain the shape and rigidity of the envelope.
The ballonet became particularly valuable as soon as airship construction became general, and it was in the course of advance in Astra Torres design that the project was introduced of using the ballonets in order to give inclination from the horizontal. In the earlier Astra Torres, trimming was accomplished by moving the car fore and aft—this in itself was an advance on the separate 'sliding weigh' principle—and this was the method followed in the Astra Torres bought by the British Government from France in 1912 for training airship pilots. Subsequently, the two ballonets fitted inside the envelope were made to serve for trimming by the extent of their inflation, and this method of securing inclination proved the best until exterior rudders, and greater engine power, supplanted it, as in the Zeppelin and, in fact, all rigid types.
In the kite balloon, the ballonet serves the purpose of a rudder, filling itself through the opening being kept pointed toward the wind—there is an ingenious type of air scoop with non-return valve which assures perfect inflation. In the S.S. type of airship, two ballonets are provided, the supply of air being taken from the propeller draught by a slanting aluminium tube to the underside of the envelope, where it meets a longitudinal fabric hose which connects the two ballonet air inlets. In this hose the non-return air valves, known as 'crab-pots,' are fitted, on either side of the junction with the air-scoop. Two automatic air valves, one for each ballonet, are fitted in the underside of the envelope, and, as the air pressure tends to open these instead of keeping them shut, the spring of the valve is set inside the envelope. Each spring is set to open at a pressure of 25 to 28 mm.
Having got off the earth, the very early balloonists set about the task of finding a means of navigating the air but, lacking steam or other accessory power to human muscle, they failed to solve the problem. Joseph Montgolfier speedily exploded the idea of propelling a balloon either by means of oars or sails, pointing out that even in a dead calm a speed of five miles an hour would be the limit achieved. Still, sailing balloons were constructed, even up to the time of Andree, the explorer, who proposed to retard the speed of the balloon by ropes dragging on the ground, and then to spread a sail which should catch the wind and permit of deviation of the course. It has been proved that slight divergences from the course of the wind can be obtained by this means, but no real navigation of the air could be thus accomplished.
Professor Wellner, of Brunn, brought up the idea of a sailing balloon in more practical fashion in 1883. He observed that surfaces inclined to the horizontal have a slight lateral motion in rising and falling, and deduced that by alternate lowering and raising of such surfaces he would be able to navigate the air, regulating ascent and descent by increasing or decreasing the temperature of his buoyant medium in the balloon. He calculated that a balloon, 50 feet in diameter and 150 feet in length, with a vertical surface in front and a horizontal surface behind, might be navigated at a speed of ten miles per hour, and in actual tests at Brunn he proved that a single rise and fall moved the balloon three miles against the wind. His ideas were further developed by Lebaudy in the construction of the early French dirigibles.
According to Hildebrandt,[*] the first sailing balloon was built in 1784 by Guyot, who made his balloon egg-shaped, with the smaller end at the back and the longer axis horizontal; oars were intended to propel the craft, and naturally it was a failure. Carra proposed the use of paddle wheels, a step in the right direction, by mounting them on the sides of the car, but the improvement was only slight. Guyton de Morveau, entrusted by the Academy of Dijon with the building of a sailing balloon, first used a vertical rudder at the rear end of his construction—it survives in the modern dirigible. His construction included sails and oars, but, lacking steam or other than human propulsive power, the airship was a failure equally with Guyot's.
[*] Airships Past and Present.
Two priests, Miollan and Janinet, proposed to drive balloons through the air by the forcible expulsion of the hot air in the envelope from the rear of the balloon. An opening was made about half-way up the envelope, through which the hot air was to escape, buoyancy being maintained by a pan of combustibles in the car. Unfortunately, this development of the Montgolfier type never got a trial, for those who were to be spectators of the first flight grew exasperated at successive delays, and in the end, thinking that the balloon would never rise, they destroyed it.
Meusnier, a French general, first conceived the idea of compensating for loss of gas by carrying an air bag inside the balloon, in order to maintain the full expansion of the envelope. The brothers Robert constructed the first balloon in which this was tried and placed the air bag near the neck of the balloon which was intended to be driven by oars, and steered by a rudder. A violent swirl of wind which was encountered on the first ascent tore away the oars and rudder and broke the ropes which held the air bag in position; the bag fell into the opening of the neck and stopped it up, preventing the escape of gas under expansion. The Duc de Chartres, who was aboard, realised the extreme danger of the envelope bursting as the balloon ascended, and at 16,000 feet he thrust a staff through the envelope—another account says that he slit it with his sword—and thus prevented disaster. The descent after this rip in the fabric was swift, but the passengers got off without injury in the landing.
Meusnier, experimenting in various ways, experimented with regard to the resistance offered by various shapes to the air, and found that an elliptical shape was best; he proposed to make the car boat—shaped, in order further to decrease the resistance, and he advocated an entirely rigid connection between the car and the body of the balloon, as indispensable to a dirigible.[*] He suggested using three propellers, which were to be driven by hand by means of pulleys, and calculated that a crew of eighty would be required to furnish sufficient motive power. Horizontal fins were to be used to assure stability, and Meusnier thoroughly investigated the pressures exerted by gases, in order to ascertain the stresses to which the envelope would be subjected. More important still, he went into detail with regard to the use of air bags, in order to retain the shape of the balloon under varying pressures of gas due to expansion and consequent losses; he proposed two separate envelopes, the inner one containing gas, and the space between it and the outer one being filled with air. Further, by compressing the air inside the air bag, the rate of ascent or descent could be regulated. Lebaudy, acting on this principle, found it possible to pump air at the rate of 35 cubic feet per second, thus making good loss of ballast which had to be thrown overboard.
[*] Hildebrandt.
Meusnier's balloon, of course, was never constructed, but his ideas have been of value to aerostation up to the present time. His career ended in the revolutionary army in 1793, when he was killed in the fighting before Mayence, and the King of Prussia ordered all firing to cease until Meusnier had been buried. No other genius came forward to carry on his work, and it was realised that human muscle could not drive a balloon with certainty through the air; experiment in this direction was abandoned for nearly sixty years, until in 1852 Giffard brought the first practicable power-driven dirigible to being.
Giffard, inventor of the steam injector, had already made balloon ascents when he turned to aeronautical propulsion, and constructed a steam engine of 5 horsepower with a weight of only 100 lbs.—a great achievement for his day. Having got his engine, he set about making the balloon which it was to drive; this he built with the aid of two other enthusiasts, diverging from Meusnier's ideas by making the ends pointed, and keeping the body narrowed from Meusnier's ellipse to a shape more resembling a rather fat cigar. The length was 144 feet, and the greatest diameter only 40 feet, while the capacity was 88,000 cubic feet. A net which covered the envelope of the balloon supported a spar, 66 feet in length, at the end of which a triangular sail was placed vertically to act as rudder. The car, slung 20 feet below the spar, carried the engine and propeller. Engine and boiler together weighed 350 lbs., and drove the 11 foot propeller at 110 revolutions per minute.
As precaution against explosion, Giffard arranged wire gauze in front of the stoke-hole of his boiler, and provided an exhaust pipe which discharged the waste gases from the engine in a downward direction. With this first dirigible he attained to a speed of between 6 and 8 feet per second, thus proving that the propulsion of a balloon was a possibility, now that steam had come to supplement human effort.
Three years later he built a second dirigible, reducing the diameter and increasing the length of the gas envelope, with a view to reducing air resistance. The length of this was 230 feet, the diameter only 33 feet, and the capacity was 113,000 cubic feet, while the upper part of the envelope, to which the covering net was attached, was specially covered to ensure a stiffening effect. The car of this dirigible was dropped rather lower than that of the first machine, in order to provide more thoroughly against the danger of explosions. Giffard, with a companion named Yon as passenger, took a trial trip on this vessel, and made a journey against the wind, though slowly. In commencing to descend, the nose of the envelope tilted upwards, and the weight of the car and its contents caused the net to slip, so that just before the dirigible reached the ground, the envelope burst. Both Giffard and his companion escaped with very slight injuries.
Plans were immediately made for the construction of a third dirigible, which was to be 1,970 feet in length, 98 feet in extreme diameter, and to have a capacity of 7,800,000 cubic feet of gas. The engine of this giant was to have weighed 30 tons, and with it Giffard expected to attain a speed of 40 miles per hour. Cost prevented the scheme being carried out, and Giffard went on designing small steam engines until his invention of the steam injector gave him the funds to turn to dirigibles again. He built a captive balloon for the great exhibition in London in 1868, at a cost of nearly L30,000, and designed a dirigible balloon which was to have held a million and three quarters cubic feet of gas, carry two boilers, and cost about L40,000. The plans were thoroughly worked out, down to the last detail, but the dirigible was never constructed. Giffard went blind, and died in 1882—he stands as the great pioneer of dirigible construction, more on the strength of the two vessels which he actually built than on that of the ambitious later conceptions of his brain.
In 1872 Dupuy de Lome, commissioned by the French government, built a dirigible which he proposed to drive by man-power—it was anticipated that the vessel would be of use in the siege of Paris, but it was not actually tested till after the conclusion of the war. The length of this vessel was 118 feet, its greatest diameter 49 feet, the ends being pointed, and the motive power was by a propeller which was revolved by the efforts of eight men. The vessel attained to about the same speed as Giffard's steam-driven airship; it was capable of carrying fourteen men, who, apart from these engaged in driving the propeller, had to manipulate the pumps which controlled the air bags inside the gas envelope.
In the same year Paul Haenlein, working in Vienna, produced an airship which was a direct forerunner of the Lebaudy type, 164 feet in length, 30 feet greatest diameter, and with a cubic capacity of 85,000 feet. Semi-rigidity was attained by placing the car as close to the envelope as possible, suspending it by crossed ropes, and the motive power was a gas engine of the Lenoir type, having four horizontal cylinders, and giving about 5 horse-power with a consumption of about 250 cubic feet of gas per hour. This gas was sucked from the envelope of the balloon, which was kept fully inflated by pumping in compensating air to the air bags inside the main envelope. A propeller, 15 feet in diameter, was driven by the Lenoir engine at 40 revolutions per minute. This was the first instance of the use of an internal combustion engine in connection with aeronautical experiments.
The envelope of this dirigible was rendered airtight by means of internal rubber coating, with a thinner film on the outside. Coal gas, used for inflation, formed a suitable fuel for the engine, but limited the height to which the dirigible could ascend. Such trials as were made were carried out with the dirigible held captive, and a speed of I 5 feet per second was attained. Full experiment was prevented through funds running low, but Haenlein's work constituted a distinct advance on all that had been done previously.
Two brothers, Albert and Gaston Tissandier, were next to enter the field of dirigible construction; they had experimented with balloons during the Franc-Prussian War, and had attempted to get into Paris by balloon during the siege, but it was not until 1882 that they produced their dirigible.
This was 92 feet in length and 32 feet in greatest diameter, with a cubic capacity of 37,500 feet, and the fabric used was varnished cambric. The car was made of bamboo rods, and in addition to its crew of three, it carried a Siemens dynamo, with 24 bichromate cells, each of which weighed 17 lbs. The motor gave out 1 1/2 horse-power, which was sufficient to drive the vessel at a speed of up to 10 feet per second. This was not so good as Haenlein's previous attempt and, after L2,000 had been spent, the Tissandier abandoned their experiments, since a 5-mile breeze was sufficient to nullify the power of the motor.
Renard, a French officer who had studied the problem of dirigible construction since 1878, associated himself first with a brother officer named La Haye, and subsequently with another officer, Krebs, in the construction of the second dirigible to be electrically-propelled. La Haye first approached Colonel Laussedat, in charge of the Engineers of the French Army, with a view to obtaining funds, but was refused, in consequence of the practical failure of all experiments since 1870. Renard, with whom Krebs had now associated himself, thereupon went to Gambetta, and succeeded in getting a promise of a grant of L8,000 for the work; with this promise Renard and Krebs set to work.
They built their airship in torpedo shape, 165 feet in length, and of just over 27 feet greatest diameter—the greatest diameter was at the front, and the cubic capacity was 66,000 feet. The car itself was 108 feet in length, and 4 1/2 feet broad, covered with silk over the bamboo framework. The 23 foot diameter propeller was of wood, and was driven by an electric motor connected to an accumulator, and yielding 8.5 horsepower. The sweep of the propeller, which might have brought it in contact with the ground in landing, was counteracted by rendering it possible to raise the axis on which the blades were mounted, and a guide rope was used to obviate damage altogether, in case of rapid descent. There was also a 'sliding weight' which was movable to any required position to shift the centre of gravity as desired. Altogether, with passengers and ballast aboard, the craft weighed two tons.
In the afternoon of August 8th, 1884, Renard and Krebs ascended in the dirigible—which they had named 'La France,' from the military ballooning ground at Chalais-Meudon, making a circular flight of about five miles, the latter part of which was in the face of a slight wind. They found that the vessel answered well to her rudder, and the five-mile flight was made successfully in a period of 23 minutes. Subsequent experimental flights determined that the air speed of the dirigible was no less than 14 1/2 miles per hour, by far the best that had so far been accomplished in dirigible flight. Seven flights in all were made, and of these five were completely successful, the dirigible returning to its starting point with no difficulty. On the other two flights it had to be towed back.
Renard attempted to repeat his construction on a larger scale, but funds would not permit, and the type was abandoned; the motive power was not sufficient to permit of more than short flights, and even to the present time electric motors, with their necessary accumulators, are far too cumbrous to compete with the self-contained internal combustion engine. France had to wait for the Lebaudy brothers, just as Germany had to wait for Zeppelin and Parseval.
Two German experimenters, Baumgarten and Wolfert, fitted a Daimler motor to a dirigible balloon which made its first ascent at Leipzig in 1880. This vessel had three cars, and placing a passenger in one of the outer cars[*] distributed the load unevenly, so that the whole vessel tilted over and crashed to the earth, the occupants luckily escaping without injury. After Baumgarten's death, Wolfert determined to carry on with his experiments, and, having achieved a certain measure of success, he announced an ascent to take place on the Tempelhofer Field, near Berlin, on June 12th, 1897. The vessel, travelling with the wind, reached a height of 600 feet, when the exhaust of the motor communicated flame to the envelope of the balloon, and Wolfert, together with a passenger he carried, was either killed by the fall or burnt to death on the ground. Giffard had taken special precautions to avoid an accident of this nature, and Wolfert, failing to observe equal care, paid the full penalty.
[*] Hildebrandt.
Platz, a German soldier, attempting an ascent on the Tempelhofer Field in the Schwartz airship in 1897, merely proved the dirigible a failure. The vessel was of aluminium, 0.008 inch in thickness, strengthened by an aluminium lattice work; the motor was two-cylindered petrol-driven; at the first trial the metal developed such leaks that the vessel came to the ground within four miles of its starting point. Platz, who was aboard alone as crew, succeeded in escaping by jumping clear before the car touched earth, but the shock of alighting broke up the balloon, and a following high wind completed the work of full destruction. A second account says that Platz, finding the propellers insufficient to drive the vessel against the wind, opened the valve and descended too rapidly.
The envelope of this dirigible was 156 feet in length, and the method of filling was that of pushing in bags, fill them with gas, and then pulling them to pieces and tearing them out of the body of the balloon. A second contemplated method of filling was by placing a linen envelope inside the aluminium casing, blowing it out with air, and then admitting the gas between the linen and the aluminium outer casing. This would compress the air out of the linen envelope, which was to be withdrawn when the aluminium casing had been completely filled with gas.
All this, however, assumes that the Schwartz type—the first rigid dirigible, by the way—would prove successful. As it proved a failure on the first trial, the problem of filling it did not arise again.
By this time Zeppelin, retired from the German army, had begun to devote himself to the study of dirigible construction, and, a year after Schwartz had made his experiment and had failed, he got together sufficient funds for the formation of a limitedliability company, and started on the construction of the first of his series of airships. The age of tentative experiment was over, and, forerunner of the success of the heavier-than-air type of flying machine, successful dirigible flight was accomplished by Zeppelin in Germany, and by Santos-Dumont in France.