DEVELOPING THE AEROPLANE
THE opening of the twentieth century found the world well prepared for actual conquest of the air. Aviation has been developed to an exact science. It had taken centuries of failure to teach man that he could not fly by flapping his wings like the birds but the idea was at last abandoned. The birds were still the models of the heavier-than-air machines, but man had at last learned to study them more intelligently. The marvellous development of modern mechanics, especially the building of light and efficient motors, was also of great importance. The theory of the aëroplane was rapidly gaining in favor.
It was thought at one time that since no birds weighed more than fifty pounds no flying machine heavier than this could ever fly. Some years ago Hiram S. Maxim pointed out, however, that if we had built our steam engines to imitate the horse, as we then hoped to build flying machines like the birds, we would have built locomotives which weighed only five tons, the weight of an elephant, which walked five miles an hour. The secret of flight evidently did not lie in closely imitating the familiar forms of flight. So far as man was interested it lay clearly in the soaring flights. When a bird flies with extended wings it does two things. It forms an aëroplane which supports its body, much the same as a kite, and it operates a propeller for driving this aëroplane forward. And so men finally learned to fly by borrowing a single principle from the birds.
It is claimed by some that the theory, and largely the form, of the modern successful aëroplane was first suggested by an English inventor, Sir George Cayley, as early as 1796. Cayley argued that a flat plane or surface when driven through the air inclined slightly upward would lift a considerable weight. He also suggested that a tail would help to steady the plane as well as steer it upward or downward. His ideas of propelling the aëroplane by screws driven by motors was also far in advance of his time, but the engines then in existence were much too heavy for the purpose and he never built a model.
Fifty years later, when the steam engine had been highly developed, these old plans were remembered and two engineers, Hensen and Stringfellow, actually built a flying machine on Cayley’s principles. This early aëroplane was of the monoplane form, made of oiled silk stretched over a frame of bamboo. A car to carry a steam engine, and presumably the passengers, was hung below this plane. The motive power was supplied by two propellers at the rear. The aëroplane carried a fan-shaped tail with a rudder for steering it sideways, placed beneath. The model is said to have actually flown for a short distance, but proved to be unstable.
From this time onward the experiments became more scientific and accurate. Reliable scientific data was accumulated which later enabled the aviators to build practical aëroplanes. A number of interesting experiments were made shortly afterward by a scientist named Wenham to prove that the lifting powers of a carrying surface might be increased by arranging small surfaces in tiers one above another. Wenham had watched the birds to some purpose, and decided that a single plane, large enough to support a man would be too large to control, but that a number of small surfaces would make the bird flight possible. Wenham built and patented a machine in 1866. He never flew but he collected a great deal of valuable information about the behavior of planes.
PLATE XIV.
The Under Body of the Monoplane Shown, Plate XIII.
The slow, but on the whole, encouraging movement toward the successful flying machine was given a serious set back in 1872 by a book written by H. Von Humboldt announcing the result of his experiments. This well known scientist, whose name carried great weight, wrote that mechanical flight was impossible. He based his idea on the discovery that as the body increased in size the work or power required to lift it increased more rapidly than the size of the body. In other words, a very large bird or flying machine could not contain muscles strong enough or machinery strong enough to enable it to fly. He argued that no bird larger than the albatross, for instance, had ever lived, therefore no flying machines could ever be more than toys. The book was so discouraging that many aviators gave up their experiments and the science of aviation stood still.
It may be said to have been awakened, however, by the German scientist, Otto Lilenthal, whose book, published in 1886, at once attracted world wide attention. It was this book, incidentally, which inspired the Wright Brothers to begin their experiments. Lilenthal was not only a great scientist, but he worked on the principle that an ounce of actual experience was worth a ton of theory. In aviation, where the weight is all important, this saving was naturally of the greatest importance. Lilenthal built gliders, many of them, and put to actual test the theories which others had merely talked and figured about. Finally he set up an engine on a glider but the machine turned over and he was instantly killed. The scientific information he collected, however, proved of the highest value to those who later actually conquered the air.
Lilenthal built a hill fifty feet in height and shaped like a cone with sides slanting at an angle of thirty degrees. Here he proved by actual tests that he might fly no matter which way the wind blew and that an arched surface, driven against the wind, would rise from the ground and support his weight. A great deal of scientific information was collected and tabulated as well as the exact effect of the pressure of the air. He also changed the shape of the gliding surfaces, making them very long and narrow and driving them edgewise as in the first form of aëroplane. The aëroplane took shape in his hands. The success of these experiments encouraged aviators in many countries to imitate him, and so great was the interest aroused that even his fatal accident in 1896 did not discourage them. The successful flying machine was now actually in sight.
PLATE XV.
A Simple Model which Proves Steady in Flight.
For a time it was believed that Hiram S. Maxim would be the first to construct a flying machine which would actually fly. He had gone about the problem in a thoroughly scientific manner, sparing neither time nor expense. An elaborate apparatus was first constructed like a revolving derrick, to test accurately the lifting powers of various aëroplanes of various sizes and shapes flying at different angles, as well as the propelling force of many kinds of screws. The horizontal arm of this machine was thirty feet, nine inches long, so that it described a circle of 200 feet in circumference. The arm was driven by an engine at high speed.
The various aëroplane forms to be tested were attached to the extreme end of this arm, and driven by propellers of various shapes and sizes, exactly as they would be in actual flight. Every part of the machine, meanwhile, was so adjusted that the readings of the speed of the aëroplane, its lifting power, the exact force of the propeller, in fact, every detail, could be measured and recorded with scientific accuracy. This preliminary work proved to be of the highest value. The test showed, for instance, just what size the propeller should be for different size planes, and the exact pitch of the screw which would give the best results, the proper angle of elevation for the front plane, the resistance offered by various shaped planes, and the exact amount of power required for planes of different sizes. A delicate machine was also built to test the different kinds of fabrics used for covering the planes. The fabric was stretched over a small steel frame, mounted at a slight angle, in a blast of air. The tendency of the cloth to lift or drift was then accurately measured. The material which gave the greatest amount of lift and the least drift was used.
A large aëroplane was finally built in 1893. It weighed 7500 pounds, measured 104 feet in width, and was driven by a 360 horsepower engine. Compared with the clear cut, ship-shape air-craft of to-day this early model appears crude and cumbersome. The main plane was almost square in shape, while stability planes extended out from the sides. A series of four narrow planes, one above another, were carried below on either side. The machinery for driving was carried far below the main plane. The two large propellers were placed in the stern. The aëroplane was run along a double-tracked railroad 1800 feet in length, to gather sufficient momentum to cause it to rise. Almost any school-boy of to-day familiar with the aëroplane models could have told at a glance that the machine could not rise. When it was finally sent down the track at a good clip, the front wheels did actually rise a trifle but it immediately came down with a bad smash.
Not in the least discouraged, Maxim at once designed a new machine. This measured fifty feet in width and forty feet in length in the middle, but with the corners cut off, so that it was sharpened both fore and aft. The wings were made long and narrow, extending out twenty-seven feet beyond the main plane, and large fore and aft rudders were attached. It was not even expected that the machine would fly. All that was hoped for was that it would lift somewhat so that its upward tendency might be accurately measured.
The most successful “flight” of this model will seem a very tame affair indeed to the boys of to-day who are daily reading of the marvellous voyages in air across sea and land. The “airship” was run over its track and the steam pressure run up to 329 pounds per square inch. The speed increased and the upward thrust began to be felt. Finally the front wheels of the machine actually lifted from the track. The rear axle rose three or four feet above its normal position. When it alighted, the delighted aëronauts found that the wheels of the machine had passed over the turf for a very short distance, without making any marks, showing that for a second or so the machine was actually off the earth. It seems curious to us to-day that this “flight” should have been considered remarkable.
PLATE XVI.
The Propeller and Shaft of the Model Shown, Plate XV.
The experiments carried out by S. P. Langley, beginning in 1887 and lasting for four years, placed a great deal of valuable, scientific data in the hands of the aviators. Thousands of tests were made with an apparatus similar to that used by Maxim. In one class of these experiments solid metal planes were attached to the end of the revolving arm in such a way that they were free to fall for a fixed distance. When in rapid, horizontal motion, the metal seemed to part with its weight, and the material, though one thousand times heavier than the air, was found to be actually supported by it. It was proven, for instance, that one horse power would support over 200 pounds weight of planes driven at a speed of fifty miles an hour.
All this preliminary work, or nearly all, we now see, was necessary before a practical aëroplane could be constructed. The early aviators, although they did not fly, at least showed what not to do, and several paid the price of their lives for this knowledge. Lilenthal had mapped out the aëroplane in the rough, and determined the general shape it must take. The experiments of Maxim and Langley enabled the successful aviators to calculate the size of the machine necessary to carry them and the amount of power required to drive it.