Part 5
Almost as soon as they began their trials of the glider, the brothers got another surprise. According to the Lilienthal tables of air pressures, their machine of 165 square feet needed a wind of only from seventeen to twenty-one miles an hour to support it as a kite with a pilot aboard. But they found that much stronger winds were needed to lift it. Since suitable winds would not be plentiful, their plan of practicing by the hour aboard the glider while flying it as a kite would have to be postponed. Instead, they flew it as a kite, loaded with about fifty pounds of chain, but with no man aboard. They held it with two ropes, and operated the balancing system by cords from the ground. Though the results were promising, inspiring confidence in the system of maintaining equilibrium, the brothers knew that only by actual gliding experience could they confirm what the kite experiments had indicated as being true.
One thing that puzzled them was that the machine appeared to be greatly deficient in lifting power as compared with calculated lift of curved surfaces of its size. In wondering what might be the cause of this wide discrepancy between expected and actual lifts, the Wrights considered the possibility that it might be because the curvature of the wings was less than that used by Lilienthal. Or could it be that the cloth covering was too porous and permitted some of the lifting power of the wind to be lost? They wondered, too, if the Lilienthal tables they had followed, relating to air pressure on wing surfaces, could be in error.
They next determined to try gliding on the side of a hill. That meant toting their machine four miles south of their camp to a great sand dune about one hundred feet high, called Kill Devil Hill. On their first day at the hill, the wind was about twenty-five miles an hour. As they lacked previous experience at gliding they decided to wait for less of a blow for their first attempt. The next day the wind had subsided to fourteen miles an hour, and they made about a dozen glides. “Bill” Tate was there and assisted them.
In making these glides, the machine was usually only two or three feet from the soft, sandy ground, and though the brothers repeatedly made landings while moving at a speed of twenty miles an hour, neither operator nor machine was harmed. The slope of the hill toward the northeast was about 9½ degrees, or a drop of approximately one foot in six. After moving at a rate of about twenty-five to thirty miles an hour with reference to the wind, or ten to fifteen miles over the ground, the machine, while keeping its course parallel to the slope, increased its speed, thus indicating that it could glide on a slope less steep.
Their control of the machine was even better than they had dared to expect. They got quick response to the slightest movement of the front elevator, which promised to be satisfactory in maintaining fore and aft balance. At first, they fastened the warping mechanism, to make it inoperable, and had only the elevator to manipulate, for they feared that, inexperienced as they were, if they tried to use both, then they might be unsuccessful with either. But even without the use of the warping mechanism it was possible to make glides of from five to ten seconds before the sidewise tilt of the machine forced a landing. Before making the last three or four flights the Wrights loosened the warping wires to permit the sidewise control to be used.
When these experiments of 1900 ended, instead of the hours of practice in the air the Wrights had hoped to have, they had flown the machine as a kite with a man aboard barely ten minutes, and had had only two minutes of actual gliding.
Now that the experiments for that year were ended and they had no further use for the glider, the brothers weighted the machine with sand and left it on the hill. When “Bill” Tate saw that they were through with the glider he asked if he might have it, and they gladly gave it to him. Mrs. Tate used the sateen that covered the wings to make dresses for her two small daughters. She noted that it appeared to be unusually good fabric, more closely woven and better than she had seen in the stores. Some of her neighbors, when they saw the dresses she made of it, remarked that it seemed too bad to use such excellent material on a kite.
Though the amount of practice was less than they had expected, all the Wrights had learned in that season of 1900 seemed to confirm the correctness of certain opinions held at the beginning. Their method of warping or twisting the wings to maintain lateral balance was better than dependence on either the dihedral angle or shifting the weight of the operator; better than any method yet tried. And their front elevator had been highly satisfactory as a means for directing the machine up and down. Before leaving Kitty Hawk they decided that their next experiments would be with a glider large enough to be flown as a kite, with an operator aboard, in winds ordinarily to be counted on.
When the brothers set to work on their glider for the experiments of 1901, they decided to make it of the same general design as the first one, and with the same system of control. But they carried out their plan to give it considerably more area, to provide greater lifting power. Another change they made was to increase the curvature of the wings to conform to the shape on which Lilienthal had based his tables of air pressures. It had wings of about seven-foot chord (the straight line distance between the front and rear edges) with a total span of twenty-two feet, and weighed ninety-eight pounds. After a section twenty inches wide had been removed from the middle of the lower wing, and the rear corners of the wings rounded off, the total lifting area was 290 square feet, as compared with 165 in the previous glider. The front elevator, with its rear edge about two and one half feet away from the front edge of the wings, had a four-and-one-half foot chord and an area of eighteen square feet.
This was a much larger machine than anyone had ever tried to fly. The Wrights knew it could not be controlled simply by shifting the pilot’s weight, as others had done, but they had faith in their own operable front elevator and believed they could manage it. If their calculations were correct, it would be supported in a wind of seventeen miles an hour, with the wing surfaces at an “angle of incidence” of only three degrees. (“Angle of incidence,” now more often called “angle of attack,” has been defined as the angle at which the plane presents itself to the air in advancing against it.)
As it would be impractical to keep so large a machine with them in the tent, as they had done with the smaller glider, the brothers built near Kill Devil Hill a rough frame shed, twenty-five feet long, sixteen feet wide, and seven feet high at the eaves. Both ends of the building except the gable parts were made into doors, hinged above. When open, the doors provided an awning at each end of the building. For living quarters they still used a tent. By driving a pipe ten or twelve feet into the sand they got a water supply.
Though the great stretch of sandy waste seemed too desolate for anyone to bother about owning, yet it was all under the ownership of one person or another and the Wrights took the precaution to obtain permission to erect their buildings.
This year they were to have company in camp. Octave Chanute, with whom they had been in correspondence for about a year, stopped in Dayton in June, 1901, at their invitation, to get better acquainted. When he learned that the Wrights had carried on their experiments in 1900 without the presence of a doctor in camp, and were intending to do so again, he told them he thought that was too risky, considering the kind of work and the isolation of the experiment ground. He said he knew a young man in Coatesville, Pennsylvania, George A. Spratt, “an amateur” in aeronautics, who had had some medical training. Spratt had never seen any gliding experiments, and Chanute thought he would be eager for the opportunity. If the Wrights would board him at camp, Chanute said, he would be glad to pay Spratt’s traveling expenses to Kitty Hawk and would consider himself “compensated by the pleasure given to him.” Chanute also proposed that they have in camp with them E. C. Huffaker, of Chuckey City, Tennessee, who was building a glider that Chanute was financing, and the Wrights consented. Thus there were four regularly in camp that season, and for a time Chanute himself was with them as a guest.
The new machine was completed and ready for trial on the afternoon of July 27. Since it was designed to be flown in a wind of seventeen miles, and there was but thirteen miles of wind on that day, the brothers took the machine to the big Kill Devil Hill for its first trial. After five or six short tuning-up flights they made a glide of 315 feet in nineteen seconds. Although several flights on this first day of experiments in 1901 exceeded the best made the year before, yet it was soon evident that in several respects the machine was not as good as the first one. It was found that the wings, with a camber of one to twelve--the camber recommended by Lilienthal, and used by Chanute and others--was not so good as the camber of one to twenty-two, used by the Wrights in 1900. (Camber of one to twenty-two means that the length of the chord, the straight-line distance between the front and rear edges of the wings, is twenty-two times the distance _from_ the chord to the deepest part of the wing curve.) This was demonstrated by the fact that the 1901 machine could not glide on a slope as nearly level as had the earlier machine. The Wrights found, too, that a machine with wings of one to twelve camber was not so easily controlled fore and aft as when the wings were of one to twenty-two camber. They decided therefore to reduce the camber of the wings to make them more like those of the earlier machine. When they resumed their gliding, after the camber had been reduced (one to eighteen), the control of the machine appeared to be as good as it was the year before, and they then made flights in winds of twenty-two to twenty-seven miles an hour, without accident. Though in most of these flights the lateral control was highly effective, in a few others--under conditions seemingly the same--the wing warping appeared to have no effect at all.
The Wrights now made the discovery that in free flight, when the wing on one side of the machine was presented to the wind at a greater angle than that on the other side, the wing with the greater angle, instead of rising as it was expected to do, sometimes descended. The explanation was that the greater angle of the wing at one side gave more resistance to forward motion and thus reduced the speed on that side. This decrease in speed more than counter-balanced the effect of the larger angle of the wing in producing lift. (The Wrights had not discovered this when flying the glider as a kite, because, when held by ropes, the wings always maintained equal air speeds, even when their resistances were unbalanced.)
It was evident to the brothers that their present method of controlling equilibrium was not yet complete. Something was needed to maintain equal speeds at the two wing tips. The idea occurred to them that the addition of a vertical fin attached to the machine at some distance in the rear of the wings might be the solution of the problem. But the test of such a fin had to be left until another season.
The behavior of the glider in these various flights forced the Wrights to give thought to another scientific problem, that regarding the center of air pressure on curved surfaces. Contrary to the teachings of scientific books on the subject, it was becoming more and more evident that the travel of the center of pressure on a cambered surface is not always in the same direction as the travel on a plane surface. When the angle of attack on a plane surface is decreased, the center of pressure moves toward the front edge; but on a cambered surface this is true only when larger angles are being decreased. When the angle of attack on a cambered surface is decreased from, say, thirty degrees to twenty-five degrees, the center of pressure moves forward, as it does on a plane surface; but when a certain angle (between twelve and fifteen degrees) is reached, then the movement of the center of pressure is reversed. From there on, the center of pressure moves toward the rear so long as any further decrease is made in the angle of attack. The Wrights proved this by a series of experiments with a single surface from their plane. Knowledge of the phenomenon of this reversal of center of pressure was of great importance to them in their later work of designing aeroplanes.
Scientific problems were not the only ones to perplex the Wrights. A sore trial were the mosquitoes and sandfleas, particularly numerous and aggressive in that summer of 1901. As Orville Wright recalled in later years, there were times when he thought, while fighting mosquitoes through the night, that if he could just survive until morning he would pack up and return home. Those mosquitoes might have caused a long postponement of the conquest of the air.
By the time they left Kitty Hawk on August 20, the brothers had satisfied themselves that a glider of large surfaces could be controlled almost as easily as a smaller one, provided the control is by manipulation of the surfaces themselves instead of by movements of the operator’s body. So far as they knew, judging from figures previously published, they had broken all records for distance in gliding. Chanute, who had witnessed part of the 1901 experiments, insisted that the results were better than had ever been attained before. All that was encouraging. But, on the other hand, if most of the supposedly scientific information available was worthless, then their task was even more formidable than they had expected. With no dependable previous knowledge to guide them, who were they to determine how man should fly? Wilbur seemed much discouraged. Possibly he had entertained hopes of actually flying, though he had always disclaimed having such an idea. He was ready to drop the experiments altogether. On the way home, Wilbur declared his belief: Not within a thousand years would man ever fly!
Chanute urged the brothers not to drop their experiments, arguing that if they did it would be a long time before anyone else would come as near to understanding the problem or how to work toward its solution. Without knowing it, Chanute made a great contribution to aviation history, for the Wrights heeded his repeated admonitions against ceasing their efforts. Without the proddings of Chanute they might not have gone on.
Chanute performed another great service for aeronautics when he, as president of the Western Society of Engineers, invited Wilbur Wright to address that body at a meeting in Chicago, September 18, 1901, on the subject: Some Aeronautical Experiments.
Wilbur shrank from the idea of making such a talk and would hardly have done so except to oblige his friend. He cautioned Chanute, though, not to make the speech a prominent feature of the program, because, he said, he made no pretense of being a public speaker. Chanute did nevertheless plan to use the announcement of the talk as a means to help make the meeting a big success. He wanted to know if it would be all right to make the occasion “Ladies’ Night.” Wilbur decided that he would already be as badly scared as a man could be and the presence of women would not make the situation much worse. But he insisted on one thing, that he must not be expected to appear in formal evening dress.
In this speech Wilbur boldly declared that the best sets of figures obtainable regarding air pressure against airplane surfaces appeared to contain many serious errors. Orville, at the shop in Dayton, was a little alarmed about that part of the speech. What if something about their own work had been wrong and the figures compiled by various scientists should finally be proved correct? Certainly it was no small responsibility for anyone so little known as Wilbur or he to denounce publicly the work of eminent scientists, dignified by preservation in books long regarded as authoritative. It would be both presumptuous and risky to brand supposedly established facts as untrue unless the person doing so could be unassailably sure of his ground.
In this cautious state of mind Orville rigged up a little wind-tunnel for the purpose of making a series of tests. This tunnel consisted simply of what appeared to be an old starch box, not more than eighteen inches long, that was lying in the shop. In it he placed a hastily constructed apparatus, a main part of which was simply a metal rod pivoted in the manner of a weather vane. Without attempting to give technical details of the method used, it may be said that a curved surface was balanced against a plane surface in an air current passing through the box. As Orville had provided the box with a glass top he could measure the angles to the wind at which the curved surface and the plane surface of equal area produced equal pressures.
The experiments with this crude apparatus lasted only one day. They were conclusive enough so far as they went, indicating errors in published figures relating to air pressure on curved surfaces. But as Orville was later to learn, the published errors were greatest in regard to wing surfaces set at small angles, such as would be used in flying, and he had tested thus far only larger angles. With the tests thus incomplete, Orville and Wilbur decided, on the latter’s return from Chicago, that it might be prudent to stay on the safe side and omit from the published record of Wilbur’s speech the more severe part of his criticism of available figures. They would wait until further wind-tunnel experiments could give more detailed knowledge. Consequently, when Wilbur’s speech appeared in the December, 1901, issue of the _Journal of the Western Society of Engineers_, it was a bit less startling than the one he had actually delivered--though, even after the deletions, there still remained strong hints that accepted tables of figures might be wrong. And the record of the speech was treated as of great importance. It has probably been reprinted and quoted as often as any other article ever written on the subject of flying.
The Wrights were not sure they would ever build another glider. But their curiosity, their passion for getting at truth, had now been too much aroused for them to quit studying the problem of air pressures. They decided to build another wind-tunnel, less crude than the one Orville had hastily used, and continue their experiments. The new tunnel consisted of an open-ended wooden box about sixteen inches square on the inside by six feet long. Into one end would come a current of air and the draft thus created would be “straightened,” as well as made uniform, by having to pass through a set of small pigeon-holes. It would have been a great convenience to use an electric fan for sending the air into the tunnel. But the Wrights had no electric current in their shop--still lighted by gas--and the fan was driven by a one-cylinder gas engine they had previously made. They attached the fan to a spindle that had held an emery wheel. A new measuring device, or balance, was built of wire intended for bicycle spokes, and pieces of hacksaw blades. These experiments were now done with much more refinement than at first, and the measurements were for both “lift” and “drift.” But as each curved surface measured was balanced against the pressure on a square plane, exposed at ninety degrees to the same air current, it was not necessary to know the precise speed of the air current.
During that autumn and early winter of 1901, the brothers tested in the wind-tunnel more than two hundred types of wing surfaces. They set these at different angles, starting with the angle at which the surface begins to lift, and then at 2½ degree intervals, up to twenty; and at five degree intervals up to forty-five degrees. They measured monoplane, biplane, and triplane models; also models in which one wing followed the other, as used by Langley in his experiments. They measured the lift produced by different “aspect ratios”--that is, the ratio of the span of the wing to its chord. They found that the greater the span in proportion to the chord the more easily the wing may be supported. They measured thick and thin surfaces. One surface had a thickness of nearly one-sixth of its chord.
Among other things, these experiments proved the fallacy of the sharp edge at the front of an airplane wing and the inefficiency of deeply cambered wings as then generally advocated by others. Sometimes they got a result so unexpected that they could hardly believe their own measurements--as, for example, when they discovered that, contrary to all previously published figures by students of the subject, a square plane gave a greater pressure when set at thirty degrees than at forty-five degrees.
These wind-tunnel experiments in the bicycle shop were carried on for only a little more than two months, and were ended before Christmas, 1901. The Wrights discontinued them with great reluctance; but, after all, they were still in the bicycle business, still obliged to give thought to their means for earning a living, and with no idea that this scientific research could ever be financially profitable. In those few weeks, however, they had accomplished something of almost incalculable importance. They had not only made the first wind-tunnel in which miniature wings were accurately tested, but were the first men in all the world to compile tables of figures from which one might design an airplane that could fly. Even today, in wind-tunnels used in various aeronautical laboratories, equipped with the most elaborate and delicate instruments modern science can provide, the refinements obtained over the Wrights’ figures for the same shapes of surfaces are surprisingly small. But it is doubtful if the difficulties and full value of the Wrights’ scientific researches within their bicycle shop are yet appreciated. The world knows they were the first to build a machine capable of sustained flight and the first actually to fly; but it is not fully aware of all the tedious, grueling scientific laboratory work they had to do before flight was possible. Important as was the system of control with which the Wrights’ name has been connected, it would not have given them success without their wind-tunnel work which enabled them to design a machine that would lift itself.
The Wrights had a double reason for making sure of their figures. With little money to spend on a hobby, it was much cheaper to rectify mistakes on paper than after the idea was put into material form. They knew that if they should decide to go on to further gliding attempts, they could not afford to spend much more money on apparatus built according to unreliable data.
After compiling their own tables of figures, the Wrights gave copies of them to their friend Chanute and others interested in the problem of aerodynamics. Chanute well knew that the Wrights now had knowledge of aeronautics far beyond that of anyone else in the world, and he felt that for them to go on with their experiments was almost a duty. He much regretted, in the interest of science, he said, that they had reached a stopping-place, for he was sure further experiments on their part promised “important results.”