Authors: Tom D. Crouch
The development of a system to control an airplane in flight rested on this foundation—Wilbur’s understanding of how a bicycle is turned to the left.
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ontrol was everything. But what sort of experimental approach would enable Wilbur to balance a machine in the air?
“After reading the pamphlets sent to us by the Smithsonian,” Orville recalled, “we became highly enthusiastic with the idea of gliding as a sport.” While the notion of rushing through the air at breakneck speed must have appealed to these two cyclists, there were better reasons to follow Lilienthal and Pilcher, the two most experienced gliding pioneers. Gliding was perfectly suited to Wilbur’s intuitive grasp of the links between flying and cycling. Continued practice was the only way to devise, test, and refine an effective control system, while at the same time exploring the remaining aerodynamic and structural problems.
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“Now there are two ways of learning how to ride a fractious horse,” he explained to the Chicago engineers in 1901.
One is to get on him and learn by actual practice how each motion and trick may be best met; the other is to sit on a fence and watch the beast a while, and then retire to the house and at leisure figure out the best way of overcoming his jumps and kicks. The latter system is the safest; but the former, on the whole, turns out the larger proportion of good riders. It is very much the same in learning to ride a flying machine; if you are looking for perfect safety, you will do well to sit on the fence and watch the birds; but if you really wish to learn, you must mount a machine and become acquainted with its tricks by actual trial.
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It was easy enough to speak of bucking horses, or bicycles, but Lilienthal and Pilcher had both died in accidents. The problem lay in the means of control. The pioneers had flown hang gliders, with weight shifting as their only means of control. It was an imprecise, uncertain, and dangerous technique. The pilot who threw his legs in the wrong direction in a moment of panic or confusion might send his machine skittering into an irreversible attitude. That had happened to Lilienthal.
Moreover, weight shifting placed an absolute limit on the span and area of the wings. With a craft any larger than Lilienthal’s, a shift in the position of the pilot’s legs would not alter the center of gravity sufficiently to change the attitude of the glider.
Unwilling to risk his life aboard an unsafe machine of limited size, Wilbur was determined to build his glider around an effective mechanical control system. Roll control would be the major problem. There were no mechanical analogies on which he could draw.
His first clues came from observations of bird flight. “I … conceive Lilienthal’s apparatus to be inadequate,” he told Chanute, “not only from the fact that he failed, but my observations … convince me that birds use more positive and energetic methods of regaining equilibrium than that of shifting the center of gravity.”
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Wilbur believed that birds balanced themselves in roll by altering the aerodynamic characteristics of their wings. But how? Could the shape or position of a wing be altered so as to mimic the attitude of a bird in flight? It did not take him long to come up with the answer.
The thought came to me that possibly it adjusted the tips of its wings … so as to present one tip at a positive angle and the other at a negative angle, thus … turning itself into an animated windmill, and that when its body had revolved … as far as it wished, it reversed the process and started turning the other way. The balance was controlled by utilizing dynamic reactions of the air instead of shifting weight.
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A bird balanced by turning the forward edge of one wingtip up and the other down. The image of “an animated windmill” stuck in Wilbur’s mind. “Here,” he noted, “was the silent birth of all that underlies human flight.”
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Now there was a new problem. How could the same thing be achieved with man-made wings? It came to Wilbur while working alone in the bicycle shop one day in July 1899. He picked up a rectangular inner-tube box from which the end tabs had been ripped, and began idly twisting it in his hand. Twisting … that was the answer.
Rather than treating each wingtip as an independent unit, he would throw a complete helical twist across the entire wing structure in either direction.
A simple cardboard box suggested a mechanical analogue to the twist of a bird’s wing. Wilbur’s graphic imagination, the extraordinary path that led from his hand and eye to his mind, was at work once again.
Orville was taking Katharine and Harriet Silliman, a visiting Oberlin classmate, around town that day. Wilbur explained the principle when they returned that evening. “We began construction of a model embodying the principle demonstrated with the paper box within a day or two,” Orville recalled.
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The small model of split bamboo, paper, and strings convinced Wilbur that he could achieve the required torsion with the sort of structure he had in mind. After playing with it for a few days, he set to work on a biplane kite with a span of five feet and a chord (straight-line distance from the leading edge to the trailing edge) of thirteen inches. The two wings were trussed together with six struts, jointed at the top and bottom, and wires crisscrossing between the wings along the leading and trailing edges. There was no fore and aft trussing. A fixed elevator was attached to the midpoint of the central trailing edge strut.
He would control the kite with a stick held upright in each hand. Fixed lengths of line ran from the top and bottom of each forward outboard strut to the bottom and top of the control sticks. When the operator tipped the upper end of both sticks toward the machine in the air, the top surface would move to the rear, causing the average center of pressure on the wings to shift behind the center of gravity. The kite would nose up. An opposite motion of the sticks would result in a dive. Pointing the top of one stick toward the kite while pointing the other away from it would cause the little craft to roll off to one side. Reverse the sticks and it would roll off to the other side.
Work on the kite was under way at the end of the first week in July, when Bishop Wright brought his young grandson Milton up the backstairs of the bike shop for an inspection. On July 24, Orville, Katharine, Harriet Silliman, Agnes Osborn, and a few of their friends set out on a chaperoned camping trip. When the tent housing the male campers blew down in a storm, everyone crowded into the remaining shelter for the rest of the night. From that point on, Camp Rain-in-the-Face was enshrined in West Side legend.
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Wilbur visited the camp on Sunday, August 6. He drew Orville
aside, the others recalled, engaging him in animated conversation. The kite worked. Wilbur had flown it the week before. It dived, climbed, and rolled to the right and left on command. John and Walt Reinieger and some of the other neighborhood boys had tagged along. At one point Wilbur put the kite into too steep a dive, causing the lines to go slack. The boys threw themselves to the ground as the little craft swooped down at them.
“We felt that the model had demonstrated the efficiency of our system of control,” Orville noted. “After a little time we decided to experiment with a man-carrying machine embodying the principles of lateral control used in the kite model already flown.”
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It was a major decision, not lightly made. It was one thing to send a small kite darting about the sky, and quite another to risk one’s life aboard such a craft. Still, if they moved slowly, the danger might be contained.
Wilbur outlined his plan confidently to Chanute:
I shall in a suitable locality erect a light tower of about one hundred and fifty feet high. A rope passing over a pulley at the top will serve as a sort of kite string. It will be so counterbalanced that when the rope is drawn out one hundred & fifty feet it will sustain a pull equal to the weight of the operator and apparatus, or nearly so. The wind will blow the machine out from the base of the tower and the weight will be sustained partly by the upward pull of the rope and partly by the lift of the wind….
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“In this way,” Orville noted, “we thought we would be able to stay in the air for hours at a time, getting … a maximum of practice with a minimum of effort.”
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Lilienthal had spent a total of several hours in the air—ten seconds at a time over a period of six years. He had made two thousand sweeps down a hundred hills, each flight over in the blink of an eye.
Wilbur hoped to soar at the end of a kite line until he became adept at the controls. While kiting offered “no guarantees,” he thought it would enable him to “escape accident long enough to acquire skill sufficient to prevent accident.”
That was the plan. Accomplishing it was another matter. Wilbur at least knew where to begin. During his research he had uncovered two equations and some precise data that would allow him to engineer his flying machine.
Engineering was the key. The Wright brothers functioned as engineers, not as scientists. Science, the drive to understand the ultimate
principles at work in the universe, had little to do with the invention of the airplane. A scientist would have asked the most basic questions. How does the wing of a bird generate lift? What are the physical laws that explain the phenomena of flight?
The answers to those questions were not available to Wilbur and Orville Wright, or to anyone else at the turn of the century. Airplanes would be flying for a full quarter century before physicists and mathematicians could explain why wings worked.
How was it possible to build a flying machine without first understanding the principles involved? In the late twentieth century, we regard the flow of technological marvels from basic scientific research as the natural order of things. But this relationship between what one scholar, Edwin Layton, has described as the “mirror image twins” of science and technology is a relatively new phenomenon. Historically, technological advance has more often preceded and even inspired scientific understanding.
The roots of the flying machine lie not in scientific theory, but in the experimental work of a group of eighteenth-century engineers who were interested in windmills. The Englishman John Smeaton—a founder of modern engineering—was the most important. A native of Whitlock, near Leeds, his list of achievements includes the rebuilding of the Eddystone Light; navigational improvements to rivers and the draining of the Fens; strengthening the piers of the old London Bridge; the design of a pumping engine to provide water for London; the construction of the bridges at Perth, Coldstream, Banff, and Hexham; water-supply systems for Edinburgh, Deptford, and Creenwich; preliminary work on the Forth and Clyde Canal; and the design of lighthouses and harbor improvements for St. Ives in Cornwall, and Ramsgate.
Smeaton also made important contributions to engineering research. His experimental studies of optimum piston size and stroke, cylinder volume, and engine operating temperatures led to basic improvements in steam engine design. He was not concerned with underlying physical principles—the why of the thing. Instead, he produced tables of precise engineering data to assist his colleagues in designing more efficient engines. In so doing, he provided a starting point for the theoreticians who would found the science of thermodynamics.
Smeaton took precisely the same approach to increasing the efficiency of windmill blades. His most important contributions were
embodied in a 1759 paper for which he received the Royal Society’s Gold Medal. Smeaton had no interest in flying machines, yet his work was crucial to the early history of aeronautics.
For Smeaton, precision measurement was the key to understanding. Like Wilbur, he asked specific questions. How much pressure was exerted on a plate immersed in a fluid stream? How much lift was generated? How much resistance was encountered? Did a change in the size or shape of a plate affect its efficiency? Did the magnitude of the forces alter with a change in the velocity of the stream?
He conceived instruments that would measure minute shifts in speed and pressure, devised whirling arms that would rotate their various test surfaces rapidly through the air, and observed the changing patterns of the water flowing past objects placed in test tanks. Clever fingers and a quick eye could coax a surprising amount of information out of such primitive apparatus.
Smeaton did not gather random information. His goal was to help engineers design blades that would extract a maximum amount of work from the energy of wind and water. Such a man would want to know the total amount of fluid pressure on his blade. Smeaton could help him there. As an appendix to his paper, he included a table showing the coefficient of air pressure (.005). This number was a standard multiplying factor used to calculate the total air pressure on a surface set at an angle of 90 degrees to a fluid stream.
Smeaton also uncovered the basic relationship between the variables of speed, surface area, and angle of attack. And he studied the efficiency of various blade shapes. The fact that the wings of a bird are cambered, or arched, had been known for centuries, but Smeaton was the first researcher to measure the difference. He had no idea why a cambered surface provided more lift, or upward force, than a flat plate, nor did he really care. It was enough to demonstrate that it was so, and to make the information available to other engineers who could put it to good use.
Sir George Cayley of Brompton Hall, in Kent, was the first man to use the data collected by Smeaton in designing a flying machine. Cayley not only built and flew the first gliders, he also conducted research that confirmed and expanded the findings of his predecessors. From his time to that of the Wright brothers, flying-machine experimenters would continue to depend on the original work of the English engineers.
The most important line of research during the nineteenth century involved the study of the lift and drag (resistance) encountered by
specific airfoil (wing cross-sectional) shapes through a range of angles of attack. Unlike the coefficient of air pressure, these figures (soon to be known as the coefficients of lift and drift, or drag) were not constant multiplying factors, but experimentally determined numbers that varied for each wing shape at each angle of attack.