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The Origins of the First Powered, Man-carrying Airplane

The Wright brothers' "Flyer'' of 1903 was not just a lucky effort by two bicycle mechanics from Dayton but the outcome of an intensive program of research, engineering and testing

by F. E. C. Culick
©F. E. C. Culick

CONTENTS
Flight Testing
The Wind Tunnel
Providing Power
The First Flight

In a series of flights on December 17, 1903, Wilbur and Orville Wright became the first to pilot a powered airplane. As is well known, they had designed and built the craft themselves. To this day, however, the notion persists that the Wright brothers were essentially bicycle mechanics who more or less stumbled on their successful design. Even among aeronautical engineers ignorance of what the Wrights accomplished is widespread. In tracing the steps by which the brothers arrived at their success, and in building a model of their 1903 "Flyer" for tests in a wind tunnel, I have come to recognize how remarkable their achievements in research, engineering and testing were. Their work deserves to be set in a richer historical context.

On the day of the first flights the brothers took turns as pilot. Wilbur was the pilot on the fourth, last and longest flight of the day, staying aloft for 59 seconds. Nearly four years passed before anyone else was able to fly for as long as a minute, and even then the machine was not fully controllable. By that time the Wrights had developed a maneuverable airplane capable of flying for more than an hour.

As late as 1908, when the Wrights first flew publicly, no one else yet understood the need for lateral control, much less the function of the vertical tail. Therefore no one else could execute proper turns. No one else knew how to make propellers correctly. Above all, no one else had pursued a comparable program: doing the necessary research, constructing his own aircraft and doing his own flying, so that he understood the entire problem. The Wrights were able to outstrip all the other people who were trying to fly because they recognized the problems that had to be faced and solved.

The brothers began their work in 1899, when Wilbur was 32 and Orville was 28. Wilbur, who was the informal leader in much of the work, realized they would have to follow a systematic program of research and engineering. He summarized his view in a lecture he gave in September, 1901, at a meeting of the Western Society of Engineers: "The problems of land and water travel were solved in the 19th century because it was possible to begin with small achievements and gradually work up to our present success. The flying problem was left over to the 20th century, because in this case the art must be highly developed before any flight of any considerable duration at all can be obtained." Four years later the Wrights would have a practical airplane.

Their invention of the powered, mancarrying airplane happened partly as a natural development from the work of others. To an extraordinary degree, however, the Wrights' own contributions were crucial. Their technical achievements were stunning, particularly when one considers where the effort to achieve human flight stood in 1899.

Four Pioneers

The people trying to build flying machines had followed several strategies. Observations of the flight of birds motivated the efforts, and so it was natural that the earliest ideas always entailed the use of flapping wings. Sir George Cayley (in 1799) was the first to understand that manned flight would be more easily achieved if the means of generating lift were separated from the means of propulsion.

Cayley is recognized as the originator of the airplane. His designs and conclusions were based on careful observations and experiments. He conceived the configuration now viewed as being conventional: a body or fuselage supporting one main wing, with horizontal and vertical tails positioned aft. He successfully flew gliders, one of them large enough to carry a boy on at least one occasion. Cayley himself never flew. Among his many other notable contributions were demonstrations that curved surfaces are better than flat ones for providing lift the idea that lateral balance or stability can be gained by giving the wing a dihedral angle, that is, making it with raised tips, and the concept of employing a horizontal tail to achieve longitudinal stability.

side view drawing of the 1903 Wright flyer

WRIGHTS' FLYER OF 1903, portrayed above in a view from the left side and below in views from above and in front, was the first powered, man-carrying airplane to fly successfully. Its most distinctive feature was the forward horizontal control surface. The twin vertical tail could not be moved except when the pilot warped the wings to make a turn. Flying prone, he achieved warping by means of wires attached to a hip cradle. Wing warping, which was an intention of the Wrights, fulfilled the function now uniformly done with ailerons. Wilbur and Orville Wright built the entire airplane, including the gasoline engine and propellers, which they had to design from scratch

drawings of the 1903 flyer

What Cayley began to comprehend were the intimate connections between the geometry of an aircraft, the forces acting on it and its stability in flight. An airplane has a vertical plane of symmetry passing through its longitudinal axis. Motions that do not deflect the craft in directions out of that plane are called longitudinal motions. The commonest motion is pitching, in which the nose moves up or down. Sideslip, roll about the longitudinal axis and yaw about the vertical axis are collectively called lateral motions.

The tendency of any object in steady motion to return to its initial state following a disturbance is called stability. There are two kinds of stability, longitudinal and lateral, corresponding to the two classes of motions of an airplane. In a modern aircraft longitudinal stability is provided mainly by the smaller horizontal surface, which is usually placed at the rear as Cayley advocated. Lateral stability is the effect of many causes, including the dihedral angle of the wings and the vertical tail. The designs of early flying machines often included a vertical tail for steering, by analogy with the rudder of a ship. The true function of the vertical tail as a contribution to stability was not apparent until the work of the Wrights.

The giant of early French aeronautics was Alphonse Penaud. He was the first to use wound strip rubber to power a propeller-driven model airplane. Independently of Cayley he conceived of the horizontal tail as a means of achieving longitudinal stability. Moreover, he understood how the tail worked and gave the first explanation of its function in producing stable flight.

Otto Lilienthal, a mechanical engineer working in Germany, made major contributions that directly influenced the Wrights. In 1889 he published a book, The Flight of Birds as the Basis for the Art of Aviation, that contained the results of extensive experiments he had carried out with the help of his brother. That work provided the first useful data on the lift and drag of curved airfoils. Lilienthal also experimented in gliding, beginning in 1891, and became the first successful hang- glider pilot. He tried both monoplane and multiplane gliders and was killed while testing a monoplane.

In the U.S. the most serious student of aeronautics during this period was Octave Chanute, an eminent civil engineer. With his writings and his own experiments he brought Lilienthal's work to the U.S. Because of his age (he was 68 in 1900) he did no flying himself. He had several proteges who tested his gliders and their own designs.

Chanute designed a biplane glider that represents the beginnings of modern aircraft. (The design has served recently as the basis for several hang gliders.) Chanute adopted Penaud's aft tail for longitudinal stability, and for better lift he designed a cambered wing section similar to one tested by Lilienthal. Less obvious is Chanute's most important contribution: his choice of structure for the biplane. It is the Pratt truss, which had been patented in 1844 as a design for railroad bridges. The two wings were connected by solid vertical struts, which carried compressive loads. Tensile loads were transmitted by crossed diagonal wires joining the struts in both the lateral and the fore-and-aft plane. The result was a rigid, lightweight structure that became the basis for all externally braced biplanes.

Lilienthal and Chanute were trying to solve the problem of longitudinal stability. Although they were able to design their gliders to generate sufficient lift, they had great difficulty maintaining balance in flight. They understood that achieving balance meant making the center of lift coincide with the center of gravity. The difficulty arises because the center of lift moves if the attitude of the aircraft is disturbed, as by a gust. In a hang glider the pilot can restore balance by shifting his position in order to move his center of gravity and change the attitude of the aircraft. If this maneuver is accomplished correctly and consistently, the combination of the glider and the pilot forms a stable system.

When an aft horizontal tail is properly installed, it will (as Penaud understood) act to restore balance. The reason is that the effective center of lift of the wing and tail together remains fixed even if the attitude of the airplane is disturbed. The airplane is inherently stable, and the pilot need not shift his position. A model airplane is stable for this reason. A fullsize aircraft requires more control, and so the tail is made movable and the pilot can change its lift at will. With a movable tail an unstable airplane can be flown by a sufficiently skillful pilot.

The Wright brothers were introduced to flying when as young boys they were given a small toy helicopter similar to one invented by Penaud. News of Lilienthal's death in 1896 aroused their interest in the problem of flying. In May, 1899, Wilbur wrote to the secretary of the Smithsonian Institution asking for a list of the available literature on flight. Among the items he decided to buy was Chanute's book Progress in Flying Machines, written in 1894. After studying the book Wilbur wrote to Chanute, initiating an exchange of letters that continued for nearly 10 years. For most of that period Chanute was a source of both information and encouragement to the Wrights. The letters and the diaries of the Wrights constitute a detailed chronicle from which one can reconstruct the events leading to powered flight.

a  photo of a glider

EARLY GLIDER was designed by Otto Lilienthal, who is seen here flying it over a brickyard on the outskirts of Berlin in 1894 Lilienthal was the first pilot of what would now be called a hang glider. He was killed in 1896 when one of his gliders lost lift and went into a sudden dive.

 

The Wrights Begin

By the time the brothers began experimenting seriously they had acquainted themselves thoroughly with the aeronautical literature. They understood the prevailing views that longitudinal stability required a Penaud tail, that lateral stability should be achieved by having a dihedral angle in the wings and that steering could be accomplished with a vertical tail or rudder. Yet they eventually rejected all three notions, producing designs that were quite different from the basic one conceived by Cayley and generally accepted by their contemporaries. Their first powered airplane, the 1903 Flyer, had a forward horizontal surface, the tips of the wings drooped with negative dihedral and the vertical tail moved only when the wings were flexed to turn the aircraft. The reasons for the configuration adopted by the Wright brothers tell the story of their research and engineering.

Probably the single most important motivation in the Wrights' program was that they themselves were resolved to learn to fly. That determination influenced virtually all their decisions. Since they were to be pilots, they had to give careful consideration to stability, control and safety. They were not inclined to build a machine, hope it would work and hire somebody else to do the testing. Moreover, they were clearly not going to be satisfied just with gliding straight ahead.

Lilienthal's death and a similar accident in England convinced Wilbur at the outset that to be safe an airplane should have the horizontal stabilizing surface in front of the main wing (a system that came to be called the canard configuration). No one at the time understood stalling (the loss of lift), and the Wrights had no scientific basis for determining the relative merits of aft and forward surfaces. While they were experimenting, the theoretical conditions for stability were being developed by others, but not until after 1903 was it widely known that an airplane can be made longitudinally stable with either an aft or a forward horizontal surface. The forward surface, which was the most unusual feature of the early Wright aircraft, had for the brothers the advantage that the pilot could see it, thereby gaining a measure of safety by being able to respond quickly if something went wrong with it in flight.

Unlike his predecessors, Wilbur did not assume that the problem of lateral stability had been solved by Cayley's invention of dihedral. "My observations of the flight of buzzards," Wilbur wrote to Chanute, "lead me to believe that they regain their lateral balance, when partly overturned by a gust of wind, by a torsion of the tips of the wings." Having made this fundamental observation, Wilbur then had to solve the problem of inventing a comparable method of control for a man-carrying aircraft.

He eventually realized the idea as wing warping, which was the predecessor of the ailerons that fulfill the function in modern aircraft. With wing warping the Wrights were able not only to stabilize the airplane in a straight path but also to turn it. And that is how they advanced far ahead of their competitors. By employing a movable horizontal surface and wing warping the Wright brothers fused the tasks of stability and control, thereby making their most important contribution to the development of the modern airplane.

Several months after Wilbur had corresponded with the Smithsonian Institution, he built a biplane kite with a five-foot wingspan. Kites as they are now known (particularly box kites) had been developed quite recently. Wilbur followed the latest construction practices but the basic structural design of his kite was the Pratt truss, which he had heard about from Chanute. He removed the fore-and-aft bracing wires in order to have the wings free to slide with respect to each other, thereby producing the wing warping. The only rigid parts of the structure were the connections between the control surface and the uprights to which it was attached. The four control cords were arranged so that the operator could exercise both longitudinal and lateral control of the kite, independently or simultaneously.

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