Chapter IV: Prologue (3)

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.”

Chanute might well have felt pride in the effectiveness of his insistence that the Wrights should go on experimenting, as well as in the results of his invitation to Wilbur to make that Chicago speech. Except for that speech and its daring statements that Orville thought needed more confirmation, there probably would have been no wind-tunnel tests; and without the kind of knowledge then obtained, neither the Wrights nor anyone else could have built a practical flying-machine. Those wind-tunnel experiments marked one of the great turning points in the long history of attempts at human flight.

It still remained for the Wrights to put their new knowledge to actual test in gliding, and they set out on August 25, 1902, for their third stay at Kitty Hawk. But not until September 8 were they able to begin the work of assembling their new glider, for the camp, battered by winter gales, needed much repairing; and they decided to build an addition to it for living quarters. They did not have their machine ready for its first trial until September 19.

This new glider was of not much greater lifting area than that of the previous year, though the wing span had been increased from twenty-two to thirty-two feet. But since the wind-tunnel experiments had demonstrated the importance of the “aspect ratio,” the total span was now about six times the chord instead of three. One minor change also may be noted. In the earlier gliders, the wing-warping mechanism had been worked by movement of the operator’s feet; but now in this 1902 glider it was done by sidewise movement of one’s hips resting on a “cradle.” The most noticeable change was the addition of a tail, consisting of fixed twin vertical vanes, with a total area of a trifle less than twelve square feet. Its purpose was to correct certain difficulties encountered in some of the flights with the 1901 machine. When the wing surfaces at the right and left sides were warped to present different angles toward the wind, the wing that had the greater angle, and therefore the more resistance, tended to lag behind, and then the slower speed offset what otherwise would have been the greater lifting power of that wing. The tail was expected to counter-balance that difference in resistance of the wing tips. If the wing on one side tended to swerve forward, on a vertical axis, then the tail, more exposed to the wind on that same side, should, it was thought, stop the machine from further turning.

Entirely apart from any advantages to be gained from the use of the tail, the first trials of the new machine were highly encouraging for another reason. It was soon evident that by disregarding all tables of air pressures used by their predecessors and building according to the figures obtained from their wind-tunnel experiments, the Wrights had made a big advance toward flight. Because of the knowledge they now had, not possessed by any previous experimenter, of how the wings should be shaped, this 1902 machine was of just about twice the “dynamic efficiency” of any other glider ever built; it could have been flown with probably less than half the power required for any other glider.

Altogether the Wrights made more than one thousand gliding flights in September and October, 1902. Several glides were of more than six hundred feet, and a number of them were against a thirty-six-mile-an-hour wind. No previous experimenter had ever dared to try gliding in so stiff a wind. That the Wrights were successful at such feats gave proof of the effectiveness of their devices for control. Some of their flights lasted more than a minute and at times it was possible to soar in one spot without any descent. So impressive were such exhibitions that Bill Tate’s brother Dan solemnly offered the opinion: “All she needs is a coat of feathers to make her light and she will stay in the air indefinitely.”

About one time in fifty, however, the machine behaved in a manner quite mysterious. It would turn up sidewise and come sliding to the ground in spite of all the warp the operator could give to the wing tips. At one trial the lateral control would work perfectly and then the next time, under conditions that seemed to be about the same, it was impossible to prevent one wing end from striking the sand with a kind of spinning movement that the brothers called “well-digging.”

This new problem, that had not occurred in their previous gliders, came from the fact that the machine had a tail. Those “well-digging” accidents were tail-spins--though that term did not come into use until several years afterward. But even after it was evident that the tail had something to do with the machine’s peculiar behavior, neither brother was prepared to explain _why_. Then one night Orville drank more than his customary amount of coffee. Instead of going to sleep as usual the moment he got into bed, he lay awake for several hours. Those extra cups of coffee may have been important for the future of practical flight for, as he tossed about, he figured out the explanation of the phenomenon caused by the tail. Here it is, as he eagerly gave it to Wilbur, and to their brother Lorin, who was visiting them, at breakfast the next morning:

When the machine became tilted laterally it began to slide sidewise while advancing, just as a sled slides downhill or a ball rolls down an inclined plane, the speed increasing in an accelerated ratio. If the tilt happened to be a little worse than usual, or if the operator were a little slow in getting the balance corrected, the machine slid sidewise so fast that this movement caused the vertical vanes to strike the wind on the side toward the low wing instead of on the side toward the high wing, as it was expected to do. In this state of affairs the vertical vanes did not counteract the turning of the machine about a vertical axis, caused by the difference of resistance of the warped wings on the right and left sides; on the contrary, the vanes assisted in the turning movement, and the result was worse than if there were no fixed vertical tail.

If his explanation was sound, as Orville felt sure it was, then, he said, it would be necessary to make the vertical tail movable, to permit the operator to bring pressure to bear on the side toward the higher wing. (This is the form of the Wright system of control generally used today--the independent control of aileron and rudder.)

Wilbur promptly saw that the explanation was probably correct and nodded approvingly. And he immediately made a suggestion. A particular relation existed, he said, in the desired pressures on the tail, no matter whether the trouble was due to difference of resistance of the wing tips or on account of sliding. Whatever the reason, it was desirable to get rid of the pressure on the side toward the low wing, to which a greater angle of incidence must be imparted in restoring lateral balance, and bring pressure on the side of the tail toward the high wing where there must be a reduced angle. So why not have the mechanism that controlled the wing warping and that which moved the tail operated in conjunction? Then the pilot, instead of having to control three things at once, would need to attend only to the front elevator and the wing-warping device. The brothers at once attached the wires controlling the tail to those that warped the wings--and they also changed the tail from two vertical fins to a single vertical rudder.

After the changes in the 1902 glider, the Wrights had their machine in about the form pictured and described in the drawings and specifications of their patent, applied for on the 23rd of the next March.

With their accurate data for making calculations, and a system of balance effective in winds as well as in calms, the brothers believed that they now could build a successful power-flyer.

VI

FIRST POWER FLIGHT

Immediately on their return to Dayton after the 1902 glider flights, the Wrights set to work to carry out plans, already begun at Kitty Hawk, for a power machine. The satisfactory performance of the glider had demonstrated the accuracy of the laboratory work on which its design was based, and they now felt sure they could calculate in advance the performance of any machine they built with a degree of accuracy not possible with the data available to their predecessors.

Early in their preparations, they took steps to obtain a suitable engine. They knew that a steam engine might do well enough for their purpose, but a gasoline engine would be simpler and better. Some time previously they had built an air-cooled, one-cylinder gas engine for operating the machinery of their small workshop; but they did not feel experienced enough to build the kind they now needed and preferred to buy one.

They wanted a motor to produce at least eight horsepower and to weigh, without accessories, not more than twenty pounds per horsepower. It seemed doubtful if such a motor as they required was then being manufactured; but perhaps one of the automobile companies could build one light enough by reducing the weight of the flywheel and using more aluminum than in the regular output. On December 3, 1902, they sent letters to a number of automobile companies, and to gasoline motor manufacturers, altogether to nearly a dozen, asking if they could furnish a motor that would develop eight brake horsepower and weigh not more than 200 pounds. Orville Wright was not sure in after years whether he and Wilbur revealed in their letters the use they planned for the motor they were seeking; but most of the companies replied that they were too busy with their regular business to undertake such a special order. There is reason to suspect the companies may have got wind of the purpose to which the motor would be put and were afraid to become implicated in the project. If a company provided a motor for a so-called flying-machine, and this fact should leak out, it could hurt their business prestige, because it might look as if they considered human flight a possibility.

Thursday, Dec. 17th

When we got up a wind of between 20 and 25 miles was blowing
from the north. We got the machine out early and put out the
signal for the men at the station. Before we were quite ready,
John T. Daniels, W. S. Dough, A. D. Etheridge, W. C. Brinkley
of Manteo, and Johnny Moore of Nags Head arrived. After running
the engine and propellers a few minutes to get them in working
order, I got on the machine at 10:35 for the first trial. The
wind, according to our anemometers at this time, was blowing
a little over 20 miles (corrected) 27 miles according to the
Government anemometer at Kitty Hawk. On slipping the rope
the machine started off increasing in speed to probably 7 or
8 miles. The machine lifted from the truck just as it was
entering on the fourth rail. Mr. Daniels took a picture just
as it left the tracks. I found the control of the front rudder
quite difficult on account of its being balanced too near the
center and thus had a tendency to turn itself when started so
that the rudder was turned too far on one side and then too
]

One company replied, however, that they had motors, rated at eight horsepower, according to the French system of ratings, which weighed only 135 pounds, and if the Wrights thought this would develop enough power for their purpose, they could buy one. After an examination of the particulars of this motor, from which they learned that it had but a single cylinder, of four-inch bore and five-inch stroke, the Wrights decided that its power was probably much overrated.

Finally the brothers decided that they would have to build their motor themselves. They estimated that they could make one of four cylinders, of four-inch bore and four-inch stroke, weighing not more than two hundred pounds, with accessories included. Their mechanic, Charlie Taylor, gave them enthusiastic help. In its final form, the bare engine, without magneto, weighed 152 pounds; with accessories, 170 pounds. At 1,200 revolutions per minute, it developed sixteen horsepower--but only for the first fifteen seconds after starting; after a minute or two it did not give more than about twelve horsepower. Since, however, they had not counted on more than eight horsepower, for a machine of a total weight of 600 pounds, now they could add 150 pounds for strengthening wings and other parts. Not yet knowing how much power an engine of that size ought to have developed, the Wrights were much pleased with its performance. Long afterward they found out that the engine should have provided about twice as much power as it did. The trouble, as they later said, was their “lack of experience in building gasoline motors.”

The wings of this new power machine had a total span of a few inches more than forty feet, and the upper and lower wing surfaces were six feet apart. To reduce the danger of the engine ever falling on the pilot, it was placed on the lower wing a little to right of center. The pilot would ride lying flat, as on the glider, but to the left of center, to balance the weight. To guard against the machine rolling over in landing, the sled-like runners were extended farther out in front of the main surfaces than on the glider. These two runners were four feet, eight inches apart. The tail of the machine had twin movable vanes instead of a single vane as in the 1902 glider.

The Wrights left the designing of the propellers until the last, because they felt sure that part of the job would be easy enough. Their tables of air pressures, derived from wind-tunnel experiments, would enable them, they thought, to calculate exactly the thrust necessary to sustain the machine in flight. But to design a propeller that would give the needed amount of thrust, with the power at their command, was a problem they had not yet considered. No data on air propellers were available, but the Wrights had always understood that it was not difficult to obtain an efficiency of fifty per cent with marine propellers. All that should be necessary would be to learn the theory of the operation of propellers from books on marine engineering and then substitute air pressures for water pressures. What could be simpler or easier? Accordingly, the brothers got several such books from the Dayton Public Library. But when they began to read those books, they discovered to their surprise that much less was known about propellers than they had supposed.

All the formulae on propellers in the books were found to be based on experiment and observation rather than on theory. The marine engineers, when they saw that a propeller would not move a boat fast enough, had then tried one larger, or of a different pitch, until they got one that would serve their purpose. But they could not design a propeller on paper and foresee exactly what its performance on a certain type of motor-boat would be. Exact knowledge of the action of the screw propeller, though it had been in use for a century, was still lacking.

The Wrights knew that rough estimates, which might be near enough for a motor-boat, would not do for an airplane. On a boat a propeller having only a fraction of one per cent of the desired efficiency could move the boat a little; but on an airplane, unless the propeller had the full amount of thrust needed, it would be worthless, for it couldn’t lift the plane into the air at all! In short, the Wrights had to have a propeller that would do exactly what was expected of it. And they had neither the time nor money to carry on a long series of experiments with different kinds of propellers until they could hit on one suitable. They couldn’t afford to make mistakes except on paper. They must somehow learn enough about how propellers acted, and why, to enable them to make accurate calculations.

It was apparent to the Wrights that a propeller was simply an airfoil traveling in a spiral course. As they could calculate the effect of an airfoil traveling in a straight course, why should they not be able to calculate the effect in a spiral course? At first thought that did not seem too difficult, but they soon found that they had let themselves into a tough job. Since nothing about a propeller, or the medium in which it acts, would be standing still, it was not easy to find even a point from which to make a start. The more they studied it, the more complex the problem became. “The thrust depends upon the speed and the angle at which the blade strikes the air; the angle at which the blade strikes the air depends upon the speed at which the propeller is turning, the speed the machine is traveling forward, and the speed at which the air is slipping backward; the slip of the air backward depends upon the thrust exerted by the propeller, and the amount of air acted upon.” It was not exactly as simple as some of the problems in the school arithmetic--to determine how many sheep a man had or how many leaps a hound must make to overtake a hare.

In trying to work out a theory about the action of screw propellers, Wilbur and Orville got into many arguments. Right here it may be noted that this habit the brothers had of arguing technical points was one of the reasons why they were able to accomplish all they finally did in a relatively short time. Neither was a “yes” man to the other. But in their arguments about propellers a peculiar thing happened. “Often,” Orville later reported, “after an hour or so of heated argument, we would discover that we were as far from agreement as when we started, but that _each had changed to the other’s original position_.”

Many months passed before the intricacies of the problem began to untangle themselves. The Wrights finally got a better understanding of the action of screw propellers than anyone had ever had before. The time came when they felt sure of their ability to design propellers of exactly the right diameter, pitch, and area for their needs.

A calculation indicated that 305 revolutions of the propeller would be required to produce 100 pounds thrust. Later, actual measurement showed that only 302 instead of 305 propeller turns were required, or just under one per cent of the calculated amount. The propellers delivered in useful work 66 per cent of the power expended. That was about one-third more than either Hiram Maxim or Professor Langley in their attempts at flying had ever been able to attain.

For two reasons the Wrights decided to use two propellers. First, they could in that way obtain a reaction against a greater quantity of air, and at the same time use a larger pitch angle; and, by having the propellers run in opposite directions, the gyroscopic action of one would neutralize that of the other. The propellers were on tubular shafts about ten feet apart, both driven by chains running over sprockets, somewhat as on a bicycle.