Part 6

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.

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.

L. M. Wainwright, president of the Diamond Chain Company, of Indianapolis, became interested in the Wrights’ transmission problem, and gave them valuable advice.

The Wrights found that the chains would have to be run through guides to prevent slapping and to overcome undue stresses on the machine. They adopted tubular guides and found that they could cross one of the chains in a figure eight and thus have the propellers running in opposite directions.

Not until September 23 was all in readiness for the Wrights to set out for Kitty Hawk. They were able to make good connections with a boat and arrived at camp two days later, on a Friday. Discussing en route what they hoped to accomplish, neither had the slightest doubt about the fulfillment of their dreams. Besides being full of confidence they also felt the exuberance of excellent physical condition. Orville was now thirty-two years old and Wilbur thirty-six. Five foot ten and a quarter inches in height, Wilbur was the taller of the two by a little more than an inch and a half. Orville weighed 145 pounds, about five more than Wilbur. Each of them had grayish-blue eyes and they might have been recognized as brothers, though in their own family Wilbur at that time was considered “more of a Wright” in his facial conformation. Orville looked a little more like his mother. Both were suitably built for bird men.

Plenty of annoyances, difficulties, and delays were still to be faced. When they reached their camp near Kill Devil Hill, the Wrights found that a storm had blown it from its foundation posts. They repaired the shed and also built a new one. With two sheds they had enough space for housing both the 1902 glider and the power machine, and also for a better workshop.

Just as the new building was nearing completion, the Kitty Hawk region had one of the worst storms in years. It came without warning, soon blowing forty miles an hour, and increased during the night until the next day the wind was more than seventy-five miles an hour. Orville risked climbing to the roof to nail down some of the more exposed parts. But by the time he got to the roof edge, the wind had blown his coat about him in a manner to pinion his arms and leave him helpless. Wilbur rushed to his assistance and held down his coat, but the wind was so strong that it was almost impossible to swing a hammer accurately enough to hit a nail.

Three weeks were needed for assembling the new machine. From time to time, also, they took out the 1902 glider, still in fairly good condition in the shed where they had left it, and got practice. After the first few trials each brother was able to make a new world’s record by gliding for more than a minute.

It was hoped to have the power machine ready for its first trial early in November. But at the first run of the motor on the completed machine, an unexpected strain from back-firing twisted one of the propeller shafts and tore loose the cross-arm to which the propeller was fastened. Both shafts were then sent back to the bicycle shop at Dayton to be made stronger. Dr. Spratt had arrived on October 23 to witness tests of the new machine, but the weather had become so wintry that he started home on November 5, taking with him as far as Norfolk the shafts for shipment to Dayton.

Octave Chanute came, on invitation, the next day, but he too found it difficult to be comfortable with the weather increasingly wintry and he stayed less than a week. Before leaving camp, Chanute had unintentionally given them something else to worry about. He had remarked that at least twenty per cent usually must be allowed in chain transmission for loss in power. As the Wrights had allowed only five per cent, they felt considerable alarm.

Since Chanute was a capable and famous engineer, it seemed prudent to find out whose estimates were more nearly correct. After Chanute had gone, the brothers suspended one of the drive chains over a sprocket and hung a bag of sand at each end of the chain. By measuring the amount of weight on one side needed to lift that on the other, they calculated the loss in transmission. As nearly as they could tell, this loss was even less than the five per cent they had estimated.