FROM ORVILLE WRIGHT’S DIARY. Part of the entry for December 17, 1903—the day of the first power flight.
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.