|BIPLANE GLIDER was designed in 1896 by Octave Chanute, a civil engineer who was the most serious student of aeronautics in the U.S. before the Wrights began their work They carried on a long correspondence with him, and he was on hand for many of their early flights The pilot for this flight in 1897 in Indiana was A. M. Herring, one of Chanute's associates.|
The Wrights spent less than a month near Kitty Hawk in 1900, experimenting with their glider between October 5 and 18. Most of the time they operated the glider as a kite, first from a 10-foot tower but usually from the ground with hand-held ropes. The total time in free flight was about two minutes, of which roughly 10 seconds was spent with a man on board. Only Wilbur flew. The controls worked well when the glider was flown as a kite, but in the brothers' limited amount of piloted-flight time they did not master the adaptation of the controls to direct operation.
It is apparent from the diaries that the first experiments were often frustrating. Orville noted: "We tried it with tail in front, behind, and every other way. When we got through, Wil was so mixed up he couldn't even theorize. It has been with considerable effort that I have succeeded in keeping him in the flying business at all." The flights were short and there were many hard landings. Fortunately the structure was simple and easily repaired.
With spring scales attached to the restraining ropes the brothers made measurements of lift and drag. They determined the speed of the wind with an anemometer. These were the first quantitative data for the performance of any full-scale lifting structure.
Although the Wrights' flying program in 1900 was short, it yielded fundamental results and served to "confirm the correctness of our original opinions." The brothers proved their techniques of longitudinal and lateral control; they believed even more strongly that they had to practice flying, and from their measurements they learned that flying in a prone position greatly reduced the drag on the machine.
Results of Testing
Not all their conclusions were so positive. It was at this time that Wilbur decided not to make the wings with a dihedral angle. Although the configuration aided stability in still air, the slightly raised tips of the wings tended to accentuate the effect of gusts from the side. Since the Wrights had lateral control through wing warping, they did not need the stabilizing effect of dihedral.
By far the most disappointing results of the experiments in 1900 were the values for lift and drag that the brothers found. The drag of the structural frame when no lift was being generated was much less than they had expected. Yet for a given angle between the wing and the wind the total drag was greater and the lift smaller than they had anticipated. (The Wrights were working from data on a flat plate perpendicular to an airstream reported by John Smeaton in 1759 and on measurements for cambered airfoils made by Lilienthal.) The brothers observed in their flight tests that the glider operated at an angle of incidence much larger than the one they had expected. Lift increases with angle of incidence and must equal the weight of the glider for steady flight. At a given angle of attack the Wrights' wings produced less lift than the brothers had predicted on the basis of Lilienthal's data.
That was a surprising result, but the Wrights believed their own data and considered three possible explanations: (1) the fabric covering was not sufflciently airtight, (2) the camber of their airfoil (1:22) was too shallow for Lilienthal's data to be applicable or (3) Lilienthal's data were wrong. They favored the second reason and designed their next glider, which was to be flown in the fall of 1901, with a more highly cambered airfoil.
With the interpretation of their results in 1900 the Wrights stopped depending on the work of others. From then on their progress was to be entirely the result of their own discoveries. They began work on their 1901 glider during the winter. To get more lift they increased the wingspan to 22 feet. With a chord of seven feet the total covered wing area was 290 square feet; the stabilizer area was 18 square feet. This was by far the largest glider anyone had tried to fly. The weight of the structure had become 98 pounds and the wing loading, with a pilot, was .78 pound per square foot. The only other major changes were the increase of the camber from 1: 22 to 1: 12 and modifications of the controls so that the pilot could more easily operate both the stabilizer and the wing warping. Although the glider did not look much different from the 1900 model, it was in some respects the most important of the Wrights' research devices. Indeed, the year 1901 was the watershed for almost all their later accomplishments.
Now the test program was expanding. The brothers built a wooden shed at Kill Devil Hills, four miles from Kitty Hawk, where large sand dunes afforded good launching platforms. Because of business pressures they conducted their flying experiments earlier than they had originally planned, from July 10 to August 20.
According to their diaries, Wilbur did all *e flying in 1901. On the first day of testing, July 17, he made 17 glides without having tested the machine as a kite. These first flights were nearly disastrous, mainly because of the problem of longitudinal control. Wilbur found that full deflections of the forward horizontal control (the canard) were often required and that the flight path of the glider was severely undulating. Twice the angle of incidence was so high that the machine lost forward motion and stalled. Each time the glider simply mushed slowly to the ground rather than diving as Lilienthal's had. These two recoveries from the conditions Wilbur had feared satisfied the brothers of the worth of the canard configuration. From then until 1910 they had no interest in trying the aft tail.
Wilbur was faced with the problem of determining why the glider was behaving so erratically. No noticeable improvement followed a reduction of the area of the control surface from 18 square feet to 10. The brothers then flew the glider as a kite and in one afternoon found that the center of lift was traveling backward as the angle of attack was decreased.
It was known at the time *at for a flat plate the center of lift progresses continuously from the center, when the plate is perpendicular to the airstream, to the leading edge, when the angle of incidence is zero. In testing a full-scale wing the Wrights discovered a fundamental property of airfoils: camber causes the motion of the center of lift to reverse as the angle of incidence is changed.
The reversal of the motion of the center of pressure with angle of incidence was the cause of the difficulties Wilbur had in controlling the glider. The solution lay in reducing the camber of the wings. The brothers went back to the camber of the 1900 glider by installing king posts on the lower wing and rigging truss wires to pulI the middle sections of the ribs downward on both wings. The results were immediate and satisfying. Wilbur made some 30 glides after the modification; the longest one lasted 17.5 seconds and the greatest distance was 390 feet.
During the last week of testing Wilbur made a fundamental observation about the behavior of an aircraft when it is turned. He recorded in his diary that the "upturned wing seems to fall behind, but at first rises." On August 22 he wrote to Chanute: "The last week was without very great results though we proved that our machine does not turn (i.e., circle) toward the lowest wing under all circumstances, a very unlooked for result and one which completely upsets our theories as to the causes which produce the turning to right or left."
Turning an airplane is done not by steering with a rudder but by rolling about the longitudinal axis. To turn to the right, for example, the right wing is lowered and the left one is raised. The direction of the total lift is thus tilted to the right of the vertical and part of the lift acts to accelerate the airplane to the right. It was the Wrights who discovered the correct method of turning, a discovery made possible by their invention of wing warping for lateral control.
|F1RST AIRCRAFT built by the Wright brothers was this biplane kite of 1899. It had a wingspan of five feet and embodied as its basic structure the Pratt truss, which had been designed for railroad bridges and adapted for aircraft by Chanute. The kite also embodied Wilbur Wright's concept of wing warping. The four cords gave the operator lateral and longitudinal control|
GLIDER-KITE OF 1900 was the first Wright aircraft large enough to support a man. It was a scale-up version of the 1899 kite, having a wingspan of 17 feet and a chord (wing width) of five feet. The Wrights tested it, mainly as a kite, at Kitty Hawk, N.C., in October of 1900. Only Wilbur flew as the pilot, for a total of about 10 seconds. Although the controls worked well when the Wrights flew the aircraft as a kite, Wilbur found in his brief attempt at piloting that he could not master the adaptation of the controls to direct operation.
While Wilbur was practicing turns he discovered what is now called adverse yaw. Since initially the lift on the raised wing is greater than that on the lowered one, so is the drag. The different drag forces tend to turn the airplane in the direction opposite to the one intended when the rolling motion is initiated. Wilbur's observations led to the installation of the vertical tail on the 1902 glider.
Because Wilbur was simultaneously a theorist, an inventor, a builder and a test pilot he had in three weeks of flying been able to make two crucial discoveries, one being the influence of camber on the motion of the center of lift and the other being adverse yaw. His responses essentially completed the configuration of the aircraft.