The Wrights were also dismayed to discover that the fore-and-aft control was not as effective in a machine with wings of 1-in-12 camber. At times when gliding, they were required to use all their skill and the full power of the rudder to prevent the glider from rearing up so sharply as to lose all headway and then to plunge toward the ground (a dangerous condition which they later referred to as “stalling”—an aeronautical term still in use). The brothers reduced the camber of the wings by adding little “trussing posts” to wires to depress the ribs and flatten the curvature from that used by others to 1 in 18 to make the wings more like those of their 1900 glider. This change resulted in control as good as it had been the year before. Several hundred glides were made by Wilbur and Orville during the 1901 season of experiments. Using the slopes of Kill Devil Hill and West Hill, they sailed along in winds up to 27 miles an hour, breaking all records for distance in gliding. But the brothers were far from satisfied. They had learned a great deal about control, though their glider was still too feeble in lifting itself off the ground and staying aloft. Occasionally in free flight, the warping of the wings to increase the angle of attack to recover lateral balance did not produce the desired result. The wing having the greater angle sometimes lost speed as it lifted, compared with the opposite wing having a lesser angle. The brothers then realized that the greater angle of the wing on one side gave more resistance to forward motion and reduced the relative speed of that wing. This decrease in speed more than counterbalanced the effect of the larger angle of the wing in producing lift. The Wrights determined that they must add something to their method of controlling equilibrium to insure that equal speeds at the wingtips would be maintained. However, a vertical tail as a solution to the problem was left for the next glider. Contrary to the scientific texts they had read, it was becoming evident to the Wrights that the travel of the center of pressure on curved or cambered surfaces was not always in the forward direction as on a plane surface. They observed that when the angle of attack on a plane surface was decreased, the center of pressure did move toward the front edge; but on a cambered surface this was true _only when large angles were being decreased_. Wilbur and Orville were discouraged that the ideas about pressures on curved surfaces and travel of center of pressure, concepts advanced by the most reputable writers on the subject, including Langley, were unreliable. So perplexing did the problem seem that the Wrights considered dropping their experiments altogether. It was apparent, then, that better scientific data were needed before the problems of flight could be solved.