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1901

Before returning to Kitty Hawk in the summer of 1901, Wilbur published two articles, "The Angle of Incidence" in The Aeronautical Journal, and "The Horizontal Position During Gliding Flight" in Illustrierte Aeronautische Mitteilungen. The brothers brought all of the material they thought was needed to be self-sufficient at Kitty Hawk. Besides living in tents once again, they built a combination workshop and hangar. Measuring 25 feet (7.6 m) long by 16 feet (4.9 m) wide, the ends opened upward for easy glider access.

Hoping to improve lift, they built the 1901 glider with a much larger wing area and made dozens of flights in July and August for distances of 50 to 400 ft (15 to 122 m). The glider stalled a few times, but the parachute effect of the forward elevator allowed Wilbur to make a safe flat landing, instead of a nose-dive. These incidents wedded the Wrights even more strongly to the canard design, which they did not give up until 1910. The glider, however, delivered two major disappointments. It produced only about one-third the lift calculated and sometimes pointed opposite the intended direction of a turn – a problem later known as adverse yaw – when Wilbur used the wing-warping control. On the trip home a deeply dejected Wilbur remarked to Orville that man would not fly in a thousand years.

The poor lift of the gliders led the Wrights to question the accuracy of Lilienthal's data, as well as the " Smeaton coefficient" of air pressure, a value which had been in use for over 100 years and was part of the accepted equation for lift.

The Wrights used this equation to calculate the amount of lift that a wing would produce. Over the years a wide variety of values had been measured for the Smeaton coefficient; Chanute identified up to 50 of them. Wilbur knew that Langley, for example, had used a lower number than the traditional one. Intent on confirming the correct Smeaton value, Wilbur performed his own calculations using measurements collected during kite and free flights of the 1901 glider. His results correctly showed that the coefficient was very close to 0.0033 (similar to the number Langley used), not the traditional 0.0054, which would significantly exaggerate predicted lift.

The brothers decided to find out if Lilienthal's data for lift coefficients were correct. They devised an experimental apparatus which consisted of a freely rotating bicycle wheel mounted horizontally in front of the handlebars of a bicycle. The brothers took turns pedaling the bicycle vigorously, creating air flow over the horizontal wheel. Attached vertically to the wheel were an airfoil and a flat plate mounted 90° away. As air passed by the airfoil, the lift it generated, if unopposed, would cause the wheel to rotate.

The flat plate was oriented so its drag would push the wheel in the opposite direction of the airfoil. The airfoil and flat plate were made in specific sizes such that, according to Lilienthal's measurements, the lift generated by the airfoil would exactly counterbalance the drag generated by the flat plate and the wheel would not turn. However, when the brothers tested the device, the wheel did turn. The experiment confirmed their suspicion that either the standard Smeaton coefficient or Lilienthal's coefficients of lift and drag – or all of them – were in error.

They then built a six-foot (1.8 m) wind tunnel in their shop, and between October and December 1901 conducted systematic tests on dozens of miniature wings. The "balances" they devised and mounted inside the tunnel to hold the wings looked crude, made of bicycle spokes and scrap metal, but were "as critical to the ultimate success of the Wright brothers as were the gliders." The devices allowed the brothers to balance lift against drag and accurately calculate the performance of each wing. They could also see which wings worked well as they looked through the viewing window in the top of the tunnel. The tests yielded a trove of valuable data never before known and showed that the poor lift of the 1900 and 1901 gliders was entirely due to an incorrect Smeaton value, and that Lilienthal's published data were fairly accurate for the tests he had done.

Before the detailed wind tunnel tests, Wilbur traveled to Chicago at Chanute's invitation to give a lecture to the Western Society of Engineers on September 18, 1901. He presented a thorough report about the 1900–1901 glider experiments and complemented his talk with a lantern slide show of photographs. Wilbur's speech was the first public account of the brothers' experiments. A report was published in the Journal of the society, which was then separately published as an offprint titled Some Aeronautical Experiments in a 300 copy printing.