Further Gliding and Wind Tunnel Experiments - 1901
The Wright brothers were so enthused about returning to their flying in the summer of 1901, that they hired Charlie Taylor, a talented machinist, to take care of the repairs and sales in their bicycle shop in Dayton. Their sister Katharine was lined up to manage the company, allowing Wilbur and Orville to return to North Carolina in early Julythe middle of the cycling season.
They took the makings of a new 22-foot (6.7-meter) wingspan machine with them. This time the brothers set up their camp near Kill Devil Hills, four miles south of Kitty Hawk and also sunk a well. It took them several weeks to build the new glider and hangar in which to store it.To provide more lift, its wing surface was almost twice the area of their 1900 glider. The glider's total surface area was 315 square feet (28 square meters), including the elevator, and was the largest glider ever flown. It also incorporated a foot actuated wing-warping control system that the pilot used to turn the aircraft. The brothers also made sure the wing camber matched Otto Lilienthal's calculations.
Two flying enthusiastsEdward Huffaker and George Sprattjoined the brothers. Octave Chanute, the Wrights-friend, thought the two could benefit from the experience by helping Orville and Wilbur. With these guests arrived a huge swarm of aggressive mosquitoes-making all their lives there miserable. When Chanute arrived at the camp a few weeks later, he brought extremely fine-meshed mosquito netting with him .
Wilbur and Orville began another season of flying, but the new glider did not perform nearly as well as the first. The glider would nose-dive into the ground, and, once, only the front-located elevator saved Wilbur's skin. The brothers realized how Lilienthal had been killed when his glider crashed on his final flight. They began questioning the validity of Lilienthal's aerodynamic calculations they were using and suspected Lilienthal's data for camber contributed to the problem. They reverted to the wing camber of the previous year, which resulted in more successful glides, but that still did not solve the problems they were experiencing with the craft's lift to drag ratio.
A close-up of Wilbur.
Although the brothers succeeded in making one flight of 389 feet (120 meters), outdistancing Chanute's gliders of 1896, they decided to return to Dayton and perform their own calculations for airfoil lift and drag. They left North Carolina feeling pessimistic about their efforts. At the time, Wilbur made the prediction that men would sometime fly, but it would not be within our lifetime. Orville's recollection of Wilbur's comment was, "Not within a thousand years would man ever fly."
Back in Dayton, they felt almost ready to abandon aeronautics. They were saved by an invitation that Chanute sent at the end of August 1901, inviting them to speak at the distinguished Western Society of Engineers meeting. Although Wilbur had to be persuaded to accept the invitation by his sister, he found that preparing for the speech forced him to examine everything that had occurred up to that time. Wilbur's speech, titled 'Some Aeronautical Experiments,' was made to an appreciative crowd of society members and their wives, and showed slides of his machines in the air. He also suggested that Lilienthal's lift and drag tables were wrong. Once Wilbur had publicly stated they felt that Lilienthal’s data was wrong, they had to find a way to determine the correct data. They devised an unconventional testing machine that called upon their bicycle-making talents.
Wrights' unconventional testing machine.
Peter Jakob says it best: "They mounted two small surfaces vertically on a bicycle wheel laid on its side. The wheel was free to turn in response to the wind striking the surfaces. One surface was a flat plate mounted perpendicular to the flow. The other was a model wing patterned after the curve used by Lilienthal. According to Lilienthal's table, the model wing set at a five-degree angle of attack would generate enough lift to balance the flat plate exactly." Exposing the device to the natural wind did not yield good results, "so the Wrights mounted the horizontal wheel to the front of a bicycle to create a steady flow."  In a steady wind, which was produced by riding the bicycle up and down the streets of Dayton as fast as possible, the lift generated by air rushing over the surface of the curved airfoil would be balanced against the drag on the flat airfoil. If the lift on the curved airfoil and the drag on the straight airfoil were equal, the wheel would not revolve. If the lift were greater than the drag, the wheel would revolve in one direction; if the drag were greater than the lift, it would revolve in the other direction.
They set the curved airfoil at the angle stated in Lilienthal's tables. The wheel revolvedproving the lift and drag were not equal and the drag exceeded the lift that was generated. An error existed either somewhere in Lilienthal's work or the multiplier he had used in his equation to describe lift, called Smeaton's coefficient, was incorrect. This number, with a value of 0.005, had been developed by the 18th-century English engineer John Smeaton to design windmills and had been accepted as true for almost 150 years.
Although their device indicated that something was amiss, it was too crude for accurate measurements, so the brothers designed and constructed a wind tunnel to double-check the results of the bicycle-wheel experiment. This wind tunnel consisted of a square tube for aiming the air, a fan that drove the air, and a balance mounted in the airstream. The preliminary results from this tunnel were so promising, the brothers built a second, larger apparatus with a square test section 16 inches (103 centimeter) on a side. This tunnel produced the critical data they needed for their 1902 glider and the powered aircraft that would follow.
During late October and early November, Orville and Wilbur conducted tests on some 200 different wing shapes in the tunnel. These tests served primarily to validate and perfect the operation of the tunnel. By November 22, they were ready to carry out their formal tests that would prove to be so important.
They used 38 different model airfoils for these tests. These airfoils had an assortment of cambers, thickness, and shapes, including squares, rectangles, ellipses, uneven tips, and half circles, which they tested in different combinations. They tested single-wing and multiple-wing configurations. It was tedious and exacting work, requiring each wing shape be tested at 45 different angles. But by mid-December 1901, they had discovered, much to their surprise, that Lilienthal's tables were largely correct. It was Smeaton's coefficient that was wrong. The brothers also found the camber, or curvature, of Lilienthal's wings was inefficient. To remedy this, they designed wings with more of a parabolic curve that placed the high point of the wing about one-fourth of the way back down the chord from the leading edge rather than at its center, as Lilienthal had.
There was one other area where they had erroneously relied on Lilienthal's tables. They did not correct for the differences in the aspect ratio between Lilienthal's wing and the wings of their gliders. In other words, the proportion between the wingspan and the wing's chord length was different. This also affected the amount of lift generated.
Wilbur and Orville spent the rest of 1901 using their wind tunnel to answer some remaining questions regarding the shape and location of the wings. Through their methodical approach, they had achieved what no one had done before and answered questions that had remained unanswered for years. The brothers were sure they were right. Now they were ready to return to Kill Devil Hills and fly.
 Peter L. Jakab. Visions of a Flying Machine – The Wright Brothers and the Process of Invention, Washington and London: Smithsonian Institution, 1990, pp. 122-123.