The Wrights based the design of their first full-size glider (as well as the 1899 kite) on the work of their recent predecessors, chiefly the Chanute-Herring biplane hang glider ("double-decker", as the Wrights called it), which flew well in the 1896 experiments near Chicago; and aeronautical data on lift that Lilienthal had published. The Wrights designed the wings with camber, a curvature of the top surface. The brothers did not discover this principle, but took advantage of it. The better lift of a cambered surface compared to a flat one was first discussed scientifically by Sir George Cayley. Lilienthal, whose work the Wrights carefully studied, used cambered wings in his gliders, proving in flight the advantage over flat surfaces. The wooden uprights between the wings of the Wright glider were braced by wires in their own adaptation of Chanute's modified "Pratt truss", a bridge-building design he used in his 1896 glider. The Wrights mounted the horizontal elevator in front of the wings rather than behind, apparently believing this feature would help avoid a nosedive and crash like the one that killed Lilienthal. (Later, when the Brazilian aviation pioneer, Santos-Dumont, flew his 14-bis in Paris in 1906, French newspapers dubbed the tail-first arrangement a "canard", because of the supposed resemblance to a duck in flight.) Wilbur incorrectly believed a tail was not necessary, and their first two gliders did not have one. According to some Wright biographers, Wilbur probably did all the gliding until 1902, perhaps to exercise his authority as older brother and to protect Orville from harm.
Glider Vital Statistics
The brothers flew the glider only a few days in the early autumn of 1900 at Kitty Hawk. In the first tests, probably October 3, Wilbur was aboard while the glider flew as a kite not far above the ground with men below holding tether ropes. Most of the kite tests were unpiloted with sandbags or chains (and even a local boy) as onboard ballast.
The 1900 glider. No photo was taken with a pilot aboard.
They tested wing-warping using control ropes from the ground. The glider was also tested unmanned while suspended from a small homemade tower. Wilbur (but not Orville) made about a dozen free glides on only a single day. For those tests, the brothers trekked four miles (6 km) south to the Kill Devil Hills, a group of sand dunes up to 100 feet (30 m) high (where they made camp in each of the next three years). Although the glider's lift was less than expected (causing most tests to be unmanned), the brothers were encouraged because the craft's front elevator worked well and they had no accidents. However, the small number of free glides meant they were not able to give wing-warping a true test.
The pilot lay flat on the lower wing, as planned, to reduce aerodynamic drag. As a glide ended, the pilot was supposed to lower himself to a vertical position through an opening in the wing and land on his feet with his arms wrapped over the framework. Within a few glides, however, they discovered the pilot could remain prone on the wing, headfirst, without undue danger when landing. They made all their flights in that position for the next five years.
Orville at Kitty Hawk with the 1901 glider, its nose pointed skyward; it had no tail.
Hoping to improve lift, they built the 1901 glider with a much larger wing area and made 50 to 100 flights in July and August for distances of 20 to 400 ft (6 to 122 m). The glider stalled a few times, but the parachute effect of the forward elevator allowed Wilbur to make a safe flat or "pancake" 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 failed to respond properly to wing-warping, turning opposite the direction intended—a problem later known as adverse yaw. On the trip home after their second season, Wilbur, stung with disappointment, remarked to Orville that man would fly, but not in their lifetimes.
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, which had been in existence for over 100 years and was part of the accepted equation for lift.
The Lift Equation
L = lift in pounds
k = coefficient of air pressure (Smeaton coefficient)
S = total area of lifting surface in square feet
V = velocity (headwind plus ground speed) in miles per hour
CL = coefficient of lift (varies with wing shape)
Replica of the Wright brothers' wind tunnel at the Virginia Air and Space Center
The Wrights—and Lilienthal—used the equation to calculate the amount of lift that wings of various sizes would produce. On the basis of measurements of lift and wind during the 1901 glider's kite and free flights, Wilbur believed (correctly, as tests later showed) that the Smeaton number was very close to 0.0033, not the traditionally used 60 percent larger 0.0054, which would exaggerate predicted lift.
Back home, furiously pedaling a strange-looking bicycle on neighborhood streets, they conducted makeshift open-air tests with a miniature Lilienthal airfoil and a counter-acting flat plate, which were both attached to a freely rotating third bicycle wheel mounted horizontally in front of the handlebars. Because the third wheel rotated against the airfoil instead of remaining motionless as the calculations predicted, the Wrights confirmed their suspicion that published data on lift were unreliable, and they decided to expand their investigation. They also realized that trial-and-error with different wings on full-size gliders was too costly and time-consuming. Putting aside the three-wheel bicycle, they built a six-foot wind tunnel in their shop and conducted systematic tests on miniature wings from October to December 1901. 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. Prior to beginning their wind tunnel experiments, Wilbur, at Chanute's invitation, traveled to Chicago to give a speech to the Western Society of Engineers on September 18, 1901. Wilbur's speech consisted of detailed accounts of his and Orville's glider experiments at Kitty Hawk up to the fall of 1901 and was complemented by a lantern slide show of photographs. Wilbur's speech was the first public account of the brothers' experiments.
A Big Improvement
At left, 1901 glider flown by Wilbur (left) and Orville. At right, 1902 glider flown by Wilbur (right) and Dan Tate, their helper. Dramatic improvement in performance is apparent. The 1901 glider flies at a steep angle of attack due to poor lift and high drag. In contrast, the 1902 glider flies at a much flatter angle and holds up its tether lines almost vertically, clearly demonstrating a much better lift-to-drag ratio.
Lilienthal had made "whirling arm" tests on only a few wing shapes, and the Wrights mistakenly assumed the data would apply to their wings, which had a different shape. The Wrights took a huge step forward and made basic wind tunnel tests on 200 wings of many shapes and airfoil curves, followed by detailed tests on 38 of them. The tests, according to biographer Howard, "were the most crucial and fruitful aeronautical experiments ever conducted in so short a time with so few materials and at so little expense". An important discovery was the benefit of longer narrower wings: in aeronautical terms, wings with a larger aspect ratio (wingspan divided by chord—the wing's front-to-back dimension). Such shapes offered much better lift-to-drag ratio than the broader wings the brothers had tried so far.
With this knowledge, and a more accurate Smeaton number, the Wrights designed their 1902 glider. Using another crucial discovery from the wind tunnel, they made the airfoil flatter, reducing the camber (the depth of the wing's curvature divided by its chord). The 1901 wings had significantly greater curvature, a highly inefficient feature the Wrights copied directly from Lilienthal. Fully confident in their new wind tunnel results, the Wrights discarded Lilienthal's data, now basing their designs on their own calculations.
Wilbur Wright pilots the 1902 glider over the Kill Devil Hills, October 10, 1902. The single rear rudder is steerable; it replaced the original fixed double rudder.
With characteristic caution, the brothers first flew the 1902 glider as an unmanned kite, as they had done with their two previous versions. Rewarding their wind tunnel work, the glider produced the expected lift. It also had a new structural feature: a fixed, rear vertical rudder, which the brothers hoped would eliminate turning problems.
By 1902 they realized that wing-warping created "differential drag" at the wingtips. Greater lift at one end of the wing also increased drag, which slowed that end of the wing, making the aircraft swivel—or "yaw"—so the nose pointed away from the turn. That was how the tailless 1901 glider behaved.
The improved wing design enabled consistently longer glides, and the rear rudder prevented adverse yaw—so effectively that it introduced a new problem. Sometimes when the pilot attempted to level off from a turn, the glider failed to respond to corrective wing-warping and persisted into a tighter turn. The glider would slide toward the lower wing, which hit the ground, spinning the aircraft around. The Wrights called this "well digging".
Orville apparently visualized that the fixed rudder resisted the effect of corrective wing-warping when attempting to level off from a turn. He wrote in his diary that on the night of October 2, "I studied out a new vertical rudder". The brothers then decided to make the rear rudder movable to solve the problem. They hinged the rudder and connected it to the pilot's warping "cradle", so a single movement by the pilot simultaneously controlled wing-warping and rudder deflection. Tests while gliding proved that the trailing edge of the rudder should be turned away from whichever end of the wings had more drag (and lift) due to warping. The opposing pressure produced by turning the rudder enabled corrective wing-warping to reliably restore level flight after a turn or a wind disturbance. Furthermore, when the glider banked into a turn, rudder pressure overcame the effect of differential drag and pointed the nose of the aircraft in the direction of the turn, eliminating adverse yaw.
In short, the Wrights discovered the true purpose of the movable vertical rudder. Its role was not to change the direction of flight, but rather, to aim or align the aircraft correctly during banking turns and when leveling off from turns and wind disturbances. The actual turn—the change in direction—was done with roll control using wing-warping. The principles remained the same when ailerons superseded wing-warping.
Wilbur makes a turn using wing-warping and the movable rudder, October 24, 1902.
With their new method the Wrights achieved true control in turns for the first time on October 8, 1902, a major milestone. During September and October they made between 700 and 1,000 glides, the longest lasting 26 seconds and covering 622.5 feet (189.7 m). Hundreds of well-controlled glides after they made the rudder steerable convinced them they were ready to build a powered flying machine.
Thus did three-axis control evolve: wing-warping for roll (lateral motion), forward elevator for pitch (up and down) and rear rudder for yaw (side to side). On March 23, 1903, the Wrights applied for their famous patent for a "Flying Machine", based on their successful 1902 glider. Some aviation historians believe that applying the system of three-axis flight control on the 1902 glider was equal to, or even more significant, than the addition of power to the 1903 Flyer. Peter Jakab of the Smithsonian asserts that perfection of the 1902 glider essentially represents invention of the airplane.