Gliders
The Wrights based the design of their kite and full-size gliders on work done in the 1890s by other aviation pioneers. They adopted the basic design of the Chanute-Herring biplane hang glider ("double-decker" as the Wrights called it), which flew well in the 1896 experiments near Chicago, and used aeronautical data on lift that Otto 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 version of Chanute's modified Pratt truss, a bridge-building design he used for his biplane glider (initially built as a triplane). The Wrights mounted the horizontal elevator in front of the wings rather than behind, apparently believing this feature would help to avoid, or protect them from, a nosedive and crash like the one that killed Lilienthal. 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 as he did not want to have to explain to their father, Bishop Wright, if Orville got injured.
The brothers flew the glider for only a few days in early autumn 1900 at Kitty Hawk. In the first tests, probably on 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 ballast.
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, October 20. 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, 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.
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 the following lift equation to calculate the amount of lift that a wing would produce:
L
=
k
S
V
2
C
L
,
{\displaystyle L=kSV^{2}C_{\mathrm {L} },}
where:
L
{\displaystyle L}
is the lift, in pounds;
k
{\displaystyle k}
is the coefficient of air pressure (the Smeaton coefficient);
S
{\displaystyle S}
is the total area of lifting surface, in square feet;
V
{\displaystyle V}
is the air velocity over the wing (headwind plus ground speed), in miles per hour; and
C
L
{\displaystyle C_{\mathrm {L} }}
is the coefficient of lift (varies with wing shape).
Over the years a wide variety of values had been measured for the Smeaton coefficient
k
{\displaystyle k}
; 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 6-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.
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 scale-model wings of many shapes and airfoil curves, followed by detailed tests on 38 of them. 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 stubbier wings the brothers had tried so far. With this knowledge, and a more accurate Smeaton number, the Wrights designed their 1902 glider.
The wind tunnel tests, made from October to December 1901, were described by biographer Fred Howard as "the most crucial and fruitful aeronautical experiments ever conducted in so short a time with so few materials and at so little expense". In their September 1908 Century Magazine article, the Wrights explained, "The calculations on which all flying machines had been based were unreliable, and ... every experiment was simply groping in the dark ... We cast it all aside and decided to rely entirely upon our own investigations."
The 1902 glider wing had a flatter airfoil, with the camber reduced to a ratio of 1-in-24, in contrast to the previous thicker wing. The larger aspect ratio was achieved by increasing the wingspan and shortening the chord. The glider also had a new structural feature: A fixed, rear vertical rudder, which the brothers hoped would eliminate turning problems. However, the 1902 glider encountered trouble in crosswinds and steep banked turns, when it sometimes spiraled into the ground – a phenomenon the brothers called "well digging". According to Combs, "They knew that when the earlier 1901 glider banked, it would begin to slide sideways through the air, and if the side motion was left uncorrected, or took place too quickly, the glider would go into an uncontrolled pivoting motion. Now, with vertical fins added to correct this, the glider again went into a pivoting motion, but in the opposite direction, with the nose swinging downward."
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. The apparatus made the trailing edge of the rudder turn 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, as a rudder does in sailing, 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.
With their new method, the Wrights achieved true control in turns for the first time on October 9, a major milestone. From September 20 until the last weeks of October, they flew over a thousand flights. The longest duration was up to 26 seconds, and the longest distance more than 600 feet (180 m). Having demonstrated lift, control, and stability, the brothers now turned their focus to the problem of power.
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.
