A Warped Experiment

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mong the many scientific books and papers that Wilbur waded through was James Bell Pettigrew's Animal Locomotion, or Walking , Swimming, and Flying, With a Dissertation on Aeronautics. Pettigrew confirmed Will's observation that birds twisted their wings in flight. "The wing is jointed to the upper part of the body by a universal joint which admits every variety of motion..." wrote Pettigrew. He went on to describe how the wings of both birds and flying insects twisted along their long dimension "like a screw." He was mostly concerned about describing how wings produce thrust and lift, but Wilbur reasoned that the same motions could also be used for aerodynamic control.

The Aileron Hypothesis

 "The thought came to me," wrote Wilbur, "that possibly [a bird] adjusted the tips of its wings…so as to present one tip at a positive angle and the other at a negative angle, thus…turning itself into an animated windmill, and that when its body had revolved…as far as it wished, it reversed the process."

But how could they imitate this in a mechanical flying machine? Orville suggested a system of intermeshed gears that would rotate the left and right wings in opposite directions. But this seemed too complex and too heavy to be of much use, as did other mechanical devices that Wilbur and Orville envisioned.  Additionally, if the wings were rotated at their roots, they would have to be sufficiently strong to support the aircraft and remain stiff without bracing. This, too, would add weight. The machine and its control system had to be simple, strong, and light.

The Elegant Solution

In July of 1899, Wilbur had just heard from the Smithsonian and his mind was full of aeronautics and aerodynamics. While talking to a customer at the bike shop, he picked up a long, slender cardboard box that had once held an inner tube and idly began to toy with it. He happened to place the thumb and forefinger of one hand on diagonal corners at one end of the box, and the other thumb and forefinger on the opposite diagonal corners at the other end. He noticed that when he squeezed his thumbs and forefingers together, the box twisted. The surfaces at each end rotated in opposite directions. In his mind’s eye, Wilbur saw the Chanute-Herring glider. The biplane was essentially a box with open sides. With a  set of cables, he could twist the wings just as he twisted the box. When one wing tip turned turned up, this would increase the lift at that end.  Where the other tip turned down, the lift would decrease. The difference in lift would cause the biplane to roll to the right or left. (To repeat this experiment for yourself, see the Inner Tube Box Experiment in our Adventure Wing.)

Wing warping also solved the weight problem. The control cables would weigh very little. And the wings could be lightly built; the rigging would provide the strength and stiffness needed. Wilbur's idea was what engineer's often refer to as the "elegant solution" – a design or idea that appears remarkable for its simplicity and the number of problems it solves. The elegant solution is to a scientist or engineer what an inspiration is to an artist, and is just emotionally satisfying and intellectually captivating. No doubt Wilbur's customer left wondering what had suddenly distracted him.

The Wright Kite

The brothers had gotten into the habit of testing their aeronautical ideas on kites or "model gliders," as they referred to them. Within a few weeks, Wilbur built a large biplane kite with a wingspan of 72 inches (183 centimeters) and a chord of 18 inches (46 centimeters). It was the same aspect ratio as the Chanute-Herring glider. (Aspect ratio is the wingspan or the length of the wing divided by the chord or width.) You can see this kite in detail in our Virtual Hangar. Or you can build and fly a smaller version yourself – see the Not Quite Wright Kite.

Wilbur fitted the kite with controls that allowed him to twist or "warp" the wings from the ground – this would roll the kite.  It also had a movable tail to control pitch. Control lines ran between the four forward corners of the kite to the ends of two sticks, one in each of Wilbur's hands. When he angled the sticks in opposite directions, the wings would twist one way or the other, hopefully causing the kite to roll right or left. When he angled his hands in the same direction, the tail would turn up or down, pitching the kite up or down.

On a windy day in late July or early August, Wilbur took the kite to a nearby open space at the Union Biblical Seminary – a school his father had helped establish. With him was a rag-tag collection of neighborhood boys who came to see what he was up to. Later, several of them would remember that when Wilbur flew the kite, it would swoop at them and they dived for cover. But aside from scaring the neighbors' kids, the controls worked just as Wilbur had expected. His hypothesis would become an important aerodynamic design principle.

Orville was camping with friends at the time and Wilbur, too excited to wait for him to come home, rode his bicycle out to the camp to tell his brother the good news. Immediately, the Wrights began to plan a glider with "wing warping" controls.

These illustrations from Pettigrew's Animal Locomotion show how a bird's wing twists in flight. The primary feathers are indicated by the label "b." To read the book, click on the title.

Click on the inner tube box to see how Wilbur twisted it. If you'd like to try this experiment yourself, click HERE for instructions and a downloadable inner tube box you can print on card stock.

WBAC Director Nick Engler tells the story of the inner tube box and shows how to twist it in this video.

In 1912, Wilbur made these sketches of the 1899 kite from memory.

Flying a replica of the 1899 Wright Kite. To see more details, visit the 1899 Kite in our Virtual Hangar. For plans and instructions to build and fly a smaller version, see the Not Quite Wright Kite.

To see a video of the 1899 Wright Kite in flight, click on the image above.

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The Wright Story/Inventing the Airplane/A Warped Experiment

Part of a biography of the Wright Brothers


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