Data and calculations on pressures on an airfoil to achieve equilibrium in an air plane as worked our by the Wright Brothers in experiments in 1899.

First Experiments, 1899

Wilbur and Orville realized that the motion of the air on a flying machine is frequently variable and tricky, causing the machine to rear up or down, or one wing to rise higher than the other, and the machine to become unstable. The problem—how to control a flying machine—was to find a method of restoring the machine's equilibrium both up and down and to each side.

Most pre-Wright experimenters had relied on human control to balance flying machines. The operator simply shifted the weight of his body to tilt the wings in the direction opposite from adverse action of the wind. But the continual contortions and acrobatics required to maintain equilibrium by this method were not within the skill of many experimenters. While using it, both Lilienthal and Percy S. Pilcher, an English experimenter, were killed in nose dives.

Chanute sought to effect "automatic stability" independent of the operator by causing the flying machine's structurally automatic supporting surfaces to adjust positions by flexible joints automatically with changes in the wind. Wilbur and Orville were to conceive a different method of control than that sought by Chanute, though they themselves later designed and patented an "automatic" device—a pendulum analogous to Sperry's gyroscope.

At Dayton, in 1899, the Wrights were ready to move beyond the first phase of study, speculation, and discussion. Their combined attack on the problem of equilibrium resulted in the conception of one of the fundamental principles of aeronautics. Their reasoned principle for lateral control of a flying machine was that the movement of an airfoil about its longitudinal axis could be controlled by means of a pressure differential exerted on its opposing lateral extremities (the principle known today as aileron control). Both modern-day ailerons and the Wrights' wing-warping are merely arbitrary mechanical devices for applying this principle. The brothers' first achievement was the conception of the principle itself.

Wilbur and Orville decided first to test their principle of control in a small model glider to see if it worked, thus sparing themselves from being injured if it did not. At first it occurred to them to effect the result of their principle by pivoting the right and left wings on geared shafts at the stable center of a glider. One wing would turn upward in front when the other turned down, and the balance would readjust. But there seemed to be no way to make this device strong enough without making the glider too heavy. They finally decided on warping or twisting the wings as the simplest and most effective method to effect the result of their principle. (It still would be effective if used today.) The wingtips were to be warped by means of cables controlled by the operator. By warping the wingtips, they expected to vary the inclination of sections of the wings at the tips, and obtain force for restoring balance from the difference in the lifts of the two wingtips.

While twisting a small pasteboard box with opposite ends removed, Wilbur observed that though the vertical sides were rigid endwise, the top and bottom sides could be twisted to have different angles at the opposite ends. Here was a simple means of warping the wings as they intended. They decided that a biplane's wings could be twisted or warped in like manner, enabling them while flying in a glider to warp the wings on the right and left sides to present their surfaces to the air at different angles. By warping the wingtips the operator would be able to increase the angle of attack on one wingtip and decrease it on the other. Thus, they believed, the operator could obtain a greater lift on whichever side he needed it and less lift on the other side in order to assure lateral equilibrium. (They later had to modify this by adding a movable vertical tail.)

To test their principle safely, the brothers built a model glider—actually a kite—with a 5-foot wingspan. Flown as a kite at Dayton, the model glider's wing surfaces were warped by the use of four cords reaching from the upper and lower wingtips on each side to the operator on the ground. Balance from front to rear was maintained in part by an elevator tested variously at the front and rear, as well as by other means. The Wrights believed after the tests that the model glider had demonstrated the efficiency of their system of obtaining both lateral and longitudinal control.