The utility of the wind tunnel is obvious today, but it was not the first aerodynamic test device. Early experimenters realized that they needed a machine to replace nature's capricious winds with a steady, controllable flow of air. They recognized, as Leonardo da Vinci and Isaac Newton had before them, that they could either move their test model through the air at the required velocity or they could blow the air past a stationary model. Both approaches were employed in the early days of aeronautics.
First, relatively steady natural wind sources were searched out. Models were mounted above windswept ridges and in the mouths of blowing caves. Even here, the perversity of nature finally forced experimenters to turn to various mechanical schemes for moving their test models through still air. The simplest and cheapest contrivance for moving models at high speeds was the whirling arm-a sort of aeronautical centrifuge.
Benjamin Robins (1707-1751), a brilliant English mathematician, was the first to employ a whirling arm. His first machine had an arm 4 feet long. Spun by a falling weight acting on a pulley and spindle arrangement, the arm tip reached velocities of only a few feet per second.
Robins mounted various blunt shapes-pyramids, oblong plates, etc.-on the arm tip and spun them in different orientations. He concluded that "all the theories of resistance hitherto established are extremely defective." Different shapes, even though they presented the same area to the airstream, did not always have the same air resistance or drag. The manifestly complex relationship between drag, model shape, model orientation, and air velocity defied the simple theory propounded earlier by Newton and made ground testing of aircraft all the more important to the infant science of flight.
Sir George Cayley (1773-1857) also used a whirling arm to measure the drag and lift of various airfoils. His whirling arm was 5 feet long and attained tip speeds between 10 and 20 feet per second. Armed with test data from the arm, Cayley built a small glider that is believed to have been the first successful heavier-than-air vehicle in history. In 1804 Cayley built and flew an unmanned glider with a wing area of 200 square feet. By 1852 he had a triplane glider design that incorporated many features of modern aircraft, but manned, powered aircraft were still half a century away.
Although Cayley performed many aerodynamic experiments and designed precocious airplane models,...
...his major contribution to flight was one of design philosophy. Before Cayley, would-be aeronauts believed that the propulsion system should generate both lift and forward motion at the same time, as birds and helicopters do. Cayley said, "Make a surface support a given weight by the application of power to the resistance of air." In other words, use an engine to create forward motion and let the motion develop lift via the wings. This separation of propulsion and lift functions, simple though it sounds, was a revolutionary change in the way people thought about aircraft. One need not build planes with flapping wings! A whole new horizon in aircraft design opened up.