Wind Tunnel | Research & Encyclopedia Articles

Wind Tunnel

Amateur nineteenth-century aviators typically studied the flight behavior of birds and then built flying machines accordingly. The resulting bird-like craft failed miserably because the builders had no knowledge of aerodynamics and aeronautics, particularly of lift and drag forces acting on surfaces cutting through the air.

The earliest invention for testing flight characteristics was a whirling arm. Benjamin Robbins, an English mathematician, first employed such a device in the eighteenth century. He mounted such shapes as pyramids and oblong plates on the arm tip and spun them in different orientations. He found that no simple theory would account for the complex forces acting on moving objects. George Cayley also used a whirling arm to measure drag and lift in the early nineteenth century. The major drawback to whirling arms was the disturbance of air created; the aircraft models flew into their own wakes, thus precluding any clear findings.

Frank Wenham of England is credited with designing and operating the first wind tunnel in 1871. It was 12 ft. (4 m) long and 18 in. square. A fan driven by a steam engine propelled air down the tube to the model. Wenham soon discovered that wings could support much heavier loads than had been thought previously; thus flight was seen as a real possibility.

Orville and Wilbur Wright used a wind tunnel to improve their planes. The first two gliders they built were quirky and unpredictable in the air. They realized that all scientific information they were using for flight data, gathered from previous experimenters, was incorrect and that they would have to conduct their own investigations. The Wright brothers first built a square tube for channeling the air, a driving fan, and a two-element balance mounted in the airstream. They attached various wings to the balance and, as the contraption revolved, observed the relative lifting forces. They went on to build a larger and more sophisticated tunnel, in which they obtained the critical data they needed for their first manned, powered aircraft.

By the time World War I began, leadership in aerodynamic research had shifted to Europe. Many wind tunnels were built there while facilities in the United States were almost nonexistent. In the 1920s, however, aviation's growth spurred renewed interest in American research. The National Advisory Committee for Aeronautics (NACA) built new wind tunnels, one of which was constructed vertically to test aircraft spinning.

The 1930s saw increased speed in the tunnels; by the end of the decade NACA had created tunnels that enabled engineers to study forces acting on an airfoil near Mach 1 (or, one times the speed of sound) speed. In 1939 NACA built a 19 ft. (6 m) in diameter, high-pressure tunnel that used an 800 horsepower electric motor to drive a 34.5 ft. (11 m) propeller, which created a wind speed of 300 miles (48 km) per hour. After World War II, there was a real need for supersonic wind tunnels. They were complicated, involving narrowed tunnel walls and compressor fans.

In 1948 a 4x4 ft. supersonic tunnel began providing data which led to the development of the B-58 bomber, the X-2 research plane, and several fighters, including the F-102 and F-105. Beginning in the 1950s, hypersonic tunnels were created to test flight at Mach 5 (five times the speed of sound). They required unusually high temperatures and speeds, and, because of power restrictions, the runs had to be of short duration.

In the last 30 years there have been several refinements in wind tunnels. Hypersonic tunnels can now operate much longer (one can provide Mach 10 flow continuously); helium has been used to create a Mach-50 blast of wind; hydrogen and oxygen have been exploded to provide a blast wave of air; and electric explosions have created similar shock waves. With extensive computerization, self-streamlining walls, laser-based instrumentation, cryogenic operations, and magnetic model suspension, wind tunnel technology is unquestionably keeping pace with the rapid advances in scientific research.