Just as its name suggests, a wind tunnel is a tube or tunnel that has man-made wind blown through it at a certain speed. Scientists and engineers put a model of an airplane in the tunnel and then study the way air moves around the model. By looking at the way this smaller model acts in the wind tunnel, they get a pretty good idea of how a real life-sized airplane of the same design will probably fly. It is a lot easier, cheaper, and safer to build and test a model than to build and fly a real airplane.
Wind tunnels work on the idea that a stationary model with air moving around it behaves the same way a real, full-scale airplane moving through stationary air does. Sometimes only a part of an airplane, like a wing or an engine, is tested in a wind tunnel. The models, usually made out of steel or aluminum, that are tested are loaded with many instruments and sensors that report back to the computers in the control room. It's there that scientists, engineers, and technicians can begin to understand how the airplane is performing.
Scientists and engineers use wind tunnels to study the pressures, forces, and air flow direction affecting an airplane. Pressure is measured by small devices called pressure taps that are placed at various locations on the surface of the model. Forces are recorded by sensors in the structures that support the model in the test section. The direction that air flows around the model can be seen by the way tufts, small yarn-like strands attached to the model, flap around. Sometimes smoke is blown into the test section to make it easier to see how the air is flowing. From these different kinds of measurements, a great deal can be learned about the model being tested.
Wind tunnels vary in size according to their function. Some of the smallest wind tunnels have test sections that are only a few inches large and therefore can only be used with tiny models. Other wind tunnels have test sections that are several feet big. The largest wind tunnel in the world is at the National Full-Scale Aerodynamics Complex at NASA Ames Research Center, in the United States. Its 80 foot by 120 foot test section can fit a life-sized Boeing 737 inside! Wind tunnels aren't just used to test airplanes. Anything that has air blowing around or past it can be tested in a wind tunnel. Some engineers have put models of spacecraft, cars, trucks, trains, even road signs, buildings, or entire cities in wind tunnels to see how to improve their designs.
(Some resources on this page are provided courtesy of NASA.)