A wind tunnel is a facility used for testing aircraft: everything from helicopter rotor assemblies to space shuttles. Most wind tunnels are equipped with large fans which force air across the surface of the craft under test, in order to simulate high-speed (up to supersonic in some cases) movement. The aircraft in the tunnel is generally a model rather than an actual flying aircraft; for instance, a model might not have see-through glass windows or a pilot's seat as an actual plane would.
Models can be full-size (often the case in large wind tunnels, such as NASA's 40x80 and 80x120 foot tunnels), or extremely small (often the case for supersonic or shock wave tests which take place in smaller tunnels where it is easier to build up high pressure. One of NASA's tunnels is only 2x2 feet!
Wind tunnel tests are often employed by the military, where aircraft safety and structural integrity is crucial. An interesting test was also done on a full-scale replica of the Wright Flyer in 1999; this test was undertaken in the interest of historical curiosity.
Data from wind tunnel tests is gathered and evaluated in several ways. In flow visualization tests, the object of the test is to observe the aerodynamics of the aircraft. Airflow patterns over the wings and body can be seen by filling the tunnel's test section with theatrical smoke and using strategically placed lasers for illumination. Flow visualization can also be achieved by using masking tape or duct tape to attach small pieces of string all over the plane's surface. When air is applied to the plane, you end up with a real-life vector field that shows the directions of the air currents across the plane's entire surface. Flow visualization data is normally photographic; that is, pictures are taken of the plane's air currents as the tunnel's speed and pressure are varied.
Other important test data is gathered by transducers, which are attached to the plane at key points. Transducers help to analyze mechanical stresses that the plane might be subjected to in actual flight.