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Wind Tunnel for Aircraft

Various forces and moments to which an aircraft is subjected by the airflow cannot be accurately determined by purely theoretical calculations. The aircraft designer must therefore have good knowledge about experimental aerodynamics, which from the earliest days has contributed much to the progress made in aeronautical science.

Wind tunnels have been used for studying the elements of flight since 1871. Initially they were small-scale, open-loop devices such as the Wright Brothers’ tunnel with its 16-inch test section. Wind tunnels grew in size and complexity, particularly after the Ludwig Prandtl first closed-loop tunnel in 1909. Tunnels were built in a variety of sizes and shapes with varying speeds depending on the current technology and their intended areas of study. The Altitude Wind Tunnel (AWT) was the first wind tunnel built to study engine performance in altitude conditions.

Like aircraft, wind tunnels have come a long way in their technological development. Their sophistication has kept pacewith the needs of designers. The first major U.S. Government wind tunnel was built at NASA’s Langley Research Center and became operational in 1921. The Center was the first major research facility of the U.S. National Advisory Committee for Aeronautics (NACA), which was founded in 1915. The NACA later became a part of NASA when it was established on October 1,1958, to carry out space research and exploration and to continue the NACA’s aeronautical work.

The first major U.S. wind tunnel was built at NASA‘s Langley Research Center; Hampton, Virginia in 1920. Late in the last century, however, the first wind tunnels were little more than boxes or pipes. A fan or other device propelled air over a model of an aircraft or of a wing suspended in the pipe or box. Observation instruments were crude. The researchers had to gather many of the test results with their own eyes.

The Wright brothers designed and used such primitive tunnels to develop the wing configurations and control surfaces with which they achieved the first powered human flight early in this century. Today’s aircraft are larger, cruise faster and higher, carry more passengers and cargo, and use less fuel per mile than most of their predecessors. Aircraft now being developed are expected to show significant improvements in all of these performance characteristics.

One of the most important experimental aids is the wind tunnel, a device whereby the reactions of a carefully controlled airstream on scale models of airplanes or their component parts can be studied. The first condition that a model for testing in the wind tunnel must satisfy is that of geometric similarity with the full scale prototype.

In addition, certain other important relating to flow conditions and velocity must be satisfied to enable valid measurements to be performed on the model. The Reynolds number is a correction factor applied to the analysis of the flow around the model; it corrects for the scale effect resulting from the difference in size between model and prototype. When the fluid flow around the model is the same as that around the prototype, there is said to be dynamic similarity. For complete similarity between the full-scale airplane and a model that is, say one-tenth its linear size, the air velocity in the wind tunnel would have to be ten times as high as the speed for which the airplane is to be designed.
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