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.

For high-speed aircraft this would require impracticably high wind velocities in the tunnel and impracticably strong models to withstand the high pressures associated with such velocities. For these reasons, the tests are usually made on models at Reynolds numbers well below those for the full-scale conditions; in the interpretation of the results, due allowance is made for this difference in dynamic conditions

Various methods and devices are employed for performing the measurement of the forces, moments, torques and pressures to which the models, attached to special balances or rigidly supported, are subjected in the wind tunnel (Fig.1). The airflow pattern can be made visible by a number of methods.

There are several categories of wind tunnels: low-speed tunnels, high-speed subsonic tunnels, and transonic tunnels. Up to the late 1920s, wind tunnels were all of the low speed type, producing maximum air speeds of about 120 mph. High-speed subsonic tunnels and supersonic tunnels were developed in the following decade.

For a time there was a gap between the subsonic and the supersonic speed ranges, which was bridged by the transonic wind tunnel, a postwar development, enabling tests to be made right through the transonic range approximately between mach 0.8 and mach 1.2. The hypersonic wind tunnel, most recent development, is used for studying the conditions associated with the launching and flight of rocket-propelled missiles and earth satellites.

In the subsonic wind tunnel, as described in the forging, the test section is located at the narrowest part of the duct, where the highest speeds below the speed of sound – are produced. In the supersonic wind tunnel (Fig.4), the test section is preceded by a construction, a so-called convergent-divergent nozzle, in which the very high speeds are attained. Each different supersonic speed requires the use of a differently shaped nozzle; in some tunnels the nozzle has a flexible wall so that can be varied in shape by hydraulic adjusting equipment instead of having to be exchanged for another. Beyond the test section is a second constriction, in which the ultrasonic speed diminishes to subsonic values.

The wind is produced by a multi-stage axial-flow compressor or by the high-speed jet from a set of gas turbines. The friction of the wind against the tunnel walls generates heat, which is removed by a cooler incorporated into the circuit, so as to maintain a constant temperature in the test section. The power requirement to maintain a continuous flow of air at supersonic speeds is very high. For very high speeds this becomes a very uneconomical method of operation, and to overcome this problem intermittently operated wind tunnels have been developed.

Power is stored in the form of compressed air or vacuum, the wind being produced in short blasts, whereby a considerable saving in power input for operating the tunnel is effected. Broadly speaking, there are two types of intermittent wind tunnel. In one type the measurements are performed during the time when a valve between the test section and the pressure storage vessel (e.g., vacuum vessel, Fig.5) is open.

This vessel is connected to the wind tunnel through a quick-closing valve; the actual tunnel comprises a convergent-divergent nozzle, the test section, and a second constriction (the diffuser). Before the test commences, the vacuum vessel is evacuated; when the valve is opened, air rushes into the vessel so that a supersonic speed is attained in the test section, depending on the shape of the nozzle and the degree of vacuum in the vessel.

So long as this vacuum is sufficient to maintain sonic speed in the throat of the nozzle, the supersonic speed in the test section remains constant. An air drier is installed at the intake to intercept any moisture that might condense into droplets in the test section, where they could disturb the flow conditions. The second type of intermittent wind tunnel (Fig.6) is a tube along which gas is driven by various means for a very short time (a friction of a second) during which the force acting on the model is measured by special techniques.

The tube, which is of constant cross section and closed at both ends, is divided into a high-pressure and a low-pressure part by a gastight diaphragm. The test section located behind a convergent-divergent nozzle is in the high-pressure part. Before the test is started, the appropriate pressures are produced in the two parts of the tube by pumping in and pumping out air respectively. When the diaphragm is ruptured, a constant airflow speed will very briefly exist in the test section.