The Evolution of the Wind Tunnel

Not every piece of aviation-related technology is actually part of the airplane. Many important technological developments that either support aircraft operations or support the design of aircraft never leave the ground. Wind tunnels are an excellent example of a technological innovation that supports aircraft design. Wind tunnels have enabled designers to develop many of the major early aviation technologies, such as the low-drag cowling, more efficient airfoils, retractable landing gear, anti-icing equipment, and better propellers and engines. Wind tunnels themselves eventually became complicated pieces of technology. They are not simply a tube or box with a fan blowing air but are carefully designed and constructed (and often very large) laboratory instruments. Without constant improvements in wind tunnel technology, aeronautical research would have ground to a standstill decades ago.

Early aviation researchers realized that they could test their craft by flying them or by flowing air past them. The earliest experimenters used a whirling arm to fly a craft under controlled conditions. But problems that others had encountered with the whirling arm convinced the Wright brothers in 1903 that they should use a wind tunnel rather than a whirling arm as their primary test device. They developed an early wind tunnel in their laboratory.

For approximately the next two decades, wind tunnels were built in Europe but very few were built in the United States. Albert Zahm, a professor at Catholic University in Washington, D.C., built an impressive wind tunnel in 1901 with the support of a wealthy industrialist. Zahm's wind tunnel had many advanced features such as special devices and structures to straighten the airflow and extremely sensitive pressure gauges. He made some important contributions to aerodynamics, such as proving that the friction of air over an aircraft's skin was a major cause of drag at subsonic speeds. But his financial supporter died, the money dried up, and Zahm's work stopped in 1908. A few years later, he built an 8 x 8-foot (2.4-meter x 2.4-meter) tunnel at the Washington Navy Yard that allowed models to be tested at speeds of 160 miles per hour (257 kilometers per hour), which many military aircraft could reach in a dive.

While American wind tunnel research stagnated, the European wind tunnels developed between 1903 and the start of World War I grew increasingly larger and more sophisticated. For instance, Gustave Eiffel, who built the famous tower in Paris, built a private aerodynamics laboratory with his own money and constructed several wind tunnels. Many of these early wind tunnels, however, were simply a long tube with a propeller mounted inside, usually behind the section for mounting the model.

In 1908, Ludwig Prandtl built the first wind tunnel of "continuous circuit" design, which meant that the tunnel's front and back were connected, like a racetrack. He used vanes at the corners of the tunnel to turn the airflow and screens and honeycomb structures to smooth the flow. This made the tunnel very efficient. In 1916 Prandtl built his second-generation wind tunnel, which serves as the basis for virtually all modern wind tunnels. In this kind of tunnel, the tunnel's cross section becomes larger as the air comes closer to the testing area (which means that you can look at the exterior of a wind tunnel and tell which way the air flows inside). The tunnel includes a "stilling chamber" just before the section where the air reaches the model. This chamber reduces turbulence. The air is then sped up by entering a contracting cone or nozzle.

In 1920, the National Advisory Committee for Aeronautics (NACA) completed its first wind tunnel at Langley Field, Virginia. It was essentially a copy of a decade-old English wind tunnel and did not perform much useful research, but it allowed NACA engineers to understand the problems associated with building and operating a wind tunnel.

By 1921, more than 20 wind tunnels were in operation throughout the world, but all operated at normal atmospheric pressures. However, Osborne Reynolds had shown that airflow conditions could be radically different for model and full-scale aircraft. One way to compensate for this was to build large tunnels able to hold full-size aircraft, but this was expensive and impractical. Another way to compensate was to build a wind tunnel that could operate at high pressures. A subscale model tested at higher pressures could provide the same exact data as a full-scale model tested at normal atmospheric pressure. In 1921, the NACA started work on the Langley Laboratory's Variable Density Tunnel (VDT), which placed the wind tunnel inside a strong pressurized tank. The VDT became operational in 1923 and quickly became the primary source of aerodynamic data for high Reynolds numbers. It was the most advanced wind tunnel of its day and helped the United States become a leader in aeronautical research.

The NACA developed the Propeller Research Tunnel (PRT) at Langley that became operational in 1927. This was a huge tunnel for the day and had a nozzle with a throat diameter of 20 feet (six meters). It allowed NACA engineers to test full-size aircraft fuselages, with their propellers attached. Using the PRT, researchers soon realized that landing gear and the engine cylinders of airplanes caused tremendous drag and led them to develop the low-drag cowling and retractable landing gear.

In 1927, the NACA also started work on its first high-speed wind tunnel (HST). Although most aircraft flew no more than 200 miles per hour (322 kilometers per hour) at this time, their propeller tips approached the speed of sound. Some racing planes were also reaching about Mach 0.5 and a few aerodynamicists realized that planes would continue to go faster. The first HSTs were small and could test models that were only a few inches long. By 1936 a Langley HST entered service with a diameter of 8 feet (2.4 meters) capable of producing airspeeds of Mach 0.75 (575 miles per hour) (925 kilometers per hour). By February 1945, this tunnel was achieving airspeeds of Mach 1. The tunnel demonstrated that rivet heads and other irregularities on the surface of an airplane created much drag, leading designers to switch to new methods that did not produce protrusions affecting the airflow.

One big question for many of these early tunnels was how to mount the model. There were two main problems associated with this. First, how could the model be mounted so that accurate readings could be taken? Second, how could the model be mounted without affecting the test data? Some early tunnels mounted the models upside down, on wires. It was easier to measure lift, which in this case pulled the plane down toward the ground (with the wing upside down, the areas of low and high pressure were reversed, pulling the plane toward the ground rather than away from it). Another development was the "stinger," which was a metal pole that stuck out from the top or bottom of the tunnel and into the rear of the model. By being located behind the model, this reduced its interference with the airflow, although it still had some effect. Later on, even less intrusive methods of supporting a model were developed, although they too had their limitations.

--Dwayne A. Day

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