Advanced Wind Tunnels

By the late 1940s, aircraft were becoming increasingly expensive to develop and the costs of designing an unsuccessful aircraft were also growing. As a result, aircraft designers sought to model mathematically and to simulate as much of an aircraft's performance as they could without having to build the airplane itself. This, combined with the increasing speed of aircraft, created a great demand for new and more sophisticated wind tunnels. In particular, supersonic wind tunnels were in great demand during the post-World War II period.

Supersonic tunnels work in a way that seems contrary to logic. As the throat of a wind tunnel constricts, one expects the velocity of the air rushing through it to increase. It therefore seems logical that a model should be placed at the constricted part of such a tunnel in order to take advantage of the high-velocity airflow. But the reality is that as the airspeed approaches Mach 1, the air compresses and also heats up as it piles up at this constricted part. Only when the air gets past this constriction does it actually move faster than Mach 1, as the energy stored in both the compression and in the heat of the air converts to kinetic energy. Put another way, all of this stored energy has to convert to another form and this form consists of large amounts of air moving very fast through the wind tunnel. This is how a supersonic wind tunnel works, with the model placed in a section of the tunnel where the throat actually expands.

Numerous small supersonic tunnels were in use by the 1940s, but aircraft designers wanted bigger tunnels for their models. By 1948, the National Advisory Committee for Aeronautics (NACA) began operating a 4-foot by 4-foot (1.2-meter by 1.2-meter) supersonic tunnel at Langley, Virginia, on the Atlantic coast. Another NACA facility, Ames, located in California, also began operating a slightly larger and more sophisticated supersonic wind tunnel around this time. Because even the slightest imperfection in the tunnel walls would cause the air to pile up and create shock waves, supersonic tunnels require very smooth interior surfaces.

Many of the same principles used in supersonic tunnels were also used in hypersonic tunnels to explore speeds greater than Mach 5. But several other problems occur in these types of tunnels. One is that the power requirements to accelerate the air are tremendous, so most hypersonic wind tunnels do not operate continuously but store up tremendous amounts of compressed air and release it in a brief burst. This is why many hypersonic tunnels have large storage tanks for holding compressed air. Another problem is that as the air moves out of the constriction chamber it cools as its heat energy is converted to kinetic energy. In a hypersonic tunnel, the air can cool so much that it actually liquefies. This is not simply a case of the moisture in the air condensing. The air itself turns to liquid. In order to prevent this from happening, the air is deliberately heated as it is compressed in a "settling chamber" before being released. In a Mach 10 wind tunnel, for example, air is heated to 3,000 degrees Fahrenheit (1649 degrees Celsius) so that it does not liquefy when it is released.

Another method of obtaining high velocities is to fire models out of the barrel of a gun inside of a supersonic wind tunnel. In this way, the speed of the model combines with the speed of the moving air to produce a greater simulated velocity. The models are photographed as they streak by. Because the air itself is not moving at hypersonic velocities, this does not create any of the problems associated with liquefication of the air, but the models are destroyed in the process of testing them.

A major development during this period was the slotted wall wind tunnel. A big problem with wind tunnels is that the air flowing off a model can hit the tunnel wall, and flow back toward the model and/or interfere with the test measurements. Ray Wright, a researcher at Langley, proposed putting slots in the walls of a wind tunnel so that the air could move more freely around the model. Another group of aerodynamicists, led by John Stack, applied this technique to the transonic wind tunnel, which instantly solved many of the problems that they were encountering as air speeds approached Mach 1. As a result, in 1951 Stack and his group were awarded the Collier Trophy, which honors the most important advance in aeronautics for the year.

In addition to being used to design new planes, wind tunnels are also used to solve many other problems that affect existing aircraft once they become operational. One problem that plagues aircraft that fly in cold temperatures is ice. Ice builds up on propellers and on aircraft surfaces, particularly wings, and can affect performance in dangerous ways. Ice buildup on wings is particularly bad, for it can destroy lift and cause the plane to lose altitude and crash, or can block control surfaces and make it impossible for the pilot to fly the plane.

Icing tunnels were developed beginning in the 1940s to study this problem. They are similar to conventional subsonic wind tunnels but are equipped with refrigeration systems that can cool the air to well below freezing. Water droplets are then sprayed into the airflow so that they can freeze on aircraft surfaces. Engineers monitor the buildup of ice on the aircraft. Anti-icing devices such as electric heaters or pipes containing a heated liquid such as alcohol are installed in the parts of aircraft that generate the most ice. When ice begins to build up on a model in the icing tunnel, the heaters are turned on and researchers then study how effective they are at stopping the buildup of ice.

There are many other different kinds of wind tunnels. There are "spin tunnels" that test how aircraft behave when they fly out of control and start spinning, a situation that is commonly referred to by pilots as "departure from controlled flight." These tunnels test whether the pilot can recover in this situation or needs to parachute out of the airplane. There are "free flight" tunnels where models are actually "flown" by remote control by a pilot sitting in a control booth and sending signals to the model through a wire tether. There are also blast-furnace-type tunnels for testing how spacecraft and missiles act in high temperature airflows such as they encounter when reentering the Earth's atmosphere. And there are magnetic tunnels, where the model is held stable inside the tunnel by powerful magnetic fields so that more accurate measurements can be taken.

Before the 1950s, most of the wind tunnels operated in the United States were run by the NACA. But in 1946, a study of American wind tunnels resulted in a recommendation that industry and universities play a greater role in operating wind tunnels. This led to the National Unitary Wind Tunnel Plan Act of 1949. The Act established new supersonic wind tunnels at the three major NACA facilities, but also pushed for the creation of supersonic wind tunnels at universities. The development of a university wind-tunnel base was important both to serve as a check on NACA research and to train new aeronautical engineers. The NACA tunnels were also directed to perform more industry research, symbolizing a decreased emphasis on government-sponsored wind tunnel research.

For years wind tunnels represented a less expensive way of testing an airplane than building the full-size vehicle. But wind tunnel research was and still is expensive. Testing a new airplane design in a wind tunnel costs millions of dollars. As a result, aircraft designers have increasingly shifted to computers and a method called computational fluid dynamics (air, after all, is a fluid, like water), which simulates airflow entirely within a computer. Computing power is relatively cheap, and computer models can be changed much more easily than physical models made of plastic, metal, and wood.

Today, wind tunnels are used less and less and the giant wind tunnels that dominated so many aeronautical research centers starting in the 1930s and 1940s are now often called upon only to serve as backups to the computer simulations, to prove that their predictions are sound. In several important cases, however, aircraft designers have had to use wind tunnels to test their designs after computer simulations have proven inadequate. For example, the Pegasus XL air-launched rocket suffered an in-flight aerodynamic failure that was not predicted by a computer-generated aerodynamic model. But in a matter of years, most of the large NACA-built wind tunnels may become totally silent, their roar replaced by the hum of a supercomputer.

--Dwayne A. Day

Sources and further reading: