Flow channels through which air or another gas is passed over a model of an aircraft or other object to study the effects of the airflow on the forces acting on the aircraft model.
Wind tunnels are based on the principle of relative motion, which states that the forces acting on an aircraft or aircraft model are dependent only on the relative motion between the aircraft and the air. It does not matter whether the aircraft is moving at a certain velocity through still air, or whether the aircraft is fixed and the air is moving over it at an equal and opposite velocity to the aircraft speed.
Although there are many types of wind tunnels, most of them share common characteristics. They all typically have an inlet section, which is a contracting passage called a venturi, or contraction cone, to speed up the flow of air. The air then enters the test section, where an aircraft model is mounted and where the effects of the air flowing over the model are measured. The most common measurements are of the forces acting on the model. The air then enters an expanding section called a diffuser, where it slows down again. One or more fans are located at the end of the diffuser to draw the air through the tunnel. At the entrance to the tunnel, flow straighteners are mounted to reduce the swirl imparted to the air by the fan, and to damp out most of the turbulence in the air. These flow straighteners consist of screens, or short sections of honeycomb material not unlike a cross section of cardboard box material. Tunnels of this design are called open-circuit, closed-test-section, or National Physical Laboratory (NPL) tunnels. The test section dimensions may vary in size, ranging from a few square inches to 80 feet by 120 feet.
Airplane models used in wind tunnels vary greatly in type and size, ranging from a few inches long to full-scale airplanes in the largest tunnels. Scales from one-twenty-fourth to one-third actual size are often used. Most commonly, a scale model of a complete airplane is tested in the wind tunnel. Sometimes components of the airplane, such as the wing, fuselage, or engine nacelles with scaled engines operating, are tested. Wind tunnels are also used by automotive engineers to test automobiles for their flow characteristics and by civil engineers to test wind effects on buildings and other structures. Numerous other things are tested in wind tunnels: parachute opening dynamics, effects of control surfaces on aircraft, helicopter rotor behavior, propellers, human ski jumpers, golf balls, and even the flight characteristics of birds and insects.
The most common measurements made in a wind tunnel are of the forces and moments, or torques caused by forces that tend to rotate the model, acting on the model. The model is usually mounted from below on a strut-type mount or from behind on a sting mount. The strut or sting mounting is connected to a force measuring balance mounted outside the wind-tunnel test section, where the forces and moments are electronically or mechanically measured. The forces measured are usually relative to the airstream direction. The upward component of force is called the lift force; the rearward force, which must be overcome by the aircraft propulsion system, is called the drag force; and the sideward component of force is called the yaw force. The turning tendency, or torque, about the vertical axis through the aircraft’s center of gravity is called the yawing moment; the torque about the lateral axis though the airplane wings is called the pitching moment; and the torque about the airplane’s longitudinal axis, through the fuselage from front to rear, is called the rolling moment. All these forces and moments must be measured. The rolling, pitching, and yawing moments measured are important for predicting the airplane’s response to deflections of its aerodynamic control surfaces.
In addition to forces and moments, other types of measurements are often made in wind tunnels. An important one is the pressure distribution across various sections of an airplane or other model. Measurements are also made for the local flow field in the vicinity of the model, in other words, the air velocity and direction along various portions of the model and very close to the model. These measurements are often made as part of a flow visualization study, where laser beams, injected smoke, or streamers or tufts are used to study local flow directions. Wind tunnels are also used for aeroelastic studies, where control surface flutter may be examined or aeroelastic twisting or bending of helicopter rotor blades may be investigated. The list of variables that can be studied is endless. For this reason, when a new aircraft is being designed, it is not unusual for several different types of models to be built and tested in different tunnels to determine all the necessary characteristics of the airplane’s aerodynamic and structural response.
There are a number of problems that occur with wind tunnel testing, which engineers must constantly strive to overcome. One of these is the scale effect, which occurs because the model is usually smaller than the full-scale prototype. The difference in size causes some important differences in flow between the model and the prototype. These flow differences must be accounted for using various techniques at the engineer’s disposal, before the model data can be used to predict the prototype behavior. The air temperature, pressure, and velocity used in the tunnel test may have to be somewhat different from actual flight conditions. Again, there are ways to compensate for this, but they must be carefully evaluated.
Another reason for differences that occur between measurements made on a wind-tunnel model and what may occur in flight is due to tunnel wall effect. An airplane flies through open air that is free to move out of the way as the airplane passes through it. Such is not usually the case with a wind-tunnel model. The walls of a wind tunnel provide a partial constraint to the motion of the air as it passes over the model, somewhat altering the forces that would occur in free flight. This effect is especially important in transonic and supersonic aircraft that are generating shock waves. In actual flight, shock waves are free to extend from an actual aircraft as far as necessary. In a wind tunnel model test, these shock waves will strike the tunnel walls and be reflected in complicated patterns that reach the aircraft model or model support system. These interactions are accounted for by complex techniques that ensure the validity of the data measured.
The most common variation to the open-circuit tunnel is the closed-circuit design, which has a return flow path that recirculates the air from behind the fan back around to the inlet section. Thus the same air continually recirculates through the tunnel. This type of tunnel is called a Prandtl- or Göttingen-type tunnel. The main advantage of this design is a reduction in the power required for a given test-section velocity, because the air is not wasted as it leaves the diffuser, but much of its kinetic energy is recovered by recirculating it back to the inlet. Other advantages are lowered noise, due to elimination of the open exhaust, and the ability to control tunnel test-section conditions better by heating, cooling, or pressurizing the air. In a closed-circuit, closed-test-section tunnel, it is even possible to change the test gas from air to some other gas to simulate certain conditions. For example, extremely low-temperature nitrogen is sometimes used to simulate high-density air, and helium or Freon have been used for other kinds of simulations.
Sometimes, open-test-section tunnels are used to eliminate the tunnel wall effects. This can only be done in tunnels using air. Other types of tunnels are especially designed for testing aircraft in supersonic or hypersonic flight where aerodynamic heating effects may be important. Free flight tunnels are sometimes used to check aircraft response to control surface inputs or recovery characteristics from unusual attitudes. Propulsion tunnels are used to study airframe-engine integration. These tunnels may use actual operating jet or rocket engines, or devices designed to simulate the intake or exhaust flow of a propulsion system.
Finally, civil engineers often do studies in what is known as a boundary-layer wind tunnel. This type of tunnel usually has a test section that is very long, compared to its width and height. The long test section is used to simulate the growth of the earth’s atmospheric boundary layer, the region near the ground where wind velocity increases with height. Boundary-layer wind tunnels are used to study wind effects around buildings or skyscrapers, the dispersion of smoke plumes from smokestacks, and the interaction of smoke with buildings located downwind.
Wind tunnels are usually operated by research organizations such as universities, aerospace companies, or government research laboratories. Prominent university wind tunnels include those of the Georgia Institute of Technology, which operates a test-section tunnel, and the Massachusetts Institute of Technology, which operates the Wright Brothers Wind Tunnel with an elliptically shaped test section. Other noteworthy wind tunnels are located at the Texas A&M University, the University of Michigan, and the University of Washington.
Well-known company wind tunnels include the Boeing Research Wind Tunnel, with a 5-foot-wide-by-8-foot-high test section, the McDonnell Douglas 8-foot-by-12-foot tunnel, and the General Motors Automotive Wind Tunnel with an 18-foot-by-34-foot test section.
The U.S. government operates wind tunnels at many of the National Aeronautics and Space Administration (NASA) facilities, predominately the NASA Langley Research Center in Hampton, Virginia, and the NASA Ames Research Center in Mountain View, California. Much of the airframe-engine integration testing is done in propulsion wind tunnels located at the United States Air Force Arnold Engineering and Development Center (AEDC) in Tullahoma, Tennessee.
One of the most famous wind tunnels in the world is the 80-foot-by-120-foot cross-section wind tunnel located at the NASA-Ames Research Center. The largest wind tunnel in the world, it was originally built as a closed-return wind tunnel with a 40-foot-by-80-foot oval cross section capable of a test speed of 230 miles per hour. In the early 1980’s, the tunnel was modified also to include an 80-foot-high-by-120-foot-wide test section operating as an open-circuit tunnel. In this mode, the maximum speed drops down to 115 miles per hour, but the test section is large enough to fit a full-size Boeing 737 airliner inside it. The six fans that drive the tunnel are powered by electric motors totaling 135,000 horsepower. With this power, the original 40-foot-by-80-foot test section can now operate at 345 miles per hour. Virtually every fighter plane and transport plane produced in the United States since the 1970’s has had one of its models tested in the NASA-Ames 40-foot-by-80-foot tunnel.
Another unique wind tunnel facility is the National Transonic Facility (NTF) located at the NASA-Langley Research Center. This is a cryogenic wind tunnel that uses very low temperature nitrogen gas at -253 degrees Fahrenheit. This tunnel can simulate flight speeds as much as 20 percent greater than the speed of sound, which is 760 miles per hour at sea level. The use of the high density nitrogen gas in the tunnel’s 8-foot-by-8-foot cross section makes it possible to simulate flight conditions very close to those encountered by full-scale craft with a relatively low power requirement compared to other tunnels.
Goin, K. L., “The History, Evolution, and Use of Wind Tunnels.” American Institute for Aeronautics and Astronautics Student Journal 9, no. 1 (February, 1971). A good nontechnical summary of wind tunnels. Pope, A., and K. L. Goin. High-Speed Wind Tunnel Testing. Huntington, N.Y.: Robert E. Krieger, 1978. Chapter 1 illustrates some transonic and supersonic wind tunnels. The remainder of the book is technical and designed for college students and practicing engineers. Rae, W. H., Jr., and A. Pope. Low-Speed Wind Tunnel Testing. New York: John Wiley & Sons, 1984. Chapter 1 describes and illustrates various types of wind tunnels in layperson’s language and contains a list of more than two hundred wind tunnels located throughout the world. The rest of the book is a technical, mathematical treatment of wind tunnel design, operation, and model testing. Shevell, R. S. Fundamentals of Flight. 2d ed. Englewood Cliffs, N.J.: Prentice Hall, 1989. Chapter 4 has a short history and summary of wind tunnels, starting with the first tunnel built by Orville and Wilbur Wright.
Aerospace industry, U.S.
Forces of flight
NASA operates a number of wind tunnels for testing the designs of experimental aircraft. The world’s largest wind tunnel is located at NASA’s Ames Research Center in Mountain View, California, where this parafoil is being tested.