Wind tunnels have always had a unique advantage over flight testing in that absolute forces and moments could be measured with respect to a fixed reference-the tunnel itself, which is firmly footed in the earth. The standard instrument for determining the three force components (lift, drag, and side force) and the three moments (pitch, roll, and yaw) is the internal balance. In modern practice strain gages inside the balance register forces and moments as changes in electrical resistance. One might surmise that the design of such an instrument would provide little challenge, but in the wind tunnel temperatures may range from those of liquid nitrogen to the incandescence of reentry. Further, the balance must be small enough to fit inside tiny models. Balance errors must be held to 0.1 percent or less, whether measuring ounces or several tons. Thus the wind tunnel internal balance is no ordinary instrument.
Typical of the latest generation of internal balance is the one designed for the National Transonic Facility. Only 3.5 inches in diameter, it can register vertical loads as high as 10 tons and axial loads of 1 ton. (Such forces are encountered in high speed pressurized tunnels even though the tests are with small models.)
Even though it is built like a fine watch, the internal balance measures only whole body forces. The aerodynamicist also wants to know how well individual aircraft components are performing. Where has flow separation occurred? Do the lift forces along the wing conform to theoretical predictions? A highly instrumented model may require 500 or more tiny pressure sensors located strategically over its surface. In early wind tunnel experimentation, a myriad of tiny pressure orifices, each connected by a small, flexible tube to fluid filled glass tube manometers outside the tunnel, registered the pressures. The thousands of feet of thin, flexible tubing and long pipe organ banks of glass manometers made this approach clumsy, eye straining, and tedious for the clipboard toting technicians. Happily, the electronic revolution swept away spaghetti like lines feeding manometer tubes and replaced them with solid state pressure sensors that can be electronically scanned at rates of 10 000 readings per second. This development alone has greatly reduced the cost of wind tunnel research.
The aerodynamicist has long bemoaned the invisibility of air. He has gone to great lengths to make airflow patterns visible by introducing into the airstream filaments of smoke, tufts of thread, and, more recently, neutrally buoyant, luminescent bubbles of helium. If the changes in air density are large enough, as in shock waves, schlieren photography or shadowgraphs can help visualize airflow. Now, arriving almost hand in hand with today's electronic wizardry, the laser hologram has added a new dimension to flow visualization in wind tunnels. Using relatively simple optics, the light from a powerful laser can be split into two separate beams, one passing through the tunnel test section and the other bypassing it. When recombined on a photographic plate, the beams form an interference pattern (called a hologram) that captures the pattern of density gradients within the test section at that instant. Later, the hologram can be rendered into a shadowgraph, schlieren photo, or interferogram and examined at leisure. Holographic flow visualization is now used routinely in small research facilities. But the greatest benefit may well be found in energy savings in the larger tunnels, where the mass of data stored in a single hologram eliminates the need for repeat runs to acquire the same data by more conventional methods.
That extraordinary device, the laser, also functions like radar in that its light can be reflected from objects to fix their positions and velocities. But what would a laser radar see of interest in a wind tunnel? Lasers have such high resolution that the light reflected from fine particles normally caught up in the airstream can be detected and converted via the Doppler effect into a measurement of air velocity. Although normal contaminants in air are frequently sufficient, stronger reflections result when oil....
....droplets a few microns in diameter are added to the airstream. The laser velocimeter, as it is called, possesses a great advantage over conventional velocity sensors, such as pitot tubes and yaw heads, because the beams of laser light do not disturb airflow at all. This is the long sought ideal of nonintrusive instrumentation.
A laser velocimeter has been installed in the Langley V/STOL tunnel to survey the flow fields around airfoils and aircraft fuselages. The laser beam....
....is first split into two components, which then cross at adjustable points within the tunnel. By using two beams of laser light, two velocity components in two different planes can be determined at the same time. With proper adjustment of the optics, air velocities can be scanned at will throughout the region of interest. During recent experiments with wings at high angles of attack, velocimeter data quantitatively mapped the flow field above the wing, where the traditional smoke flow patterns provided only a qualitative picture. With its high sampling rate, the laser velocimeter can even provide a "moving picture" of changing flow fields, but with numbers rather than images. It is a powerful diagnostic tool that does not distort airflow and can be connected directly to a computer, thus bypassing error prone humans.
The hallmark of modern scientific research is the computerization of all measuring devices, the direct analysis of the data gathered, and the automatic display of digested information in forms palatable to human observers. The Wrights were able to make do with the visual observation of test airfoils mounted on a bicycle, but aerodynamic research depends more and more heavily on computers-so much so that a  wind tunnel must now shut down when its computer falters. Computers not only collect and process the data but they also control the tunnel itself. This is now the norm, not an extreme example of computerization.
The National Transonic Facility typifies the modern trend toward tunnel / computer symbiosis. Four computers handle the functions of data acquisition and display, data base management, process monitoring and communication, and tunnel and model control. Tunnel control means just that. The various aspects of tunnel operation are monitored continuously and automatically. When these parameters stray too far from the norm, the computer sounds the alarm and takes appropriate remedial action, even shutting the tunnel down when personnel and equipment are in danger.
A test in the tunnel can be completely programmed from tunnel startup to shutdown. Changes in Mach number, air temperature, and the attitude and configuration of the model are also brought under the jurisdiction of the computers. Up to 384 channels of data flow into the computer at rates up to 50 000 points per second. At the behest of the test operator, various plots and displays can be generated almost instantaneously for real time evaluation. Parameters being measured can be compared within 2 seconds with theoretical expectations or with data from previous runs by calling in information stored in memory banks.
Old timers in wind tunnel research can recall how, back in the 1920s, the two engineers with the sharpest eyes would peer through tunnel observation ports, read the balance scales, and call out their readings to the recorder. It would be days, sometimes weeks, before the data were processed and the test director knew what his tunnel had wrought. Of course, there were computers of sorts in those days, but they were slow, error prone, and also went out to lunch.
The control room of a big, up to date wind tunnel resembles that of an electric power plant-buttons, lights, switches, and displays everywhere. Bit by bit, though, the computer is taking over the monitoring of displays and the pushing of buttons. Today, the impact of the marriage of the wind tunnel and computer is large-tomorrow it will be profound. As Leo Cherne has asserted, "The computer is incredibly fast, accurate, and stupid. Man is unbelievably slow, inaccurate, and brilliant. The marriage of the two is a force beyond comprehension."