High Velocity Wind Tunnels Test the Pace of Technology

Outside the Arnold Engineering Development Center's cavernous wind tunnels during testing, you can hear the engine rumbling like a large air conditioning unit. But inside the control room all is quiet, except for the humming of computers.

For more than four decades, AEDC's wind tunnels have tested aircraft like the F-15 Eagle, F-16 Fighting Falcon, F-35 Lightning II Joint Strike Fighter and the B-1 Lancer. Wind tunnel testing led to the development of the nation's top aerospace programs, including the Atlas, Titan, Minuteman and Peacekeeper ICBMs, and the space shuttle, the Global Positioning System and GOES-M weather satellites. While the testing pace has slowed in recent years, the focus behind the transonic and continuous flow wind tunnels in the Von Karman Gas Dynamics Facility is the same--to make air and space travel more effective, reliable and safe.

"Everything we do here makes the lives of flyers safer--the lives of military and commercial pilots, commercial passengers and our astronauts," said Capt. Brandon P. Herndon, a test project manager with the 716th Test Squadron. "We accomplish this through the testing we do in our wind tunnels--predicting and evaluating aerodynamic effects before they are actually flown," he said.

"Although I've only been assigned at AEDC for about nine months, one can realize quickly how the wind tunnel data helps us design and modify aircraft, spacecraft ... before we make costly design mistakes on a larger scale," Captain Herndon said.

The center is the nation's largest aerospace ground test complex and supports propulsion, aerodynamic, re-entry and space flight systems testing. This is the home of 58 aerodynamic and propulsion wind tunnels. Fourteen of them are the only tunnels of their kind, and 27 are unique to the United States. Although most of the wind tunnels are at Arnold Air Force Base, Tenn., AEDC has other facilities, such as Hypersonic Wind Tunnel 9 in White Oak, Md.

When the PWF was built, the electric drive motors were the largest in the world. The facility has two 16 feet by 40 feet transonic and supersonic tunnels, which are still among the largest in the world. An aerodynamic transonic tunnel, 4 feet by 16 feet, is also used to test most of the modern Air Force flight assets. The 16T transonic tunnel began operating in 1956, with the 16S supersonic following in 1960 and the 4T transonic tunnel opened in 1967. Transonic tunnels test aircraft that fly at speeds near the speed of sound, while the supersonic version is for aircraft that can fly faster than Mach 1, or about 717 mph.

Small-scale models are usually used in wind tunnel testing because full-scale versions would require tunnels substantially larger than any tunnel currently in existence, except for those that are limited to low speeds. Building large high-speed wind tunnels would result in high construction costs and takes considerably more electricity.

The 16T uses about 120 million watts of power for a typical test. "A full-scale tunnel would need an 80 feet x 80 feet test section and 3,000 million watts to operate," said Mike Mills, an AEDC aerospace engineer.

While the 16S hasn't been used for testing since 1997, the 16T remains what Captain Herndon calls "the workhorse of the facility," with testing continuing year-round. Its size can accommodate large-scale models for testing conventional aerodynamic and combined aerodynamic/propulsion systems.

Testing in the 16T supported almost every Department of Defense and government flight vehicle program in the last half-century, including the B-2 Spirit stealth bomber, RQ-4 Global Hawk remotely piloted aircraft, space shuttle and X-33 reusable launch vehicle.

The 4T transonic tunnel is used mostly for small-scale aerodynamic testing and can simulate altitudes from sea level to 98,000 feet. This continuous flow tunnel tested almost every major national flight vehicle developed.

Most higher speed air and spacecraft are tested in the smaller A, B and C tunnels of the Von Karman Facility across the street. Here, everything from space capsules to winged planes and the most sensitive of experimental craft, or X-planes, are tested. The facility is named for Dr. Theodore von Karman who led a scientific advisory group that began this long-range research and development method.

Several models of the space shuttle were tested in the VKF tunnels to determine the aerodynamic relationship between the main components. In recent years, improved foam insulation, replacing the version blamed for the Space Shuttle Columbia disaster, was tested repeatedly for its ability to handle the heat during launch and re-entry.

With each safe, successful launch and return of a shuttle crew, outside machinists Scott Pogue and Jimmie Sanders can't help but feel proud of their work. They know how important it is.

"When I saw the last shuttle launch on TV, I was nervous, but I felt really good about having a part in helping the crew come back safely," said Sanders, who spent six of his 30 years at AEDC on active duty in the Air Force.

Mills arrived in 1978 and said wind tunnel testing solved many operational problems, made pilot ejection safer and helped the Air Force field the F-22 and the F-35 Lightning II.

One of the first test programs Mills worked on was a wind tunnel fly-off competition between air-launched cruise missiles before selecting the contractor.

"During these tests, full-scale cruise missiles were in the tunnel," Mills said. "The sequence of events, just after release from a B-52, was tested. The wings and tail fins were deployed and the engine started to demonstrate that the missiles were ready for flight."

About a month before testing begins, scale models are built and installed in the test section on support struts that can be moved through a range of angles to simulate those in flight. In the 16S and 16T, the models are installed in a removable test section and moved to the tunnel on railroad tracks. The trip usually takes about an hour because the combined weight of the test section and transfer car is about 1 million pounds. The test section is brought in through a large clamshell door and wind tunnel workers spend the next few hours making utility hook-ups and checking out the data systems.

Next, the pressure in the tunnel is dropped to about one-half that of the earth's atmosphere, the equivalent of an altitude of 20,000 feet. Then it's time to start the large electric motors that drive the main compressor. The 16T's main compressor is more than 30 feet wide and rotates at 600 rpm. This compressor moves air at a rate that could fill a 2,500 square foot house in 0.1 seconds said Mills. Once the motors are at full power, the test conditions are set including the speed, pressure and temperature.

Inside the tunnel, air is propelled around by another compressor and can be cooled if needed. The 16T's cooler flows water at a rate that could fill an Olympic-sized swimming pool in seven minutes, Mills said.

A typical series of tests take from one to four weeks. Individual tests can be completed in a few hours. The time depends on how much data is needed.

Testing in the VKF tunnels is much the same, except for a different process of installing the models and making configuration changes. The continuous flow tunnels can work for hours at a time with air that is supplied by a nine-stage compressor system. The entire system is an eighth of a mile long and driven by electric motors that provide up to 92,500 horsepower.

"The basic process is the same for all of our tunnels, whether running in the subsonic, supersonic or hypersonic regimes," said Dr. Richard A. Roberts, 716th Test Squadron project manager.

AEDC's wind tunnels also provide testing for store separation, pressure-sensitive paint capability and captive trajectory support testing. Store separation testing ensures that bombs and missiles separate cleanly from the aircraft when released. If this doesn't happen, the object could veer upward and hit the aircraft.

"We can simulate store separation by positioning a missile, bomb or fuel tank next to the flight vehicle in the test section," Roberts said. "Using feedback from the data acquisition system, the path of the store can be simulated as it leaves the aircraft."

AEDC is also the home of the Mark 1 Space Facility, a 42-feet-by-82-feet space environment test chamber. Built in 1966 for the Air Force's Dyna-Soar developmental space program, it is now used for testing satellites and other space systems.

"The Air Force had a program called the Manned Orbital Laboratory Program back when we wanted astronauts to do our own missions in space," said Capt. Catercia Isaac, a physicist with the 718th Test Squadron. "So, they built this. But, later on NASA decided to use this chamber to test satellites. We've tested the GPS and weather satellites, which are still in orbit. We put them through all of the rigors in space in this chamber. If it can survive here, it can survive in space."

Through the years, testing activity slowed as fewer new craft were built. Though still an essential part of fielding any new air or space asset, demand is down at the tunnels.

"There were many more project engineers then, because there were many more projects," said Dr. Stan Powell, an Aerospace Testing Alliance engineering scientist. "We probably had 25 to 30 project engineers, and each engineer would have two or three projects.

"At that time, all of the problems of re-entry had not been worked out," he said. "Apollo, Gemini and Mercury had flown and the intercontinental ballistic missiles were good enough to hat a [target], but there were still a lot of problems that had not been worked out yet. As a community, we are smarter than we were in 1978. We know how to make airplanes that fly pretty efficiently. The things we are testing now are more in terms of getting the next 1 to 2 percent of performance, rather than merely demonstrating whether the thing will work or not."

"As good as we may be, we are not to the point where the work that AEDC does can be done any other way," said Powell. "Aerospace systems are too complicated to just build them and let them fly."

"When the warfighter is doing his lob, he's got enough to worry about without worrying about whether his equipment will perform." Powell said. "When he punches a button, he has an expectation of what is going to happen, and he deserves for that expectation to be correct. We need to provide him with dependable. reliable systems."