PROPULSION WIND TUNNEL FACILITY (PWT)
Arnold Engineering Development Center (AEDC) is the nation's most advanced and largest complex of flight simulation test facilities. The test facilities include nearly 60 aerodynamic and propulsion wind tunnels, rocket and turbine engine test cells, space environmental chambers, arc heaters, ballistic ranges and other specialized units. Twenty-seven of the test units have capabilities unmatched elsewhere. Facilities can simulate flight conditions from sea level to altitudes around 100,000 feet, and from subsonic velocities to those well over Mach 20.
The Propulsion Wind Tunnel (PWT) facility is devoted to aerodynamic and propulsion integration testing of large-scale aircraft models. In some cases, the propulsion systems and inlets are tested simultaneously to make sure they are aerodynamically designed to provide adequate airflow. Other tests involve store separation investigations-making sure the bombs, missiles or other stores separate cleanly from the parent aircraft when released.
PWT's two closed-circuit wind tunnels (one to simulate transonic speeds and the other supersonic), with 16-foot-square test sections, are also used for conventional aerodynamic tests. They offer a unique capability to combine simultaneous aerodynamic studies with propulsion testing. The facility also includes a four-foot transonic tunnel used primarily to support design, development and improvement of aircraft stores. 4T also supports facility improvement using a one-foot scale model of the 16-foot transonic test section which helps evaluate technology that could be applied in the full-scale tunnel.
PWT boasts some of the most powerful electric motors ever built-tall as a two-story house and as heavy as a railroad locomotive. Four motors-two rated at 83,000 horsepower each and a smaller pair rated at 35,000 horsepower each-were built to drive five compressors that generate wind speed in PWT's transonic and supersonic tunnels. Both 83,000-hp motors stand 21-1/2 feet high and weigh 225 tons, with 31 miles of copper wire used in the motor windings. When put together in tandem, the motor drive system is almost two football fields long- among the largest rotating machines in the world.
In 1989, PWT was designated by the American Society of Mechanical Engineers as an International Historical Mechanical Engineering Landmark.
The origin of PWT may be traced back to the end of World War II, when Gen. H.H. "Hap" Arnold-then Army Air Forces commander and the man for whom the center was named-sought to determine how the Germans had made such rapid progress developing high-performance jet aircraft and rocket-powered missiles. He enlisted the help of Dr. Theodore von Karman, one of history's great aeronautical scientists, to conduct a survey of the German facilities as soon as the war was over.
Dr. von Karman's subsequent report recommended the Air Force create a center with"...wind tunnel facilities to attain speeds up to three times the velocity of sound, with large enough test sections to accommodate models of reasonable size, including jet propulsion units, and one ultrasonic wind tunnel for exploration of the upper frontier of the supersonic speed range. Ample facilities for the study of combustion and other characteristics of propulsion systems at very high altitudes should be provided..."
Planning for PWT began in January 1950, when the Air Force Research and Development Board on Facilities met with representatives of aircraft propulsion companies and agreed that industry needed a supersonic propulsion wind tunnel with a 15-foot-diameter test section. By December 1951, the commanding general at AEDC had approved a proposal for design, construction and operation of a scale model of the PWT transonic circuit. The initial test facility was a one-foot cross-section prototype transonic tunnel, and the first test was performed in June 1953 on a 0.03-scale model of the Bomarc missile for the Boeing Co. In 1956, the transonic circuit, with its 16-foot test section, underwent its first powered operation preliminary to calibration. The entire PWT complex was accepted by the Air Force in January 1961. The approximate cost of the 39 contracts to construct the facility was $78.7 million.
The two 16-foot tunnels are used both for conventional aerodynamic tests and for combined aerodynamic/propulsion system tests. Large and full-scale models of aircraft, missiles and rockets are tested with propulsion systems installed and operating. Pressure of the airflow through the test sections can be varied to simulate altitude conditions from sea level to about 150,000 feet.
The two large tunnels have interchangeable test sections, allowing preparations for one test to be made while another is being run in the tunnel. To eliminate swirling and turbulence, which could affect test results, the airflow is guided smoothly around the right-angle turns of the closed-circuit tunnels by giant turning vanes that resemble huge, vertical venetian blinds. A flexible nozzle regulates the velocity of the airflow as it enters the test section.
These tunnels are also equipped with moveable supports, called stings, for mounting test models. To simulate change in flight attitudes or maneuvers, the support is yawed, rolled or pitched up or down.
The 16-foot tunnels are often used to examine the relationship between engine air inlets and the corresponding performance of and compatibility with the engine itself. This is done to determine the most efficient air-induction system design or to study how varying the geometrical shape of an inlet or propulsion nozzle can affect the aerodynamics of the flight vehicle.
Large tunnel size allows for full-scale missile installations to test engine performance and airframe aerodynamics. Rocket propulsion systems and problems associated with their external aerodynamics are also investigated.
Moreover, because of their large size, the PWT 16-foot tunnels are adaptable to the testing of parachutes or other decelerators. Measurements have been made of the aerodynamic forces acting on the human body during emergency ejection from an aircraft traveling at speeds up to 900 mph. Tunnel conditions were created to simulate the high dynamic pressures acting on the pilot in and near the cockpit during an emergency ejection at a wide range of altitudes and flight speeds.
STORE SEPARATION TESTING
At transonic speeds, and at certain altitudes or maneuver conditions, an aircraft's stores-bombs, missiles or drop tanks carried externally-may veer upward when released, and collide with the aircraft.
In years past, bombs were carried and dropped out of bomb bays. But as high-thrust engines became available, the weapons could be shifted outside and carried in considerable numbers on pylons attached to the lower surface of the wings. Problems became evident when the aircraft speed became progressively faster. Large, unexpected aerodynamic forces were sometimes generated, resulting in vibrations violent enough to render the aircraft uncontrollable by the pilot. This aerodynamic condition, known as "flutter," can lead to breakup of the craft. It has been extensively investigated in the 16-foot tunnels, which can accommodate large-scale models.
The problem of clean store separation-investigations of aerodynamic forces that can alter the planned trajectory of air-launched bombs or missiles-is explored in the 16-foot tunnels and in the four-foot transonic tunnel.
The aircraft model is mounted upside down in the tunnel on a special support system attached to the floor of the test section. The store model is mounted on another sting attached to the top of the test section, and the two models are mated as they would be in flight. When the desired simulated flight conditions are established in the tunnel, the store model is "released" from the parent aircraft model by activating a computer that controls movement of the sting-supported store as it traces the trajectory.
Information obtained in these tests is used to design new stores or to modify existing ones, to ensure they separate cleanly, do not damage the parent aircraft, and stay on their intended flight path in the proper attitude.
Office of Public Affairs, Arnold Engineering Development
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Cleared for Public Release Current as of April 1998