40- by 80-/80- by 120-Foot Wind Tunnels
The National Full-Scale Aerodynamics Complex (NFAC) 40- by 80-/80- by 120-Foot Wind Tunnels are managed and operated by the U.S. Air Force’s Arnold Engineering Development Center under a long-term lease agreement with NASA. The Air Force conducts NFAC test programs for the Department of Defense, NASA, other government agencies, and industry. The wind tunnels are primarily used for aerodynamic and acoustic tests of rotorcraft and fixed wing, powered-lift V/STOL aircraft, and developing advanced technologies for these vehicles.
NASA uses the 40- by 80-/80- by 120-Foot Wind Tunnels to support an active research program in aerodynamics, dynamics, model noise, and full-scale aircraft and their components. The aerodynamic characteristics of new configurations are investigated, with an emphasis on validating computational methods. Aeromechanical stability boundaries of advanced rotorcraft are explored as well as rotor-fuselage interactions. Stability and control derivatives are also determined, including the static and dynamic characteristics of new aircraft configurations. The acoustic characteristics of most of the full-scale vehicles are also determined, as well as acoustic research aimed at discovering and reducing aerodynamic sources of noise. In addition to the normal data gathering methods (e.g., balance system, pressure measuring transducers, and temperature sensing thermocouples), state-of-the-art, non-intrusive instrumentation (e.g., laser velocimeters and shadowgraphs) are available to help determine flow direction and velocity in and around the lifting surfaces of models or aircraft undergoing investigation.
The 40- by 80-Foot Wind Tunnel has a closed test section with semicircular sides of 20 foot radius and a closed-circuit air return passage. The 80- by 120-Foot Wind Tunnel also has a closed test section which is rectangular in cross-section. It is, however, a non-return tunnel. The 40- by 80- and 80- by 120-Foot Wind Tunnels share a common fan drive system. By adjusting the position of vane sets three (3) and four (4), the airflow can be directed through one test section (40x80 or the 80x120) or the other.
Both the 40- by 80- and 80- by 120-Foot Wind Tunnel test sections are lined with sound-absorptive material to permit acoustic research to progress simultaneously while aerodynamic research is in progress. The lining in the 40- by 80-Foot Wind Tunnel is forty-six (46) inches deep over the entire test section while the lining in the 80- by 120-Foot Wind Tunnel test section is ten (10) inches deep on the walls and six (6) inches deep on the floor and ceiling. These linings permit near-anechoic (without reflections) testing above 75 and 500 Hz, respectively. With the acoustic liner installed, test section dimensions for the 40- by 80-Foot Wind Tunnel are 39 feet high, 79 feet wide (at the horizontal center line), and 80 feet long. The test section dimensions for the 80- by 120-Foot Wind Tunnel are 79 feet high, 118.3 feet wide, and 190 feet long with the acoustic liner installed.
The air in the tunnel is driven by six 40-foot diameter, 15-bladed, variable-pitch fans powered by 40-pole, 6600V, three-phase, synchronous motors each rated at 12 MW (18,000 hp) continuous with 2-hour 25% overload capability (22,500 hp). The speed range in the 40- by 80-Foot Wind Tunnel test section is continuously variable from 0 to 300 knots while the speed range in the 80- by 120-Foot Wind Tunnel test section is continuously variable from 0 to 100 knots. The stagnation pressure in the tunnel is atmospheric, and stagnation temperature is uncontrolled, dependent on such things as outdoor temperature and the temperature rise that operation of internal-combustion engines in models or aircraft contribute.