The 3 ft x 3 ft. Subsonic Wind Tunnel (SWT) is a highly sought after resource. It is a closed circuit design capable of producing velocities well into the compressible range (M = 0.6). An air exchange system allows the tunnel to maintain constant temperature (M<0.5). This wind tunnel became operational in 1986 and has been continually modernized to include new variable frequency drive (VFD) system and programmable logic controller that controls safety and tunnel operations. It has optical windows with 3 sq ft of viewing area. Multiple measurement techniques can be used to include a sting-mounted force and moment balance, external balance, pressure transducers, hot film, pressure-sensitive paint, and particle image velocimetry, to name a few.
An example of work performed in the SWT is the study of C-130 Drag Reduction Using Aft Body Beavertail Strakes for USAF Special Operations Command. The overall objective of this study was to investigate the use of beavertail strakes to reduce drag. Prior phases proved that the beavertail strake provide the optimum method for reducing drag. Further testing proved the strake would mitigate the vortex influence of the airflow as it approached the aft cargo door. The last study focused on quantifying the drag potential at high angles of attack, and at various sideslip angles, as well as the corresponding stability effects. Pressure-sensitive paint was used to analyze forces of the airflow on the strakes.