Tech Briefs

Hardware and Software for Air-to-Air Schlieren Imaging

Accurate positioning and camera control are necessary for success in schlieren imaging.

Software and electronic hardware are being developed to provide cockpit guidance and camera control for an air-to-air schlieren photography system that is to be used to take high-resolution pictures of shock waves generated by a full-scale airplane (see Figure 1). For success in schlieren imaging, it will be necessary to position two airplanes — an observing airplane and the one generating the shock waves — precisely along a line of sight to the Sun, which will be used for illumination. The shock-wave-generating airplane will fly at supersonic speed, while the observing airplane will have to fly at a much lower speed of 250 knots (129 m/s).

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Figure 1. This is a Ground-to-AirSchlieren Image of a T-38 airplane.The air-to-air schlierenimages are expected to have resolutionsof about 1 in. (≈2.5 cm).
The schlieren images will be captured by a charge-coupled-device time-delay integration (TDI) camera. Global Positioning System (GPS) receivers are mounted in both airplanes. The schlieren calculations will be performed by a personal computer with dimensions of about 5 by 8 by 8 in. (about 13 by 20 by 20 cm), ruggedized to withstand acceleration of as much as 5.4 g’s.

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Figure 2. A Moving Map and a Terminal Needle Display will guide the pilot in positioning the observing airplane to acquire schlieren images.
A two-seat F-18 airplane has been configured for performing the observations. The schlieren computer is located in the rear cockpit. The schlieren camera and its optics are housed in a forward-looking infrared (FLIR) pod. Six-in. (15-cm) monitors are mounted in both cockpits. The computer is equipped with a frame-grabber board and runs Windows NT, Matlab, and Extended Real Time Toolbox software plus the XCAP imaging software.

In addition to positioning of both airplanes precisely, it will be necessary to generate accurate line-scan-rate information, based on the relative positions and velocities of the airplanes and orientations of the airplanes relative to the Sun, to prevent blurring of the schlieren images. Matlab-based software was developed to compute the positions, velocities, orientations, and the corresponding required line-scan rates from the GPS data. The software will display trajectory-guidance information to the pilot of the observing airplane (see Figure 2), will control the operation of the camera, and will cause all trajectory and image data to be recorded in the computer.

A moving map showing both airplanes, local landmarks, and restricted airspaces will guide the F-18 pilot in following a trajectory needed for acquiring a schlieren image. This map display includes a scrolling heading indication of the actual and desired headings of both airplanes. Inset guidance needles show the offset to the required flight track and indicate the required bank angle to capture the track from the downwind leg. The rear-seat operator can select different flight-path separations and map scaling via a graphical user interface or by pressing keys.