By Graham Warwick, Aviation Week
Schlieren photography has been used for decades to visualize shockwaves generated by supersonic aircraft, but usually it is used with scale models in wind tunnels. Now NASA has captured the shockwaves from full-size aircraft flying supersonically, using ground-to-air schlieren photography.
F-15B at Mach 1.2 and 40,000ft (All photos and video: NASA)
Optical schlieren uses a collimated light source that shines on the target from behind. Variations in the density of air as it flows over the object, and is compressed and expanded by shockwaves, change its refractive index and distort the beam, causing changes in the light intensity that can be visualised using a shadowgraph.
The Ground-to-Air Schlieren Photography System (GASPS), developed by MetroLaser and tested by NASA Dryden FLight Research Center, uses two telescopes, digital cameras - and the Sun as the collimated light source. The test subjects were a NASA F-15 and F/A-18 flying supersonically over Edwards AFB in California.
F/A-18B at Mach 1.1 and 44,000ft
The tests are part of NASA's program to develop technology for a low-boom supersonic transport, with a carefully shaped shockwave signature, and ground-to-air schlieren of full-size aircraft will allow the flow around the engine inlet and tail region to be visualized and modeled more accurately than with windtunnel model tests.
Schlieren photography was invented in the 1860s, and NASA says ground-to-air schlieren was used in the 1990s, but required elaborate equipment and precisely aligned optics. GASPS "uses just a telescope and a digital camera, leaving the difficult aspects to be performed post-flight using image processing software," the agency says.
F-15B at Mach 1.38 and 44,000ft
The images shown here were taken manually when the aircraft passed in front of the Sun. Later tests will likely use aircraft GPS data transmitted to the system on the ground to automatically activate the shutters for more precise imaging, the agency says.