Elusive schlieren images of supersonic shockwaves emanating from NASA F-15 and F/A-18 aircraft flying at supersonic speeds were captured during recent pilot proficiency flights.
The images were gathered by a twin telescope and digital camera system on the ground at NASA’s Dryden Flight Research Center to manually test the Ground-to-Air Schlieren Photography System, or GASPS, that was developed by MetroLaser, Inc. under a NASA Small Business Innovation Research project. Schlieren photography is a technique that enables imaging of airflow, with special illumination making changes in air density—in this case the density of the shockwaves—apparent.
"Our team was able to photograph truly spectacular images showing the shockwaves of full-scale supersonic aircraft in flight,” said Ed Haering, Dryden’s GASPS principal investigator.
“For new quiet supersonic aircraft designs, computer simulations and wind tunnel tests are used to model how to minimize the loudness of the sonic booms, but the simulations and wind tunnel tests have challenges in accurately modeling the flow around engine inlet and tail regions,” Haering explained.
"We can use these images to validate our computer simulations and wind tunnel tests, giving us confidence that we can properly design supersonic civil aircraft of the future. Then we will be able to fly over land at about double the speed of current civil aircraft without bothering people on the ground," he added.
Engineers used a thumb trigger to manually snap digital images when the aircraft passed in front of the sun. Later tests will most likely use aircraft GPS tracking transmitted to the GASPS system on the ground to automatically activate their shutters for more precise imaging.
Previous schlieren photography used an elaborate series of lenses, bright backlighting, and other devices to capture supersonic shockwaves on film as darker or lighter streaks against high-contrast backgrounds like the edge of the sun. Versions of this ground-to-air technique used in the 1990’s required extremely precise alignment of the optics as well.
In contrast, the GASPS project uses just a telescope and a digital camera, leaving the difficult aspects of the work to be performed post-flight using image processing software. This improved method greatly relaxes the precision needed, with the post-mission digital processing of the imagery employed to visualize the shock wave patterns.
Schlieren imaging provides a clearer understanding of the location and relative strength of supersonic shockwaves. This represents another tool in the growing toolbox of techniques used by NASA researchers designed to characterize sonic booms.
This latest project continues a long series of sonic boom reduction research by NASA. The 10th anniversary of the NASA/Northrop Grumman F-5E Shaped Sonic Boom Demonstration (SSBD) project's first reduced sonic boom flight was Aug. 27, marking another milestone in NASA and industry’s path to lower sonic booms. A new NASA aeronautics book, “Quieting the Boom: the Shaped Sonic Boom Demonstrator and the Quest for Quiet Supersonic Flight,” by Lawrence R. Benson details the project. The book was recently posted on-line as a NASA eBook here.