Details of an unusual collaborative project between the US Air Force and NASA have been revealed, in which tests of a manned glider demonstrated the feasibility of dynamic soaring techniques that could be used by future airborne robotic planetary explorer vehicles being studied by NASA for missions to Mars.

Dynamic soaring differs from the traditional static soaring techniques normally used by glider pilots. Unlike static soaring, which uses vertical motion such as riding thermals, dynamic soaring involves extracting energy from horizontal windshears. This method has been perfected in nature by far-ranging seabirds like the albatross, which can fly up to 72ft (22m) for just a 3ft loss in altitude without wing flapping, but is thought never to have been quantifiably demonstrated before using full-scale, instrumented manned gliders. Researchers hoped the tests would prove large gliders could use dynamic soaring, and this would help in the design of future explorer vehicles.

Dynamic soaring

During the tests, which involved the USAF Test Pilot School and Air Force Institute of Technology (AFIT) together with NASA's Dryden Flight Research Center, a heavily instrumented Let L-23 Super Blanik sailplane was flown at high speeds and low altitudes in boundary-layer windshears over the dry lake bed at Edwards AFB, California.

Nearly 140 test sorties were completed over 27 flight hours between mid-March and May, says USAF test pilot and project lead Capt Randel Gordon. "We were the first to go out there and make it work, and we came up with what we called 'hairpin' and 'anti-hairpin' manoeuvres," he said at the recent Society of Experimental Test Pilots symposium in Anaheim, California. Entry speeds for the manoeuvres, including the opposite anti-hairpin, were 85kt (160km/h), 95kt and 105kt. "Our tests showed that in dynamic flying, the hairpin does better than the anti-hairpin," says Gordon.

Source: Flight International