Glimpse of the Future: Advanced Civil Aircraft

Most of the free-flight model research conducted by NASA to evaluate dynamic stability and control within the flight envelope has focused on military configurations and a few radical civil aviation designs. This sit­uation resulted from advances in the state of the art for design methods for conventional subsonic configurations over the years and many expe­riences correlating results of model and airplane tests. As a result, trans­port design teams have collected massive data and experience bases for transports that serve as the corporate knowledge base for derivative air­craft. For example, companies now have considerable experience with the accuracy of their conventional static wind tunnel model tests for the prediction of full-scale aircraft characteristics, including the effects of Reynolds number. Consequently, testing techniques such as free-flight tests do not have high technical priority for such organizations.

The radical Blended Wing-Body (BWB) flying wing configuration has been a notable exception to the foregoing trend. Initiated with NASA sponsorship at McDonnell-Douglas (now Boeing) in 1993, the subsonic BWB concept carries passengers or payload within its wing structure to minimize drag and maximize aerodynamic efficiency.[503] Over the past 16 years, wind tunnel research and computational studies of various BWB configurations have been conducted by NASA-Boeing teams to assess cruise conditions at high subsonic speeds, takeoff and landing charac­teristics, spinning and tumbling tendencies, emergency spin/tumble recovery parachute systems, and dynamic stability and control.

By 2005, the BWB team had conducted static and dynamic force tests of models in the 12-Foot Low-Speed Tunnel and the 14- by 22-Foot Tunnel to define aerodynamic data used to develop control laws and con­trol limits, as well as trade studies of various control effectors available

on the trailing edge of the wing. Free-flight testing then occurred in the Full-Scale Tunnel with a 12-foot-span model.[504] Results of the flight test indicated satisfactory flight behavior, including assessments of engine – out asymmetric thrust conditions.

In 2002, Boeing contracted with Cranfield Aerospace, Ltd., for the design and production of a pair of 21-foot-span remotely piloted models of BWB vehicles known as the X-48B configuration. After con­ventional wind tunnel tests of the first X-48B vehicle in the Langley Full – Scale Tunnel in 2006, the second X-48B underwent its first flight in July 2007 at the Dryden Flight Research Center. The BWB flight-test team is a cooperative venture between NASA, Boeing Phantom Works, and the Air Force Research Laboratory. The first 11 flight tests of the 8.5- percent-scale vehicle in 2007 focused on low-speed dynamic stability and control with wing leading-edge slats deployed. In a second series of flights, which began in April 2008, the slats were retracted, and higher speed studies were conducted. Powered by three model aircraft turbojet engines, the 500-pound X-48B is expected to have a top speed of about 140 mph. A sequence of flight phases is scheduled for the X-48B with various objectives within each study directed at the technology issues facing the implementation of the innovative concept.