NASA is set to begin a series of supersonic flights that will examine efforts to improve the efficiency of future supersonic aircraft. The flights, which are expected to begin in March, will take place at NASA's Armstrong Flight Research Center in Edwards, California, and will follow developments identified by high-speed wind tunnel testing conducted at NASA's Langley Research Center in Virginia.

As NASA proceeds toward the possible development of a proposed Low-Boom Flight Demonstration aircraft, or LBFD, research done by the agency's Commercial Supersonic Technology project, or CST, continues to investigate ways to mitigate or minimize the disruptive sonic boom associated with supersonic flight, as well as approaches to overcome other technical barriers to innovation in commercial supersonic flight.

One such barrier is fuel efficiency. At supersonic speeds, the force of drag that must be overcome is large. Due to the interaction of flow with the aircraft's surface, this friction drag contributes about half of the total drag at supersonic speeds. This particular series of flights will explore ways of reducing friction drag and increasing efficiency through new and innovative methods of achieving swept wing laminar flow.

As an aircraft flies, there is a thin layer of air, called the boundary layer, which exists between the surface of a wing and the fast-moving air around it. This boundary layer generally begins as a smooth, or laminar, flow, which creates minimal friction drag. However, as air flow progresses over the aircraft's surfaces, tiny disturbances begin to affect the boundary layer, and it eventually transitions into a more turbulent flow, which produces much more friction drag. On swept wing aircraft, this turbulence presents the aerodynamic challenge of overcoming crossflow on the wing.

Future supersonic aircraft seeking to achieve a low-boom, such as NASA's proposed LBFD, will rely on a swept wing design in order to fly at supersonic speeds without producing a loud sonic boom. The swept wing design generally produces crossflow, which is a name for air flow disturbances that runs along the span of the wing, resulting in turbulent flow, increased drag, and ultimately, higher fuel consumption.

NASA believes this obstacle may be overcome through the use of an array of small dots, called distributed roughness elements, or DREs. "Swept wings do not have much laminar flow naturally at supersonic speeds, so in order to create a smoother flow over the wing, we're putting the DREs along the leading edge of the wing," says CST subproject manager Brett Pauer. "These DREs can create small disturbances that lead to a greater extent of laminar flow."

The DREs work by alleviating the crossflow, and delaying the transition to turbulent air flow. The crossflow is essentially crowded out, and is not allowed to grow. The boundary layer flow eventually does transition, but it occurs much further along the path of the wing flow, and thus maintains laminar flow for a longer period of time, and over more of the wing. The more laminar flow, the lower the overall drag, leading to a more efficient aircraft.

The use of DRE's was first explored by Prof. William Saric. A different configuration of the DRE's than that which was expected to work at these high-speed conditions was recently discovered during wind tunnel testing of a wing model at NASA Langley.

"We recently completed testing the 65-degree swept wing model at Langley," NASA Armstrong principal investigator Dan Banks said. "Part of the purpose for the flight tests will be to document the differences in crossflow transition between that which occurs in the wind tunnel and that occurring in flight. Flight testing the exact same test article that was tested in the wind tunnel gives us the best possible comparison."

NASA engineers have integrated the 65-degree wing test article that had been previously tested in the wind tunnel, to the underside of a NASA F-15. The swept wing model will test several configurations of DREs along the test article's leading edge at speeds up to Mach 2. This will allow researchers to examine how different configurations of DREs impact laminar flow.

This will be done by monitoring the flow during flight through the use of an infrared camera mounted under the right inlet of the F-15, which will help interpret which DRE configurations produce the most laminar flow. The camera will monitor flow by picking up signatures of heat produced by air flow, with more heat indicating more friction.

The swept wing laminar flow efforts continue previous NASA research, performed using two F-16XL aircraft between 1988 and 1996. Those tests investigated the use of suction to maintain laminar flow, using slots, perforations, and porous titanium material under the surface of the wing. If successful, the DRE's are a much simpler and elegant solution. The wind tunnel tests and NASA Langley were instrumental in discovering the potential for DREs to increase the fuel efficiency of future supersonic aircraft.

"In these wind tunnel tests, we studied a large number of DRE patterns based on subsonic research approaches and none worked at supersonic speeds," NASA Langley principal investigator Lewis Owens said. "The real breakthrough came when we finally abandoned the idea that DRE heights needed to be kept very small and this counter-intuitive approach opened the door to new DRE patterns with the potential to produce the desired supersonic boundary-layer control effect."

Swept wing laminar flow technology allows NASA to consider wing designs that have low boom characteristics, yet can be more efficient. In the past, a large extent of laminar flow had only been practically achieved on wing designs with very little sweep. Such designs, however are not workable in NASA's efforts to produce a soft thump in place of the sonic boom. The direction of future supersonic aircraft also depends, in part, on their potential to be more fuel efficient.

If environmental noise standards are identified and met, and are acceptable to the community, the future could be opened to commercial supersonic flight over land, which is currently restricted due to the loud sonic boom.

"Supersonic laminar flow is something of an elusive holy grail for aerodynamicists," states CST project manager Peter Coen. "This test, while still exploring fundamentals, is an important step toward achieving CST's fuel efficiency goals for quiet supersonic overland airliners."

Flights are expected to continue through May.

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