March 18, 2014
NASA’s High Speed Project Aims To Tackle Sonic Boom
April Flowers for redOrbit.com - Your Universe Online
Nearly a decade ago, the French Concorde landed for the last time at Heathrow Airport. Commercial supersonic air travel has been as elusive as your lost luggage ever since. This hasn't stopped NASA from searching for solutions that will get supersonic passenger travel up and going once again. Aerospace engineers have made significant progress in their understanding of supersonic flight since the Concorde folded up its wings, but one challenge remains: the loud sonic boom.
“There are three barriers particular to civil supersonic flight; sonic boom, high altitude emissions and airport noise. Of the three, boom is the most significant problem,” said Peter Coen, manager of NASA’s High Speed Project with the agency’s Aeronautics Research Mission Directorate’s Fundamental Aeronautics Program.
The sonic boom became such an annoyance that the Federal Aviation Administration (FAA) prohibited domestic civil supersonic flight over land in 1973, which helped to quiet the skies and reduce potential impacts on the environment. The hope of introducing supersonic overland passenger service within US airspace was dashed during the Concorde era.
Engineers at all four NASA centers that conduct aeronautics research were kept busy trying to overcome the sonic boom prohibition.
The current FAA regulation does not specifically define the maximum acceptable loudness of a sonic boom, so NASA and its aviation partners have been researching ways to identify a loudness level that is acceptable to both the public and the FAA. They also want to reduce the noise created by supersonic aircraft. The researchers have been exploring "low-boom" aircraft designs, along with other strategies, that are showing promise for reducing sonic boom levels using cutting-edge testing.
Prior studies conducted by NASA, the military and the aircraft industry determined that a variety of factors — including shape and position of aircraft components, and the propulsion system's characteristics — create the makeup of a supersonic aircraft's sonic boom. These factors allow engineers to tune or "shape" a boom signature through design to minimize the loudness of the boom it produces in flight.
NASA's low-boom requirements are being met by the latest possible supersonic designs. Those requirements include which specify targets for boom loudness, aerodynamic efficiency, and airport noise for an N+2 aircraft design that might be flying by the years 2020 – 2025. The N+2 aircraft is a second generation beyond current technology.
The new concepts share characteristics with their predecessors: a needle-like nose, a sleek fuselage, and a delta wing (highly-swept wings). The small details of how the new designs are shaped, however, create the reduction in the sonic boom. Lockheed Martin proposed one design that mounts two engines under the wing in a traditional configuration. The tweak to this design is an additional centerline engine above the wing. The Boeing Company, the second partner working with the NASA High Speed Project, proposed two top-mounted engines in a departure from historical aircraft design.
“Engine installation is a critical part of achieving an overall low boom design," Coen, who is located at NASA's Langley Research Center, told NASA's Frank Jennings, Jr. and Karen L. Rugg. "If we mount the engines in a conventional manner, we need to carefully tailor the shape of the wing to diffuse the shock waves. If we mount the engines above the wing, the shock wave can be directed upward and not affect the ground signature. However, such installations may have performance penalties."
As part of the project’s Experimental Systems Validations for N+2 Supersonic Commercial Transport Aircraft effort, NASA’s recent focus on supersonic research testing began in November 2010 with a goal of capturing boom-relevant data from supersonic scale models built by Boeing and Lockheed.
Industry engineers designed full-sized aircraft computer models to prepare for the research. These computer models were then scaled down to build wind tunnel models that exhibit the same flight characteristics during testing as do their full-size counterparts in actual flight. The scale models were sent to NASA's wind tunnel facilities at the Ames and Glenn research centers.
Once the scale models arrived at NASA, the engineering team focused on obtaining data from two distinct aspects of supersonic design — the measurement of the sonic boom pressure signature at various distances around the aircraft, and the measurement of engine inlet performance for the top-mounted engines. The wind tunnel data is being used to validate the computer-based design tools for continued use in future low-boom aircraft design research.
The wind tunnel tests began in late 2010 at Ames' 9-by-7 Foot Supersonic Wind Tunnel. They continued through mid-2012 with initial tests of Lockheed’s and Boeing’s Phase I supersonic aircraft concepts, focusing on the boom signature measurements and development of test techniques. Phase I designs were also tested in late 2012 at Glenn's 8- by 6-Foot Supersonic Wind Tunnel.
Both Boeing and Lockheed Martin refined their designs for better boom characteristics and improved aerodynamic performance. Phase II designs have been tested through 2012 and 2013, focusing on the engine nacelle integration with the overall vehicle. The nacelle houses the engine, and is generally mounted directly on the wings or fuselage, or on pylons attached to the aircraft.
A propulsion integration test at Glenn's 8- by 6-Foot supersonic wind tunnel was conducted in March of 2013 as part of the Phase II testing. It consisted of a 43-inch long, 1.79-percent scale model built by Boeing, and focused on capturing performance data from the engine air inlets — the components through which air enters the aircraft engines. NASA created several iterations of this test: with the inlets integrated on the overall aircraft, mounted above the wings, and with one of the inlets by itself. They measured the inlet air flow and pressure recovery (the pressure level at the engine face after losses from the flow turning and shock waves in the inlet) during each test. A series of pressure and temperature probes deep inside the inlet, where the first set of blades for the engine would be located, captured the measurements. To give the engineers the capability to vary the rate of air flow through the inlet in order to capture data throughout the test "flight" in the tunnel, the team fitted a remotely-controlled mass-flow plug assembly — a movable cone that varied the size of the nacelle exit area.
“Capturing this flow rate is important because it directly impacts a supersonic aircraft’s thrust performance in flight, as well as cruise efficiency,” said Coen.
The engineers were able to capture inlet performance data, without the influence of the rest of the aircraft, during a test consisting of a stand-alone air inlet, which was mounted on a support cone within the wind tunnel. Comparing the data from the two configurations will allow NASA and Boeing to learn if the shape of the airframe has a significant effect — good or bad — on the performance of the inlet.
"High levels of inlet performance are desirable to keep the vehicle’s engines running smoothly and able to provide thrust,” said Raymond Castner, Glenn’s Inlet and Nozzle Branch Propulsion Technical Lead for the High Speed Project. “The inlet data collected was used to increase our knowledge and to validate both design and analysis tools. This knowledge was needed across a range of flight conditions at Mach numbers from 0.25 to 1.8, and at various angles occurring between the airflow and the aircraft as it flies."
After the testing at Glenn, further testing was performed at Ames Research Center. The engineers at Ames worked with the 43-inch as well as 16-inch scale models provided by Boeing, similar to a test the year prior with a 19-inch scale model provided by Lockheed Martin. The new tests sought to capture data indicating how well the nacelles were integrated with the overall design, as well as how they affected the aircraft's boom characteristics and aerodynamic drag.
Two different nacelle shapes were tested on the Boeing scale models, as well as being tested with the nacelles not installed. The Lockheed Martin scale, on the other hand, underwent one set of tests with nacelles installed and one without. Data relating to the influence nacelle configurations had on the overall boom signatures and aerodynamic performance of the models was measured by the engineering team.
“The purpose of our testing was to measure the impact of the nacelle configurations on the boom signatures,” said Don Durston, a High Speed Project engineer at Ames Research Center. “Preliminary results showed that as expected, with Boeing's nacelles being on top of the wing, any small changes there had negligible effects on the boom, Lockheed’s model having the two of the nacelles under the wing, did show a measurable impact on boom; however, that effect was predicted, and could be accounted for in the design process Lockheed used.”
Engineers subjected each scale model to a series of tests using Ames’ 9-by 7-Foot supersonic wind tunnel. The tests were designed to capture the design's overall boom signature, or sound personality.
In the coming months, engineers at NASA — including the Armstrong Flight Research Center — will pore through the test data with industry partners to prepare for future research and additional testing. In the short term, the engineering team will focus on how shock waves in the engine exhaust flow impact the overall boom signature.
To help foster further innovation, NASA will add additional boom research discoveries to the growing repository of supersonic data that is currently available to the civil aviation community.
Coen believes the research performed over the past year is bringing engineers closer to realizing a viable low-boom, civil supersonic aircraft transport design.
“We've convinced ourselves that we have the design tools and we've validated the level we need to design to,” said Coen. “We've reached a point where quiet, low-boom overland supersonic passenger service is achievable.”
Image 2 (below): This rendering shows The Boeing Company's future supersonic advanced concept featuring two engines above the fuselage. Credit: NASA/Boeing