Lockheed bygger bemannet X-fly for NASA - Test av overlydssmell - AW&ST

Lockheed To Build NASA’s Low-Boom Supersonic X-Plane Graham Warwick and Guy Norris Lockheed Martin is to build NASA’s first clean-sheet manned X-plane in decades, to demonstrate that sonic booms can be reduced to levels that could enable lifting the ban on civil supersonic flight over land. The award of a $247.5 million contract to Lockheed’s Skunk Works to build the Low Boom Flight Demonstrator (LBFD) is a milestone for NASA’s revamped strategy for aeronautics research. Unveiled in 2015, this directive called for a return to flight research, and X-planes. “We are opening a new era, a 21st-century X-plane era,” says Jaiwon Shin, associate NASA administrator for aeronautics. Success with the low-boom supersonic demonstrator will boost NASA’s plans for an X-plane to demonstrate technologies for ultra-efficient subsonic aircraft. NASA’s first clean-sheet manned X-plane in decades to fly in 2021 Aircraft will be used for community-response testing of low booms Aimed at enabling sound-based rules for supersonic flight over land NASA began looking at how to reduce the sonic boom of supersonic airliners immediately after cancellation of the High-Speed Civil Transport program in 1999, says Peter Coen, manager of NASA’s Commercial Supersonic Technology project. The agency began working with Lockheed, and later Boeing, on developing design tools to shape the aircraft to minimize sonic booms. This led to wind tunnel tests in 2012-13 that validated the design tools and resulted in reduced shockwave signatures. “The idea when we started was not to do an X-plane, but to prove it in the wind tunnel,” says Coen. But work with Lockheed showed it was possible to build a small, affordable X-plane to gather data on public response to low booms for use by regulators in crafting sound-based rules for supersonic overland flight. The single-seat, single-engine LBFD is designed to mimic the shockwave signature of a small, quiet supersonic airliner, with a maximum boom loudness of 75 PLdB while cruising at Mach 1.4 and 55,000 ft. This compares with 105-110 PLdB for the Concorde supersonic airliner. Sonic booms occur “because the air does not know the aircraft is coming,” says Coen. Because the aircraft is flying faster than the speed of sound, “all pressure changes are through shockwaves.” The shockwaves from different parts of the airframe have different strengths and, as they propagate through the atmosphere, they catch up with each other and coalesce into strong bow and aft shocks. These are heard on the ground as the characteristic “double bang” of a classic N-wave sonic boom. Careful control of volumetric changes from nose to tail through airframe shaping produces shockwaves that do not coalesce as they propagate to the ground. This creates an S-shaped boom signature that generates a thump about as loud as a car door closing, according to NASA.

Lockheed’s Low Boom Flight Demonstrator is powered by a single GE F414 engine fed via a diverterless supersonic inlet. Credit: NASA/Lockheed Martin

The low-boom X-plane will be flown over locations across the U.S. to gather data from noise measurements and public surveys. These will be provided to the FAA and International Civil Aviation Organization (ICAO) to help them replace the supersonic overland ban with sound-based certification rules. A planned three-year period of community-response flight testing in 2023-25, “meshes up with key international meetings to which we will provide the data, so it is important we stay on schedule,” says Ed Waggoner, director of NASA’s Integrated Aviation Systems program. ICAO’s Committee on Aviation Environmental Protection (CAEP) is scheduled to establish the sonic boom standard at its 13th meeting (CAEP13), in 2025. Lockheed’s Skunk Works had produced a preliminary design for the demonstrator under a previous NASA contract and was the sole bidder to build the X-plane, says Waggoner. NASA received three inquiries after releasing a request for proposals in August 2017 and provided the preliminary design data to all interested parties, he says, but only Lockheed submitted a formal bid. Although Lockheed’s was the only bid, it was "exhaustively analyzed . . . and deemed excellent in every respect,” says Waggoner. The bid was carefully structured to ensure that NASA’s first clean-sheet X-plane in decades will be affordable and successful, says Peter Iosifidis, Skunk Works’ LBFD program manager. This includes building-in buffers at key points to allow for delays, but to remain on schedule. A program kickoff meeting is planned for May, following by a “delta” preliminary design review in July. Because NASA’s requirements have remained stable, the aircraft Skunk Works will build is essentially unchanged from the preliminary design produced for NASA, says Iosifidis. The only major change is deletion of the fiber-optic sensing system NASA planned to install to measure airframe deflections. The agency is confident it can accurately determine the aircraft’s shape in cruise by other means, he says. The critical design review is planned for September 2019. Following a first flight in the summer of 2021, the X-plane will undergo flight-clearance testing at the NASA Armstrong Flight Research Center at Edwards AFB, California, followed by an acoustic-validation phase to ensure it produces the desired boom signature, says Coen. This will include confirming the shockwave pattern emanating from the demonstrator using air-to-air Schlieren imaging techniques with the Sun as the background.

The long nose for a low boom requires a synthetic external vision system for the pilot, its camera mounted forward of the cockpit. Credit: NASA/Lockheed Martin

Immediately after the completion of these test phases in September 2022, Coen says, NASA plans to conduct its first public-response test campaign over a community in the southwest U.S. that, unlike Edwards AFB, is not routinely exposed to sonic booms. “We plan to conduct two community-response tests a year, for a total of 4-6,” says Coen. Flights to gather data from noise measurements and public surveys will be conducted over communities that range from large cities to small towns and rural areas. “The data will be representative of the diversity of communities exposed [to booms]” to meet the requirements of the regulators, says Waggoner. Lockheed is confident of staying on schedule, in part because most of the X-plane’s systems and subsystems are off-the-shelf and taken from other aircraft. The engine is a General Electric F414-400 from a Boeing F/A-18E/F, the landing gear is from a Lockheed F-16, and the cockpit, ejection seat and canopy are from a Northrop T-38. The X-plane is 94 ft. long and the nose, shaped to break up the bow shock, is essentially empty from the cockpit forward. Jim Less, a research pilot at NASA Armstrong and one of the LBFD project pilots, says the lack of forward visibility from the cockpit over the aircraft’s long nose presents several challenges. These range from ground handling, particularly taxiing, to avoiding other aircraft in the air. “We can land without being able to see most of the way down—you can look out the side, and line yourself up—but the other part is being able to see other air traffic that is out there,” he says. A forward view will be provided by the NASA-developed synthetic external vision system (XVS), which uses a high-definition camera located just forward of the cockpit, image processing and a 4K ultra-high-definition (UHD) display to enable the pilot to see other air traffic. “We have been evaluating different systems and cameras in a Beechcraft UC-12 King Air at NASA Langley,” says Less. As flown in the King Air, the system has a camera with a 33-deg. horizontal by 19-deg. vertical field of view that provides 3,480 X 2,160-pixel UHD color imagery at 60 Hz, with only 25-millisec. latency. The long nose will present ground-handling challenges. “We shall have to see what the turn radius is, but we won’t be able to operate out of little airfields,” says Less. For landing and ground maneuvering, the pilot will use a camera under the nose that looks downward when the gear is extended. The imagery is merged into the XVS display to provide a full picture. To validate its models for sonic-boom signatures in the build-up to the LBFD project, NASA has been flying F-18s on a special dive trajectory that simulates a shaped boom. In August 2017, crews from Armstrong and Langley deployed to the Kennedy Space Center in Florida for a series of F-18 flights called Sonic Boom in Atmospheric Turbulence, or SonicBAT. The tests were targeted at gathering data on the effect of humid-atmosphere turbulence on sonic booms, and illustrate the complexity of the problem. The impact of turbulence “turns out to be a little unpredictable,” says Less. “Most of the time it softens the edge of the boom, but amplifies it at other times. It is like the way light is reflected off the bottom of a pool; some of it is all broken up and mostly shadows, but every so often you will just see a flash which momentarily focuses the light.” Coen says F-18 dive flights will be used again in the near future for risk-reduction tests of the noise measurements and community surveys to be used during the LBFD program. “As the aircraft is being built, we will plan the response testing in detail,” he says. “We are working with the international community to craft questions for the surveys that will get us the right answers.”