Taming the Sonic Boom: NASA's Wind Tunnel Tests of Low-Boom Supersonic Airliners

Commercial supersonic travel ended when the Concorde made its last landing at London's Heathrow Airport. NASA has kept working on bringing it back, and of the three obstacles particular to civil supersonic flight (the sonic boom, emissions at high altitude, and airport noise), the boom is the biggest.

Concern over its noise grew so great that in 1973 the Federal Aviation Administration banned civil supersonic flight over land in the United States. That quietened the skies but ended hopes of supersonic passenger service across the country in the Concorde era. The FAA rule does not set a maximum acceptable loudness, so NASA and its partners have worked to find a level the FAA and the public could accept, and to design aircraft that stay under it. The work spans NASA's four aeronautics research centers in California, Ohio and Virginia.

Peter Coen of NASA’s Langley Research Center. NASA.

Shaping the boom

Earlier research by NASA, the military and industry showed that a boom's character depends on many things, from the shape and placement of an aircraft's parts to its engines. That means engineers can shape the boom signature through design to make it quieter. NASA set targets for boom loudness, aerodynamic efficiency and airport noise for an "N+2" aircraft, two generations beyond current technology, that could fly between 2020 and 2025.

The new concepts share the look of past supersonic designs, with a needle nose, a slender fuselage and a delta or sharply swept wing; the quieter boom lies in the details. Two industry partners in NASA's High Speed Project proposed different answers. Lockheed Martin mounted two engines under the wings in the usual way, plus a third on top. Boeing put two engines on top of the fuselage, a break with tradition.

"Engine installation is a critical part of achieving an overall low boom design," said NASA's Peter Coen. Engines in the conventional place need a carefully tailored wing to spread out the shock waves; engines on top can send the shock waves upward, away from the ground, but may cost performance.

At NASA Ames’ 9- by 7-Foot Supersonic Wind Tunnel, researcher Don Durston checks the mount of a 16-inch model of the Boeing concept. NASA.

Into the wind tunnels

The testing began in November 2010. The companies designed full-size aircraft on computers, then built scale models that behave like the real thing, and sent them to NASA's Ames and Glenn research centers. Engineers measured two things: the boom's pressure signature at different distances around the model, and how well the top-mounted engine inlets performed. The data were used to check the computer design tools.

• Phase I, late 2010 to mid-2012 at Ames, and late 2012 at Glenn's 8- by 6-Foot Supersonic Wind Tunnel, measured boom signatures and developed test methods.
• Phase II, through 2012 and 2013, tested the refined designs, focusing on how the nacelles, the housings around the engines, fit onto each aircraft.

At Glenn, technician Dan Pitts inspects Boeing’s 1.79% scale model with its two flow-through nacelles. NASA / Quentin Schwinn.

In March 2013 Glenn tested Boeing's 43-inch, 1.79 percent scale model, measuring the air flow and pressure reaching the engine face with probes deep inside the inlets, where the first engine blades would be. A remotely controlled plug behind the inlet varied the air flow during the run. The inlet was tested both on the aircraft and on its own, to see whether the airframe helped or hurt it, at speeds from Mach 0.25 to 1.8. The flow rate matters, Coen explained, because it "directly impacts a supersonic aircraft's thrust performance in flight, as well as cruise efficiency."

The flow-through inlet on its own in the Glenn tunnel. NASA / Quentin Schwinn.

A final round at Ames tested Boeing's 43-inch and 16-inch models, with two nacelle shapes and without nacelles, after a Lockheed Martin 19-inch model had been tested the year before. As expected, small changes to Boeing's top-mounted nacelles had almost no effect on the boom; Lockheed's under-wing nacelles did, but by the predicted amount, which its design process could allow for.

The Lockheed Martin model in Phase II testing at Ames. The small dots trip the air flow from smooth to turbulent, for better predictions of the boom. NASA / Dominic Hart.

Close to quiet

Next, with NASA's Armstrong Flight Research Center joining in, engineers would study how shock waves in the engine exhaust affect the boom, and NASA would add the results to the supersonic data it shares with the aviation industry. Coen's verdict on the testing: "We've reached a point where quiet, low-boom overland supersonic passenger service is achievable."

Sources

• NASA Ames Research Center, "NASA Centers Team Up to Tackle Sonic Boom"; rewritten in hubnx's own words. Photos from NASA; renderings made jointly with Lockheed Martin and Boeing are left out.