Tests of scale models like this one in a supersonic
wind tunnel at NASA's Ames Research Center help researchers understand
the forces acting on the aircraft that create sonic booms.
Credits: NASA / Dominic Hart
NASA's aeronautical innovators are one step closer to confidently
crafting a viable commercial airliner that can fly faster than the speed
of sound, yet produce a sonic boom that is quiet enough not to bother
anyone on the ground below.
This design for a possible future aircraft that
could fly at supersonic speed over land came from a team led The Boeing
Company and funded by a NASA Research Announcement. Other team members
included Boeing Phantom Works, GE Global Research, Georgia Tech, M4
Engineering, Pratt &Whitney, Rolls Royce and Wyle Laboratories.
Credits: NASA / The Boeing Company
The key to this recent advance came when wind tunnel tests of scale
model airplanes verified that new approaches to designing such aircraft
would work as hoped for when aided by improved computer tools, which
were used for the first time together in each step of the design
process.
Lockheed Martin led the team that developed this
design for a possible future aircraft that can fly at supersonic speed
over land. Funded by a NASA Research Announcement, the team also
included GE Global Research, Purdue University and Wyle Laboratories.
Credits: NASA / Lockheed Martin
"That was really the breakthrough for us. Not only that the tools
worked, but that our tests show we could do even better in terms of
reducing noise than we thought at the start of the effort," said Peter
Coen, NASA's supersonic project manager at Langley Research Center in
Virginia.
NASA personnel install a sonic boom sensor in a
residential community for tests last year that remotely measured sonic
boom levels. The data is helping researchers determine an acceptable
level for sonic booms heard over land.
Credits: NASA / Tom Tschida
Nuisance noise generated by a commercial supersonic jet's sonic booms
during cruise, and by its powerful engines at takeoff and landing, has
kept the speedy aircraft from entering service in the United States –
except for Europe's Concorde, which was limited to trans-Atlantic
flights only.
Using the computer tools, teams led by Boeing and Lockheed Martin, and
funded through a NASA Research Announcement, came up with designs for
two small supersonic airliners that would carry between 30 and 80
passengers and potentially enter service in the 2025 timeframe.
"In bringing their design expertise to the process, these companies are
not only addressing the low boom design elements, but all of the other
aspects necessary for a realistic design," Coen said.
For example, the computer tools show that one way to reduce the
perceived loudness of a supersonic jet's sonic boom is to change the
aircraft shape, in part, by lengthening the aircraft's fuselage, making
it much more slender. Theoretically, the noise issue could be solved by a
really, really long aircraft body.
Unfortunately, while an 800-foot-long airliner may lead to publicly
acceptable sonic booms, an aircraft that size still must fit at its
gate, make turns while taxiing to the runway without hitting anything
and generally not require an expensive redesign of the nation's
airports.
"The long skinny fuselage is not a practical answer. In our pursuit of
boom reductions, we examine the whole, three-dimensional shape of the
vehicle including the engine configuration," Coen said. "Even then, we
keep in mind that the airliner has to meet all of the other requirements
which are part of good design practice."
To help reach their goals, the engineers relied on earlier studies that
revealed how an aircraft's overall configuration could modify the shape
of the supersonic shockwaves coming off the airplane so that the
atmosphere then reduces the sharpness of the wave. By the time the
shockwave reached the ground the shock would be removed, resulting in a
nearly inaudible sonic boom.
"The booms are still there, but your ear is tricked into hearing a thump," Coen said.
Two other design considerations are important. The first reduces the
size of the proposed commercial airliner so it carries fewer passengers
and is lighter. The second slows the cruising speed. While the Concorde
cruised at twice the speed of sound, or Mach 2.0, this airliner would
cruise at a slightly slower Mach 1.6 to Mach 1.8.
"These design choices not only made both the sonic boom problem easier
to tackle, but make the takeoff and landing noise problem much more
solvable, much more amenable to solutions with the technologies we have
in hand," Coen said.
So how loud was the Concorde and how does that relate to NASA's goals of making a quieter supersonic airplane?
The measurement NASA researchers are using to base their work on is
called perceived decibel level, or PLdB. Like comparing apples and
oranges, PLdB is a different flavor of decibels than the measurement
(dBA) often quoted when discussing how loud, for example, a rock concert
is compared to a kitchen blender or library reading room.
Concorde's sonic boom noise level was 105 PLdB. The PLdB that
researchers believe will be acceptable for unrestricted supersonic
flight over land is 75, but NASA wants to eventually beat that and reach
70 PLdB.
"For this phase of the research, we did succeed in reducing the
perceived noise level. In fact, one of the designs reached as low as 79
PLdB," Coen said. "It was a really big step, but we still have some more
work to do to reach our ultimate goal of about 70 PLdB."
Additional studies already are under way to keep whittling away at the
supersonic noise challenge and come up with solutions that will be
acceptable to regulatory agencies such as the Federal Aviation
Administration, as well as airplane manufacturers, the airlines and the
general public.
And while a commercial supersonic airliner flying from New York to Los
Angeles over the U.S. heartland may be another decade or two away, Coen
said it's very possible that smaller supersonic business jets could
debut in the skies much sooner because lighter aircraft create weaker
shock waves, which makes the low boom design challenge easier to solve.
"The business jet would probably be the first on the market, and that
would help introduce some of the technologies that eventually would be
used on the supersonic airliner. But such product decisions belong to
others outside of NASA," Coen said. "Our job is to support the science
and technology behind those choices, eventually making supersonic flight
available to the traveling public."
Jim Banke
NASA Aeronautics Research Mission Directorate
Last Updated: July 31, 2015
Editor: Lillian Gipson
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