low-noise concepts
flightglobal.com 26 January-1 February 2016 | Flight International | 25
“CAEP has essentially said there is no
possibility of having a standard without
community overflight data to validate the
metrics that would be used for response, and
to develop the procedures that would be used
for certification,” he says. “There’s clearly a
need for a flight demonstration. That’s why
the NASA work has been moving in that
direction for the past three years.”
If NASA headquarters approves, Coen
intends his agency will deliver that data over
several years. The X-plane will need all of
2020 to clear the flight envelope for
supersonic testing over a carefully instru-
mented range spread over Edwards AFB in
the California desert. Then, beginning in
2021, NASA plans to carry out a demographi-
cally broad community noise survey,
beginning in Southern California, before de-
ploying the X-plane to other US locations
and, ideally, overseas.
This next step requires reviving memories
of Operation Bongo II, a 1964 experiment
during which the US Federal Aviation
Administration selected Oklahoma City to
perform a series of supersonic acoustic
surveys. In fact, the agency carried out more
than 1,200 sonic booms over the city over six
months to measure how the population
would react. After breaking hundreds of
windows of downtown buildings, the agency
was flooded with complaints and halted the
tests prematurely.
NASA❯❯“There’s a need for a flight
demonstration, that’s why
NASA is moving towards that”
PeTer coeN
Head, high-speed project, NASA aeronautics research
The outcry came against the backdrop of a
wider public backlash in the USA against the
damage and annoyance caused by sonic
booms as supersonic fighters became the
backbone of the US Air Force fleet. The mili-
tary received nearly 39,000 claims for damag-
es caused by sonic booms between 1956 and
1968, according to a NASA book published in
2013 called Quieting the Boom.ProhibiTioN error
The public backlash prompted the FAA to
issue a regulation in 1969 prohibiting super-
sonic flight over populated areas, a stricter
regulation than adopted in Europe, where
only making an audible sonic boom is
outlawed. Public anger would also play a rolein a decision by Congress to cancel the Boeing
2707 supersonic transport (SST) programme
in 1971.
By then the aeronautics industry was well
aware of what needed to be done. Two NASA
scientists, Richard Seabass and Albert George,
had by 1969 developed the basic mathematics
for relating aircraft size and shape to sonic
boom noise. It would still take decades to
transfer the mathematical theory into even
experimental flying aircraft, but their work
gave aerodynamicists tools for shaping an
aircraft with sonic boom noise as a predicta-
ble and primary requirement at design stage.
As Concorde was heading towards a retire-
ment date in 2003, NASA finally began apply-
ing those formulae to a flying vehicle. The
shaped sonic boom demonstration (SSBD) in
2003 used a Northrop F-5E with a heavily
modified forward fuselage, sculpted to muffle
the double-thumb signature of a sonic boom.
The SSBD programme succeeded in
demonstrating that aircraft shaping can re-
duce the pressure rise of the supersonic
shockwave, thus muffling the boom signature.
But it also proved, as expected, that more than
the forward fuselage would have to be sculpt-
ed. In fact, new software-based design tools
were needed to optimise the shaping of the
forward and aft structures to achieve the low-
est boom signature. At the same time, NASA’s
aerodynamicists also needed to solve what
was then called the “low-boom, low-drag
paradox”; shapes that reduce the sonic boom
also increase drag. Achieving a “low-boom”
noise profile might, then, gain regulatory
approval – but result in an aircraft so ineffi-
cient as to be unmarketable.
Another consideration is the human ear’sBoeing’s concept, upside
down in the wind tunnel,
opts for over-wing engines