flightglobal.com 26 January-1 February 2016 | Flight International | 27
low-noise concepts
❯❯ tolerance for sonic booms. NASA used a
Boeing F-15A test aircraft to break the sound
barrier over a range and measure human
responses to the noise. The agency also creat-
ed a ground-based acoustic chamber to repli-
cate the same noise signature in a controlled
environment, leading to a determination of
the range of tolerable noise for the human ear.
“It’s somewhere between 70-80PLdB,”
Coen says. That roughly compares to the
noise made by a passing car as heard from
inside a building, versus the explosive cracks
of the Concorde’s unmuffled double-boom.
To justify the investment of an X-plane
programme, NASA wanted proof modern
airframe design tools and propulsion systems
could achieve those goals. So-called Phase 2
reports submitted by Lockheed and Boeing
recently helped Coen make that case.
MOVING ON
More than 45 years after the first flight of
Concorde, aviation technology has clearly
progressed. Lockheed’s Model 1044-2 is
almost a direct comparison to the payload of
that 100-seat pioneer. As Lockheed reports,
the 1044-2 should fly about 40% farther than
the 3,900nm (7,200km)-range Concorde, but
its maximum take-off weight is expected to be
14% less than the 185,000kg (408,000lb) mass
of the Anglo-French jet. NASA targeted a
300% improvement in fuel efficiency per pas-
senger mile per gallon, but achieved a 250%
improvement with the Lockheed and Boeing
designs, Coen says. Most importantly, the
Lockheed and Boeing designs fall within the
perceived noise range NASA is targeting.
Both companies were allowed to use tech-
nologies that are likely to become available in
the next decade. For Lockheed’s 1044-2
model, this allowed designers to switch to a
relaxed compression and bleedless engine
inlet that improved fuel performance, espe-
cially compared to the heavy and complexbleed inlet designs developed for Concorde’s
Rolls-Royce Olympus engines.
Moreover, the 1044-2 also would benefit
greatly from two major advances in propul-
sion technology. The conceptual supersonic
airliner would be powered by three engines,
each using a GE Aviation-supplied variable
cycle engine. Developed under the US Air
Force’s versatile advanced aircraft turbine
engine programme, the engine opens a third
stream of airflow during take-off to increase
bypass airflow, thus reducing noise. At the
same time, the other two flow streams would
be reversed compared to a typical turbine;
rather than bypass flow surrounding the air
flowing through the engine core, the 1044-2’s
engines would have their core section on the
exterior, with bypass flow streaming through
the centre.
“Essentially, that milks every decibel out of
the jet you can,” Coen says. “You wind up
with a fairly low speed subsonic jet where
you bring the jet noise down to where it’s
substantially below the requirement.”
There are still many hurdles to realising
quiet supersonic propulsion. One of Coen’s
team’s biggest concerns remains meeting
community noise levels for take-offs at
airports, which are set to get tighter after 2021.
Supersonic aircraft generally require low-
bypass ratio engines, which lack the built-in
noise mufflers provided by high-bypass
systems. Opening a third stream of airflow
helps, but may not solve the problem.
“If I could point to anything that was a
challenge, it was community noise,” notes
Coen. Pushing propulsion and nozzle tech-
nology very hard has achieved a great
amount, he says, but going to such technical
extremes “kind of says to us that that’s an area
where more effort is needed to try and come
up with simpler solutions that can give us
more margin.” ■“If I could point to anything
that was a challenge, it was
community noise”
peter cOeN
Head, high-speed project, NASA aeronautics researchNASA’s 2003 activity with a modified Northrop F-5E showed that an aircraft’s shape could muffle its sonic boom
The third engine on Lockheed Martin’s 1044-2 concept rides on top of the aircraft
NASANASA