way in Langley’s Combined Loads Test
System. Prseus incorporates graphite
epoxy and resin materials similar to to-
day’s composite structure, but rather
than being laid down in large plies or
weaved, the manufacturing process
involves physically stitching together
warp-knit fabric. The material is also
infused with resin after stitching, and
although cured at around the same
350F as conventional carbo-fiber
composites, does not require simulta-neous pressurization in an autoclave.
Developed by Boeing and NASA,
the double-deck specimen structure
represents an 80% scaled hybrid wing
body (HWB) cabin section, and will be
subjected to a series of ultimate load
tests between now and the end of April
(AW&ST Dec. 15-22, 2014, p. 11). The fi rst
fi ve runs will vet the article in pristine
condition before the steps are repeated
after parts of the structure have been
deliberately damaged. The structurewill initially be loaded to 18.4 psi inter-
nal pressure, before being subjected to
downward/upward bending equivalent
to +2.9g/-2.9g while unpressurized. The
unit will then be pressurized again for
a repeat of the two simulated g-loaded
tests. Engineers will then infl ict damage
on some internal stif eners and skin, as
well as particular outside keel panels,
before restarting the load test sequence.
Results are aimed at reducing struc-
tural weight by 20% for a future largeCIVIL AIRCRAFT ADVANCES48 AVIATION WEEK & SPACE TECHNOLOGY/MARCH 30-APRIL 12, 2015 AviationWeek.com/awstA
s designers of future airliners look increasingly beyond
traditional tube-and-wing confi gurations to meet the high
ef ciency goals of the 2030s and beyond, new territory is being
carved out in the critical area of airframe-engine integration.
Unusual features ranging from recessed inlets to pylon-
mounted upper-surface engines have become familiar sights in
wind tunnels, but even seasoned researchers are surprised by
a new engine architecture proposed by Pratt & Whitney. The
concept not only physically separates the propulsor from the
gas generator, but also mounts the core backward and at an
angle. This novel arrangement is aimed at overcoming installa-
tion challenges in new confi gurations like the D8 double-bubble
airliner concept under study by NASA and the Massachusetts
Institute of Technology (MIT).
Aimed at NASA’s N+3 performance goals for an airliner that
could enter service around 2035, the D8 is designed to burn at
least 60% less fuel than the current generation of narrowbody
airliners. The secret behind this leap in performance is a con-Guy Norris Los Angeles
and Graham Warwick WashingtonNEW SLANT fi guration that clusters the engines together atop the wide tail
of a fl attened fuselage. Besides providing a clean high-aspect-
ratio wing for low drag, this enables the engines to reenergize to
slow-moving boundary layer fl ow over the fuselage, increasing
ef ciency.
But such a confi guration creates several issues. The engines
lie so close to the upper surface of the fuselage their fans must
be suf ciently robust to cope with fl ow distortion from ingest-
ing the boundary layer. Fan size will also be large because the
engines envisioned for the D8 will have a bypass ratio of at
least 20:1, and be targeted at extremely low noise levels of -52
EPNdb below current Stage 4 limits. Scale tests conducted at
NASA of a distortion-tolerant fan developed by United Tech-
nologies Research Center show the boundary-layer challenge
has been met, but other key questions remain.
Because engine cores are becoming more ef cient and op-
erating at higher pressure ratios, they are also shrinking and be-
coming disproportionately small compared to the propulsor sec-
tion as bypass ratios increase. This leads to blade heights of 0.5
in. or less at the exit of the high-pressure compressor. At this
small scale, tip clearances not only become harder to maintain,
but there is little space within the core through which to run the
driveshaft connecting the fan to the low-pressure turbine. Ad-
ditionally, because the core is proportionately longer and thinner,
designers face the issue of backbone bending which further af-
fects clearance control.
“So that’s when we had the breakthrough idea of turning the
core backward,” says Pratt & Whitney Technology and Environ-
ment Vice President Alan Epstein. Air enters the engine through
the fan as normal, but instead of continuing directly into the com-
pressor, it is ducted around the side and back of the core to enter
from the opposite direction. In an arrangement similar to Pratt &
Whitney Canada’s PT6, in which air fl ows forward through the
engine, hot gas will be discharged forward through a power (low-
pressure) turbine connected to the fan via a gear system. The
turbine, gearbox and fan will be connected via “a really short shaft,
and because the core is not connected to the power side, you can
take the core of easily for maintenance,” Epstein explains.Pratt & Whitney’s innovative reversed, separated and
angled propulsion concept could enable certifi cation of
PRATT & WHITNEY adjacent engines.