twin-aisle airliner, and will specifi cally
evaluate the damage tolerance prop-
erties of Prseus, the development of
which has been underway at Boeing
for more than a decade.
Flight tests of the Adaptive Compli-
ant Trailing Edge (ACTE), a seamless,
morphing fl ap structure attached to a
Gulfstream III , meanwhile continue at
NASA Armstrong Flight Research Cen-
ter under a second weight-related ITD.
Tests are focused on demonstrating the
viability of the ACTE system, developed
by Michigan-based FlexSys, and could
pave the way for follow-on applications
of fl exible structures to both existing
and new-build aircraft (AW&ST Feb.
2-15, p. 56). NASA believes the ACTE
system could reduce wing weight by 5%
on a transport aircraft. Additional drag
and noise benefi ts are also expected.
ERA propulsion work based at
NASA Glenn Research Center is fo-
cused on a fi nal series of rig tests of ahighly loaded front block compressor,
an ultra-high bypass (UHB) propulsor
and an advanced, low-emissions com-
bustor. Propulsion work is targeted
at reducing engine system level fuel
burn by 15%, of which 9% is expected
to come from improvements to the
propulsor and 6% from the core.
Of the savings NASA hopes to gain
in the core, 2.5% or so of the fuel-burn
reduction is expected to come from im-
proving the ef ciency of the high-pres-AviationWeek.com/awst AVIATION WEEK & SPACE TECHNOLOGY/MARCH 30-APRIL 12, 2015 49
The concept also overcomes another
challenge. The idea of nested engines, as
in the D8, does not meet current FAA certifi cation
criteria under the “1 in 20” rule. This states that there should
be only a 1 in 20 chance of debris from an uncontained engine
failure causing a second engine to fail. However, be-
cause the core and propulsor are no longer
mechanically linked, “the design-
ers have come up with an
extraordinarily clever ar-
rangement in which the
cores are angled relative to
each other,” Epstein says.
“We cant them at around 50 deg. and the exit from the
core turns via a 50-deg. duct to go into the power turbine. So
now they are more than 90 deg. of from each other. It’s simple
geometry,” he says. “It enables you to have a large bypass ratio,
and you are not turning much of the airfl ow if you are turning
just the core fl ow, so pressure losses are low.”
Pratt hopes to lay out a roadmap for future studies, pos-
sibly with NASA, to further defi ne the architecture and evaluate
related elements such as shorter-length inlets that would also
help development of the next-generation geared turbofan. Other
focus areas could include studies of the ducting to evaluate
weight and temperature requirements and whether materials
such as ceramic matrix composites might be suitable. “Then
there’s the question of how do you convince the FAA it can be
certifi ed,” says Epstein.
For the MIT-led team, Pratt’s novel engine design is a key en-
abler of the D8 confi guration. Another crucial element has been
validating the ef ciency benefi t from boundary layer ingestion
(BLI) by the clustered aft-mounted engines through large-scale
wind-tunnel testing with NASA. This has quantifi ed the power-
saving from BLI at around 8% “in a realistic confi guration,” says
Ed Greitzer, MIT professor and D8 principal investigator. “This
is the proof of concept for BLI in civil transports,” he told the
American Institute of Aeronautics and Astronautics’ SciTech
conference in Orlando in January.
Clustered beneath the pi-tail of the
NASA/MIT D8 design, the engine
location presents certifi cation and
confi guration challenges.NASAIn a conventional aircraft design, a signifi cant amount of ki-
netic energy is lost in the slow-speed wake behind the fuselage
and wasted in the high-velocity jet exhaust from the engines. This
increases the power required. By ingesting and reenergizing the
boundary layer fl ow, “BLI reduces the wasted kinetic energy in the
combined jet and wake,” says Greitzer. With its engines mounted
atop the aft fuselage, the D8 ingests about 40% of the kinetic-
energy defi cit.
By conducting back-to-back tunnel tests of BLI and con-
ventional podded-engine “non-BLI” versions of the D8, the MIT
team set out to quantify the benefi t by measuring the mechanical
power transmitted to the fl ow by the propulsors to maintain the
same conditions. The measured power reduction with BLI was
8.4% when propulsor nozzle area was held constant, increasing
to 10.4% when mass fl ow was kept equal. “That’s a signifi cant
benefi t,” Greitzer says.
“The dominant ef ect is an increase in propulsive ef ciency
through the reduction in jet velocity,” as a result of starting with
slower fl ow into the propulsor, says Alejandra Uranga, MIT tech-
nical lead. “The D8 engine has similar specifi c thrust but higher
propulsive ef ciency than current-generation engines like the
CFM56-7. That’s why we think the results are applicable to full
scale,” she notes.
The D8 model was tested at angles of attack up to 8 deg.
and slideslip angles up to 15 deg. and the fl ow behaved well,
Uranga says. Engine-out testing showed no adverse impact on
the running engine and the observed fan ef ciency loss from
ingesting distorted boundary-layer fl ow was “an order of magni-
tude less” than the BLI benefi t, says Greitzer, adding “There are
no showstoppers to the D8 confi guration.” c