More-electric aircraft z
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guaranteed never to jam, it is “not yet
possible” to use EMAs for airliner
primary controls.
Alternatively, electrohydrostatic
actuators (EHAs), driven by local
hydraulic systems and with no direct
mechanical connection between
motor and actuator arm, offer benign
failure modes, concludes Wheeler,
who sees EHAs as finding readier
industry acceptance because of
component manufacturer’s familiarity
with that technology.
Committed to meeting European
Commission flight path 2050 targets,
including a 75% reduction in aircraft
CO 2 emissions (compared with 2000),
Airbus, its research network Airbus
Group Innovations, and companies
like Rolls-Royce and Siemens, are
exploring different MEA avenues
with myriad ‘e’ epithets.E-PLANS
Airbus Group Innovations has flown
the E-Fan fully electric training aircraft
and its ‘e-roadmap’ includes the
E-Thrust concept study, based on
a distributed propulsion system
architecture, to develop electric or
hybrid technologies for an all-electric
helicopter and a 90-seat aircraft.
Working as integrator, it has developedlarger electrical systems of new aircraft
such as the A380 and Boeing 787 also
power flight-control and undercarriage
actuation systems, protection against
airframe icing, cabin environmental
control systems and fuel pumping:
“These systems help to make future
aircraft more fuel efficient and quieter,
improving the environment.”
In a 2013 Institute of Mechanical
Engineers paper, Wheeler explained
that MEA concepts assume that a
single type of engine-derived power
source is more effective and that
electrical power provides flexibility
and application options. The overall
goal, according to Wheeler, is to reduce
airlines’ operating costs, fuel burn and
environmental impact.
GETTING THE CONNECTION
The move to MEA technology greatly
increases the electrical energy
required: established airliner systems
provide 115V AC power at 400Hz,
with light aircraft and low-power
systems using 28V DC. Wheeler
points out that a Boeing 737’s typical
power rating is around 100kW, while
that for the larger 787 is over 1MW.
Higher voltages could be used to
reduce current and hence cable
weight. Many aircraft use mixed
supplies, frequently 28V DC for
equipment such as avionics, with
larger loads supplied from higher-
voltage AC or DC systems.
The need for MEAs to be
competitive in weight and system
reliability has driven fundamental
developments in how onboard
electricity is generated. For a constant
frequency 400Hz electrical supply,
most airliners use mechanical
gearboxes to create constant-speed
drive to a generator from a variable-
speed engine input. An alternative way
to generate constant frequency supply,
according to Wheeler, is to connect the
generator direct to the engine, with the
resulting output having a variable
frequency based on engine speed.
“This can be processed by a suitable
power converter to produce a fixed
frequency and voltage supply,” says
Wheeler. “Unfortunately power
electronics is not yet reliable enough to
make this option viable and it remains
a rarely chosen configuration.”
To move primary flight control
surfaces electrically (typically by
motor-driven ball screw, often through
a reduction gearbox), the obvious
choice is an electromechanical actuator
(EMA), according to Wheeler. But
because the mechanism must beCOULD BE BUSINESS
Dassault is researching and developing an MEA for
a Falcon business jet that could enter service in the
2020 to 2025 timeframe. The company stated that
engineers from the company and partners in Europe’s
Clean Sky initiative are endeavoring to use fewer
hydraulics and take less bleed-air from the engines,
with the aim of improving overall aircraft efficiency.
The installed electric power would therefore be
greater than the 50kW system on the current Falcon
7X. However, the predicted reliability of electric
systems, especially for power electronics, is an
obstacle. “The right level would be one failure every
10,000 flight hours. We are not there yet,” explained
Jean-Marc Le Peuvédic, Dassault’s onboard energy
architect for future systems. Since high temperatures
affect component reliability, the validation of a
thermal modeling software program is key, even
for transient temperatures.
Validation trials are now underway at a Fraunhofer-
IBP test facility in Stuttgart. Dassault also uses a
testbed operated near Paris by Labinal Power Systems
that was recently upgraded to implement a full DC
architecture. The need for more electrical wiring might
add weight, but choosing aluminum over copper could
minimize any increase. Le Peuvédic also singled out
the operating noise of electric systems as an issue of
specific importance for business jets. Ensuring a quiet
cabin will require designers to pay particular attention
to this issue.“THESE SYSTEMS HELP TO MAKE FUTURE
AIRCRAFT MORE FUEL EFFICIENT AND
QUIETER, IMPROVING THE ENVIRONMENT”ABOVE: The four-
engine all-electric
Cri-Cri was
developed
by EADS
innovation Works