IEEE Electrification Magazine - December 2014 - 46

VIEWPOINT

Challenges of the More Electric Aircraft
(continued from page 48)

(EMA) to replace hydraulic actuation
pumps, which are at the heart of a
continue to progress, not all applications
centralized hydraulic system, we can
are suited for replacement at this time.
take advantage of electrical systems
Demanding applications such as landfor ease of distributing the hydraulic
ing-gear retraction for large aircraft still
power. Distributed hydraulic systems
favor hydraulic actuators for three key
using remote hydraulic power sources
reasons. To begin with, the retract actua(electric motor-driven pumps) offer
tor has to be packaged in a way that
potential benefits including weight
usually results in poor mechanical
savings and safety improvements (miniadvantage, requiring very high force
mizing impact of any one hydraulic
density, on the order of hundreds of
failure). Although an electric motorthousands of newtons. This is not an
driven pump is conceptually a simple
insurmountable task to overcome for an
system, it has many of the same
EMA, but it does come at a size and
requirements of a complex flight conweight impact. Additionally, while the
trol servo system. Each must use
actuator is not designed to be in the
power-dense electric machines and
structural load path, deflections of the
power electronics that can be prolanding gear often require some compliduced for an acceptable cost.
ance in the actuator. Hydraulic actuators
Optimizing technology at the airhave a sufficient amount of compliance
craft level versus the component
to limit the peak stress when subjected
level requires detailed trade studies
to impact loads. EMAs
with the airframer
are inherently stiff by
and supplier to
The integration of
design, which drives
evaluate the pros
the EMA in the system
stress to unacceptable
and cons of a given
may produce an
levels if not taken into
t e c h n o l o g y. A s
overall lighter solution
account. Finally, hydrauanother example,
lic actuators easily fail in
an EMA sharing
or lower cost at the
a passive damping state,
the same perforaircraft level due to
whereas EMAs do not.
mance capabilities
the elimination of
While it is easy to add
of a hydraulic actuhydraulic components.
active damping to an
ator may be heaviEMA system, failure of
er and have a highthe motor controller rener initial cost but
ders damping inoperative. In the event
still be the best choice. The integraof a failure, the dampened state is pretion of the EMA in the system may
ferred for many applications on the airproduce an overall lighter solution or
craft, including the landing-gear actualower cost at the aircraft level due to
tion system.
the elimination of hydraulic compoHowever, even if it is not feasible to
nents (valves, hydraulic fuses, accureplace a hydraulic actuator with an
mulators, etc.). Even if the system is
EMA for a given application, it does
heavier with the EMA, it still may
not mean that we keep the status
represent the best choice because of
quo. Instead of large, engine-driven
the added functionality or relative

I E E E E l e c t r i f i c ati o n M agaz ine / DECEMBER 2014

ease of maintenance. The Boeing 787
airplane uses electric brakes (EMAs)
versus conventional hydraulic actuation. In general, the individual EMAs
weigh significantly more than the
equivalent hydraulic actuators at the
brake stack, but the overall system
gain surpasses the individual component difference on the 787.
One of the main goals of the MEA
trend is to increase the overall vehicle
efficiency. Vehicle efficiency is not
simply measured in terms of power
output for a given power input but rather a means of optimizing the sum of
all systems on the aircraft. None of
these challenges are new or isolated
to the aerospace community; they
just have different weighting criteria
used to assess their merits. In fact,
there are common denominators in
the push to electrify all transportation
systems. All of these systems demand
higher power densities, higher reliability, more functionality, and lower
costs. The multidisciplinary challenge
we face is how much "More" is the
right amount in the MEA.

Biography
Nick Nagel (njnagel@triumphgroup.
com) is the director of research and
development at Triumph Aerospace
Systems, Seattle, Washington, where
he works with both electromechanical
and hydraulic actuation systems. He is
also an affiliate professor at the University of Washington, where he teaches courses in electric machines, drives,
and controls. He has more than 20
years of experience in research and
teaching with a focus on control of
electric machinery.

Table of Contents for the Digital Edition of IEEE Electrification Magazine - December 2014