More Electric Aircraft

1. AEROSPACE AMERICA/SEPTEMBER 2005 35Copyright© 2005 by the American Institute of Aeronautics and Astronautics.
I
n the competitive world of airline econom-
ics, where low-cost carriers are driving
down profit margins on airline seat-miles,
techniques for reducing the direct operating
costs of aircraft are in great demand. In an effort
to meet this demand, the aircraft manufacturing
industry is placing greater emphasis on the use
of technology, which can influence mainte-
nance costs and fuel usage.
To achieve lower costs, the industry has
three technology options: optimize the aircraft
equipment systems (AES) and engines; optimize
the system of systems (the aircraft as a whole);
and optimize the solutions that enable the air-
craft to fulfill its required functions.
The Power Optimized Aircraft (POA) is a
European Union (EU) project designed to ad-
dress the first two of these options. The four-year
effort, scheduled to run through 2006, is a Tech-
nology Platform within the EU’s Fifth Frame-
work Program on R&D. With a budget of about
€100 million, the POA is jointly funded by the
EU and a 46-company consortium. Coordinat-
ing the program is Liebherr-Aerospace, a man-
ufacturer of aircraft environmental control sys-
tems, landing gear, and actuation systems.
Power optimized systems
Historically, the industry has pursued option
one: optimizing the aircraft equipment systems,
or AES (engine optimization is addressed in
other EU-funded projects). The technology in
conventional AES is a product of decades of de-
velopment. Its complexity has increased as de-
signers have worked to overcome conflicting
interactions between equipment systems by en-
hancing the efficiency of each. Nevertheless,
many of the larger power users on board re-
main inefficient. This is primarily because of
historical resistance to solving the problem at
the whole aircraft level, and partly because suit-
able technology alternatives are lacking.
Take, for example, the primary flight con-
trol system (PFCS) actuators. Conventionally, a
large civil aircraft has three lines that feed hy-
draulic fluid to all the actuators on board. The
last two decades have seen the development of
electrohydrostatic actuators (EHAs), which re-
quire only external electrical power to run.
by Lester Faleiro,
Project manager,
Power Optimised
Aircraft Project
European project demonstrates how technology
improvements can work together to multiply their
effects and lower costs for cash-strapped airlines
Electrohydrostatic actuators like
this one for the A380 require only
external electrical power to run.
Beyond the
More Electric
Aircraft

2. 36 AEROSPACE AMERICA/SEPTEMBER 2005
EHAs will be installed on the F-35 Joint Strike
Fighter and the Airbus A380. Despite this ad-
vance, the central hydraulic lines can be re-
moved only partially, because today they also
supply the landing gear and the secondary
flight control systems, to name just two. Thus,
the much talked-about More Electric Aircraft
(MEA) of today consists of no more than a con-
ventional AES layout with a few hydraulic users
replaced by electrical ones.
PFCS engineers are among the many AES
specialists who firmly believe that electrically
driven systems are more efficient than most of
their conventional counterparts. Reflecting this
philosophy, recent widebody aircraft have re-
placed some conventional systems with more
electrical alternatives. On the Airbus A380, elec-
trically driven systems include not only PFCS
actuators, but also thrust reverser actuation,
horizontal stabilizer backup, and a number of
smaller systems. The Boeing 787’s brakes, ice
protection, engine start, and environmental
control system will all be electrical. The aircraft
will also have some electrohydraulic pumps for
actuation.
A closer look
The latest aircraft equipment technologies de-
signed and prototyped under the POA fall into
four categories:
•Engine electrical systems, including em-
bedded starter/generators, direct current (DC)
power generation, high-voltage DC bus sys-
tems, active magnetic bearings, electrical fuel,
oil, and engine actuation systems.
•Aircraft electrical systems, including novel
electrical distribution architectures, network in-
teractions, protection, high-voltage DC commu-
tation, wiring, and load management.
•Actuation systems, including alternative ar-
chitectures with electrohydrostatic, hybrid, and
electromechanical actuation for primary and
secondary flight control, as well as new landing
gear, braking, nacelle actuation, and horizontal
stabilizer architectures.
•Pneumatic systems, including electrical en-
vironmental control systems (ECS), wing ice
protection systems, and the use of vapor cycle
cooling, cabin energy recovery, and fuel cells in
aircraft.
The aim of POA is to identify, optimize, and
validate which of these innovative aircraft equip-
ment systems can help to lower consumption of
nonpropulsive power, and thus fuel burn.
For each major AES, the project defined a
number of possible architectures, all of which
are feasible in terms of technology and function.
Studies and validation testing for these systems
show that at this level, there are both advantages
and disadvantages to more electrical systems.
The results so far indicate that in general,
electrical systems tend to be heavier than their
conventional counterparts. This is true for most
large systems. The weight increase is based on
conservative estimates related mainly to heavy
power electronics and heavy drives, both of
which are absent in a conventional aircraft. This
is the price to be paid today for transferring
hundreds of kilowatts of electric power through
the aircraft.
A major finding of the study was that elec-
trical systems tend to be more energy efficient.
This is due to one of three closely related effects:
First, many electrical systems are inherently
more efficient than their conventional counter-
parts. Losses in electrical cabling are lower than
those in hydraulic or pneumatic piping.
Second, electrical systems can be designed
to provide the right function at the right time,
and only at the right time. Today, the central
hydraulic lines are kept energized during the
entire flight. However, some of the larger users
of hydraulic flow, such as landing gear and sec-
ondary flight control, require this power for
only a short time. Electrical systems can be
switched on and off as needed, thus conserving
power.
A third reason for the greater efficiency of
electrical systems is that they can be designed to
provide exactly the needed function, with no
need to worry about having too much or too lit-
Primary
controls
Primary
controls
Secondary
controls
Central
hydraulics
Engine
systems
Environmental
control
Landing
gear Engine
Ice
protection
Gearbox
Generator
Electrical
distribution
Commercial
loads
APU
Mechanical power
Pneumatic power
Hydraulic power
Electrical power
CONVENTIONAL POWER DISTRIBUTION

3. AEROSPACE AMERICA/SEPTEMBER 2005 37
tle available power. The POA has validated large
reductions in energy losses between the power
source and end user compared to conventional
aircraft. This has been seen in configurations
that no longer need to use a precooler (which
wastes bleed air pressure and temperature), no
longer require restrictors in the hydraulic sys-
tems (which today have to reduce hydraulic
pressure and flow from the excess provided by
the central hydraulic lines), and no longer re-
quire engine systems to be dependent on engine
speed or thrust. All these savings can be trans-
lated into reduced fuel burn.
Another major finding was that electrical
systems tend to be more reliable. The replace-
ment of a bleed-air-based ECS by an electrical
ECS, for instance, has shown benefits in this re-
gard. POA studies of landing gear show that fur-
ther simplification of MEA systems is achievable
and leads to higher reliability. This all eventually
translates into lower airline maintenance costs.
Individually, none of these AES have dem-
onstrated overall benefits to the aircraft except
the ECS. It is clear that electrical systems are
more efficient, because they either use less en-
ergy or waste less energy than their conven-
tional counterparts. However, because most of
them are heavier, they yield no real overall im-
provement in fuel consumption. Thus it may
seem as if this system-level development is lim-
ited in scope. But a clear message from POA is
that electrical AES have far more potential for
improvement in the future than conventional
systems. We now have enough experience with
various electrical AES to be able to speculate on
how to change the layout—or architecture—of
these systems.
Taking on the whole aircraft
The second option available to industry is to
optimize the AES all together. Assembling elec-
trical systems to create an MEA is no longer suf-
ficient. The increase in electrical loads (in-flight
entertainment systems, for instance), and the
fact that effective improvements in many con-
ventional AES are becoming difficult to achieve,
has led to a need for readdressing the MEA at
the aircraft level.
The goals of POA are to demonstrate a re-
duction in peak nonpropulsive power usage, in
fuel consumption, and in equipment weight,
and to demonstrate no reduction in production
costs, maintenance costs, or reliability. These
goals are achieved by completely altering the
way in which the architecture of AES is de-
signed. The effects of the new systems in terms
of safety, cost, reliability, maintenance, power
management, and fuel usage at the total aircraft
level are all juggled against the operational ben-
efits of implementing the systems. These issues
are treated as simultaneous goals, and the air-
craft is being optimized to achieve them all.
Analyses of future POAs are being carried
out via a huge simulation called the Virtual Iron
Bird. Models of the more electric AES have been
brought together in various aircraft-level archi-
tectures, and current results from the simulation
show that large power savings can be achieved
with an MEA.
The full results will not be known until the
end of the project, but the trends show some
important aspects of MEA, many of which imply
a philosophical change in the way we consider
aircraft systems:
•Decreasing engine autonomy. The engine is
no longer an independent powerplant; it must
be fully integrated with the aircraft electrical
concept. Consider that a conventional engine
can still function autonomously if the aircraft
systems fail, but a more electric engine is po-
tentially dependent on the aircraft electrical sys-
tem (and vice versa) for its power.
•Increasing availability. The use of multiple
generation sources of electric power (POA has
two in each engine) increases the amount of
power available to each system. This implies
that electrical systems in an MEA may have
greater availability of power than a conven-
tional aircraft, potentially facilitating fulfillment
of system safety requirements.
•Importance of consequential effects. The power
off-takes at the engine from all the aircraft sys-
tems are typically responsible for 3-5% of the
total power produced by the engines (which
No gearbox
Reduced engine bleed
Local hydraulic source
More electrical power
Primary
controls
Secondary
controls
Primary
controls
Cabin expansion
generator
Commercial
loads
Electrical
distribution
Environmental
control
Starter
generator Ice
protectionLanding
gear
Engine
systems
Engine
A POTENTIAL OPTIMIZED ARCHITECTURE

4. 38 AEROSPACE AMERICA/SEPTEMBER 2005
varies by flight phase, engine, and aircraft type).
AES-level improvements can affect only a small
part of this already small percentage. To make a
substantial improvement in airline direct oper-
ating costs, the systems must also be lighter and
more reliable.
However, the POA has also determined that
most of the major power savings do not come
from electrification of the systems themselves,
but from the resulting “consequential effects” it
can have. For example, using electrical flight
control systems in combination with a local hy-
draulic power concept for landing gear actuation
means that the central hydraulic lines can be re-
moved. Not only is the aircraft lighter, but power
also will be generated only when it is needed,
which leads to a saving in fuel. Furthermore, it
has been shown in POA that such effects, com-
bined with optimization of all AES, can “snow-
ball” to produce further weight reductions.
More important, consequential effects on the
engine are being assessed. For instance, the ben-
efits of an electric ECS are related mainly to the
potential they hold for engine improvements.
One such benefit is that there is no direct in-
tervention of ECS requirements in the opera-
tional cycles of the engine. An electrical ECS will
allow the engine compressor to be designed sep-
arately from the pneumatic systems, indepen-
dent of their needs.
Another benefit is easier realization of a higher
bypass ratio engine. This is because the mass
flow of air required by the ECS is “bled” from
the engine high-pressure compressor airflow,
which always has to provide the aircraft cabin
(and hence the ECS) with a minimum amount.
The more this core airflow is reduced via use of
alternative sources of compressed air, the greater
the improvement in fuel consumption.
During some flight phases, the minimum
thrust produced by the engine can be dictated
by the ECS, which requires a minimum pres-
sure to fulfill its tasks. This leads to additional
fuel consumption and prevents the plane from
flying a fuel-efficient mission profile. An electri-
cal ECS would allow the segregation of engine
thrust from ECS airflow needs.
•Effects of load distribution. The power re-
quired by the AES varies considerably depend-
ing on which system is active and in which
flight phase it is active. The landing gear, flaps,
and slats are prime examples of systems that are
used only in particular phases.
A conventional allocation of power to these
systems could lead to the oversizing of some
generators. This in turn means extra weight is
carried on board to generate power that is used
for only part of the flight. Balancing all the safety
and loading requirements in order to make the
generators as small and as fully utilized as pos-
sible is thus necessary to realize the full poten-
tial of an MEA.
•Power electronics and motor drives. These are
major MEA components that could potentially
add much weight. Consolidating these systems
by standardizing them and/or specifying their
technology will be among the future tasks fac-
ing an MEA manufacturer and its AES suppli-
ers. Current developments of these compo-
nents in other manufacturing fields show that
there is huge potential for improving their
power density. Future projects are bound to
address this issue.
The move to more solid-state electronics,
such as those on the Boeing 787, is also a must.
The packaging and cooling of these drive sys-
tems (for instance, the 787 has a propylene gly-
col cooling system for high power drives), and
most significantly their reliability, are becoming
ever more important to the feasibility of MEA.
The other aspect of MEA that is being vali-
dated in POA is that these new systems will all
work together safely and efficiently. Many of
the prototyped systems are being placed on two
test rigs, called the Aircraft System Validation
Rig (ASVR) and the Engine System Validation
The Boeing 787’s brakes, ice
protection, engine start, and
environmental control system
will all be electrical. The aircraft
will also have some electro-
hydraulic pumps for actuation.

5. AEROSPACE AMERICA/SEPTEMBER 2005 39
Rig (ESVR). True to its name, the ASVR will ex-
amine the integration of an aircraft electrical
network, from the generators to electrical distri-
bution and cabling, to electrical actuators, mo-
tor compressor, aircraft fan, fuel pump, and
other loads. Research will focus on the quality of
electricity, how the generators and other com-
ponents react to electrical phenomena, and ad-
ditional aspects of aircraft electrical behavior.
Similarly, the ESVR will use an engine with
an embedded electric starter generator, electric
fuel pump, actuators, oil system, and bearing, to
validate the integration and functional behavior
of a more electric engine. The results of testing
on these two platforms will take us one large
step closer to understanding how more electri-
cal systems can be better on a future MEA.
Energy optimization
The third approach to improving direct operat-
ing costs through technology is to optimize the
solutions to the set of functions the aircraft
must perform. So far, the industry has concen-
trated on producing potentially revolutionary
ideas by taking evolutionary steps.
Things change if we begin to look at the air-
craft as a functional solution to a problem: how
to transport a certain payload a certain distance
at a certain speed in the most efficient manner.
This is primarily a question of energy optimiza-
tion, not power optimization. Theorists working
in fields such as energy and energy optimiza-
tion, and emerging fields such as “constructal
theory,” have a role to play here.
Science is able to show engineers what an
energy optimized aircraft really could look like,
based on its mis-
sion(s). The engi-
neers must then
show the scientists
what their func-
tional constraints
are (cabins require
pressurization,
temperature con-
trol, and provision
of oxygen; energy
must be expended
to maneuver the aircraft, and so on). Eventu-
ally, various solutions, and combinations of so-
lutions, that meet each of the functions of the
aircraft, can be found. These may or may not
look like the solutions we have in today’s air-
craft in the form of conventional AES.
The beauty of this approach is that instead
of providing a number of potential solutions to
a problem, engineers can be shown the ideal di-
rection in which they have to take their develop-
ments, at both the aircraft level and the AES level.
Engineering has taken us to a stage where we are
beginning to rationalize and reintegrate things
that we have spent many decades separating and
“optimizing.” Now, the potential of science can
enable us to take the next step in this journey.
Such a step could eventually lead to far-reaching
changes in the aircraft manufacturing industry.
Toward AES
The diversity of aeronautical manufacturing sec-
tors represented in POA is vital to achieving the
program’s goals. The major POA contractors in-
clude Airbus and Alenia Aeronautica, Germany’s
DLR, Goodrich Actuation Systems, Hispano-
Suiza, Liebherr-Aerospace, Rolls-Royce, Snecma
Moteurs, and Thales Avionics Electrical Systems.
One industrial trend necessary for POA’s
success was that focus in the design of integrated
AES had to move down the supply chain. What
was once the sole domain of the aircraft makers
is now also the focus of the AES and engine
manufacturers. Rather than dealing with myr-
iad suppliers who provide thousands of com-
ponents, aircraft manufacturers realize that
there are benefits in dealing with a few large
AES and engine companies as “total systems so-
lution” providers, rather than product providers.
The aircraft manufacturer is now an “integrator
of systems.”
Current aircraft programs mirror this trend.
In regional aircraft manufacture, products such
as landing gear and flight control systems are
very often contracted to a single company. On
recent Embraer, Bombardier, and Sukhoi air-
craft, all aspects of the landing gear, from the
cockpit controls to
the tires, are handled
as one product. The
Russian Regional Jet
also has a single con-
tractor to provide
everything from the
cockpit control to the
control surface attach-
ment points for pri-
mary and secondary
flight control, includ-
ing spoilers and horizontal stabilizer actuators.
The Aerospace Industries Association estimates
that by 2010, 80% of what was traditionally air-
craft manufacturers’ work will be outsourced,
compared to 50% in the 1970s.
With this trend, opportunities are evolving
from the development of new configurations, as
AES suppliers are encouraged to provide alter-
native solutions to conventional systems func-
tions. Electrical AES are accelerating the trend
“Engineering has taken us to a stage
where we are beginning to rationalize
and reintegrate things that we have
spent many decades separating and
‘optimizing.’ Now, the potential of
science can enable us to take the
next step in this journey.”

6. 40 AEROSPACE AMERICA/SEPTEMBER 2005
further, because when various power supply
types are consolidated into electricity, previously
separated systems can be combined in new
ways. On the 787, one company is responsible
for several major systems (electrical ECS, auxil-
iary electrical power generation, main electrical
power generation, and electrical power distrib-
ution). This has enabled it to achieve a huge
amount of integration of these electrical systems.
Such integrated solutions add to the flexi-
bility of the supply chain and improve both ef-
ficiencies and costs. All of this creates a win-win
situation: The integrated AES suppliers obtain
more business; the aircraft manufacturers obtain
a simpler, lighter, and more reliable product and
have fewer subcontractors; and the airlines can
improve their bottom lines and competitiveness
by passing these gains on to their passengers.
Partnerships for the future
New business opportunities are emerging, and
many companies are taking advantage of them
by forming partnerships or acquiring comple-
mentary technologies. Only a decade ago, it was
unthinkable that electrical systems would be
implemented on aircraft to the extent we are
seeing today. It is clear that, unlike conventional
AES, electrical systems hold great potential for
improvement, both at the level of individual
components such as power electronics, and at
the level of increased integration of previously
separated systems and products.
These issues are so important to the future
of the industry that an AIAA Program Commit-
tee on Energy Optimized Aircraft and Equip-
ment Systems has been formed to address them.
The committee brings together major industrial
partners, research institutions, and academia.
Activities that will see a greater focus on power
and energy optimized aircraft will include the
2006 International Council of Aerospace Sci-
ences conference.
In the meantime, the POA project will con-
tinue to validate the integration of more electri-
cal systems in order to confirm that the positive
results shown so far really can be put onto an
aircraft in the not too distant future (see www.
poa-project.com). This will take the industry a
step further than the A380 and 787, toward the
dream of an “All Electric Aircraft.”
Information technology and assurance
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Computational techniques
Embedded systems
Communication systems
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