IEEE Electrification Magazine - December 2017 - 39

of energy storage to power these machines exceeds current
technology. With aviation fuel having roughly 50 times the
energy storage density of electric batteries (Adams 2017),
leaps in battery technology are required to close this gap so
that electric propulsion can expand to larger aircraft.
In addition to propulsion, there are numerous other
systems that can be electrified to increase overall efficiency, reliability, availability, or maintainability. The
Boeing 787 is a good MEA example. Boeing increased
electrical power generation and distribution more
than fourfold over previous commercial aircraft. By
increasing electrical power  generation, Boeing
was able to electrify the wing anti-icing and the
environmental control systems. This allowed
the elimination of the bleed-air system (although
they still used engine-air bleed for nacelle
anti-icing), significantly increasing fuel efficiency (Sinnett 2007).
One of the main emphases in the MEA has
been to increase the utilization of electric
power instead of hydraulic power in actuation
systems. This article focuses on actuation challenges in the MEA. It seems like a simple problem: replace hydraulic actuation systems with
electrohydraulic or electromechanical actuation
(EMA) systems. In other words, use actuators
that are powered from a centralized electric
power source instead of a centralized hydraulic
source. To understand the challenges in solving this
problem, we first examine the conventional aircraft
actuation systems architecture.

Aircraft Hydraulic Systems
Conventional aircraft hydraulic systems consist of sources (hydraulic pumps) and consumers (actuators). There
are also reservoirs, accumulators, filters, heat exchangers, multiple valves, hydraulic fuses, tubing, and hoses.
Aircraft commonly use three separate hydraulic systems
for redundancy, and hydraulic pumps provide fluid
power to various actuation systems such as the following
(FAA 2012a):
x flight controls
x landing-gear retraction/extension
x main gear brakes
x main and nose landing-gear steering
x leading edge slats
x trailing edge flaps
x thrust reversers.
The hydraulic system architecture is configured such
that two additional systems provide backup capabilities if
any one system fails. Despite having many components,
hydraulic systems are mature designs and have proven to
be safe, reliable, and efficient energy-transfer media.
Like any other technology, conventional hydraulic actuation has advantages and disadvantages. Some disadvantages of hydraulic actuation are as follows:

x assembly time and maintenance (requires bleeding of

air on installation)
x fluid leakage
x fluid contamination
x fluid property changes (viscosity) over tempera-

ture range
x system sizing for transient applications.

Fire resistant phosphate-ester hydraulic fluid (Skydrol)
used in commercial aerospace has additional health concerns, including being a suspected carcinogen. It also damages many paints, organic coatings, and elastomers, often
triggering expensive overhauls if fluids from hydraulic systems leak on other equipment. Finally, it is not totally fireproof and burns at a very high temperature if ignited, as by
an electrical arc.
An example of a transient application would be during
takeoff, where both primary flight controls and landing
gear demand the highest hydraulic flow. The entire hydraulic system must be sized to accommodate such a demand
with accumulators and reservoirs, which increases the system size and weight.
Despite their limitations, hydraulic systems have been
the mainstay in aerospace for more than 80 years due to
the following advantages:
x power density
x force density
x control bandwidth (relatively stiff, low-inertia systems
that allow fast response)
x thermal capacity
x application heritage (billions of flight hours of successful operation)
x relative ease of control (simple, low-power valves to
port fluid).
To gain an understanding of the power density in
hydraulic systems, conventional electric motor pumps
(EMPs) can be examined. Until recently, commercial aircrafts used fixed-frequency, 400-Hz induction motors in
EMPs. The induction motor is typically two-to-four times
larger than the hydraulic pump it is driving when sized for
continuous operation. While the line-feed induction motor
design has not changed significantly in the past 50 years,
neither has the pump design. A more favorable comparison would be between modern permanent-magnet (PM)
machines and hydraulic pumps, but the PM machine can
only handle a similar power output for transient periods
due to thermal limitations. The pump can handle these
power output levels continuously. Additionally, the PM
machine requires a controller that increases the overall
EMP size, weight, and complexity.
Force density is another important benefit to hydraulics for primary flight control and landing-gear extension/retraction actuation. Primary flight-control actuators
are required to hold high loads for extended periods of
time while the output position is dithering (changing in
small amounts). This dithering action is a more challenging task for EMAs because of the high motor currents
IEEE Elec trific ation Magazine / D EC EM BE R 2 0 1 7

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