IEEE Electrification Magazine - December 2017 - 43

xx
Extensive field experience provides data on the wear

life of hydraulic actuators. Predicting wear life on
EMAs is more difficult, since little field experience
exists in primary flight-control applications. Although
there is extensive experience with mechanical transmissions (ball screws, gearing) in secondary flight controls, the duty cycle and fatigue loads on primary
flight-controls can be substantially different.
xx
EHAs and EBHAs are designed with common hydraulic
devices and also designed to fail with damped control
(a desirable failure mode in flight-controls actuation).
The adoption of EMAs into primary flight controls has
been challenging for the reasons highlighted by van den
Bossche. Despite these challenges, Boeing introduced the
first primary flight-control EMAs on a large commercial aircraft, the spoilers on the 787. This introduction of
EMAs in commercial aircraft primary flight controls was
far from the first application of this type. Similar to many
application firsts, the military was an early adopter of
EMA technology. The U.S. Air Force started exploring
rotary EMAs for primary flight controls in the mid-1970s
(Rubertus et al. 1984). They published a report at Wright-
Patterson Air Force Base on the development and flight
test of an EMA in the left aileron on a modified C-141 aircraft (Norton 1986). The report documented that the EMA
seamlessly replaced the existing hydraulic actuator and
that the three test pilots who flew the plane could not
distinguish the difference in operation.
In addition to the military, NASA also played a role in
the early development and application of EMAs in flight
controls. Figure 5 shows the modified F/A-18B of NASA's
systems research aircraft that was used as a test bed for
the electrically powered actuation design (EPAD) program.
The EPAD program replaced a conventional hydraulic servo
actuator with an EHA and then an EMA. Both the EHA and
EMA replaced hydraulic power with electric power for the
flight-control application. Both electrically powered actuators performed equivalently to the hydraulic actuator, but
the EMA was smaller, lighter, and less complex than the
EHA (Jensen et al. 2000).

constraints, the retract actuator is often designed to apply
force close to the landing-gear trunnion mount (the hinge
that the landing-gear pivots around). Applying force close
to the pivot point means that the mechanical advantage is
low; therefore, the actuator must be appropriately sized. In
large commercial airplanes, this results in several hundred
thousand newtons of force for retraction (extension is
lower because it is aided by gravity). Figure 6 shows the
kinematic constraints of a typical commercial aircraft
landing gear. This figure highlights the short moment arm
and corresponding poor mechanical advantage faced by
the retract actuator (Luculescu and Prisacariu 2015).
One additional challenge with the retract actuator is
the requirement for a damped free fall in the event of a
system power failure. In the event of a failure, the landing-gear doors and uplocks are mechanically released
(opened), and the gear is allowed to free fall. The weight
of the gear falling without a resistive damping force
would induce excessive impact loads into the structure.
Hydraulic actuators can be easily designed with fluid
porting through an orifice that damps the gear's free fall
in the event of hydraulic power system failure. While
EMA systems can easily add active damping, they do not
inherently have a simple means of passive (unpowered)
damping. Despite landing-gear electric extension/retraction being typically only viable in small aircraft (FAA
2012b), there has been a concerted effort to develop
EHA and EMA solutions for large commercial aircraft
(Safran 2017, Liebherr-Aerospace 2017).

Nose-Wheel Steering
Nose-wheel steering presents another challenge to conventional EMA actuation due to periodic high-impact
loads from runway obstructions. The more compliant
hydraulic actuators can withstand impact loads better
than the typically stiffer EMAs. In addition, this compliance allows greater relative motion of the actuator, which
spreads the impact energy over a larger distance and
therefore reduces the peak loads transmitted.

Landing-Gear Systems
Since the 1930s, centralized, redundant hydraulic systems
have been used to power the landing-gear system. The
landing-gear system consumes a significant portion of the
overall hydraulic power generated by the hydraulic pumps
to power the landing-gear extension/retraction actuators,
braking, and steering. As the MEA moves toward the
reduction and/or elimination of hydraulic power for aircraft systems, electrification of the landing-gear system
presents additional challenges.

Landing-Gear Extension/Retraction Actuation
Actuation of the landing gear requires high forces for
extension/retraction. The extension/retraction actuator is
commonly termed the retract actuator. Due to space

Figure 5. The systems research aircraft flight test bed. (Photo courtesy of Brown, NASA Dryden Flight Research Center Photo Collection.)

	

IEEE Elec trific ation Magazine / D EC EM BE R 2 0 1 7

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