IEEE Electrification Magazine - June 2020 - 19

storage scheme, electrified propulsion systems offer an
ability to reduce direct operating costs. Previous decades
have demonstrated a great deal of volatility in jet fuel prices, while electrical energy costs have increased only at a
rate proportional to inflation. A comparison of equivalent
energy costs between jet fuel and electricity from the U.S.
national grid is shown in Figure 3, which takes into
account estimated differences in overall drivetrain efficiency between these two approaches.
Distributed electric propulsion can also be used with
many electric drivetrain architectures to produce large
system-level benefits on the vehicle. These concepts permit improvements to aerodynamic efficiency, decreased
noise, and increased tolerance to failure scenarios of one
or more propulsors. This role of distributed propulsion in
the design of electric aircraft will be further described in
the "Electric Aircraft Architectures" section. In addition,
electric machines are known to have superior reliability,
reduced maintenance, and lower operating costs as compared to most combustion engines. Noise generation from
electric motors is also significantly lower than that

World Annual Traffic (Trillion
Revenue Passenger-Kilometers)

systems. In this way, as more wind, solar, nuclear, hydroelectric, and geothermal power plants come online in the
terrestrial grid, the proportion of greenhouse gas emissions directly attributable to aviation may see a rapid rise
over the coming decades.
In response to this need for greener aviation, several
U.S. and international organizations have set aggressive
benchmarks for future aircraft systems. The International
Air Transport Association has committed to reducing CO2
emissions of commercial aircraft by 50% relative to values
produced in 2005. The International Civil Aviation Organization similarly committed to ensuring carbon-neutral
growth in aviation beyond 2020. NASA has also established goals of reducing landing and takeoff nitrogen
oxide emissions by more than 75%, reducing fuel burn by
70%, decreasing the noise impact of aircraft, and ensuring
the use capability of future aircraft to operate across a
range of airport runway classes and infrastructures.
Recent efforts in aircraft propulsion electrification seek
to disrupt the current paradigm and decrease community
dependence on fossil fuels by developing novel means of
aircraft energy storage, power generation and management, and integration. Before describing propulsion electrification efforts, however, we must discuss the current
systems used for air transportation and their associated
fuel burn impact.
Electric flight is already possible across certain types
of aircraft platforms and is even common for certain
unmanned aerial systems. Recent developments by a
number of aircraft manufacturers have paved the way
for small, one- and two-passenger experimental aircraft
that operate solely on battery energy storage and a fully
electric drivetrain. These developments for small-scale
aircraft serve as a noteworthy start to the aircraft pro--
pulsion electrification process, though several key challenges exist when moving toward larger, commercial
aircraft platforms.
Despite these difficulties, the electrification of propulsion at larger-aircraft scales is necessary to see the
greatest impact on the environmental sustainability of
aviation into the future, as these platforms are associated with the greatest CO2 emissions (Figure 2). A total of
93% of commercial aviation-related fuel burn is directly
attributed to aircraft that are designed to carry more
than 150 passengers or feature a maximum takeoff
mass of greater than 45 t. Across category, 36% of global
fuel consumption is associated with single-aisle transport aircraft, and 57% is attributed to twin-aisle transport aircraft. The remaining 7% of fuel burn is associated
with business jets (1%), turboprop aircraft (1%), and
regional jets (5%).
While the electrification of aircraft propulsion can act
to displace the greenhouse gas emissions associated with
fuel burn by changing the energy source used for propulsion, there are other benefits to using electrically driven
propulsion systems as well. Depending on the energy

10
9
8
7
6
5
4
3
2
1
0
1970

1980

1990
2000
Year

2010

2020

Figure 1. The historical growth in global commercial air traffic. Data
are from the International Civil Aviation Organization and Airbus.

Regional Jet
5%

Turboprop
1%

Business Jet
1%

Single Aisle
36%

Twin Aisle
57%

Figure 2. The proportion of jet fuel burn based on aircraft class for
global civil aviation. Data are from Yutko and Hansman (2011).

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IEEE Electrification Magazine - June 2020

Table of Contents for the Digital Edition of IEEE Electrification Magazine - June 2020