IEEE Electrification - June 2022 - 13

a belly landing due to gear-actuation
failure, the risk of ignition of fuel
vapors in the vicinity of the aircraft
undercarriage will be significantly
mitigated. The high mounting of the
hydrogen fuel system also accommodates
upward venting of hydrogen
vapor from the apex of aircraft
structures and surfaces.
In addition to being mounted
above the passenger cabin, the tanks
are mounted laterally outside of the
region occupied by the aircraft's
pressurized main cabin. As such, the
only regions where fuel flow pathways
cross over the aircraft's centerline
are within a forward firewall
region and at the aft end of the aircraft. Limiting flow lines
of cryogenic LH2 to occur outside of passenger and flight
crew occupancy areas ensures that large breaks or leaks in
LH2 flow will not pose frostbite hazards to occupants of
the aircraft. A gap in the longitudinal placement of the
tanks can also be observed near the trailing edge of the
wing-body interface. The absence of tanks in this vicinity
is motivated by the need to reduce the probability of tank
puncture in the event of a propulsor fan-blade off event.
Another notable characteristic illustrated in Figure 2 is
the wide centerbody blended into the aircraft's fuselage.
On a conventional kerosene-based aircraft, fuel is typically
stored within the wing structure. However, with the significant
increase in volume required for an LH2 system,
increasing the wing area to accommodate this storage
requirement would result in a wing configuration with a
very low aspect ratio (AR). Small-AR wings typically have
poor aerodynamic performance and, as such, would result
in increased energy requirements of the aircraft's platform.
Conversely, isolated external tanks alleviate these
limitations on wing aerodynamic performance but also
represent an appreciable drag penalty without significant
lift benefits. Instead, for the CHEETA configuration, the
large-volume centerbody is intended to serve as both an
unpressurized fuel storage region and an intentional liftgenerating
component of the aircraft. This lifting centerbody
can actually be configured to improve aerodynamic
efficiency of the entire aircraft configuration, relative to
modern tube-and-wing designs, by allowing lift distribution
across the span of the aircraft to be more ideally configured.
The quasi-cylindrical fuselages used today do
generate a nonzero amount of lift during a typical cruising
flight stage but also introduce a local decrease in the overall
lift profile. The defect produced in this " carryover lift " of
a typical fuselage reduces the aerodynamic efficiency of
the configuration, relative to an ideal lift distribution. By
utilizing the hydrogen storage centerbody region as an
active lift producer, lift distribution can be returned to a
more ideal state for maximum aerodynamic performance.
The quasi-cylindrical
fuselages used today
do generate a
nonzero amount of
lift during a typical
cruising flight stage
but also introduce a
local decrease in the
overall lift profile.
Section 2.4-Additional Benefits
of Distributed Electric Propulsion
Utilization of the aforementioned
lifting centerbody does result in
improvements to the lift distribution,
although volumetrically driven in -
creases to the aircraft-exposed surface
area (referred to technically as
the wetted area) are unavoidable. This
increase in wetted area is typically
associated with undesired additional
drag due to an overall increase in the
aerodynamic skin friction applied
across the aircraft's surface. For this
reason, a bank of propulsors are configured
across the downstream end
of the lifting centerbody as this configuration
allows the benefits of boundary-layer ingestion
to improve propulsive efficiency and partially offset skinfriction
drag penalties.
Stated broadly, boundary-layer ingestion leverages the
low-momentum state of the slow-moving air present in a
region immediately adjacent to the vehicle, known as the
viscous boundary layer, to improve the efficiency of doing
work on the flow by the propulsion system. In simplified
terms, if the flow entering the propulsion system begins
with a large flow velocity, a large increase in kinetic energy
is required by the propulsion system to produce a
given increment in flow momentum. Conversely, as the
boundary-layer flow is already in a low-momentum state,
a smaller increment in kinetic energy of the flow is
required to produce a given amount of thrust. As such,
boundary-layer ingestion can act to reduce the power
required by the propulsion system to deliver a given
thrust requirement. To be clear, the improvements in propulsive
efficiency provided by boundary-layer ingestion
do not indicate that momentum should be purposefully
removed from the flow as much as possible, but rather
that this serves as a useful approach to offsetting the
undesired momentum decreases (i.e., drag) imposed by
large surface-area regions like hydrogen storage volumes
and fuselages.
In addition to centerbody-integrated propulsors, a
series of wing-integrated propulsor banks are also presented
in Figure 2. Although boundary-layer ingestion
benefits can be expected for these wing-integrated
propulsors, the associated reductions in power are
appreciably less aggressive than those anticipated for centerbody-integrated
propulsors. Instead, the primary motivation
for wing integration of these propulsor banks is to
allow a high-momentum nozzle flow of the fan units to be
used for augmenting maximum lift characteristics of the
wing system at low speeds. Blown flaps have been used
on a number of aircraft platforms, such as the Lockheed
F-104 Starfighter, McDonnell Douglas C-17 Globemaster,
and numerous others. Coupling the propulsion system to
IEEE Electrification Magazine / JUNE 2022
13
IEEE Electrification - June 2022

Table of Contents for the Digital Edition of IEEE Electrification - June 2022
