Electrical Cable Design for Urban Air Mobility Aircraft

Urban Air Mobility (UAM) describes a new type of aviation focused on efficient flight within urban areas for moving people and goods. There are many different configurations of UAM vehicles, but they generally use an electric motor driving a propeller or ducted fan powered by batteries or a hybrid electric power generation system. Transmission cables are used to move energy from the storage or generation system to the electric motors. Though terrestrial power transmission cables are well established technology, aviation applications bring a whole host of new design challenges that are not typical considerations in terrestrial applications. Aircraft power transmission cable designs must compromise between resistance-per-length, weight-per-length, volume constraints, and other essential qualities. In this paper we use a multidisciplinary design optimization to explore the sensitivity of these qualities to a representative tiltwing turboelectric UAM aircraft concept. This is performed by coupling propulsion and thermal models for a given mission criteria. Results presented indicate that decreasing cable weight at the expense of increasing cable volume or cooling demand is effective at minimizing maximum takeoff weight (MTO). These findings indicate that subsystem designers should update their modeling approach in order to contribute to system-level optimality for highly-coupled novel aircraft.

Mobility (UAM) vehicles have the potential to change urban and intra-urban transport in
new and interesting ways. In a series of two papers Johnson et al.1 and Silva et al.2 presented four
reference vehicle configurations that could service different niches in the UAM aviation category. Of those,
this paper focuses on the Vertical Take-off and Landing (VTOL) tiltwing configuration shown in Figure 1.
This configuration uses a turboelectric power system, feeding power from a turbo-generator through a system
of transmission cables to four motors spinning large propellers on the wings. Previous work on electric cable subsystems leaves much yet to be explored, especially in the realm of
subsystem coupling. Several aircraft optimization studies1, 3, 4 only considered aircraft electrical cable weight
and ignored thermal effects. Electric and hybrid-electric aircraft studies by Mueller et al.5 and Hoelzen
et al.6 selected a cable material but did not investigate alternative materials. Advanced cable materials
have been examined by a number of authors: Alvarenga7 examined carbon nanotube (CNT) conductors for
low-power applications. De Groh8, 9 examined CNT conductors for motor winding applications. Behabtu
et al.,10 and Zhao et al.11 examined CNT conductors for a general applications. There were some studies
that examined the thermal effects of cables but they did not allow the cable material to change; El-Kady12
optimized ground-cable insulation and cooling subject constraints. Vratny13 selected cable material based
on vehicle power demand, and required resulting cable heat to be dissipated by the Thermal Management
System (TMS). None of these previous studies allowed for the selection of the cable material based on a
system level optimization goal. Instead, they focused on sub-system optimality such as minimum weight,
which comes at the expense of incurring additional costs for other subsystems. Dama14 selected overhead
transmission line materials using a weighting function and thermal constraints. However, that work was not
coupled with any aircraft subsystems like a TMS.
The traditional aircraft design approach, which relies on assembling groups of optimal subsystems, breaks
down when considering novel aircraft concepts like the tiltwing vehicle. In a large part, this is because novel
concepts have a much higher degree of interaction or coupling between subsystems. For example, when a
cable creates heat, this heat needs to be dissipated by the TMS, which needs power supplied by the turbine,
and delivering the power creates more heat. The cable, the TMS, and the turbine are all coupled. A change
to one subsystem will affect all the other subsystems, much to the consternation of subsystem design experts.
Multidisciplinary optimization is the design approach that can address these challenges. However, to fully
take advantage of this, we must change the way we think about subsystem design. Specifically, we must
move away from point design, and focus on creating solution spaces.
The work presented in this paper uses the multidisciplinary optimization approach with aircraft level
models to study the system-level sensitivity of cable traits: weight-per-length and resistance-per-length.
Additionally, we examined the effects of vehicle imposed volume constraints on these traits. This is useful
for three purposes: (1) to demonstrate a framework that can perform a coupled analysis between the aircraft
thermal and propulsion systems, (2) to provide a method by which future cable designs can be evaluated
against each other given a system-level design goal, (3) to provide insight into what cable properties may
be promising for future research. This last element is explored given the caveat that the models contained
in this analysis do not represent high-fidelity systems. Thus, while we can demonstrate coupling in between
systems, the exact system-level sensitivity to a given parameter may change if a subsystem model or the
assumptions governing that model change.
The organization of this paper is as follows, in Sec II we outline a method to combine the VTOL vehicle
design and cable information in order to produce cables sensitivity studies. Results analysis and discussion
are contained in Sec III. Conclusions are presented in Sec IV.