IEEE - Aerospace and Electronic Systems - December 2022 - 15

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once for the entire mission profile, limiting multiple transitions
in any realistic multimodal mission. Recently, bioinspired
designs for multimedium vehicles focused on
flapping-wing flight behavior for locomotion. However,
flapping wings' flight behavior with variable stroke length
and wing-beat frequency could only be implemented in
microaerial underwater vehicles, where energy efficiency
is not a major concern. The main demerit ofsuch microhybrid
robots with piezoelectric actuators for flapping is that
they are not portable and energy efficient since tethers
connect them to external power sources.
Despite the apparent limitations in the existing locomotion
approaches adopted earlier, some conceptual multicopter-based
designs such as Loon-copter [16] successfully
implemented aerial actuators in underwater locomotion.
However, there was no parametric study of the tradeoffs in
utilizing a single propulsion strategy for both of these
mediums. The optimal parameters for the propulsion subsystem
were based on the static thrust tests, which evaluated the
optimal thrust to power ratio for a particular thruster configuration
[16]. Moreover, efficient propulsion in amphibious
operation depends on multiple factors such as propeller
geometry, motor efficiency, and specific thrust to power ratio,
apart from the physical constraints like blade deflection, cavitation,
maximum current drain, andmotor power rating.
In this article, we consider all these criteria and propose
a three-step framework for choosing the right combination
ofpropeller-motor pair to get optimal performance in aerial
and underwater operations. Propeller thrust and required torque
are analyzed using blade elemental momentum theory
and compared with the first-order three-constant motor
model to predict the operational points of the motor-propeller
pair. Finally, numerical simulations in QPROP [23] help
us validate the accuracy of theoretical models and identify
the feasible operating range in both the mediums while considering
the various constraints and limitations imposed on
its working. Experimental studies are performed in the feasible
operating region to demonstrate the application of the
proposed thruster setup in multimediumapplications.
Based on the above analysis, the overall framework
proposed for the propeller-motor selection in amphibious
DECEMBER 2022
operation is presented in Figure 1. The entire methodology
has three major steps as shown in Figure 1. In the first
step, prescreening of the propellers is conducted over the
entire propeller range available in the database. This was
done by comparing the thrust profile obtained by BEMT
methodology with the absolute aerial thrust requirement.
In the second step, the right set of propellers is evaluated
based on multiple factors such as thrust profile, required
torque to spin the propeller, possibility of cavitation, and
extent of propeller tip deflection. This leads to narrowing
down to the best set of propellers obtained from the proposed
propeller ranking score, further discussed in the
" Propulsion System Design-Key Requirements " section.
The third stage is motor-propeller matching, which uses
torque equilibrium to choose the best set of motor-propeller
pairs, which are then validated using computational
simulations and experimental studies.
PROPULSION SYSTEM DESIGN-KEY REQUIREMENTS
Propeller blade is a series of airfoil or hydrofoil cross sections
that generate thrust and drag forces because of pressure
differences between top and bottom surfaces when
accelerated rotationally. Each propeller blade is defined in
terms of two-dimensional (2D) cross sections throughout
the span length. This individual cross section is an airfoil or
hydrofoil, which, when coupled with other parameters
such as propeller diameter, pitch, and overall geometry,
provides us with the complete 3D shape of the propeller.
There is a substantial amount of literature focusing on the
design of propellers and optimization of propeller performance
for operation either in aerial or aquatic mediums
[16], [24]. Various nondimensional coefficients are defined
for a given propeller to quantify its performance characteristics.
These nondimensional parameters generally include
the advance ratio (J), thrust coefficient (Ct), torque coefficient
(Cq), power coefficient (Cp), and efficiency (h)
J ¼
V
nD
;Ct ¼
Cp ¼
IEEE A&E SYSTEMS MAGAZINE
rn2D4 ;Cq ¼
P
T
rn3D5 ; h ¼
Q
rn2D5 ;
Ct:J
Cq:2p
:
(1)
15
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IEEE - Aerospace and Electronic Systems - December 2022

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