Hybrid Power and Propulsion for Unmanned Aerial Systems
Stanley D. Pebsworth
Bachelors of Science in Aeronautics
Embry-Riddle Aeronautical University
July 2016
A Research
Project Submitted to the Worldwide Campus in partial fulfillment of the
requirements for course UNSY 615, Unmanned Systems Power, Propulsion, and
Maneuvering
Abstract
With an
increased demanded for unmanned aerial systems (UAS), advanced technological
alternatives must be found to meet the expanding mission requirements. Developers and designers are often challenged
with balancing the customer’s mission
requirements with the customer’s budget constraints. A
new type of power and propulsion system must be found that will meet
requirements while maximizing cost savings, improving safety, decreasing
failure rates, and reducing acoustic signature.
The demand is for UAS to have longer mission times, utilize weight
saving measures, and incorporate cost saving fuel sources. An innovative research strategy that combats
these issues is the combination of multiple power, propulsion, and storage
technologies. Research and development
in hybrid technology is ongoing in the automobile industry and therefore, must
be considered. This research paper proposes
the use of hybrid technology to satisfy the requirements of UAS greater than 50
pounds. Through quantitative research,
this paper will analyze current power and propulsion options and provide
alternative hybrid solution that will fulfill the needs of various UAS. This paper will also recommend research
strategies that will concentrate on the need for the future development of alternative
power and propulsion sources for UAS.
Keywords: unmanned
systems, hybrid systems, propulsion, power storage, power generation
Hybrid Power and Propulsion for
Unmanned Aerial Systems
Since the early days of aviation,
the need to balance the requirement to fly farther and longer with the
requirement to reduce weight and increase payload has been ongoing. An unmanned aerial system with the ability to
fly for 24 hours and carry no payload is useless to the customer. As fuel cost fluctuates, the need to improve
fuel economy also exists. In addition, we
must also consider the primary role of unmanned system to be reconnaissance and
therefore must address the need for a reduced acoustic signature.
Currently, there is no replacement
for hydrocarbon fuels that can match its energy density known at this time. Hybrid technology, in some cases, will be
less efficient than heavy fuel systems.
However, there are several scenarios in which hybrid systems could be
optimized and meet the requirements of its mission such as during long cruise
flight and descents (Rupe, 2010).
The research and development of
hybrid technology has the ability to address all of these issues confronting UAS
today. Hybrid systems have the potential
to maximize power to weight ratios which will increase range and reduce fuel
usage. With hybrid technology, the
potential also exists to reduce acoustic signature due to the reduced operating
RPM of the engine. Improved safety can
also be accomplished with hybrid technology by removing the single point
failure system of a hydrocarbon engine and replacing it with a lighter engine
that generates electrical current to charge onboard batteries as well as
operate an electrical propulsion engine.
The primary scope of this research
paper is to compare and contrast current power and propulsion system with
comparable hybrid power and propulsion systems.
Furthermore, this paper will address the tradeoffs between hydrocarbon
fueled systems and that of hybrid systems.
This paper will focus primarily on hybrid systems for use in UAS larger
than 50 pounds and with a real world reconnaissance role.
Literature Review
In an effort to find viable power
and propulsion solutions for UAS, this research analyzed past and current
research on hybrid automobile technology, power sources options, as well as
current hybrid aircraft systems both in use and in development. Hybrid technology has matured over the last
decade and therefore has become a feasible solution for power and propulsion
issues facing the UAS used in today’s military and civilian world security
missions.
The use of hybrid propulsion systems
combining a heavy fuel engine with a battery storage system has the ability to
produce significantly higher endurance for small unmanned aerial systems. Aerial platforms typically require a large
power range to support all modes of flight such as take-offs, climbs, descents,
and cruise profiles. A hybrid system
that has the capability to provide short term increased power profiles through
the combination of both a fuel engine and battery storage could produce an
operationally viable system (Verstraete, Lehmkuehler, Gong, Harvey, Brian,
& Palmer, (2014).
The Department of Defense (DoD)
produced its plans for unmanned systems through the year 2032. The DoD discussed their four primary needs as
being reconnaissance and surveillance, target identification and designation,
counter improvised explosive device (IED) warfare, and Chemical, Biological,
Radiological, Nuclear, Explosive (CBRNE) reconnaissance (Rupe, 2014). In order to be mission effective in these
needs, range and endurance must be addressed (Rupe, 2014).
On today’s battlefield, commanders
have an appetite for constant intelligence updates. Commands today list reconnaissance as their
number one priority (Hiserote, & Air Force Inst of Tech Wright Patterson
AFB OH School of Engineering and Management, 2010).
“The
intrinsic characteristics of UAS are unmatched by their manned counterparts.
“The attributes of persistence, efficiency, flexibility of mission, information
collection and attack capability have repeatedly proven to be force multipliers
across the spectrum of global Joint military operations.”6 In the asymmetric warfare
of GWOT, the abilities of UAS have proven to be mission essential. The Air Force
is currently posturing itself to develop and harness unmanned system
capabilities to maximize current and future contributions to the Joint Force”
(Hiserote, et al, 2010).
Mission effectiveness could be
accomplished through the use of a parallel hybrid/electric power and propulsion
system. Current battery powered UAS do
not offer the much needed mission time and their counterpart, the internal
combustion engine driven UAS, often produce an unwanted and compromising acoustic
signature. A parallel hybrid/electric
system for UAS has the potential to combat both of these issues (Hiserote, et
al, 2010).
Serial and parallel hybrid solutions
were the topic of research conducted by Fusaro, (2016). In her paper she describes serial hybrid
solutions as one that has an electric motor connected directly to the propeller
as the main element in the system and can be powered by the generator of an
internal combustion engine or through a battery storage system. In addition, she describes parallel hybrid
solutions as one that has an internal combustion engine and an electric motor
connected to the propeller through a gearbox (Fusaro, 2016).
The aviation industry is projected
to grow by over 10% in the next 20 years (Friedrich, & Robertson,
(2015). Coupled with the rising fossil
fuel prices, alternatives to meet the propulsion requirements. Hybrid-electric systems provide synergy
between fuel and electric systems which improve performance over internal combustion
engines alone. This improved performance
includes: reduced fuel consumption, reduced CO2 emissions, reduced
noise signature, and an increase in power to weight ratio (Friedrich, et al,
2015).
Design Overview
There are currently several designs in
development that could provide research as well as solutions for a conceptual
design. In 2009, Flight Design presented
an aircraft that used a combination of a 115 HP engine coupled with a 40 HP
electric drive motor to replace the normal 160 HP internal combustion
engine. This design allows for a more
efficient fuel engine and incorporates electric boost power for climbs and
take-off (Rupe, 2014).
Another design idea was developed by students from Embry
Riddle University in 2011 called the Eco-Eagle.
This design used the smaller Rotax 912 100 HP engine coupled with a 40
HP electric motor. This hybrid
propulsion system was tested in the Stemme S10 motor glider (Rupe, 2014).
Hybrid electric distributed propulsion (HEDP) could be a
possible solution to the design and mission needs of UAS as prescribed by the
DoD’s plans for unmanned aerial systems through 2032. HEDP combines powers sources and can provide
supplemental power for takeoff and climbs (Schiltgen, Green, Freeman, &
Gibson, 2014). Having multiple power
sources also adds a safety factor in the event of a one of the propulsion
sources. The following diagram
illustrates the options of both direct drive systems with power supplement and
distributed propulsion with a power supplement.
Design Decisions
In the development stages of a
hybrid system that would compare to the power output of its internal combustion
engine counterpart, the ratio of weight to power output must be
considered. In research it has been
stated through statistical data that the average internal combustion engine has
a power to weight ratio of approximately 1:1 and an electrical motor has a
power to weight ration of approximately .65:1 (Fusaro, 2016). In comparison, a 195kW electric motor is
similar in power to that of a 260hp internal combustion engine (Fusaro, 2016). Therefore, full analysis of the propulsion
requirements must be fully understood in order to apply the proper hybrid
solution for the design.
The batteries to be used also play a
key role in the design decisions. The
batteries capacity and current rating provide critical response requirements
during power load changes. The battery
recharging rate must be addressed as well to determine the best balance between
the time it takes to fully recharge the batteries and the available time that
the batteries can provide either complete or supplement power. A compromise must be met between the fuel
consumption required for recharging the batteries and the required endurance
time (Verstraete, Gong, Lu, & Palmer, (2014).
In a research paper by
Jiménez-Espadafor, Guerrero, Trujillo, García, & Wideberg, (2015), research
was conducted to produce an energy management system for serial hybrid
systems. Although this research was
conducted to manage the power consumption of a wheeled vehicle, critical
information was addressed that can provide a starting point for aerial
systems. In their research, the
management of thermal energy is critical to maximize fuel efficiency.
Acoustic signature is another design
choice that must be addressed. The
desire to conduct covert reconnaissance requires certain aspects of the design
to be taken into consideration. First of
which is the noise produced by the exhaust system of the internal combustion
engine. Design choices must be made to
determine the desired location in which the systems acoustic signature must be
minimized. It is assumed that this
requirement would be during cruise flight while the system is conduct
reconnaissance missions.
A possibility to reduce acoustic signature is maximizing the
noise reducing capability of the exhaust system. In a hybrid system, the internal combustion
engine runs at a constant RPM. This constant
RPM will produce a known acoustic signature and therefore, the muffling
characteristics of the exhaust system can be optimized to focus primarily on
targeting that signature.
Additional research must be
conducted on propeller design and how it effects audible signature in the
design decisions as well. The desire is
to produce a propeller design that optimizes the reduction in radiated noise,
but does not significantly reduce cruise performance.
Addressing
this criterion consists in research that addresses several design variables so
that the objectives can be met.
“Although
broadband noise contributions for rotors and propellers are generally non-negligible,
in the present study, the tonal noise contribution due to the high-speed
flow in
the tip region and the blade–exhaust interaction in the inner part of the blade
plays a dominant role. The shape optimization revealed that the overall
acoustic energy of the pusher propeller can be reduced up to a value of 3.5 dB.
This reduction is nearly equally due to an optimal design of the blade planform
in the tip and inner
regions.
Compared with the five-blade production propeller, the new optimized six-blade propeller
results in a 5.5 dB overall noise reduction” (Pagano, Barbarino, Casalino,
& Federico, 2010).
Conclusion
Baring entry of hybrid systems into
current UAS design is the cost of implementation. Future systems are believed to use this
technology giving its recent technological advances in both ground and aerial
platforms. There are a number of
tradeoffs that must be considered between heavy fuel and hybrid systems such as
endurance verses stealth. Hybrid systems
offer a complete new range of options in design options as well as
limitations. Mission requirements must
be carefully analyzed and options presented to the customer in order to
determine if hybrid power and propulsion are the best option.
This research paper presented a review of current literature
found on hybrid propulsion technology and formulated a design idea based on
this research. Through this design idea,
limitations were addressed and made available that will give design engineers
the tools necessary to formulate goals for specific UAS power and propulsion
design. Results found in this research
show that hybrid propulsion is a viable option for UAS due to its fuel savings
of 6.5% compared to internal combustion engines (Hung, & Gonzalez,
2012). This fuel savings results in
increased payload capability or increased endurance.
Additional research is necessary to
provide quantifiable data on the increased safety aspect hybrid system over
that of internal combustion systems. It
is proposed that hybrid systems have an increased safety factor due to the
addition of optional power sources in comparison. It is also recommended that this increased
safety margin be considered in additional research into the acceptance of UAS
into the national airspace system.
References
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