Introduction
Our company is responsible for designing a
precision crop-dusting Unmanned Aircraft System (UAS) for the domestic U.S.
market. We were chosen for the project due to our experience in UAS. However,
unlike our other successful projects, the task of a integrating in an ad-hoc payload
design has had its challenges (Loewen, 2013). Time still exists to correct this
problem as the project hasn’t completed the final system-level critical design
review (CDR), that is required for fabrication, integration, and developmental
test and evaluation of our final product (Loewen, 2013).
Background
The current guidance, navigation and
control (GNC) for our baseline UAS is a commercial off-the-shelf (COTS) design
that allows us to save money on research, development, and manufacturing costs.
We decided to stick with this option as continued investment with the current
system allowed lower manufacturing costs, and easier support/sustainment from
an existing contract. The payload is comprised of several components that allow
storage and distribution of insecticide or fertilizer for crop-dusting
operations. In an effort to reduplicate success that we had with our GNC system,
we decided to take the same approach and purchase a COTS system for the
payload. However, the use of ad-hoc systems has increased the weight of the
baseline UAS, which might require a newer engine or force us to reduce weight
by decreasing storage capacity. The systems engineering department has been
tasked with figuring out a solution that keeps the program on schedule with
minimal delays.
Problem Statement
The addition of a COTS payload onto the
baseline configuration of our UAS has created a significant challenge for the
design process. Because the payload was not custom designed and engineered with
weight limitations in mind (we have no control of how much a COTS system
weighs), the amount of advertised spraying capability that was presented to the
customer is degraded by 20%. This is a significant reduction in capability that
could mean the difference between two flights, rather than one, being necessary
to get the job done.
Where
Systems Engineer Fits Into Solving the Problem
As the systems engineer, I approached the
situation with an unbiased view and coordinated with the respective Propulsion,
Guidance, Navigation and Control, Payload, Safety and Aerodynamics areas to
determine a solution to the weight problem. Even though the main problem is
focused with the weight of COTS GNC and payload systems, bringing in other
areas could help determine an out-of-box approach to our dilemma. The first task
I accomplished was setting priorities to guide the respective engineering teams
in developing a solution. There are three high-level priorities that would be
given and they are as follows: (1) Try to find other payload COTS systems or
sub-components that meet or exceed operational requirements but are less weight
than the current system, (2) what other components, or systems can we
change/delete on the baseline UAS that may be excessive for this requirement or
a substitute available at a lower weight, and (3) what options do we have for
increasing propulsion efficiency to maintain the same thrust-to-weight ratio (using
less fuel capacity, etc) but with no changes in endurance and minimal changes
to propeller design.
After setting these three priorities, I gave
each team a week to research alternatives or courses of action to solve the problem,
then reconvened them for a second meeting to address
specific
solutions and look at how those solutions impact cost and effect the program’s
schedule. This effort fell under the system critical design review (CDR) as
this would be a collective effort from all of the program’s engineers (ACQuipedia,
2016). The Safety Engineer was primarily responsible for ensuring a good margin
of safety with any and all design changes (Loewen, 2013). Once an acceptable
solution was agreed upon, the CDR would be finalized and the program would
progress into the next phase.
The results of the CDR revealed that the
propulsion system, specifically the propellers, could be updated with a newer
design that uses less fuel from take-off to landing. The dual counter rotating
propellers, thinner propeller blades and more blades per propeller offered a
significant advantage of the single propeller design of the baseline UAS (Jha,
2008). These advantages included greater thrust, lower fuel consumption with
lower engine/propeller RPMs (Jha, 2008). This design is highly desirable as the
aircraft will not need higher power thrust for sustained cruise flight to
execute crop-dusting operations. Sufficient thrust would be retained for maximum
weight take-offs. Due to the 30% reduction in fuel consumption over the course
of a typical 45-minute flight time cross dusting operation, fuel capacity was
reduced enough to allow only a modest 2% increase in overall weight of the
vehicle, as compared to the 10% increase observed with the COTS GNC and payload
systems together and no other weight modifications (Jha, 2008). Thus, the GNC
and payload systems remain 100% COTS, allowing us to simplify our production
process with overhead lower costs and decreased risk from an in-house design
and development program. The small 2% increase in weight over the baseline UAS
configuration was well within the safety margin, as determined by our safety
engineer, for maintaining proper weight and balance
for optimal flight control performance and dynamic stability during flight
operations. In addition, and as a result of the new propeller design, the
propulsion engineers suggested that future iterations of our UAS could employ
twin engines and larger a payload for crop dusting operations with greater
endurance. This would equate to larger fields or even multiple fields being
serviced vice a single field the current UAS is design to support.
References:
ACQuipedia. (2016). Critical Design Review. Defense
Acquisition University. Retrieved from https://dap.dau.mil/acquipedia/Pages/ArticleDetails.aspx?aid=dcc068fd-9994-44ed-9500-3a7ec7f81876
Jha, Alok. (2008). Rolls-Royce
brings propeller engines back in vogue. The Guardian. Retrieved
from http://www.theguardian.com/environment/2008/oct/20/travelandtransport-rollsroyce
Loewen, H. (2013). Requirements‐based UAV design process
explained: A UAV manufacturer’s guide. Micropilot.com.
Retrieved from http://www.micropilot.com/ pdf/requiremenv.3ts-based-uav.pdf
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