INGENIEUR
build a MUAV can be purchased off-the-shelf from
hobby shops with complete ready-to-fly (RTF) kits.
It is just like buying a traditional remote-controlled
aircraft including spare-parts and accessories.
Most developers or owners are willing to accept
the risk of failures or crashes because the MUAVs
are considered to be very low cost drones.
However, the mini to medium sized UAVs are
generally of higher performance and capability
and are used for more serious commercial or
military applications. They usually carry on-
board equipment and payloads worth millions of
dollars. It is not feasible to design and construct
a medium-altitude, long endurance (MALE) or a
high-altitude, long-endurance (HALE) UAV using
the “trial-and-error” approach. Employing such
an approach may result in non-optimised aircraft
design and systems. This is due to the high risk
of failure (initial crashes, uneconomic utilisation,
and uncontrollable flights). For these UAVs, a full
understanding of aircraft aerodynamics, flight
stability and control, structures and materials,
propulsion systems and avionics are prerequisite
to successful UAV design and development.
Typical UAV Categories
Table 1 shows typical categories of UAVs in relation
to range, endurance, flight altitude and maximum
take-off weight (MTOW). As mentioned above, the
majority of all UAVs in Malaysia are in the micro
UAV category and most of them are developed
using off-the-shelf parts and components. As of
today, there is only one mini UAV called Aludra
built locally in 2003, weighing around 250kg.
Design Method
In this article, a systematic approach to UAV
design and development is presented for a large
sized UAV, especially for more than 20 kg all up
maximum take-off weight.
For a micro UAV, designers with a minimum
knowledge of aerodynamics, stability and control,
and strength of the airframe, can build and test
their own UAVs using a trial-and-error approach.
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In contrary, for large UAVs of more than 20
kg all up weight, the UAVs has to be designed
systematically. The design process is not just to
ensure the UAV can fly safely to meet the mission
requirements, but it is very important for the
design process to be documented systematically,
such that the documentation will subsequently
be required for airworthiness and certification
processes.
Accurate information on UAV aerodynamics,
stability and control are crucial steps to be
taken before we can design the autopilot
system of a UAV to fulfil its mission requirements
autonomously. Flight control and autopilot
systems fitted in the UAVs have to be designed
to meet specific airframe configurations (i.e.
shape, geometry and dimensions), stability
and performances. The stability of the UAV
has to include accurate and precise flying
responses. This ensures the autopilot can
follow and maintain the mission navigation
including precisely following the paths over the
programmed waypoints. The best method to get
accurate aerodynamics data, stability and control
derivatives is from wind tunnel testing.
Most local developers use popular and well
established airframes (platforms) to build their UAVs
in order to save costs on new airframe development
and autopilot design. Most of the off-the-shelf UAVs
have an embedded autopilot applicable to that
particular platform (commonly using a Proportional-
Integral-Derivative or PID feedback controller)
which has limited capability and performance.
Any modification to the platform configuration, for
example altering the shape and weight, may change
the location of the centre of gravity, moment of
inertia, aerodynamics and dynamic responses of
the aircraft. This will upset the original flight control
gain values of the embedded controller, which then
need to be re-tuned. Due to this required re-tuning,
most of the time the UAV developers will face
limitations and constraints to modify the airframe/
platform. So, the performance and capability of the
UAV are limited and difficult to predict which can
lead to failures and crashes. For that, a systematic
design process, proper fabrication methods
and flight testing activities have to be conducted
professionally.