THESIS
2022
1 online resource (xvii, 103 pages) : illustrations (some color)
Abstract
The dramatic development of the Micro Air Vehicle (MAV) has aroused a lot of research interest
on the flapping wing aerodynamics. Flapping wings with proper kinematics, wing shapes, and
flexible structures can improve flapping performance by utilizing large-scale vortical structures
under various circumstances. Because most flying animals in nature use flapping wings for flight,
much can be learnt from nature in the design of flapping wing MAVs. Birds and bats are able to
actively flex their wings with wing musculature to vary the lifting surface and the angle of local
incident flow to their wings as their flight maneuvering may require. By contrast, the wings of
insects are actuated solely by the wing root musculature connected to the exoskeleton and the wing
root itself. The complete...[
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The dramatic development of the Micro Air Vehicle (MAV) has aroused a lot of research interest
on the flapping wing aerodynamics. Flapping wings with proper kinematics, wing shapes, and
flexible structures can improve flapping performance by utilizing large-scale vortical structures
under various circumstances. Because most flying animals in nature use flapping wings for flight,
much can be learnt from nature in the design of flapping wing MAVs. Birds and bats are able to
actively flex their wings with wing musculature to vary the lifting surface and the angle of local
incident flow to their wings as their flight maneuvering may require. By contrast, the wings of
insects are actuated solely by the wing root musculature connected to the exoskeleton and the wing
root itself. The complete wing stroke of an insect is typically divided into four stages: two
translational stages (upstroke and downstroke) and two rotational stages during stroke reversal
(supination and pronation). The wing rotation, measured by the pitching angle, is a combination
of the flexible wing deformation and wing root rotation. The wing root rotation can be passively
driven by the combination of aerodynamic force and inertial force on the wing. Meanwhile, it can
also be actively controlled by the flight musculature. Dragonflies, which are the focus of this study,
as aerial predators, exhibit excellent flight abilities and are special for flapping with two pairs of
independently controlled wings. The studies of the dragonfly’s thoracic musculature found that
different direct muscle groups connecting the frontal plate and backward plate to the thorax can
activate wing depression and elevation together with wing supination and pronation. This thesis contains my effort to identify the passive and active components in the wing rotation of dragonfly
flight and then investigate the effects of dragonfly’s muscle control on flapping kinematics and
aerodynamics. I designed the passive rotation structure which enabled the observation of the purely
passive pitching flight of a freshly cut dragonfly wing. This gave us the basis to compare purely
passive means of pitching with the flight musculature assisted active and passive pitching of the
live dragonfly, to then isolate and quantify when and how strongly the wing rotation is assisted by
active flight control. By comparing it with the live tethered dragonfly flight of the exact same wing,
the passive mechanism was found to dominate wing rotation in the natural flight. However,
dragonfly rotated their wing faster than in the motor wing case at the upstroke reversal, which
should be due to active muscle control. As a result, it was found that LEV of the live dragonfly
was noticeably stronger than that of the motor actuated wing throughout the entire downstroke.
After observing the potential benefits brought by active pitching, a motor control platform was
designed and built, which could control the wing’s flapping and pitching motions simultaneously.
Inspired by the difference in pitching angle curves between dragonfly’s different flight modes, a
new parameter, average wing-root rotational angle (AWRA), was proposed and investigated. The
wing-root rotational angle (WRA), which could be controlled directly by the motor, was set to be
a sinusoidal shape with constant amplitude and frequency. ARWA is the average WRA over a
flapping cycle, and it measures the vertical shifting of the WRA temporal curve. By exploring
cases with different ARWAs, some of which were even impossible for the dragonfly to achieve
because of biological constraints, it is found that AWRA was an efficient control parameter. In the
flight regime of similar flapping/pitching amplitudes and phase lag to dragonflies’ natural flight,
AWRA could change the direction and magnitude of the thrust while keeping a steady lift.
In addition to the studies of wing pitching, artificial dragonfly wings were proposed and
fabricated with MEMS technology. It had a mass of three times that of the natural wing but its
spanwise stiffness was only a third of the natural counterpart, suggesting that corrugation could
significantly increase the spanwise stiffness.
It is hoped that the findings of this work will inspire further studies on dragonfly aerodynamics
and facilitate future MAV design.
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