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...[ Read more ]

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.