Wing in the Loop: Integrating the Wing into Dynamics of Insect Flight

Insects were the first animals to evolve active flight and remain unsurpassed in many features of flight dynamics and performance. The capacity to move in 3D space provided them with superior abilities
in predator-prey interactions and in exploring novel habitats.
Among insects, we find animals capable of precise hovering, taking off backwards, flying sideways and landing upside down.
Some insects are capable of generating forces many times their body weight.
Others perform intricate mating rituals in flight. The outstanding flight capabilities of insects has intrigued scientists for centuries.
However, only recently, with the advent of high speed videography and accurate tracking techniques, have researchers been able to appreciate the elegant motion of
the wings and body of insects in flight.
Over the past few decades, researchers have explored insect flight, searching for the clues to these unique flight capabilities.
Much progress has and is being made toward understanding the unsteady aerodynamics of flapping flight.
Yet many aspects of how insects maneuver and control their flight is still poorly understood.

The capacity to control and alter the flight is as essential for aerial locomotion as the generation of aerodynamic force.
To control their flight, insects alter the motion of their rapidly beating wings. The wings of insects with high flapping frequencies, such as fruit flies, move more than 10 times faster than
their bodies. For other insects, such as dragonflies and damselflies, this number is only 3-5 times. Currently, it is not known how the time scales of the wing and body motions are related
and how the frequency of flapping affects the dynamics and control of flight. Toward addressing these questions, this work is specifically aimed at investigating the dynamics of the insect
wing and integrating it into the open-loop dynamics of flight.

In this work, we focused on low flapping frequency insect flight, where the time scales of the wing and body motions are close.
The pursuit of our goal is carried out via two different branches. First, we gathered accurate data on the free flight of several species of insects during different aerial maneuvers.
Quantitative analysis of the flight behavior was then conducted and comparisons were made with maneuvering flight of high flapping frequency insects.
In the second branch of this work, we developed a physics-based model of the insect wing and the mechanical properties of its hinge, and combined it with a model of the unsteady aerodynamics of
flapping flight to study actuation of the wing pitch. Generation of the aerodynamic force is sensitive to the wing pitch and its dynamics and therefore this motion plays a unique role in
executing aerial maneuvers. Quantitative investigations of the wing dynamics were then carried out by varying the wing geometry, kinematics and structural properties. The ratio of the aerodynamic
to elastic force, termed as \textit{Cauchy number}, was identified as the most important parameter in governing wing motion.
The analysis of the wing dynamics also revealed a mechanism by which the motions of the wing and body are coupled together.
During aerial maneuvers, the body motion alters the balance of forces on the wing, causing the wing kinematics to passively change in response.
Furthermore, the changes in the wing kinematics results in altering the force and consequently the body motion. This chain of actions and reactions is the source of coupling in the dynamics of
the wing and the body. This phenomenon, which was predicted by our theoretical model and verified by our experimental measurements, cannot be explained by the currently accepted model of the insect flight dynamics.
The modifications that were made to the current model based on our analysis and findings resulted in an improved model of the insect flight dynamics that can successfully predict the connection
between the wing and body motions during aerial maneuvers.