Unsteady Aerodynamics and Mechanics of Force Generation in Insect Free Flight

Engineers are interested in designing unmanned aerial vehicles (UAVs) for surveillance, environmental monitoring, assessment, and package delivery, etc. However, as the sizes of these vehicles become smaller, conventional fixed-wing and rotorcraft designs do not scale down well regarding aerodynamics and performance of components needed for propulsion. In the last two decades, bio-inspired flapping flight has offered an approach to bypass the challenges associated with the scalability of rotorcraft. Since the small dimensions and low flight speeds requirements for UAVs are similar to those of most flying insects (nature’s most advanced fliers), understanding insect flight serves as a primary source of inspiration. However, our understanding of the flight these agile organisms is still incomplete — the goal of this work to bridge some of the gaps in our understanding of insect flight.
This dissertation describes efforts toward understanding how insects generate forces for flight, in particular, force generation during the upstroke (upstroke effects) which is less known. To transition from hovering to forward flight, a tilt of the path of the wings (stroke plane) is necessary. However, this tilt induces an asymmetry in the half stroke kinematics and aerodynamics so that the downstroke is dominant, being more aerodynamically active, while the upstroke is inactive or less active, playing a supporting role. Using high-speed photogrammetry to capture free-flying insects, high-fidelity three-dimensional surface reconstructions, kinematics quantification, and computational fluid dynamics (CFD) simulations, the coordination between wing and body motion, the techniques of force generation, and use of unsteady aerodynamics, force orientation, and reorientation, and wing half stroke function were unraveled. We elucidated how the upstroke can generate large forces, in particular, lift and even dominate the downstroke forces in free flight. Results indicate that the coordination between wing and body via body postural adjustments leads to stroke plane adjustments, which in turn influences the wing kinematics and aerodynamics. Our investigations also indicate that the upstroke is instrumental in extending the flight envelope of insects in free flight. The aerodynamic activity of the upstroke was found to increase as flight transitioned from positive to negative advance ratios. Flights with negative advance ratios have not been quantitatively characterized in the literature before this work. The mechanisms associated with the upstroke were found to robust among many flying species from complex to simple fliers.
The primary contributions of this dissertation are in the discovery and characterization of a novel flight mode among vastly different species spanning the entire spectrum of Reynolds numbers of small to large-sized insects with varying complexity, quantitative measurement of flight kinematics, discovery of novel upstroke lift and associated unsteady aerodynamics, clarification of the facultative nature of wing half stroke function, elucidation of the importance of body on wing aerodynamics and finally extraction of simple techniques to extend the flight envelope for additional maneuverability.
By enumerating these techniques across diverse species and flight conditions, our fundamental understanding of flapping flight was substantially improved, and the findings from this research are relevant for highly versatile next-generation small-scale flying robots.