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1. Enhanced flight performance in non-uniformly flexible wings

   https://doi.org/10.1098/rsif.2020.0352  

   Vincent, Lionel; Zheng, Min; Costello, John H.; Kanso, Eva (July 2020, Journal of The Royal Society Interface)

   null (Ed.)

   The flexibility of biological propulsors such as wings and fins is believed to contribute to the higher performance of flying and swimming animals compared with their engineered peers. Flexibility seems to follow a universal design rule that induces bending patterns at about one-third from the distal tip of the propulsor’s span. However, the aerodynamic mechanisms that shaped this convergent design and the potential improvement in performance are not well understood. Here, we analyse the effect of heterogeneous flexibility on the flight performance (range and descent angle) of passively tumbling wings. Using experiments, numerical simulations, and scaling analysis, we demonstrate that spanwise tip flexibility that follows this empirical rule leads to improved flight performance. Improvement in flight range seems to be related to flutter-induced drag reduction. This mechanism is independent of the wing’s auto-rotation and represents a more general trait of wings with non-uniform tip flexibility. 

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2. Flapping Flight of Tiny Insects: Role of Bristled Wings

   https://doi.org/10.2514/6.2018-2916  

   Kasoju, Vishwa T.; Ford, Mitchell P.; Santhanakrishnan, Arvind (January 2018, 2018 Fluid Dynamics Conference, AIAA AVIATION Forum, (AIAA 2018-2916))
3. Springs and Wings: A robotic study of the insect flight system

   Lynch, James (July 2023, escholarship.org)

   In the last decade, roboticists have had significant success building centimeter-scale flapping wing micro aerial vehicles (FWMAVs) inspired by the flight of insects. Evidence suggests that insects store and release energy in the thoracic exoskeleton to improve energy efficiency by flapping at resonance. Insect-inspired micro flying robots have also leveraged resonance to improve efficiency, but they have discovered that operating at the resonant frequency leads to issues with flight control. This research seeks to investigate the roles that elasticity, aerodynamics, and muscle dynamics play in the emergent dynamics of flapping flight by studying elastic flapping spring-wing systems using dynamically-scaled robophysical models of springwings. Studying the dynamics of a robot with comparable features enables the validation of models from biology that are otherwise difficult to test in living insects, the generation of new hypotheses, and the development of novel FWMAV designs. In Chapter 1, the spring-wing system is characterized via a nonlinear spring-mass-damper model. A robophysical model validates that such systems gain energetic benefits from operating at resonance, but reveals that the benefit scales with an underappreciated dimensionless ratio of inertial to aerodynamic forces, the Weis-Fogh number. We show through dimensional analysis that any real system, living or robotic, must balance the mechanical advantage gained from operating at resonance with diminishing returns in efficiency. Chapter 2 further explores the impact of the Weis-Fogh number on flapping dynamics, showing that responsiveness to control inputs is reduced and resistance to environmental perturbations is increased as the dimensionless ratio increases. Together with calculations of Weis-Fogh number in insects, these studies illustrate tradeoffs that drive evolution of resonant flight in nature and guide development of future FWMAVs with elastic energy exchange. In the second half of the thesis, muscle dynamics are introduced in the form of a simplified model of self-excited asynchronous insect muscle. In Chapter 3, a form of velocity feedback, adapted from experiments on insect flight muscle, is developed and integrated with the springwing model, producing a system that generates steady flapping via limit-cycle oscillations despite the absence of periodic control inputs. The model is explored analytically, in simulation, and via implementation on the robotic spring-wing. Novel dynamic characteristics that enable adaptation to damage and passive response to wing collisions are described. Chapter 4 leverages the asynchronous feedback model as part of an interdisciplinary study of the evolution of asynchronous muscle. Phylogenetic analysis, direct measurement of insect muscle dynamics, and experiments on the robophysical system show that evolutionary transitions between periodicallyforced and self-excited insect muscle were likely made possible by a ”bridge” in the dynamic parameter space that could be traversed under specific conditions. The asynchronous spring-wing model provides new insight into the flight and evolution of some of the most agile insects in nature, and presents a novel adaptive control scheme for future FWMAVs. 

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4. Wings and whiffs: Understanding the role of aerodynamics in odor-guided flapping flight

   https://doi.org/10.1063/5.0174377  

   Lei, Menglong; Li, Chengyu (December 2023, Physics of Fluids)

   Odor-guided navigation is an indispensable aspect of flying insects' behavior, facilitating crucial activities such as foraging and mating. The interaction between aerodynamics and olfaction plays a pivotal role in the odor-guided flight behaviors of insects, yet the interplay of these two functions remains incompletely understood. In this study, we developed a fully coupled three-way numerical solver, which solves the three-dimensional Navier–Stokes equations coupled with equations of motion for the passive flapping wings, and the odorant advection–diffusion equation. This numerical solver is applied to investigate the unsteady flow field and the odorant transport phenomena of a fruit fly model in odor-guided upwind surge flight over a broad spectrum of reduced frequencies (0.325–1.3) and Reynolds numbers (90–360). Our results uncover a complex dependency between flight velocity and odor plume perception, modulated by the reduced frequency of flapping flight. At low reduced frequencies, the flapping wings disrupt the odor plume, creating a saddle point of air flow near the insect's thorax. Conversely, at high reduced frequencies, the wing-induced flow generates a stagnation point, in addition to the saddle point, that alters the aerodynamic environment around the insect's antennae, thereby reducing odor sensitivity but increasing the sampling range. Moreover, an increase in Reynolds number was found to significantly enhance odor sensitivity due to the synergistic effects of greater odor diffusivity and stronger wing-induced flow. These insights hold considerable implications for the design of bio-inspired, odor-guided micro air vehicles in applications like surveillance and detection. 

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5. Role of Active Morphing in the Aerodynamic Performance of Flapping Wings in Formation Flight

   https://doi.org/10.3390/drones5030090  

   Billingsley, Ethan; Ghommem, Mehdi; Vasconcellos, Rui; Abdelkefi, Abdessattar (September 2021, Drones)

   Migratory birds have the ability to save energy during flight by arranging themselves in a V-formation. This arrangement enables an increase in the overall efficiency of the group because the wake vortices shed by each of the birds provide additional lift and thrust to every member. Therefore, the aerodynamic advantages of such a flight arrangement can be exploited in the design process of micro air vehicles. One significant difference when comparing the anatomy of birds to the design of most micro air vehicles is that bird wings are not completely rigid. Birds have the ability to actively morph their wings during the flapping cycle. Given these aspects of avian flight, the objective of this work is to incorporate active bending and torsion into multiple pairs of flapping wings arranged in a V-formation and to investigate their aerodynamic behavior using the unsteady vortex lattice method. To do so, the first two bending and torsional mode shapes of a cantilever beam are considered and the aerodynamic characteristics of morphed wings for a range of V-formation angles, while changing the group size in order to determine the optimal configuration that results in maximum propulsive efficiency, are examined. The aerodynamic simulator incorporating the prescribed morphing is qualitatively verified using experimental data taken from trained kestrel flights. The simulation results demonstrate that coupled bending and twisting of the first mode shape yields the highest propulsive efficiency over a range of formation angles. Furthermore, the optimal configuration in terms of propulsive efficiency is found to be a five-body V-formation incorporating coupled bending and twisting of the first mode at a formation angle of 140 degrees. These results indicate the potential improvement in the aerodynamic performance of the formation flight when introducing active morphing and bioinspiration. 

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