We designed and built a three degrees-of-freedom (DOF) flapping wing robot, Flapperoo, to study the aerodynamic benefits of wing folding and twisting. Forces and moments of this physical model are measured in wind tunnel experiments over a Strouhal number range of St = 0.2–0.4 - typical for animal flight. We perform particle image velocimetry (PIV) measurements to visualize the air jet produced by wing clapping under the ventral side of the body when wing folding is at the extreme. The results show that this jet can be directed by controlling the wing twist at the moment of clapping, which leads to greatly enhanced cycle-averaged thrust, especially at high St or low flight speeds. Additional benefits of more thrust and less negative lift are gained during upstroke using wing twist. Remarkably, less total actuating force, or less total power, is required during upstroke with wing twist. These findings emphasize the benefits of critical wing articulation for the future flapping wing/fin robots and for an accurate test platform to study natural flapping wing flight or underwater vehicles.

Inspired by bat flight performance, we explore the advantages of wing twist and fold for flapping wing robots. For this purpose, we develop a dynamical model that incorporates these two degrees of freedom to the wing. The twist is assumed to be linearly-increasing along the wing, while the wing fold is modeled as a relative rotation of the handwing with respect to the armwing. An optimization scheme parameterizes the wing kinematics for 2, 5 and 8 m/s forward flight velocities. The intricate interplay between wing orientation, effective angle of attack and the ensuing lift and thrust generation are discussed. The results show that wing twist and fold alleviate negative lift and thrust in the upstroke, and in some cases producing persistent positive thrust throughout cycle for handwing. As a result, power consumption drops precipitously compared to the base case of a rigid flat plate. Another crucial realization is the relative importance of wing twist and fold in achieving efficient flight strongly depends on speeds. At slow flight, twist is significantly more effective in minimizing the power, but becomes energetically inefficient for fast speeds. The results also show that a 45° wing fold during upstroke is energetically beneficial for all speeds. The synergy of wing twist and fold are most prominent at slow flight. These findings provides useful guidelines for designing flapping wing robots.

We study the dynamics of hovering mechanisms inspired by hummingbirds and bats to extract practical guidelines for the design and control of aerial robots. Using a dynamical model that incorporates wing flapping, pronation and folding, we determine the parameters for which hovering is possible. We show that the pronation angle (rotational degree of freedom for the wing about its span) significantly effects energy consumption of the system and demonstrate an optimal wing-beat frequency that minimizes the average power consumption. Furthermore, we show that increased body rotational inertia can significantly increase the energy consumption of hovering and can lead to implausible hovering trajectories. Last, we show that a bat-inspired hovering mechanism that allows for wing folding can generate plausible hovering behavior for larger aerial systems.

Introduction: We present visualizations that emphasize vortices in simulated airflow around a motion-captured bat model. These visualizations aim to help biologists gain understanding on the mechanics of bat flight. As suggested by our fluid dynamics collaborators, studying the formation and shedding of vortices in the flight of bats will help scientists understand the efficient mechanisms bats employ in generating lift. By understanding bat flight, we hope to make discoveries in areas such as biomechanics, aerodynamics, and evolutionary biology. This work is the time varying extension of R. Weinstein’s simulation and visualization of a still bat [7].