Covert-inspired flaps: an experimental study to understand the interactions between upperwing and underwing covert feathers.

Birds are agile flyers that can maintain flight at high angles of attack (AoA). Such maneuverability is partially enabled by the articulation of wing feathers. Coverts are one of the feather systems that has been observed to deploy simultaneously on both the upper and lower wing sides during flight. This study uses a feather-inspired flap system to investigate the effect of upper and lower side coverts on the aerodynamic forces and moments, as well as examine the interactions between both types of flaps. Results from wind tunnel experiments show that the covert-inspired flaps can modulate lift, drag, and pitching moment. Moreover, simultaneously deflecting covert-inspired flaps on the upper and lower sides of the airfoil exhibit larger force and moment modulation ranges compared to a single-sided flap alone. Data-driven models indicate significant interactions between the upper and lower side flaps, especially during the pre-stall regime for the lift and drag response. The findings from this study are also biologically relevant to the observations of covert feathers deployment during bird flight. Thus, the methods and results summarized here can be used to formulate new hypotheses about the coverts role in bird flight and develop a framework to design covert-inspired flow and flight control devices for engineered vehicles.

Nonlinear elasticity and damping govern ultrafast dynamics in click beetles.

Many small animals use springs and latches to overcome the mechanical power output limitations of their muscles. Click beetles use springs and latches to bend their bodies at the thoracic hinge and then unbend extremely quickly, resulting in a clicking motion. When unconstrained, this quick clicking motion results in a jump. While the jumping motion has been studied in depth, the physical mechanisms enabling fast unbending have not. Here, we first identify and quantify the phases of the clicking motion: latching, loading, and energy release. We detail the motion kinematics and investigate the governing dynamics (forces) of the energy release. We use high-speed synchrotron X-ray imaging to observe and analyze the motion of the hinge's internal structures of four Elater abruptus specimens. We show evidence that soft cuticle in the hinge contributes to the spring mechanism through rapid recoil. Using spectral analysis and nonlinear system identification, we determine the equation of motion and model the beetle as a nonlinear single-degree-of-freedom oscillator. Quadratic damping and snap-through buckling are identified to be the dominant damping and elastic forces, respectively, driving the angular position during the energy release phase. The methods used in this study provide experimental and analytical guidelines for the analysis of extreme motion, starting from motion observation to identifying the forces causing the movement. The tools demonstrated here can be applied to other organisms to enhance our understanding of the energy storage and release strategies small animals use to achieve extreme accelerations repeatedly.

The function of the alula on engineered wings: a detailed experimental investigation of a bioinspired leading-edge device.

Birds fly in dynamic flight conditions while maintaining aerodynamic efficiency. This agility is in part due to specialized feather systems that function as flow control devices during adverse conditions such as high-angle of attack maneuvers. In this paper, we present an engineered three-dimensional leading-edge device inspired by one of these specialized groups of feathers known as the alula. Wind tunnel results show that, similar to the biological alula, the leading-edge alula-inspired device (LEAD) increases the wing's ability to maintain higher pressure gradients by modifying the near-wall flow. It also generates tip vortices that modify the turbulence on the upper-surface of the wing, delaying flow separation. The effect of the LEAD location and morphology on lift production and wake velocity profile are investigated using force and hot-wire anemometer measurements, respectively. Results show lift improvements up to 32% and 37% under post and deep stall conditions, respectively. Despite the observed drag penalties of up to 39%, the aerodynamic efficiency, defined as the lift-to-drag ratio, is maintained and sometimes improved with the addition of the LEAD to a wing. This is to be expected as the LEAD is a post-stall device, suitable for high angles of attack maneuvers, where maintaining lift production is more critical than drag reduction. The LEAD also accelerates the flow over the wing and reduces the wake velocity deficit, indicating attenuated flow separation. This work presents a detailed experimental investigation of an engineered three dimensional leading-edge device that combines the functionality of traditional two dimensional slats and vortex generators. Such a device can be used to not only extend the flight envelope of unmanned aerial vehicles (UAVs), but to also study the role and function of the biological alula.