Covert-inspired Deployable Structures for Flow Control

Nature’s Inspiration

Birds are often observed to be superior in terms of mission adaptability; the same bird can perch, glide, and maneuver during flight with remarkable agility compared to similar engineered vehicles. During high angle-of-attack maneuvers, the covert feathers which line up the upper and lower surfaces of a bird's wings are observed to deploy passively, such as shown in the video below. The deployment of these feathers leads to several changes in the unsteady flow features, including shear layer separation and the adverse pressure gradient along the wing's surface, that lead to lift enhancement and stall mitigation.

BAM Approach

The covert feathers are modeled as either passively deployable flaps or torsionally hinged flaps that respond to the flow based on the fluid-structure interaction. A preliminary experimental model is being developed to determine the location, hinge stiffness, and mass of single and multiple covert-inspired flaps to effectively fulfill diverse mission requirements.

The bioinspired design process associated with designing and evaluating the distributed and passive covert-inspired flow control system showing: a) The inspiration of multiple rows of covert feathers on the bottom surface of a parrot wing (left) and upper surface of a heron wing (middle). Upper-surface covert feathers deploy during a high angle-of-attack maneuver (right) (Photo Credit: Pixabay), b) The Engineering analogy consisting of five covert-inspired flaps distributed on the suction side of an airfoil, c) The Evaluation Approach using wind tunnel experiments (middle) to discover and isolate the key physical principles and flow control mechanisms of covert-inspired flaps using flow field measurements (left) and integrated force-torque measurements (right), and d) Field implementation by adding the spatially-distributed covert-inspired flow control device on a remote controlled bird-scale airplane and performing power-on stall maneuvers during flight.

Example analogy of the covert-feathers as torsionally-hinged flaps. (Photo Credit: (left) Pixabay and (middle) The Feather Atlas)

Recent Results

Time-resolved force measurements demonstrate that the coverts-inspired flaps improve lift by up to 55% and reduce drag by up to 42% in the post-stall regime for the multi-flap case. For a single flap case, the flap can improve lift by up to 20% and reduce drag by up to 13%, highlighting the remarkable ability of these fully passive flow control devices in stall mitigation. Flow field measurements from time-resolved Particle Image Velocimetry (PIV) highlight the main flow physics underlying the enhancement in aerodynamic performance, including a reduction in the angle of the separation shear layer and wake size.

Lift and drag improvement due to the incopration of covert-inspired flaps to a NACA 2412 airfoil along with time-averaged flowfield snapshots.

Engineering Impact

Unmanned aerial vehicles (UAVs) are expected to fulfill increasingly complex mission requirements but are limited by their inability to efficiently perform high-angle-of-attack maneuvers at low Reynolds numbers. To evaluate the performance of coverts-inspired flaps in flight, we integrated them onto an unmanned, relevant-scale, fixed-wing aircraft. We then designed an autonomous stall sequence to assess the aircraft at high angles of attack; autonomy was used to create more consistent and repeatable conditions than manual control. Over the course of 19 power-on stalls (12x for Coverts and 7x for Baseline), we demonstrate that the flaps increase the average angle of attack α by 9% -- from 4º to 50º. Moreover, the flaps are noticed to effectively delay stall; the mean covert stall time occurs 0.15 s after the mean baseline stall time . Furthermore, the slope

∂α/∂t is noticeably shallower for the coverts both pre- and post-stall, indicating a more gradual stall, in agreement with the wind tunnel results.

max

stall

Covert-inspired flow control devices offer a scaleable and easily deployable solution to broaden the flight envelope of UAVs and improve flight stability and robustness. Covert-inspired flaps are proven effective at improving lift at post-stall conditions and for a wide range of Reynolds numbers, and thus, they can deployed on vehicles of different sizes or at various flight speeds associated with various flight missions.

Publications

Published

Othman, A., Nair, N., Goza, A., & Wissa, A. (2023). Feather-inspired flow control device across flight regimes. Bioinspiration & Biomimetics, 18(6), 066010.
Othman, A., Nirmal, N., Sandeep, A., Goza, A., & Wissa, A. (2022). Numerical and experimental study of a covert-inspired passively deployable flap for aerodynamic lift enhancement. In AIAA 2022-3980. AIAA AVIATION 2022 Forum. June 2022.
Duan, C., & Wissa, A. (2021). Covert-inspired flaps for lift enhancement and stall mitigation. Bioinspiration & Biomimetics, 16, 046020.
Duan, C., Waite, J., & Wissa, A. (2018). Design optimization of a covert feather-inspired deployable structure for increased lift. In AIAA 2018-3174. 2018 Applied Aerodynamics Conference. June 2018.

Under Review

Sedky, G., Simon, N., Wiswell, H., Othman, A., & Wissa, A. (n.d.). Distributed feather-inspired flow control mitigates stall and expands flight envelope.
Othman, A., & Wissa, A. (n.d.). Feather-inspired passive flaps for flow control on a finite rectangular wing. Under review, AIAA SciTech 2025 Forum.
Simon, N., Zekry, D., Sedky, G., \& Wissa, A. (n.d.). Aerodynamic model synthesis of an aircraft with feather-inspired flow control devices. AIAA SciTech 2025 Forum.

Acknowledgements