Wissa to create new UAV wings inspired by birds’ covert feathers

7/18/2018

Julia Stackler

Aimy Wissa
Aimy Wissa
Bird wings are highly adaptive structures that respond to changes in the surrounding flow condition using the feathers. The covert feathers—a system of feathers that has been studied by biologists and engineers—allow a bird to outperform engineered UAVs at high-angle-of-attack maneuvers and during strong gusts of wind.

Assistant Professor Aimy Wissa has conducted aerodynamic testing and structural modeling on a single covert feather, and now, a new Young Investigator award from the Air Force Office of Scientific Research (an AFOSR YIP award) will allow Wissa and her team to use this modeling to create a full wing system equipped with covert feathers in both chord and span directions. This marks the first time engineers will create a design framework based on aeroelastic experiments and simulations to implement the covert-inspired structures in a full wing system.

Her project, “Spatially Distributed Passively Deployable Structures for Stall Mitigation,” was born out of a collaboration with the Air Force Design and Analysis branch following Wissa’s 2016 summer fellowship at the Air Force Research Lab. Funding is through AFOSR’s Multi-Scale Structural Mechanics and Prognosis program.

A schematic of Wissa's proposed spatially distributed deployable flap system.
A schematic of Wissa's proposed spatially distributed deployable flap system.
Her team, in the Bioinspired Adaptive Morphology (BAM) Lab, aims to improve the performance of UAVs during high-angle-of-attack maneuvers as well as during gusty conditions. On traditional vehicles, these flight conditions usually reduce the lift produced by the wings because of flow separation. Mimicking birds’ covert feathers means the UAVs could manipulate the flow over the wing and maintain lift production.

According to Wissa, the deployment dynamics as well as the effect of the geometric and structural parameters of the covert feathers on the flight performance of birds are not well understood, even among the biology community.

As part of this effort, Wissa and her team will design spatially distributed covert-inspired deployable structures on the upper and lower surfaces of a wing. These new structures will be deployed passively (i.e. without the need for actuators or sensors) over a range of flight conditions. They will use multi-material additive manufacturing to create hard and soft materials to achieve a compliant hinge that allows and tailors the passive deployment of these feather-inspired structures.

A: 2D wing section prepared for wind tunnel experiments to investigate the deployable flap effect on lift production.  B: Wind tunnel results showing lift coefficient versus wing angle of attack. The deployable flap improves lift by up to 38% when deployed at high angles of attack.  C and D: CFD simulations showing the velocity field around the two-dimensional wing section without and with a cover-inspired flap, respectively. The covert-inspired flap reduces the velocity deficit and delays the point of flow separation.
A: 2D wing section prepared for wind tunnel experiments to investigate the deployable flap effect on lift production. B: Wind tunnel results showing lift coefficient versus wing angle of attack. The deployable flap improves lift by up to 38% when deployed at high angles of attack. C and D: CFD simulations showing the velocity field around the two-dimensional wing section without and with a cover-inspired flap, respectively. The covert-inspired flap reduces the velocity deficit and delays the point of flow separation.
Successfully implementing these deployable devices on UAVs could transform current UAVs’ mission adaptability and maneuverability, as well as enable new mission profiles—in line with the Air Force’s vision of the war fighter of the future.

“This is reflective of our BAM Lab approach to UAV design: investigating a current limitation of engineered UAVs due to aerodynamic scaling, Reynolds number effects, or challenging mission requirements and transforming it using informed design principles from nature to improve efficiency and extend the vehicle’s flight envelope. In this process, not only do we improve the engineering state of the art, but we also contribute to the understanding of the biological function,” said Wissa.

Photo, at top: An owl with the upper wing coverts deployed at a high angle of attack approach for landing.