Unmanned aerial vehicles 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, while birds seem to perform these maneuvers with little effort. Birds use a passively-deployed feather called the covert feather to correct for flow reversal over their wings during high-angle-of-attack maneuvers, thereby delaying the onset of stall.
The challenge here is to extend the understanding of the covert feather’s role in nature and learn from it to increase the mission adaptability and agility of engineered aerial vehicles during high-angle-of-attack maneuvers and during gust. This goal can be achieved by designing covert-inspired deployable structures that, when distributed along the wing span and chord, can mimic the function of the covert feathers.
Currently, an aeroelastic model is being developed that will guide the design process for the covert-inspired deployable structures. A preliminary model has been used to determine the hinge stiffness and mass of a single covert-inspired flap attached to the upper surface of an airfoil.
Wind tunnel test
Wind tunnel tests were conducted to investigate how the lift was affected by the presence of a flap at different chord-wise locations along the suction side of an airfoil. The wind tunnel test set up is shown as below:
Iterative design optimization based on vortex panel method
To optimize the flap design to provide a maximum improvement in lift, an iterative design optimization of a single covert-inspired flap that is attached to the upper surface of an
NACA 2414 airfoil was studied. An evolutionary algorithm, known as The CMA-ES (Covariance Matrix Adaptation Evolution Strategy), is used for the design optimization. The objective function is to maximize lift and the design parameter is the flap deflection angle. The lift coefficient is calculated using an unsteady discrete vortex method (DVM). Preliminary results show that the optimal flap design improves lift up to 23% comparing with the clean airfoil at high angles of attack. The design optimization methodology is shown in the flow chart below:
A novel vortex panel method is under development to account for more design parameters.
RANS-based computational fluid dynamic (CFD) simulations were conducted using COMSOL Multiphysics. These simulations were used to determine the effect of the flap on the flow field and pressure coefficient. An airfoil at angles of attack of 16 deg, 13 deg, and 10 deg were tested without a flap and the same simulations were repeated with a flap deployed at 12 deg. The SST turbulence model was used to close the RANS equations.
The velocity magnitude plots show that at angles of attack of 16 deg and 13 deg, an airfoil with a flap has significantly smaller wake region. While at an angle of attack of 10 deg, the effect of the flap on the wake region is negligible. The Cp comparison at an angle of attack of 16 deg and 13 deg show a pressure recovery step at the flap root location. Moreover, an airfoil with a flap deployed at the optimal deflection angle results in a higher suction peak when compared to the baseline airfoil.