Experimental Investigation of Bio-Inspired High Lift Effectors on a 2-D Airfoil

Abstract

JOHNSTON, JOE MONROE. Experimental Investigation of Bio-Inspired High Lift Effectors on a 2-D Airfoil. (Under the direction of Dr. Ashok Gopalarathnam). Flaps mounted on the upper surface of an airfoil, called Lift Enhancing Effectors, have been shown to increase maximum lift and stall angle in wind tunnel tests. These effectors are fabricated from 0.35 mm thick Mylar and are allowed to rotate freely about their leading edges. The tests were done in the NCSU Subsonic Wind Tunnel at a chord Reynolds number of 4 × 10 5 . The maximum lift coefficient wasincreased by up to 30% and α stall was increased from 12°to at least 16°. Effectors were also fabricated out of stiff wood, allowing the deployment angle to be fixed with respect to the airfoil surface. This was not attempted in earlier research efforts and provided increased control in the current experimental study as the free-moving effectors tend to oscillate and their deployment angle cannot be controlled.Studying multiple fixed-deployment angles provided better understanding of the aerodynamics of the effectors and helped determine the optimal deployment angles. Fixed-deployment-angle effectors caused the zero-lift angle of attack to increase in proportion to the deployment angle. Drag tests on both the free-moving and fixed-deployment effectors showed marked improvement in drag at high alpha. The fixed-deployment-angle effectors showed drag improvement at increasingly higher alpha as deployment angle was increased. Oil flow visualization was conducted on the clean airfoil and the fixed-deploymentangle effectors. The surface flow pictured by these oil flow tests proved that the effector causes the separation point to move aft on the airfoil, as compared to the clean airfoil. This is thought to be the main mechanism by which the effectors improve both lift and drag. Finally CFD simulations were run and compared to the oil flow visualization. Results for separation point agree between oil flow and CFD, for most alphas. Lift tests indicate that increasing the deployment angle past 60°amounts to very little improvement in C l . Drag tests show that the free-moving effector naturally produces a drag curve in between the curves for the 30°and 45°fixed effectors.

Effect of Flexible Flaps on Lift and Drag of Laminar Profile Flow

“…In order to prevent flutter motion or fall-over of the flaps, their length, thickness, and material properties, such as bending stiffness, are crucial. Kernstine et al [4] and Johnston et al [6] reported additional drag on airfoils caused by flaps. This is in contrast to Liu et al [9], who reported significant reduction in drag.…”

“…This interaction between feathers and flow allows for better control of lift at high angles of attack. This positive effect on the lift coefficient could be adopted for technical applications by replacing feathers with flexible flaps [2][3][4][5][6][7]. Examples for usage include small wind turbines, where strong gusts could lead to sudden drops in lift and therefore induce strong mechanical loads.…”

“…They all have a low flap weight in common. Meyer [2], Schlüter [5], and Johnston and Edwards [6] used rigid flaps made of aluminum, PET, carbon fiber, or polyester. In the cases of flexible flaps, recent publications used aluminum foil [4], silicone elastomer [8,13], and cellulose acetate [3].…”

Investigation of self-deploying high-lift effectors applied to membrane wings

“…Figure 1 shows a duck and a raven, both with pop-up flap deployed during landing. This behaviour of bird wings has been mimicked in various experiments with a flap attached to the top surface of a wing (8)(9)(10)(11)(12)(13) . The flap is hinged about its leading edge and the trailing edge of the flap is left free.…”

Application of biologically inspired "Pop-Up'' feather style high lift device on micro aerial vehicles.

“…16 showed a larger displacement in the z-direction between the upper and lower stream ribbon in flow initiated at z = 2.0 than the one initated at z = 1.0 (circled in blue); the observation shows that the flow feature that cause the displacement in stream traces is most likely initiated at the wingtip, but have little effect on flow closer to the root of the wing.By combining the observations fromFigure 7.14, 7.15 and 7.16, we can reach the conclusion that the primary three dimensional flow effect influencing the performance of the passive flap is the wingtip vortex. As the lift enhancement from the passive flap is due to its position at the optimal flap angle, the non-uniform flow field due to wingtip vortex would cause the flap to reach an equilibrium β that is smaller than the optimal β, thus reducing the additional lift generated.…”