Investigation of a bio-inspired lift-enhancing effector on a 2D airfoil

Abstract

A flap mounted on the upper surface of an airfoil, called a 'lift-enhancing effector', has been shown in wind tunnel tests to have a similar function to a bird's covert feathers, which rise off the wing's surface in response to separated flows. The effector, fabricated from a thin Mylar sheet, is allowed to rotate freely about its leading edge. The tests were performed in the NCSU subsonic wind tunnel at a chord Reynolds number of 4 × 10(5). The maximum lift coefficient with the effector was the same as that for the clean airfoil, but was maintained over an angle-of-attack range from 12° to almost 20°, resulting in a very gentle stall behavior. To better understand the aerodynamics and to estimate the deployment angle of the free-moving effector, fixed-angle effectors fabricated out of stiff wood were also tested. A progressive increase in the stall angle of attack with increasing effector angle was observed, with diminishing returns beyond the effector angle of 60°. Drag tests on both the free-moving and fixed effectors showed a marked improvement in drag at high angles of attack. Oil flow visualization on the airfoil with and without the fixed-angle effectors 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 an effector improves both lift and drag. A comparison of the fixed-effector results with those from the free-effector tests shows that the free effector's deployment angle is between 30° and 45°. When operating at and beyond the clean airfoil's stall angle, the free effector automatically deploys to progressively higher angles with increasing angles of attack. This slows down the rapid upstream movement of the separation point and avoids the severe reduction in the lift coefficient and an increase in the drag coefficient that are seen on the clean airfoil at the onset of stall. Thus, the effector postpones the stall by 4-8° and makes the stall behavior more gentle. The benefits of using the effector could include care-free operations at high angles of attack during perching and maneuvering flight, especially in gusty conditions.

Influence of self-adaptive hairy flaps on the stall delay of an airfoil in ramp-up motion

“…properly respond to flow separation (Johnston & Gopalarathnam 2012). Only the chord-wise section of the lower half of the airfoil (0.58 <= x/c <= 0.98) was therefore covered with flaps.…”

“…A comparison of the action of fixed and free-moving effectors has been drawn up by Johnston and Gopalarathnam (2012). The fixed-deployment effectors represented a useful means of studying the influence on the flow field and airfoil surface pressures while maintaining control over the deployment angle.…”

The PELskin project-part V: towards the control of the flow around aerofoils at high angle of attack using a self-activated deployable flap

“…Venkataraman and Bottaro [23] performed a numerical study of the effect of hairy coatings on an NACA0012 aerofoil at low Reynolds number and high angle of attack , and found a set of coating parameters able to deliver an increase in lift (). Finally, the effectiveness of fixed versus free-moving flaps has been studied by Johnston and Gopalarathnam [9]. They found that also fixed flaps deliver an improvement in both lift and drag at high angles of attack.…”

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

“…The flap was found to produce a 7% increase in coefficient of lift (C L ) at stall (12,15) . In the years since 1997, many researchers have advanced our understanding of the pop-up flap in many areas including: optimisation of flap size and position (12,14,(16)(17)(18) , optimisation of flap material and geometry (9,17,19) , flap deployment angle (10,(19)(20)(21)(22) , flap performance at low Reynolds Number (Re) (11) , using pop-up flaps for drag reduction (23) , dynamic testing with the pop-up flap (24) , testing of pop-up flaps on highly elliptical low AR wings (25) , kinematics and stability of the wing and flap (26)(27)(28) , the effects of having multiple flaps (18) , and how hairy surfaces can have a similar function to the pop-up flap (8,29) .…”

“…The of 1,300-1,800 MPa, according to the manufacturer's technical data sheet). The flap size and position were based on published experimental results for a rigid wing (11,20,21) . To avoid the flap from being mechanically bound by the coupling of its stiffness and the spanwise deflection of the membrane on which the flap is hinged, the flap was cut along its spanwise direction into five sections free to rotate about their hinge.…”