Effects of Gurney Flaps on an Annular Wing

Abstract

F OR a constrained wing span, nonplanar wings exhibit significant aerodynamic benefits. These are generally indicated through increased lift curve slopes and reduced drag due to lift. These benefits are accrued through thewing capturing a larger volume of air to generate the lift impulse, and consequently the downwash at a point is reduced [1]. A simple biplane is a clear embodiment of nonplanarity. As the spacing between the upper and lower wings tends to infinity, the lift curve slope of the biplanewing tends to twice that of the equivalent monoplane, and the lift dependent drag tends to half that of the monoplane. Naturally, an infinitely spaced biplane or even that with a significant gap is not a practical design solution. As the gap between the biplanewings reduces, mutual wing interference effects reduce the nonplanar benefits. An annular (or ring) wing achieves the same aerodynamic benefits as an infinitely spaced biplane but with a maximum vertical wing spacing of the diameter of the ring. However, annular wings are hindered by their significant minimum drag coefficient that is largely due to their wetted area, which is 1.57 times greater than an equivalent projected span biplane of equal aspect ratio. Annular wings also suffer from pitch-up at stall due to dissimilar stall characteristics between the upper and lower wing halves [2]. Consequently, an active design space for annular wings is where flight operations at low speed (high loading) for extended duration are a design driver, as may be required for a small reconnaissance unmanned air vehicle [3]. Gurneyflaps have received considerable attention of late as simple devices capable of augmenting the lift of a wing or airfoil [4–7]. The flap itself is usually composed of a small metal tab, typically nomore then 1–2% of the chord attached perpendicular to the trailing edge on the pressure side (generally within the pressure side’s boundary layer). The flaps essentially work by violating the Kutta condition at the trailing edge; thus greater loading is carried over the aft section as final pressure recovery occurs in thewake. This also has the benefit of reduced pressure recovery demands on the upper surface boundary layer [5]. The flaps have also been observed to increase base suction; beneficial in delaying stall but also causing a drag increase. Flap inclination has been indicated to be favorable in reducing drag, and placement at the trailing edge is generally indicated as being themost advantageous [5]. Segmented flaps have been studied as a means to attenuate the drag penalty of the flaps by introducing instabilities into the wake, which culminate in the breakdown of the two-dimensional vortex street that has been observed aft of theflap [6]. Liftmodulation of an annular wing can be complicated due to the wing’s geometric shape; flaps or ailerons may require large end gaps for clearance. Consequently, a Gurney flap may prove an effective device for force andmomentmodulation on an annular wing. Lift augmentation of an annular wing usingGurney flaps is apparently not documented in the literature. Consequently, a study has been undertaken to elucidate the effects of various Gurney flap configurations and layouts on the aerodynamic characteristics of an annular wing.