Lift-induced drag

About: Lift-induced drag is a research topic. Over the lifetime, 2861 publications have been published within this topic receiving 41094 citations.

Aerodynamic Characteristics at Low Reynolds Numbers for Wings of Various Planforms

Abstract: The aerodynamic characteristics of various wing planforms at low Reynolds numbers (about 1 x 10 4 ) were studied by conducting wind-tunnel tests. These low Reynolds numbers correspond to the flights of small creatures, such as insects. Elliptical, rectangular, and triangular planforms with various aspect ratios were used in this study, as well as a swept rectangular (parallelogram) wing with an aspect ratio of four. The wing sections of all models were thin rectangular airfoils. The aerodynamic forces (lift and drag) and the pitching moment acting on the wing were measured for a wide range of angles of attack (including the maximum of 90 deg). Nonlinear characteristics of the lift coefficient were obtained, even at low angles of attack for high-aspect-ratio wings, whereas a small lift slope and a large maximum lift coefficient were obtained for low-aspect-ratio wings. The drag and the pitching moment coefficients also exhibited nonlinear characteristics. The effect of the Reynolds number based on the wing chord was comparatively small, but a distinctive phenomenon in low-Reynolds-number flow was observed in flow visualization using oil-film and smoke-wire methods.

Aerodynamic characteristics of low-aspect-ratio wings in close proximity to the ground

Abstract: A wind-tunnel investigation has been conducted to determine the effect of ground proximity on the aerodynamic characteristics of thick highly cambered rectangular wings with aspect ratios of 1. 2, 4, and 6. The results showed that, for these aspect ratios, as the ground war, approached all wings experienced increases in lift-curve slope and reductions in induced drag which resulted in increases in lift-drag ratio. Although an increase in lift-curve slope was obtained for all aspect ratios as the ground was approached, the lift coefficient at an angle of attack of 0 deg for any given aspect ratio remained nearly constant. The experimental results were in general agreement with Wieselsberger's ground-effect theory (NACA Technical Memorandum 77). As the wings approached the ground, there was an increase in static longitudinal stability at positive angles of attack. When operating in ground effect, all the wings had stability of height at positive angles of attack and instability of height at negative angles of attack. Wing-tip fairings on the wings with aspect ratios of 1 and 2 produced small increases in lift-drag ratio in ground effect. End plates extending only below the chord plane on the wing with an aspect ratio of 1 provided increases in lift coefficient and in lift-drag ratio in ground effect.

Flow topology in the wake of a cyclist and its effect on aerodynamic drag

Abstract: Three-dimensional flows around a full-scale cyclist mannequin were investigated experimentally to explain the large variations in aerodynamic drag that are measured as the legs are positioned around the crank cycle. It is found that the dominant mechanism affecting drag is not the small variation in frontal surface area over the pedal stroke but rather due to large changes in the flow structure over the crank cycle. This is clearly shown by a series of detailed velocity field wake surveys and skin friction flow visualizations. Two characteristic flow regimes are identified, corresponding to symmetrical low-drag and asymmetrical high-drag regimes, in which the primary feature of the wake is shown to be a large trailing streamwise vortex pair, orientated asymmetrically in the centre plane of the mannequin. These primary flow structures in the wake are the dominant mechanism driving the variation in drag throughout the pedal stroke. Topological critical points have been identified on the suction surfaces of the mannequin’s back and are discussed with velocity field measurements to elucidate the time-average flow topologies, showing the primary flow structures of the low- and high-drag flow regimes. The proposed flow topologies are then related to the measured surface pressures acting on the suction surface of the mannequin’s back. These measurements show that most of the variation in drag is due to changes in the pressure distribution acting on the lower back, where the large-scale flow structures having the greatest impact on drag develop.

Minimizing induced drag with wing twist, computational-fluid-dynamics validation

Abstract: A more practical analytical solution for the effects of wing twist on the performance of a finite wing of arbitrary planform has recently been presented. This infinite series solution is based on Prandtl's classical lifting-line theory, and the Fourier coefficients are presented in a form that depends only on wing geometry. Except for the special case of an elliptic planform, this solution shows that, if properly chosen, wing twist can be used to reduce the induced drag for a wing producing finite lift. A relation for the optimum twist distribution on a wing of arbitrary planform was presented. If this optimum twist distribution is used, the new solution predicts that a wing of any planform can be designed for a given lift coefficient to produce induced drag at the same minimum level as an elliptic wing having the same aspect ratio and no twist. In the present paper, results predicted from this new lifting-line solution are compared with results predicted from computational-fluid-dynamics (CFD) solutions. In all cases, the CFD solutions showed that the drag reduction achieved with optimum twist was equal to or greater than that predicted by lifting-line theory.

Auxiliary wing tips for an aircraft

Abstract: Auxiliary winglets or control surfaces for aircraft wings are tiltable ab an axis extending in the flight direction and about an axis extending substantially perpendicularly to the flight direction. The auxiliary wing tips have a configuration which assure a wing surface continuity, especially when the tips are in their normal wing extending position, but also in any other position of the winglets. Additionally, at least the leading auxiliary winglets are located upstream of the elastic wing axis, as viewed in the flight direction and they have a forward sweep or negative sweepback. The combination of these features permits a simultaneous reduction of induced drag and of stress caused by wind gusts, and for increasing the effectiveness of the wing's ailerons. Thus, these auxiliary winglets have three advantages simultaneously.