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.

New Data on the Laws of Fluid Resistance

Abstract: Thus far, all attempts at the quantitative determination of drag, on the basis of the theory of viscous fluids, have met with but slight success. For this reason, whenever a more accurate knowledge of the drag is desirable, it must be determined by experiment. Here, a few experimental results are given on the drag of a cylinder exposed to a stream of air at right angles to its axis. It is shown that the drag depends on the absolute dimensions of the body and the velocity and viscosity of the fluid in a much more complex manner than has heretofore been supposed.

Design and experimental results for a flapped natural-laminar-flow airfoil for general aviation applications

Abstract: A natural-laminar-flow airfoil for general aviation applications, the NLF(1)-0416, was designed and analyzed theoretically and verified experimentally in the Langley Low-Turbulence Pressure Tunnel. The basic objective of combining the high maximum lift of the NASA low-speed airfoils with the low cruise drag of the NACA 6-series airfoils was achieved. The safety requirement that the maximum lift coefficient not be significantly affected with transition fixed near the leading edge was also met. Comparisons of the theoretical and experimental results show excellent agreement. Comparisons with other airfoils, both laminar flow and turbulent flow, confirm the achievement of the basic objective.

Gliding birds: reduction of induced drag by wing tip slots between the primary feathers

Abstract: 1. The feathers at the wing tips of most birds that soar over land separate both horizontally and vertically in flight to form slotted tips. The individual feathers in the slotted tips resemble the winglets used on the wing tips of some aircraft to reduce induced drag. 2. A wing that produces lift leaves a pair of vortex sheets in its wake. Wing theory shows that winglets can reduce the kinetic energy left in the vortex sheets, and hence the induced drag, by spreading vorticity both horizontally and vertically. 3. This paper describes the aerodynamic forces on a wing made of a base wing and different wing tips. The feathered wing tip was slotted and was made of four primary feathers from a Harris9 hawk (Parabuteo unicinctus). The Clark Y tip was unslotted and was made of balsa wood shaped to a Clark Y aerofoil. The balsa feather tip was slotted and was made of three balsa wood wings shaped like feathers. 4. The base wing in a wind tunnel at an air speed of 12.6 m s-1 generated upwash angles as high as 15° at the end of the wing when the angle of attack of the wing was 10.5°. The feathered tip responded to upwash by increasing its lift to drag ratio (L/D) by 107 %, from 4.9 to 10.1, as the angle of attack of the base wing increased from 4° to 14°. The L/D values of the balsa feather tip and the Clark Y tip increased by 49 % and 5 %, respectively, for the same change in angle of attack. 5. With the angle of attack of the base wing fixed at 13°, changing the angle of attack of the wing tip changed the drag of the base wing. The drag of the base wing increased by 25 % as the angle of attack of the Clark Y tip increased from 0° to 15°. The base wing drag decreased by 6 % for the same change in the angle of attack of the feathered tip. 6. The total drag of the wing with the feathered tip was 12 % less than that of a hypothetical wing with the same lift and span, but with tip feathers that did not respond to upwash at the end of the base wing. This value is consistent with wing theory predictions on drag reduction from winglets. 7. Wings with the tip and the base wing locked together had lift and drag that increased with increasing base wing angle of attack, as expected for conventional wings. Span factors were calculated from these data - a large span factor indicates that a wing has low induced drag for a given lift and wing span. The wing with the Clark Y tip had a span factor that decreased from 1 to 0.75 as the angle of attack of the base wing increased. Over the same range of angle of attack, the span factor of the wing with the feathered tip remained constant at 0.87. As the angle of attack of this wing increased, aerodynamic forces spread the feathers vertically to form slots. With fully formed slots, the wing had a higher span factor than the wing with the unslotted Clark Y tip. 8. Flow visualization with helium-filled bubbles showed that the addition of two winglets to the tip of a model wing spread vorticity both horizontally and vertically in the wake of the tip. 9. These observations taken together provide strong evidence that the tip slots of soaring birds reduce induced drag in the sense that the separated tip feathers act as winglets and increase the span factor of the wings.

Aerodynamics of a NACA4412 airfoil in ground effect

Abstract: The flow characteristics over a NACA4412 airfoil are studied in a low turbulence wind tunnel with moving ground simulation at a Reynolds number of 3.0 x 105 by varying the angle of attack from 0 to 10 deg and ground clearance of the trailing edge from 5% of chord to 100%. The pressure distribution on the airfoil surface was obtained, velocity survey over the surface was performed, wake region was explored, and lift and drag forces were measured. To ensure that the flow is 2-D, particle image velocimetry measurements were performed. A strong suction effect on the lower surface at an angle of attack of 0 deg at the smallest ground clearance caused laminar separation well ahead of the trailing edge. Interestingly, for this airfoil, a loss of upper surface suction was recorded as the airfoil approached the ground for all angles of attack. For angles up to 4 deg, the lift decreased with reducing ground clearance, whereas for higher angles, it increased due to a higher pressure on the lower surface. The drag was higher close to the ground for all angles investigated mainly due to the modification of the lower surface pressure distribution.

Aerodynamic and Static Aeroelastic Characteristics of a Variable-Span Morphing Wing

Abstract: The morphing concept for unmanned aerial vehicles is a topic of current research interest in aerospace engineering. One concept of morphing is to change the wing configuration during flight to allow for multiple flight regimes. A particular approach to planform morphing is a variable-span morphing wing to increase wingspan to reduce induced drag and increase range and endurance. The wing area and the aspect ratio of the variable-span morphing wing increase as the wingspan increases. This means that the total lift increases while the induced drag is reduced, whereas the wing-root bending moment increases, thus, requiring a larger bending stiffness of the wing structure. Therefore, a study of the variable-span morphing wing requires not only aerodynamic analysis but also an investigation of the aeroelastic characteristics of the wing. The aerodynamic characteristics of the variable-span morphing wing are investigated, and a static aeroelastic analysis is performed.