Showing papers on "Lift-induced drag published in 1995"

On the force and moment on a body in an incompressible fluid, with application to rigid bodies and bubbles at high and low reynolds numbers

Abstract: An analysis is made of the force and moment exerted on a rigid body in unsteady motion in a uniform incompressible, viscous or inviscid fluid. Integral formulae are derived which express the force and moment in terms of the velocity and vorticity of the fluid and the velocity of the body. The results extend a previous investigation of this problem (1), and permit the identification of the separate influences of added mass, normal stresses induced by free vorticity, and viscous skin friction. Illustrative applications are made to airfoil theory, to the calculation of the induced drag of a vortex street and the force on a cylinder moving in a slowly varying rotational mean flow, and to the determination of the viscous drag experienced by solid spheres and bubbles. The theory gives a direct prediction of the drag on a bubble in high-Reynolds-number motion, in contrast to the usual approach in which the drag is determined by equating the viscous dissipation to the work done by the drag force.

Drag reduction by wing tip slots in a gliding Harris' hawk, Parabuteo unicinctus

Abstract: The anterior-most primary feathers of many birds that soar over land bend upwards and separate vertically to form slotted wing tips during flight. The slots are thought to reduce aerodynamic drag, although drag reduction has never been demonstrated in living birds. Wing theory explains how the feathers that form the tip slots can reduce induced drag by spreading vorticity horizontally along the wing and by acting as winglets, which are used on aircraft to make wings non-planar and to spread vorticity vertically. This study uses the induced drag factor to measure the induced drag of a wing relative to that of a standard planar wing with the same span, lift and speed. An induced drag factor of less than 1 indicates that the wing is non-planar. The minimum drag of a Harris' hawk gliding freely in a wind tunnel was measured before and after removing the slots by clipping the tip feathers. The unclipped hawk had 70­90 % of the drag of the clipped hawk at speeds between 7.3 and 15.0 m s-1. At a wing span of 0.8 m, the unclipped hawk had a mean induced drag factor of 0.56, compared with the value of 1.10 assumed for the clipped hawk. A Monte Carlo simulation of error propagation and a sensitivity analysis to possible errors in measured and assumed values showed that the true mean value of the induced drag factor for the unclipped hawk was unlikely to be more than 0.93. These results for a living bird support the conclusions from a previous study of a feathered tip on a model wing in a wind tunnel: the feathers that form the slotted tips reduce induced drag by acting as winglets that make the wings non-planar and spread vorticity both horizontally and vertically.

Tip vortex generation technology for creating a lift enhancing and drag reducing upwash effect

Abstract: Methods for using tip generated vortices to improve performance of foils. These methods include generating a substantially streamwise beneficial vortex (74) near the outboard end (60) of a foil (82). This beneficial vortex (74) spins in the opposite direction of an induced drag vortex (62), and is used to create an upwash field (76) which neutralizes induced drag by deflecting the flow behind the trailing edge (56) at an upward angle. Upwash field (76) causes the lift vector (118) on the foil (82) to tilt forward, thereby creating a forward directed force of induced thrust upon the foil (82). Beneficial vortex (74) is also used to contain and compress the high pressure field existing along the attacking surface of the foil (82), and displace the induced drag vortex (62) inboard from the tip of the foil (82). Numerous performance parameters are improved dramatically by using beneficial vortex (74), as well as by using a double vortex pattern (124). Methods for creating, using, and controlling these vortex patterns are offered along with a variety of embodiments for employing such methods. Described embodiments include a twist (66) along the span of a foil (82b), an anhedral tip droop having a divergent axis of droop curvature (86), a movable vortex flap (102), a cone shaped vortex generator (134), a curved outboard droop (146), and a vortex flap network (162). Methods are also disclosed for reducing overall tip vorticity.

Lifting surface design using multidisciplinary optimization

Abstract: Lifting surface design is affected by many considerations; drag, weight, and high-lift are particularly important. These effects place different and often opposite requirements on wing shape, complicating the selection of a best configuration. To assist this selection, a preliminary design method using nonlinear optimization has been developed. An isolated lifting surface design problem is formulated from aircraft mission parameters, typically to calculate the platform and twist minimizing cruise drag or maximizing range, subject to constraints such as structural weight and maximum section lift. Solving this with optimization requires very fast analyses that are capable of capturing the effects of detailed changes in wing shape. This motivated significant improvements that were made to structural calculations and maximum-lift prediction methods for preliminary design. A method was developed to evaluate wing weight and stiffness considering bending and buckling strength. A critical section method was modified to enable the prediction of flaps-down maximum lift, correcting for induced camber near the flap edge. The lifting surface optimization method performs platform design while accounting for many effects: static aeroelasticity, weight evaluated from multiple structural design conditions, induced drag, profile drag, compressibility drag, maximum lift, static stability, and control power constraints. The method was used to explore the influence of these effects on optimal wings, demonstrating how strongly lifting surface design is influenced by maximum-lift constraints. The method was also applied to studies of wing tip shape and optimal wing-tail configurations. In many cases, the optimizer exploits physical effects, creating design features that are easy to interpret in hindsight but difficult to predict in advance. In creating these designs, the method has demonstrated that optimization can be a valuable tool for lifting surface design.

Drag prediction at subsonic and transonic speeds using Euler methods

Abstract: A technique for the evaluation of aerodynamic drag from flow field solutions based on Euler equations is discussed. The technique is based on the application of the momentum theorem to a control surface enclosing the configuration and it allows the decomposition of the total drag into induced drag and wave drag. Consequently, it provides more physical insight into the drag sources than the conventional surface-pressure integration technique. The induced drag is obtained from the integration of the kinetic energy of the trailing vortex system on a wake plane and the wave drag is obtained from the integration of the entropy jump over the shocks. The drag-evaluation technique is applied to three-dimensi onal steady flow field solutions for the ONERA M6 wing as well as an AR-7 wing with an elliptic spanwise chord distribution and a NACA 0012 section shape. Comparisons between the drag obtained with the present technique and the drag based on the integration of surface pressures are presented for several Euler codes.

Turbulent drag reduction using riblets on a supercritical airfoil at transonic speeds

Abstract: Numerous studies have shown that viscous drag reduction of 4-8% at low speeds can be achieved in simple two-dimensional flows at wind-tunnel Reynolds numbers. Because of the encouraging benefits realized at low speeds, an evaluation of riblet effectiveness at subsonic and transonic speeds, both in wind tunnels and flight, has been reported. Realistic applications involve, among other factors, pressure gradient (eg, airfoil and wing) and three-dimensionality. Drag reductions under these conditions are being assessed currently. This paper presents recent results of drag reduction using 3M riblets on a supercritical airfoil at transonic speeds covering an angle of attack range of -0.5 to 1 deg, which is relevant to cruise conditions. (Authors)

Improvement of vehicle aerodynamics by wake control

Abstract: The rear-end shape of a car is one of the most important parts from the view point of aerodynamics. It governs the aerodynamic characteristics of the car, especially drag and rear lift. However, a rear-end shape like a spoiler often increases drag on recent low-drag cars. In this paper, the relation of rear-end shape to drag and lift is studied by consideration of the wake structure, which is obtained by wind tunnel testing and CFD. Finally, a new rear shape which reduces rear lift without increasing drag and front lift is discussed.

Study of Potential Aerodynamic Benefits From Spanwise Blowing at Wingtip

Abstract: Comprehensive experimental and analytical studies have been conducted to assess the potential aerodynamic benefits from spanwise blowing at the tip of a moderate-aspect-ratio swept wing. Previous studies on low-aspect-ratio wings indicated that blowing from the wingtip can diffuse the tip vortex and displace it outward. The diffused and displaced vortex will induce a smaller downwash at the wing, and consequently the wing will have increased lift and decreased induced drag at a given angle of attack. Results from the present investigation indicated that blowing from jets with a short chord had little effect on lift or drag, but blowing from jets with a longer chord increased lift near the tip and reduced drag at low Mach numbers. A Navier-Stokes solver with modified boundary conditions at the tip was used to extrapolate the results to a Mach number of 0.72. Calculations indicated that lift and drag increase with increasing jet momentum coefficient. Because the momentum of the jet is typically greater than the reduction in the wing drag and the increase in the wing lift due to spanwise blowing is small, spanwise blowing at the wingtip does not appear to be a practical means of improving the aerodynamic efficiency of moderate-aspect-ratio swept wings at high subsonic Mach numbers.

On the induced drag reduction due to the propeller-wing interaction

Abstract: The aerodynamic interaction between a wing and a couple of propellers in tractor configuration is investigated by means of a model based on the lifting line concept and under the assumption of quasi-steady incompressible motion of inviscid fluid. The ways the propellers influence the wing performances, particularly the induced drag, are analysed. It turns out that the lift increase and the possible drag reduction depend strongly on the direction of the blade rotation and on the propeller distances from midspan, and result from two main effects: the direct propeller induction on the wing and the modification of the lift distribution along the span. An additional nonlinear (mixed) contribution affects the drag but has only negligible influence on the lift. The described model allows one to evaluate separately all these effects and helps understanding their influence on the global modification of the wing performances.

Drag Measurements of an Axisymmetric Nacelle Mounted on a Flat Plate at Supersonic Speeds

Abstract: An experimental investigation was conducted to determine the effect of diverter wedge half-angle and nacelle lip height on the drag characteristics of an assembly consisting of a nacelle fore cowl from a typical high-speed civil transport (HSCT) and a diverter mounted on a flat plate. Data were obtained for diverter wedge half-angles of 4.0 deg, 6.0 deg, and 8.0 deg and ratios of the nacelle lip height above a flat plate to the boundary-layer thickness (h(sub n)/delta) of approximately 0.87 to 2.45. Limited drag data were also obtained on a complete nacelle/diverter configuration that included fore and aft cowls. Although the nacelle/diverter drag data were not corrected for base pressures or internal flow drag, the data are useful for comparing the relative drag of the configuration tested. The tests were conducted in the Langley Unitary Plan Wind Tunnel at Mach numbers of 1.50, 1.80, 2.10, and 2.40 and Reynolds numbers ranging from 2.00 x 10(exp 6) to 5.00 x 10(exp 6) per foot. The results of this investigation showed that the nacelle/diverter drag essentially increased linearly with increasing h(sub n)/delta except near 1.0 where the data showed a nonlinear behavior. This nonlinear behavior was probably caused by the interaction of the shock waves from the nacelle/diverter configuration with the flat-plate boundary layer. At the lowest h(sub n)/delta tested, the diverter wedge half-angle had virtually no effect on the nacelle/diverter drag. However, as h(sub n)/delta increased, the nacelle/diverter drag increased as diverter wedge half-angle increased.

Wake integration to predict wing span loading from a numerical simulation

Abstract: Introduction A N important aspect in the application of computational fluid dynamics (CFD) is the determination of the resulting forces on the geometry of interest. The typical approach to computing forces on a body consists of integrating the pressures (and possibly skin friction) over the surface. However, the grid required to obtain an accurate resolution of the forces varies drastically with the geometry and can be memory-prohibitive in some cases. The integration of pressure can also provide some difficulties when overset grids are used due to the overlapping regions of the surface grids. A new approach to computing wing span loading from a CFD result is presented in this Note. The method uses the CFD solution to create an "equivalent" lifting line model from which the lift and induced drag are computed.

Vortex creation and pinning in high amplitude third sound waves

Abstract: Changes of macroscopic circulation induced by high amplitude, rotationally polarized waves in a third sound resonator are measured. The results are compared to a model including a simple frictional vortex drag with the substrate. A significantly larger than expected concentration of vorticity is deposited in the vicinity of a central hole piercing the resonator which other modifications to the model cannot address.

Drag reduction due to riblets on a NACA 0012 airfoil at higher angles of attack

Abstract: Previous investigations of viscous drag reduction due to riblets on a NACA 0012 by Sundaram et a1 has been extended to higher angles of attack. Tests were made at a freestream velocity of 30 m/s and angle of attack (&) ranging from 8 to 12. Total airfoil-drag was determined from wake survey while pressure drag was obtained by integrating surface pressure distributions on the model. Skin friction drag was estimated from the difference between total and pressure drag. Results showed that both total and skin friction drag reduction which were found to be about 13% and 15% respectively at cC =6 , decreased rapidly with a further increase in, there was virtually no drag reduction at cC= 12:

Application of an Euler Code on a modern combat aircraft configuration

Abstract: An Euler Code is applied on a modern combat aircraft configuration to study the effect of deflections of leading edge high lift devices and control surfaces on the aerodynamic coefficients. The calculations for deflection of Leading Edge Vortex. CONtroller (LEVCON) and Elevons are done at Mach No. = 0.7, and angle of attack = 10° and for deflection of leading edge slats at Mach No. = 0.95, angle of attack = 7.5°. It is found that the incremental change in the aerodynamic coefficients — lift, induced drag, and the pitching moment due to the deflections of LEVCON, slats and elevon are predicted reasonably accurate both quantatively and qualitatively.

On the use of a wake-integral method for computational drag analysis

Abstract: In this expository paper, a practical software implementation for calculating induced drag based on awakeintegral approach is presented. The procedures required to produce satisfactory performance predictions are disclosed. The flow solver used to numerically simulate the flowfield ahout the geometries studied herein is based on a finite-volume method to solve the three-dimensional Euler equations. Interface fluxes are determined using a high-resolution Riemann scheme. The post-processing software is versatile and is capable of handling different grid topologies on the transverse plane. To test the software, an elliptic wing is analyzed and a comparison is made with lifting-line theory. Further tests are conducted with isolated rectangular wings of varying aspect ratios under different lifting conditions. Finally, the software is applied to some wing-body combinations (tip engine configurations) and a comparison is made with experimental results.

Minimum Induced Drag of a Three-Surface Ultra High Capacity Aircraft:

Abstract: The aerodynamic properties of a particular three-surface ultra high capacity airplane are determined by means of a hybrid vortex-lattice/panel method model. The study includes effects of aerodynamic and structural design limitations. It is shown that stable or slightly unstable configurations are compatible with low values of induced drag, and that the main design constraints are the maximum allowed lift coefficient of wing sections and the canard surface loading.

The Application of Photogrammetry in the Study of the Effects of Sailboard Fin Flexibility

Abstract: In recent years, the reduction of aero- or hydrodynamic drag on fins and finite wings by passive means has been a popular research topic. Among many drag reduction devices being investigated, the flexible tip is of particular interest to non-aerospace application. In the wind-surfing industry, slender fins with flexible tips have shown high performance recently. The development of sailboard fins, however, has been on a trial-and-error basis and the physics behind the improved performance has not been fully understood, although some suggest that the flexible tip helps to reduce the induced drag. At Exeter University a series of comparative studies has thus been carried out in order to help fully understand the effects of tip flexibility on performance. We present the experiments which were aimed at observing and measuring the bending deflection and twist of the fins of various degrees of reinforcement due to fluid dynamic forces. Fin deformation was measured by means of photogrammetry which is a low-cost non-interfering technique suited for steady displacement measurement.

Calculation of all drag components for 3-D bodies in wind tunnel flows. A boundary element-wake approach

Abstract: A boundary element-wake numerical approach is developed and used to determine all drag components of a three dimensional body in a wind tunnel flow. The approach decomposes the total drag into three components; the profile drag, the cross flow drag (induced drag), and the tunnel-wall effect component, each with its own physical significance. Additionally, the cross flow drag component is divided into two components, the vortex component and the source (dilatation) component. In the present approach, the transverse kinematics relations are expressed as integral representations of the axial vorticity and the transverse dilatation (source strength). This advantage permits the vortex and the source drag computations to be performed only in the vortical area of the transverse wake and hence avoids excessive computations. Also, the procedures distinguish the contribution of the transverse dilatation to the cross flow drag. The validity of the present procedure is examined by comparing the present results against the experimental data of reference [1] for a car and wing models. The comparison shows that the present computed total drag, for the wing and the car models, agrees very well with the experimental data, provided that the wake data are measured at survey planes moderately distant from the body.

A numerical analysis of steady lifting surface problems by proposal of green function considering wave generation caused by vortex system (2nd report); an evaluation on components of the drag force

Abstract: In this paper, Green function considering wave generation caused by vortex system is proposed. The Green functions of an isolated point vortex for the various image systems present an appearance of different wave-making properties. However, composed Green function of vortex ring has uniqueness for the different image system. Accordingly, Green function with image system satisfying rigid wall conditions is applied to numerical analysis of steady lifting surface problems. Then, waves are created only by transverse component of vortex system but longitudinal one doesn't create wave system.The lift forces acting on hydrofoils, for example, some rectangular wings and delta wings are calculated considering the free surface effects by means of present method. Numerical results present the free surface effects remarkably for the rectangular wings rather than delta wings. And the effects appear strongly as aspect ratio increases. Present analysis method is practically useful because the ordinary computational program for the Green function of a point doublet with x-directional axis can be applied to the present case by slight modification.