Showing papers on "Lift-induced drag published in 1978"

Induced Drag of a Bird Flock

Abstract: It is likely that birds such as geese, which migrate in horizontal V formation, save appreciable energy from the mutual aerodynamic interaction which can decrease their induced drag, an effect recently analyzed in some detail by Lissaman and Shollenberger (1970). Many species, however, migrate in large three-dimensional flocks, so the simplest aerodynamic theory has been used to estimate the change in total induced drag for both two- and three-dimensional lattices of birds, compared with the same numbers flying individually. For a large dilute flock, a novel approximation is introduced, representation of a vertical array of trailing-vortex pairs as a continuum of dipole strength. This relatively simple model shows a total drag decrease when the flock extends farther laterally than vertically. A more detailed (three-dimensional) analysis, using a horseshoe-vortex pattern to represent each bird and neglecting the disturbance due to flapping, adds the information that extension in the flight direction can be...

The Drag Related Flow Field Characteristics of Trucks and Buses

Abstract: Non-aerodynamic factors are largely responsible for the size and shape of contemporary trucks and buses. The results of wind tunnel experiments with 1/7-scale tractor-trailer and bus models are used to identify major drag producing regions of the flow fields, and to document some of the detailed characteristics. Some modifications of both the forebody and base flow fields are made in order to explore the practical potential for drag reduction. The largest drag reductions are shown to be achievable by changing the forebody flow field. By controlling flow separation from leading edges, either by modifying body contours or by employing add-on devices, apparent minimum drag limits have been identified. The possibility of even lower drag levels within existing constraints is analyzed. At the end, non-zero yaw drag characteristics are briefly discussed.

Positive Tail Loads for Minimum Induced Drag of Subsonic Aircraft

Abstract: By applying Prandtl's relation for the induced drag of a biplane to typical wing-tail combinations, it can be shown that the minimum induced drag occurs with a positive tail upload. This fact has been overlooked because the reduction in the total induced drag by a tail download was overestimated by using the total downwash of the wing on the tail, while neglecting the downwash produced on the wing by the tail. It is proved that, regardless of the relative size of the tail, the downwash produced by a tail download increases the induced drag of the wing so as to cancel the additional "tail thrust," and keep the mutually induced drag of a wing-tail combination the same as that induced upon the tail alone when it is in the wing's Trefftz-plane. At any finite tail length the bound circulation vortex of the wing produces a downwash that increases the induced drag of a tail upload. However, the circulation vortex system of the tail upload produces an upwash on the wing that results in a "wing thrust" component that cancels the increased drag on the tail so that the total induced drag is a minimum with a positive tail load. In order to facilitate the calculation of the mutually induced drag of typical wing-tail combinations, an explicit relation is derived for the limiting case of a small-span tail at any distance above or below a large-span wing.

Subsonic Vortex-Flow Design Study for Slender Wings

Abstract: A theoretical study describing the effects of spanwise camber on the lift dependent drag of slender delta wings having leading-edge vortex flow is presented. The earlier work by Barsby, using conical flow, indicated that drag levels similar to those in attached flow could be obtained. This is re-examined and then extended to the more practical case of nonconical flow by application of the vortex-lattice method coupled with the suction analogy and the recently developed Boeing free-vortex-sheet method. Lastly, a design code is introduced which employs the suction analogy in an attempt to define "optimum" camber surfaces for minimum lift dependent drag for vortex flow conditions.

Minimum Trimmed Drag and Optimum c.g. Position

Abstract: An analytical expression is developed which shows that wing/tail interference drag is determined by wing downwash at downstream infinity. With the use of this expression, relations for minimum trimmed drag and optimum e.g. position are presented in explicit form. From this it follows that minimum induced drag is less for the combination of wing-plus-tail than for the wing alone. It is shown that this is true even for the case when the optimum tail load is a download rather than an upload. Furthermore, it is shown which are the factors that have a decisive effect on optimum e.g. position.

An aerodynamic analysis of several hypersonic research airplane concepts from M = 0.2 to 6.0

Abstract: Several conceptual hypersonic research airplanes, designed within the constraints of a B-52 launch aircraft, have been studied experimentally and analytically at Mach numbers from 0.2 to 6.0. Vehicles built to these criteria for Mach 6 cruise were shown to be feasible, if careful attention was paid to the low speed lift, drag, and high angle of attack stability to assure successful landings and transonic pitch angle maneuvers. The integrated scramjet engine drag was high at subsonic speeds and appears to be constant with Reynolds number. The variable geometry airfoil used previously to improve directional stability was shown to be equally adaptable to the improvement of longitudinal stability. The vortex lattice theory gave good subsonic predictions of lift, drag due to lift, and pitching moments. Wind tunnel tests must be relied on for the drag at zero lift, trim, static margins and lateral-directional stability. The Gentry Hypersonic Arbitrary Body Program gave good predictions of the trends of lift, drag, and pitching moments with angle of attack at Mach numbers above 3, but the level of the values were not consistently predicted. No currently available theory or program gave accurate predictions of directional stability or dihedral effects at hypersonic speeds.

Design and analytical study of a rotor airfoil

Abstract: An airfoil section for use on helicopter rotor blades was defined and analyzed by means of potential flow/boundary layer interaction and viscous transonic flow methods to meet as closely as possible a set of advanced airfoil design objectives. The design efforts showed that the first priority objectives, including selected low speed pitching moment, maximum lift and drag divergence requirements can be met, though marginally. The maximum lift requirement at M = 0.5 and most of the profile drag objectives cannot be met without some compromise of at least one of the higher order priorities.

Some contributions to aerodynamic theory for vertical-axis wind turbines

Abstract: A review and modest extensions are described of quasisteady aerodynamic theory for performance prediction on Darrieus-type turbines. Results are given for both parallel-axis and curved-blade configurations. Blade stall and variable inflow are neglected; it is hypothesized that unsteady effects support the former approximation down to lower values of tip-speed ratio than hitherto believed. Both profile and induced drag are included, and their influences on power and downwind force are expressed in terms of elliptic integrals. Comparisons are presented with power data from the Sandia 2-m turbine. Three values of profile drag coefficient are employed, and it is argued that numbers in the range CD() =0.015-0.017 are most appropriate to the example chosen. Finally, a linearized analysis of unsteady flow effects on performance is summarized. Calculations suggest that they may be larger than might be expected in view of the low operating reduced frequencies of these machines.

An investigation of drag reduction for tractor trailer vehicles

Abstract: Force and moment data were obtained from a one-twenty-fifth scale wind tunnel model of a cab-over-engine tractor trailer combination. The tests define the aerodynamic characteristics of the baseline (unmodified) vehicle and several modified configurations. The primary modifications consist of: (1) greatly increased forebody corner radii, (2) a smooth fairing over the cab-to-trailer gap, (3) a smoothed underbody, and (4) rear streamlining (boattailing)of the trailer. Tests were conducted for yaw angles from 0 deg to 30 deg. The reduction in drag, relative to the baseline, obtained by combining the modifications are compared for the zero yaw condition with full scale coast down drag results for similar configurations. The drag reductions obtained from the model and full scale tests are in good agreement.

Closed-form equations for the lift, drag, and pitching-moment coefficients of airfoil sections in subsonic flow

Abstract: Closed-form equations for the lift, drag, and pitching moment coefficients of two dimensional airfoil sections in steady subsonic flow were obtained from published theoretical and experimental results. A turbulent boundary layer was assumed to exist on the airfoil surfaces. The effects of section angle of attack, Mach number, Reynolds number, and the specific airfoil type were considered. The equations were applicable through an angle of attack range of -180 deg to +180 deg; however, above about + or - 20 deg, the section characteristics were assumed to be functions only of angle of attack. A computer program is presented which evaluates the equations for a range of Mach numbers and angles of attack. Calculated results for the NACA 23012 airfoil section were compared with experimental data.

A Wind-Tunnel Study of the Effect of Turning Vanes on the Aerodynamic Drag of Tractor-Trailer Trucks

Abstract: A wind-tunnel study was made of the reduction of the aerodynamic drag of tractor-trailer trucks due to turning vanes used to control flow separation. A variety of vane settings and positions were investigated on the tractor and on the trailer. It was found that substantial drag reductions were obtained by employing turning vanes on the lower part of the front vertical edges of the tractor. Vanes mounted elsewhere had a smaller effect on the aerodynamic drag. Vane setting, turning angle and edge radius were found to be highly important.

Numerical Modeling of Blunt-Body Flows—Problems and Prospects

Abstract: The problems associated with numerical modeling of blunt-body flows are discussed. An efficient modeling technique should ideally incorporate the interactions between the turbulent boundary layer near the body, the unsteady, highly vortical wake flow behind the body, and the potential-flow regions outside these. The incomplete understanding of vortical unsteady flow fields, in particular, turbulent boundary layers and their separation behavior, will for the foreseeable future preclude accurate modeling; but even coarse modeling methods could serve an important role in establishing cause-and-effect relationships. In particular, one should aim at finding methods which can be used to predict, at least qualitatively, the effect of small local body changes on local flow patterns and on the overall drag. A case is made for flow-field calculation methods based on the vorticity equations. Such methods have proved successful in aeronautical and meteorological applications. The overall drag and lift can be calculated in terms of the vorticity shed into the wake; in particular, the vortex drag associated with longitudinal vortices due to aerodynamic lift can be analyzed.

Comment on "Ideal Tail Load for Minimum Aircraft Drag"

Abstract: P E. V. Laitone's Engineering Note on ''Ideal Tail Load for Minimum Aircraft Drag," [J. Aircraft 15, 190-192 (1978)] is interesting in its use of the basic biplane equation and Munk's stagger theorem for estimating trim drag. However, there are errors in the conclusions and some of the basic assumptions of the note. First, in modern high-speed transport aircraft the contribution of compressibility drag to the total trim drag problem is very significant. The download on the tail increases the total lift that the wing must carry and if the airplane is operating near the Mach number for drag divergence the required increase of wing angle of attack will cause an increase in compressibility drag. This compressibility contribution is of the order of half of the total trim drag. On the other hand, when compressibility drag is not present the interference term in the biplane equation introduces a negative drag term which compensates to a large degree for the obvious induced drag penalties on the tail due to its download and on the wing due to the greater wing lift. Second, even with compressibility drag, trim drag is generally much less than the 5% mentioned by Professor Laitone. Figure 1 shows the variation of trim drag with center-of-gravity position for various lift coefficients and Mach numbers for a DC-8-54 aircraft. Typically, the DC-8 flies at a CL of about .35. At the original design cruise Mach number of 0.82 and at an average center-of-gravity position of about 26% of the mean aerodynamic chord, the trim drag is 2.2% of the total drag. With some consideration of aft loading cargo the average center of gravity might be moved to about 29%. The trim drag will then be 1.6%. Since the dramatic rise in fuel prices, cruise Mach number has been reduced to 0.8 to reduce compressibility drag. The trim drag penalties are then 1.4% at 26% center-of-gravity position and 0.9% at 29% e.g. position. These representative numbers are well below 5%. Only at high CL and Mach number and at far forward center-of-gravity positions can trim drag approach such high values. Third, Professor Laitone concludes that large trim drag savings could occur if only transport aircraft were designed with larger tails. An aircraft design is a matter of complete integration and a savings in trim drag would have to be weighed against the weight and parasite drag penalty of the larger tail. If we accept the possibility of a 1% decrease in induced drag from the zero tail load case with tail upload this corresponds to something like a 0.4% reduction in total drag since induced drag is approximately 0.4 of the total drag in cruise. Then the total trim drag gain from current practice is of the order of 2.0% of total drag. To gain this 2.0% one would have to move the center of gravity well aft and to do that would require significant increases in the horizontal tail size. Since the horizontal tail contributes about 8% of the total parasite drag or about 4.8% of the total drag it can be seen that a significant increase in this tail size would quickly

A Study of the Base Drag of Tractor-Trailer Trucks

Abstract: Measurements were made of the base pressure distribution and the aerodynamic drag of a variety of 1/8th-scale tractor-trailer truck models in a wind tunnel at yaw angles ranging from 0/sup 0/ to 20/sup 0/. Base-drag coefficients and overall aerodynamic-drag coefficients were calculated from this data. The measurements show that the base-drag coefficient of typical tractor-trailer trucks does not vary much with vehicle configuration, and that base drag constitutes approximately 13 to 15% of the total aerodynamic drag at zero yaw. The base drag increases in magnitude and also becomes a larger part of the overall aerodynamic drag as yaw angle increases, reaching about 18 to 25% of the overall drag at 20/sup 0/ yaw. Streamlining the forebody of the vehicle has little effect on the base-drag coefficient, but increases the fraction of the overall aerodynamic drag due to the base.

The Effect of Winglets on the KC-135A Aircraft

Abstract: : To investigate the effects of winglets on the aerodynamic characteristics of the KC-135 aircraft, semispan and full-span wind tunnel models with winglets have been investigated in the NASA/LRC 8-Foot Transonic Pressure Tunnel. At cruise conditions, the full-span tests indicated a total drag reduction of 5.3 percent for the model with the Boeing and NASA upper plus lower winglet configurations and 6.5 percent for the model with the NASA upper winglet configuration. A wing-tip-extension configuration tested on the semispan model had a drag reduction of about 3 percent compared to about 5 to 7 percent for the winglet configurations, however, the tip extension was not optimized for drag reduction. At cruise conditions, the wing tip extension produced the greatest increase in wing root bending moment and the upper winglets the least. The increase in wing root bending moment at cruise conditions varied from about 2.5 percent with the Boeing winglets, to about 3.5 percent with the tip extension. At cruise flight conditions, winglets on the KC-135A aircraft were estimated to reduce the drag about 8.2 percent and increase the maximum lift- drag ratio about 9.5 percent.

Deflected Thrust Effects on a Close-Coupled Canard Configuration

Abstract: The effects of deflected thrust on the stability and performance of a close-coupled canard fighter configuration are presented. These results were obtained at low speeds in the Langley V/STOL tunnel. Transonic as well as low-speed results are also presented for an unpowered close-coupled canard and a supercruiser configuration. The V/STOL tunnel data indicate an increase in maximum lift and reductions in drag due to lift with the addition of two-dimension al vectored thrust at the wing inboard trailing edge. The longitudinal pitchup associated with the unpowered configuration at higher angles of attack was significantly reduced with power.

The aerodynamic design of sailplane tail assemblies

Abstract: The aerodynamic design of a sailplane is dominated by its high aspect ratio unswept wing, which by itself is inherently unstable and uncontrollable in pitch and yaw. A tail assembly of minimum drag and weight must be provided which will permit the pilot to regulate the wing angles of attack and sideslip in normal flight, to overcome towline moments during launch, and to insure recovery from spins. Aerodynanic constraints upon stability and control are examined to determine minimum control surface size, and a simple method, based on biplane theory, is presented for estimation of the vortex drag penalty for tallplane trinming loads. It is concluded that small drag savings can be made with modest weight penalties by adopting longer tail moment arms and higher control surface aspect ratios.

Ultrasonic wing for aircraft

Abstract: PURPOSE:To prevent the lift of an aircraft from lowering and the induced drag of the aircraft from increasing by sharpening the leading edge line of the airfoil at the wind root extended forward from a swept-back wing and droping it to thereby eliminate occurrence of early vortex at the upper leading edge of the wing to thus stabilize the generated vortex.

The Influence of Aerodynamic Interference on High Angle of Attack Wind Tunnel Testing.

Abstract: : Results from an experimental investigation of strut support interference on high angle of attack aerodynamic measurements are presented. The influence of the strut support on the leeward wake structure was investigated by means of a two-dimensional experiment of a cylinder-splitter plate combination. Pressure distributions, pressure drag coefficient and wake flow visualization data for various cylinder-splitter plate combinations are presented for high subcritical Reynolds numbers. The influence of plate position and size on the pressure drag coefficient were also examined. The results show the splitter plate can alter the vortex wake formation significantly and, as a consequence, reduce the pressure drag coefficient by as much as 30% or more. Plate sizes of 0.5, 1.1 and 1.5 diameter were tested with the 1.1 diameter plate yielding the largest drag reduction. (Author)

Analytic investigation of advancing blade drag reduction by tip modifications

Abstract: Analytic techniques were applied to study the effect on the performance of the nonlifting advancing blade when the outboard 5% of the blade is modified to reduce drag. The tip modifications studied consisted of reducing airfoil thickness, sweepback, and planform taper. The reductions in instantaneous drag and torque were calculated for tip speed ratios from about 0.19 to 0.30, corresponding to advancing blade tip Mach numbers of 0.855 to 0.936, respectively. Approximations required in the analysis introduce uncertainties into the computed absolute values of drag and torque; however, the differences in the quantities should be a fairly reliable measure of the effect of changing tip geometry. For example, at the highest tip speed, instantaneous drag, and torque were reduced by 20% and 24%, respectively, for tip sweep of 40 deg on a blade using an NACA 0010 airfoil and by comparable amounts for 30-deg sweep on a blade having an NACA 0012 airfoil section. The present method should prove to be a useful, inexpensive technique for identifying promising configurations for additional study and testing.

Aerodynamic Accounting Technique for Determining Effects of Nuclear Damage to Aircraft. Volume 1, Empirical Methods

Abstract: : This volume presents the methods, equations and substantiating data for an empirically based computer program which calculates lift, moment, and drag characteristics of an undamaged baseline aircraft in addition to calculating the aerodynamics of an aircraft which has sustained nuclear damage. The input requires a simple definition of the basic undamaged configuration geometry along with parameters specifying the mode of damage, magnitude, and dimension of the damaged area. Up to fourteen different modes of damage can be specified which can model most types of damage. The computer code has the capability of assessing the aerodynamics effects of damage, such as rough, bent, and burnt skins, boundary-layer thickness effects, and loss of radomes, panels, doors and covers. Also, the computer code has the capability of analyzing changes in drag due to lift and trim caused by asymmetric loss of parts of the wing or trim surfaces. The accuracy of the program is verified through comparisons of the predicted results with experimental data for several configurations.

A note on lift and drag effects due to polymer injections from several locations on a symmetric hydrofoil

Abstract: It is well known that the injection of dilute polymer solutions into the boundary layer on two-dimensional hydrofoils produces changes in both the drag and lift of the foils, with the changes being dependent on the polymer, its concentration , the injection technique, the location of the injection slit, and the rate of injection. Previous tests have also shown that the observed drag reduction does not necessarily increase linearly with the rate of injection of the polymer. Indeed, beyond a certain injection rate, further increases lead to little or no drag reduction and, in some cases, actually lead to a drag increase. Thus, one question that is of considerable practical interest is the effect of multiple injections of a given amount of polymer from several chordwise locations as compared with that due to the injection from a single location. The present study investigates the effects on lift and drag of a 20.16-cm (8 in.) chord NACA 63 sub 4-010 two dimensional hydrofoil due to multiple injections of 200-ppm Polyox WSR 301 solution from several chordwise locations. The results indicate that, for a given flux of polymer injection, multiple injections from properly selected locations as compared with the injection from a single location result in a larger drag reduction without adversely affecting the foil lift.

Three-dimensional canard-wing shape optimization in aircraft cruise and maneuver environments

Abstract: This paper demonstrates a numerical technique for canard-wing shape optimization at two operating conditions. For purposes of simplicity, a mean surface wing paneling code is employed for the aerodynamic calculations. The optimization procedures are based on the method of feasible directions. The shape functions for describing the thickness, camber, and twist are based on polynomial representations. The primary design requirements imposed restrictions on the canard and wing volumes and on the lift coefficients at the operating conditions. Results indicate that significant improvements in minimum drag and lift-to-drag ratio are possible with reasonable aircraft geometries. Calculations were done for supersonic speeds with Mach numbers ranging from 1 to 6. Planforms were mainly of a delta shape with aspect ratio of 1.

Model support system interference on zero-lift drag at transonic speeds

Abstract: In this paper the support system interference on the zero-lift drag of an axisymmetric and an aircraft type models is discussed. Two different techniques were adopted for the two models tested to evaluate the support sting interference. It is found from these tests that the presence of a rear sting support would result in a reduction in the zero-lift drag of as much as 20 to 50 percent of the true value. This apparent reduction in drag is found to be a strong function of the free stream Mach number close to unity. Detailed pressure measurements over the aft-body of the axisymmetric model suggests that due to the positive pressure field imposed by the sting over the boat-tail region of the model the free stream Mach number at which the shocks appear in the boat-tail region will be higher when the sting is present than that without it. This will result in an increased drag divergence Mach number for the model in the presence of the sting. It is argued that because of this reason the sting effect on zero-lift drag strongly depends on the Mach number close to unity.

Drag force knotmeter

Abstract: A drag force knotmeter for indicating the speed of a boat in water, especially in the range of 1-12 knots is a device attached to the boat by a flexible line and trails an elastomeric line attached to a drag. The drag is a rigid rod having an eye on one end and a circular disk on the other end. Said circular disk being attached to the rod by its center. The drag is heavier-than-water and is attached to the elastomeric line through its eye. System friction is minimized by maintaining an overall straight line configuration from the drag to the point of attachment to the boat. The elastomeric line isolates sudden surges on the drag from the indicator on the knotmeter.

Minimum induced drag configurations with jet interaction

Abstract: A theoretical method is presented for determining the optimum camber shape and twist distribution for the minimum induced drag in the wing-alone case without prescribing the span loading shape. The same method was applied to find the corresponding minimum induced drag configuration with the upper-surface-blowing jet. Lan's quasi-vortex-lattice method and his wing-jet interaction theory was used. Comparison of the predicted results with another theoretical method shows good agreement for configurations without the flowing jet. More applicable experimental data with blowing jets are needed to establish the accuracy of the theory.

Decrease of drag in the case of detached flow past a body with a jet cavitator

Abstract: It was shown in [4, 5] on the basis of a qualitative analysis that a flow past a body with detachment of the jets according to Kirchhoff's scheme can be formed by means of a counterjet with half as much drag as in the case of detached flow past a body with a solid cavitator. However, such a result is the theoretical limit and is attainable only on removing the interior stagnation point of the interacting flows to infinity relative to the edge of the duct through which injection into the counterflow is accomplished. We will examine the range of possible variation of the drag force of a body with a jet cavitator for a plane flow of an ideal fluid.