Showing papers on "Lift-induced drag published in 1987"

The lateral force on a spinning sphere: Aerodynamics of a curveball

Abstract: The lateral force on a spinning baseball in a wind tunnel has been measured. The magnitude of the force is nearly independent of the orientation of the seams of the ball. The drag coefficient appears to be at most weakly dependent on Reynolds number and to be principally a function of the ratio of the rotational speed of the equator of the ball to the wind tunnel speed. This is to be compared to the work of Briggs, which implies a strong effect of Reynolds number on the drag coefficient.

Gliding Birds: The Effect of Variable Wing Span

Abstract: SUMMARY 1. The equilibrium gliding performance of a bird is described by the relationship between sinking speed (VB) and air speed (V). When V3 is plotted against V, the points fall in a 'performance area' because the wing span is changed during gliding. 2. The lowest V, for each V in the performance area defines a 'maximum performance curve'. This curve can be predicted by a mathematical model that changes the wing span, area and profile drag coefficient (Crj,rr) of a hypothetical bird to minimize drag. The model can be evaluated for a particular species given (a) a linear function relating wing area to wing span, and (b) a 'polar curve' that relates C Dp r and the lift coefficient (CL) of the wings. 3. For rigid wings, a single polar curve relates C Dp r to CL values at a given Reynolds number. The position and shape of the polar curve depend on the aerofoil section of the wing and the Reynolds number. In contrast, the adjustable wings of a laggar falcon (Falco jugger) and a black vulture (Coragyps atratus) gliding in a wind tunnel have CL, and CD pr values that fall in a 'polar area' rather than on a curve. The minimum values of CD pr at each CL bound the polar area and define a polar curve that is suitable for evaluating the model. 4. Although the falcon and the vulture have wings that are markedly different in appearance, the data for either bird are enclosed by the same polar area, and fitted by the same polar curve for minimum Co,pr at each CL value. This curve is a composite of the polar curves for rigid wings with aerofoils similar to those found in avian wings. These observations suggest that the polar curves of other gliding birds may be similar to that of the falcon and the vulture. 5. Other polar curves are defined by CL and CD pr values for the falcon and the vulture gliding at a constant speed but at different glide angles. Each speed has a different polar curve; but for a given speed, the same polar curve fits the data for either bird. 6. The falcon and the vulture gliding in the wind tunnel at a given speed were found to increase their drag by decreasing their wing span. This change increases induced drag and probably increases CD pr for the inner parts of the wing because of an unusual property of bird-like aerofoil sections: wings with such sections have minimum values of C Dp r at CL values near 1, while conventional wings have minimum values of CotPr at CL values near 0.

Induced-drag characteristics of crescent-moon-shaped wings

Abstract: Results are presented of a theoretical study conducted to analyze the potential drag reduction characteristics of wings with highly tapered aft-swept tips at subsonic speeds. These planar wings, also named crescent- moon-shaped wings or wings with curved planform, can produce induced drag less than the minimal value ob- tained with the classical unswept elliptical wing for a constant lift and wing span. The induced-drag reduction is the result of the nonplanar wing and wake shape at the angle of attack. For an example wing of aspect ratio 7, the crescent-moon shape provides a reduction in cruise induced drag of 8.0% as compared to an unswept elliptical wing. equal to but not less than the minimum value obtained for the classical straight wing with elliptical loading distribution can be achieved with planar curved planforms. In order to obtain the mimimum value of induced drag for the wing with backward sweep, it may be required to introduce a substantial amount of washout, while for the wing with for- ward sweep, it may be necessary to incorporate washin. The twist requirement has a severe disadvantage, however, because now the minimum induced drag is obtained only at the angle of attack for which the wing twist distribution is optimized. At off-design conditions, drag penalties are in- curred and the induced drag produced by the twisted swept wing will be higher than for the untwisted unswept elliptical wing for a constant lift coefficient. This paper will demonstrate that an untwisted planar crescent-moon wing can be more efficient than an unswept elliptical wing. A computa- tion method that correctly accounts for the influence of the trailing wake must be applied in order to obtain this somewhat unexpected result. Zimmer3'4 demonstrated that crescent-moon planform shapes can significantly reduce the level of induced drag pro- duced by a wing. The results are obtained with a vortex- lattice method that models the lifting surface by its mean camber line and takes into account the nonlinear effect of the trailing vortex sheet. Consequently, the influence of air- foil thickness is assumed to be negligible and the leading- edge suction force is modeled mathematically. Zimmer's work resulted in the Dornier New Technology Wing (Dornier Do-228), which represents a practical (and somewhat com- promised) application of the crescent-moon planform shape. In this study, a nonlinear surface panel method is applied to study the drag characteristics of a crescent-moon-shaped lifting surface of aspect ratio 7. Consequently, no approxima- tions are required to calculate the induced-drag force. Analysis Method

Aerodynamic characteristics of biplanes with winglets

Abstract: This paper examines, both theoretically and experimentally, the possibility of improving the aerodynamic characteristics of a biplane configuration by adding winglets. Theoretical calculations show good agreement with experiment in predicting inviscid drag due to lift. Theoretical and experimental results indicate that the addition of winglets to an optimized biplane configuration can increase the ideal efficiency factor by up to 13%, as well as increasing the lift-curve slope and maximum lift coefficient. A theoretical analysis comparing the biplane with an optimized winglet to an equivalent monoplane indicates that the biplane has the potential for a 6.4% increase in L/Dmsa[ and a 13% increase in CL/CD, the classical endurance parameter.

Evaluation of installed performance of a wing-tip-mounted pusher turboprop on a semispan wing

Abstract: An exploratory investigation has been conducted at the Langley Research Center to determine the effect of a wing-tip-mounted pusher turboprop on the aerodynamic characteristics of a semispan wing. Tests were conducted on a semispan model with an upswept, untapered wing and an airdriven motor that powered an SR-2 high-speed propeller located on the tip of the wing as a pusher propeller. All tests were conducted at a Mach number of 0.70 over an angle-of-attack range from approximately -2 to 4 deg at a Reynolds number of 3.82 x 10 to the 6th based on the wing reference chord of 13 in. The data indicate that, as a result of locating the propeller behind the wing trailing edge at the wing tip in the crossflow of the wing-tip vortex, it is possible to improve propeller performance and simultaneously reduce the lift-induced drag.

Wind tunnel evaluation of a truncated NACA 64-621 airfoil for wind turbine applications

Abstract: An experimental program to measure the aerodynamic performance of a NACA 64-621 airfoil with a truncated trailing edge for wind turbine applications has been conducted in the Ohio State University Aeronautical and Astronautical Research Laboratory 6 in. by 21 in. pressurized wind tunnel. The blunted or trailing edge truncated (TET) airfoil has an advantage over similar trailing edge airfoils because it is able to streamline a larger spar structure, while also providing aerodynamic properties that are quite good. Surface pressures were measured and integrated to determine the lift, pressure drag, and moment coefficients over angles of attack ranging from -14 to +90 deg at Mach 0.2 and Reynolds numbers of 1,000,000 and 600,000. Results are compared to the NACA 0025, 0030, and 0035 thick airfoils with sharp trailing edges. Comparison shows that the 30 percent thick NACA 64-621-TET airfoil has higher maximum lift, higher lift curve slope, lower drag at higher lift coefficients, and higher chordwise force coefficient than similar thick airfoils with sharp trailing edges.

The Aerodynamic Optimization of Wings at Subsonic Speeds and the Influence of Wingtip Design

Abstract: Some of the objectives of modern aircraft development are related to the achievement of reduced fuel consumption and aircraft noise. This investigation is mainly concerned with the aerodynamic aspects of aircraft development, i.e., reduction of induced drag. New studies of wing design, and in particular wing tips, are considered. Induced drag is important since, in cruising flight, it accounts for approximately one-third of the entire drag for the aircraft, and one-half while climbing. A survey is presented for the wing geometries and wing tip designs studied, and theoretical investigations of different planar wings with systematically varied wing tip forms are conducted. Attention is also paid to a theoretical study of some planar and nonplanar wings and their comparison with experimental data.

High drag airfoil apparatus

Abstract: Disclosed are airfoil structures which provide two distinct mean centers of lift and high induced drag. Also disclosed are aircraft structures employing such high-lift high-drag airfoils for both the lifting wing and the horizontal stabilizer with the center of gravity disposed between the mean centers of lift of the fore and aft airfoils.

Helicopter blade airfoil

Abstract: A family of airfoil cross sections, termed SC21xx, for use in a helicopter blade is disclosed. The airfoil (36) achieves maximum lift performance equivalent to prior art airfoil configurations without incurring increased aerodynamic drag at high velocities. The airfoil (36) was developed by thickening and drooping the leading edge region (38) of the prior art airfoil (30) improving lift in the leading edge region (38) and delaying the formation of sonic shock waves at high velocities.

Drag Reduction on a Bluff Body at Yaw Angles to 30 Degrees

Abstract: The base separation alleviation and drag reduction effectiveness of transverse rectangular grooves and longitudinal v-grooves in the afterbody shoulder region of a bluff body is investigated for body yaw angles of 0-30 deg. The grooves are found to be beneficial in reducing both freestream and axial drag coefficients at yaw angles of up to 25 deg.

Winglets on low-aspect-ratio wings

Abstract: The drag reduction potentially available from the use of winglets at the tips of low aspect ratio (1.75-2.67) wings with pronounced (45-60 deg) leading edge sweep is assessed numerically for the case of a cruise design point at Mach of 0.8 and a lift coefficient of 0.3. Both wing-winglet and wing-alone design geometries are derived from a linear-theory, minimum induced drag design methodology. Relative performance is evaluated with a nonlinear extended small disturbance potential flow analysis code. Predicted lift coefficient/pressure drag coefficient increases at equal lift for the wing-winglet configurations over the wing-alone planform are of the order of 14.6-15.8, when boundary layer interaction is included.

Analysis of a Delta Wing with Leading-Edge Flaps

Abstract: THE effect of a leading-edge flap on the aerodynamic characteristics of a low-aspect-ratio delta wing has been studied using vortex-feeding-sheets singularity systems to represent the separated flow. The analysis was performed in the crossflow plane using the Schwarz-Christoffel transformation. Particular attention was paid to the influence of flap deflection on lift and drag. It was found that both lift and drag decrease during flap deflection, while the lift-to-drag ratio increases. The main reason for lift reduction is the partial suppression of the vortex system from the leading edge as a result of flap deflection, while drag reduction originates in a propulsive component of the pressure force acting on the deflected flaps.

Influence of geometry of rear part on the aerodynamic drag and wake structure of a vehicle

Abstract: The substantial part of drag on an automobile is due to pressure. Therefore, a car must be designed which produces minimum pressure drag. The paper describes the effects of geometrical configuration of the rear part of a car on aerodynamic drag. Experiments were made on 1/5 scale models of fastback and notchback design. For a fastback car, the drag depends heavily on the angle of the rear window. At a certain critical angle, the drag shows a sharp peak. This peak drag can be reduced drastically by the tapering of plan form of the rear geometry. For the notchback design, one combination of the angle of the rear window and height of the trunk deck shows similar maximum drag. Methods of avoiding this large drag were found. The experiment was extended to the measurement of the wake structure by hot wire anemometers and total pressure tubes. The correlation between the wake structure and drag was clarified by the consideration of vorticity and circulation (a).

Reduction of Hub and Pylon Fairing Drag

Abstract: A wind tunnel test was conducted to obtain data on several drag reduction methods for rotorcraft hubs. The objective of the test was to use small-scale models to develop the technology to substantially reduce hub drag. A helicopter test model, which did not incorporate a rotor, was used to study single-rotor fairing configurations. The rotor shaft assembly was modeled with nonrotating hardware. Drag trend data was obtained for the following fairing configuration changes: hub-fairing camber, hub-fairing thickness ratio, hub-fairing surface curvature, hub-fairing height with respect to the fuselage, inclusion of blade shanks in the hub fairings, hub- and pylon-fairing gaps, pylon-fairing cross-sectional geometry, pylon-fairing thickness ratio and camber. Substantial drag reductions could potentially result from application of the test results to helicopter hub- and pylon-fairing design.

An Examination of Controlled Vortex Drag Using Stepped Afterbodies from M = 0.5 to 3.0

Abstract: : A comparison of the zero lift drag coefficients of a stepped base projectile to flat base and truncated boattail base projectiles is presented. Three model configurations were investigated during the test program. These included an experimental 20mm round with a 7 1/2 deg., truncated boattail base, a round modified with a flat base, and a round modified with a stepped base. All of the projectiles were tested at sea level conditions in an indoor ballistic free-flight facility. This paper discusses the aerodynamic experiment and the data obtained. Results show that the zero lift drag coefficient of the stepped base projectile was less than that of the flat base round for the subsonic Mach number range and approximately the same for the transonic and supersonic ranges. However, the stepped base projectile produced zero lift drag greater than that of the boattail round at each Mach number. Keywords: Base drag; Separated flow; projectile afterbody; Ballistic testing.

Hydro-numeric design of winglet keels for stars & stripes

Abstract: A new computational method is presented for the design of optimal keels for racing yachts. The method was used in the design of the 1987 America's Cup winner STARS & STRIPES. Lift and induced drag are computed with an extended version of the VSAERO panel method program, employing a far-field wake integration technique. Parametric studies of lift and induced drag performance for variations of keel and winglet geometry are presented. The optimal configuration for the expected racing conditions was chosen through a sailing performance code which accepted as input the parameterised lift and induced drag relationships. See next Abstract.

Pressure field due to drag reducing outer layer devices in turbulent boundary layers

Abstract: The wall static pressure in the vicinity of drag reducing outer layer devices in flat wall turbulent boundary layers has been measured and compared with an inviscid theory. Symmetric and cambered airfoil devices have been examined at small angles of attack and very low chord Reynolds numbers. Airfoil devices impose a sequence of strong favorable and adverse pressure gradients on the boundary layer whose drag is to be reduced. At very small angles of attack (± 2°), this pressure field extends up to about three chord lengths downstream of the trailing edge of an airfoil device. Also examined are the pressures on the upper and lower surfaces of a symmetric airfoil device in the freestream and near the wall. The freestream pressure distribution around an airfoil section is altered by the wall proximity. The relevance of lift enhancement caused by wall proximity to drag reduction has been discussed. The pressure distributions on the flat wall beneath the symmetric airfoil devices are predicted well by the inviscid theory. However, the remaining pressure distributions are predicted only qualitatively, presumably because of strong viscous effects.

Comment on "Numerical Optimization Design of Advanced Transonic Wing Configurations"

Abstract: H read Ref. 1 with great interest, I was surprised to find apparent signs of serious inaccuracy in the tables of numerical results. This can be seen from Tables 1 and 2, in which the numbers are identical with those of the paper, except for the additional lines Ci/wA and CD/(C%/TrA)< From Tables 1 and 2, it will be seen that for both wings, even before optimization, the quoted values of the inviscid drag coefficient CD are well below the ideal minimum induced drag values for the appropriate values of CL and aspect ratio A. After optimization, the improved values are only about half the theoretical minimum. Assuming that these apparent errors are not just the result of some numerical slip, this does cast doubt on the accuracy of the authors' method of calculating their "objective function" (L/D) by integrating the appropriate component of pressure over the wing surface. Earlier in the paper (p. 194, top of second column), they state correctly that "for efficient operation in the transonic regime the wave drag must be minimized ..." In my opinion it is, therefore, better to use a method of drag prediction that deals separately with the wave drag and vortex ("induced") drag components (and, in a real flow, the viscous drag). For this purpose, a method such as those suggested in Refs. 2 or 3 could be used. In mentioning these doubts about the numerical accuracy of the published results, I do not, of course, intend to detract from the undoubted value of the authors' method as a design tool. It is worth stressing, however, the importance of checking the validity of any purely inviscid method Such as this by subsequent calculations by a suitable "viscous" code, to make sure that any reductions in inviscid drag have not been outweighed by corresponding increases in viscous drag.

Reply by Authors to R. C. Lock

Abstract: H read Ref. 1 with great interest, I was surprised to find apparent signs of serious inaccuracy in the tables of numerical results. This can be seen from Tables 1 and 2, in which the numbers are identical with those of the paper, except for the additional lines Ci/wA and CD/(C%/TrA)< From Tables 1 and 2, it will be seen that for both wings, even before optimization, the quoted values of the inviscid drag coefficient CD are well below the ideal minimum induced drag values for the appropriate values of CL and aspect ratio A. After optimization, the improved values are only about half the theoretical minimum. Assuming that these apparent errors are not just the result of some numerical slip, this does cast doubt on the accuracy of the authors' method of calculating their "objective function" (L/D) by integrating the appropriate component of pressure over the wing surface. Earlier in the paper (p. 194, top of second column), they state correctly that "for efficient operation in the transonic regime the wave drag must be minimized ..." In my opinion it is, therefore, better to use a method of drag prediction that deals separately with the wave drag and vortex ("induced") drag components (and, in a real flow, the viscous drag). For this purpose, a method such as those suggested in Refs. 2 or 3 could be used. In mentioning these doubts about the numerical accuracy of the published results, I do not, of course, intend to detract from the undoubted value of the authors' method as a design tool. It is worth stressing, however, the importance of checking the validity of any purely inviscid method Such as this by subsequent calculations by a suitable "viscous" code, to make sure that any reductions in inviscid drag have not been outweighed by corresponding increases in viscous drag.

VSAERO analysis of structurally decoupled tip planforms for a semispan wing

Abstract: A numerical analysis has been conducted with the three-dimensional panel code VSAERO for two interacting lifting surfaces that are separated in the spanwise direction by a narrow gap, with the angle of attack of the outboard section being set independently of the inboard section, as in the 'free tip' rotor blade system proposed for helicopters. Computed values of tip surface lift and pitching moment coefficients are correlated with experimental data to determine the most suitable method for modeling the gap region between the surfaces. It is shown that the induced drag of the tip surface is reduced for negative incidence angles relative to the inboard section.

The effect of heavy rain on an airfoil at high lift

Abstract: No serious studies of the relationship of heavy rain to aircraft safety were made until 1981 when it was suggested that the torrential rain which often occurs at the time of severe wind shear might substantially increase the danger to aircraft operating at slow speeds and high lift in the vicinity of airports. While these data were not published until early 1983, appropriate measures were taken by NASA to study the effect of heavy rain on the lift of wings typical of commercial aircraft. One of the aspects of these tests that seemed confirmed by the data was the existence of a velocity effect on the lift data. The data seemed to indicate that when all the normal non-dimensional aerodynamic parameters were used to sort out the data, the effect of velocity was not accounted for, as it usually is, by the effect of dynamic pressure. Indeed, the measured lift coefficients at high lift indicated a dropoff in lift coefficient for the same free-stream water content as velocity was increased. indicated a drop-off in lift coefficient for the same free-stream water content as velocity was increased.

Summary of studies to reduce wing-mounted propfan installation drag on an M = 0.8 transport

Abstract: Powerplant installation losses for an advanced, high-speed, turboprop transport have been investigated in the Ames Research Center Transonic Wind Tunnels as a part of the NASA Advanced Turboprop Program (ATP) Force and pressure tests have been completed at Mach numbers from 06 to 082 on baseline and modified powered-model configurations to determine the magnitude of the losses and to what extent current design tools could be used to optimize the installed performance of turboprop propulsion systems designed to cruise at M = 08 Results of the tests indicate a large reduction in installed drag for the modified configuration The wing-mounted power plant caused destabilizing pitching moments and a negative shift in the zero-lift pitching moment

An aerodynamic model for a hemispherically-capped biconic reentry vehicle with six drag flaps

Abstract: The development of an aerodynamic model for a hemispherically-capped biconic reentry vehicle with six drag flaps is presented. The aerodynamic model is primarily based on wind tunnel test results, with the use of computational fluid dynamic codes. For Mach numbers from 4 to 15, the inviscid axial force coefficient was computed for drag flap deflections from 6 to 36. Axial force coefficient was found to vary significantly with ablating flap shape as well as with changing flight conditions. The aerodynamic model can be used for input to vehicle recovery trajectory simulations.