Spanwise Lift Distribution Research Articles

Drag reduction on aircraft configurations with adaptive lifting surfaces

An approach, applicable to multiple-lifting-surface fixed-wing aircraft operating at subcritical Mach numbers, is presented for minimizing induced and profile drag with a constraint on the pitching moment. The approach allows the designer to select surface incidence, twist, and flap angles as variables for the optimization. The numerical formulation uses superposition to construct the spanwise lift distribution from basic and additional loadings, and decomposes the flap-angle distributions for each surface into mean and variation distributions. Together, these elements enable the solution of the problem using semi-analytical methods that also provide insight. Results are presented for a three surface aircraft which highlights low drag possibilities with positive static margins, presents the trade-offs between induced and profile drag, and provides insight into the aerodynamics of multiple lifting surface configurations.

Experimental and computational analysis of stall cells on rectangular wings

ABSTRACTThe present paper is the second part of a combined (experimental and computational) study on stall cells (SCs) on a rectangular wing. In the first part, tuft data were used in order to geometrically characterize a stabilized SC resulting from a localized spanwise disturbance introduced by a zigzag tape. Here, pressure measurements on the model and in the wake and aerodynamic polars at midspan are reported. The wing model had an aspect ratio value of 2, the Reynolds number was 106 and the range of angles of attack (α) was from −6° to 16°. Experimental results confirm previous findings. Furthermore, two‐dimensional and three‐dimensional Reynolds Averaged Navier‐Stokes RANS simulations are used in order to better understand the structure of SCs. 3D simulations reproduce the experimental data with a 3° delay in α and permit a qualitative analysis. It is found that the SC vortices start normal to the wing surface and extend downstream in the wake; the evolution of the SC vortices in the wake is in strong interaction with the separation line vortex and the trailing edge line vortex; as the SC vortex develops downstream in the wake, its centreline is contracted towards the SC centre; the wing wake is pushed upstream at the centre of the SC and downstream at the sides by the SC vortices; spanwise lift and drag distributions always attain their minimum at the SC centre. Copyright © 2013 John Wiley & Sons, Ltd.

Comparing Aerodynamic Efficiency in Birds and Bats Suggests Better Flight Performance in Birds

Flight is one of the energetically most costly activities in the animal kingdom, suggesting that natural selection should work to optimize flight performance. The similar size and flight speed of birds and bats may therefore suggest convergent aerodynamic performance; alternatively, flight performance could be restricted by phylogenetic constraints. We test which of these scenarios fit to two measures of aerodynamic flight efficiency in two passerine bird species and two New World leaf-nosed bat species. Using time-resolved particle image velocimetry measurements of the wake of the animals flying in a wind tunnel, we derived the span efficiency, a metric for the efficiency of generating lift, and the lift-to-drag ratio, a metric for mechanical energetic flight efficiency. We show that the birds significantly outperform the bats in both metrics, which we ascribe to variation in aerodynamic function of body and wing upstroke: Bird bodies generated relatively more lift than bat bodies, resulting in a more uniform spanwise lift distribution and higher span efficiency. A likely explanation would be that the bat ears and nose leaf, associated with echolocation, disturb the flow over the body. During the upstroke, the birds retract their wings to make them aerodynamically inactive, while the membranous bat wings generate thrust and negative lift. Despite the differences in performance, the wake morphology of both birds and bats resemble the optimal wake for their respective lift-to-drag ratio regimes. This suggests that evolution has optimized performance relative to the respective conditions of birds and bats, but that maximum performance is possibly limited by phylogenetic constraints. Although ecological differences between birds and bats are subjected to many conspiring variables, the different aerodynamic flight efficiency for the bird and bat species studied here may help explain why birds typically fly faster, migrate more frequently and migrate longer distances than bats.

Drag Reduction Methodology for Adaptive Tailless Aircraft

An approach is presented for determining optimum lift distributions for adaptive tailless aircraft. In this study, wing adaptation is achieved using multiple trailing-edge flaps that are used to optimally distribute the lift of the wing such that drag is minimized. For tailless aircraft that are stable in pitch, the lack of a secondary lifting surface makes it necessary that the lift distribution on the wing also satisfies a pitching-moment constraint to ensure trim. The current work implements a numerical approach that solves for the optimal scheduling of multiple trailing-edge flaps on the wing of a tailless aircraft for various flight conditions with a pitching-moment constraint to reduce both induced and profile drag. The approach uses superposition to construct the spanwise lift distribution from basic and additional loadings, and decomposes the flap-angle distribution into mean and variation distributions. Together, these elements enable the solution of the problem using semi-analytical methods that also provide insight. The results are presented for a planar, swept, tapered wing with two airfoil-section choices to verify the theory and provide insight for trade studies.

Aero-Structural Optimization of Morphing Airfoils for Adaptive Wings

The design of an airfoil structure involves the disciplines of aerodynamics and structural mechanics, both of which are considered in the design methodology presented in this article. The approach described in this article starts from a requirement formulation based on a time-series of spanwise lift distributions on a morphing wing, representing the mission profile of the aircraft as a whole. This allows to specify goals based directly on aerodynamic performances instead of prescribing fixed geometrical shapes. Using the aero-structural analysis tool presented here, together with a parametrization representing the airfoil outer shape as well as its mechanical properties, allows the formulation of a combined aero-structural optimization problem. Promising aerodynamic and structural morphing performances have been obtained by applying the method to a morphing concept using Dielectric Elastomers (DEs) as actuators. Although the coupled physics are considered and a detailed material model has been used, results can be obtained within reasonable computational time by parallel evaluation of the candidate solutions. Improved aerodynamic performances have been obtained using this concurrent coupled method, in comparison to a sequential aerodynamic and structural optimization.

Preliminary Design of the Tail Unit of a Newly Designed Agricultural Aircraft

This project work undertakes the task of analyzing stresses and preliminary designing of the tail structure members of a newly designed light agricultural aircraft. Schrenk’s method has been used for estimating the span-wise lift distribution, while the tail weight was assumed to be uniformly distribution along the span. Hence, the shear force and bending moment diagrams have been obtained. Accordingly, the working shear and bending stresses within the flight envelope have been calculated. The design has been based on the limit load while the maximum bending and shear values are encountered at the tail root, with the gust load being neglected. The bending and shear margins are found to be positive along the semi-span/height, which shows good enough strength.

Wake Vortex Control Using Static Segmented Gurney Flaps

A study to assess the potential for using static segmented Gurney flaps, also known as miniature trailing edge effectors, for active wake vortex alleviation is conducted using a half-span model wing with NACA 0012 shape and an aspect ratio of 4.1. All tests are performed with the wing at an 8.9 deg angle of attack and chord-based Reynolds number around 350,000. The wing is equipped with an array of 13 miniature trailing edge effector pairs. Each miniature trailing edge effector has a flap that in the neutral position rests behind the blunt trailing edge of the wing, and in the down position extends 0.015 chord lengths perpendicular to the freestream on the pressure side of the wing. Measurements of section lift coefficient are made using pressure taps to assess the effect of various miniature trailing edge effector configurations on the spanwise lift distribution. Particle image velocimetry and five-hole probe measurements are made on the trailing vortex to assess the steady-state effects of different flap configurations

Optimal Scheduling of Control Surfaces Flexible Wings to Reduce Induced Drag

An aircraft wing structure is a complex assemblage of interconnected elements composed of both metallic and composite materials. To capture the physics of the structural response at an appropriate level of accuracy the finite element method is employed to model the aircraft structure. The external aerodynamic loads distribution in both the chord and span direction are predicted using the USSAERO paneling method. The Automated StRuctural Optimization System[1] (ASTROS) is used for modeling the structure, the steady aerodynamic loads, and carrying out the trim analysis of a free flying flexible air vehicle. Two wing models are selected for this investigation. The first model, referred to as the Goland Wing[2], has a “beam-rod” structural model. The second model considered is a built-up model typical of a fighter type aircraft[3] and is shown in Figure 7 with the corresponding aerodynamic panel model. Both models have 20 discrete control surfaces along the trailing edge of each aerodynamic model. The aerodynamic effects of gaps between each of the discrete surfaces are ignored to simulate a “so-called” conformal control surface[4]. R.T. Jones[5] has demonstrated that wings having an elliptical spanwise lift distribution ensures minimum induced drag. Therefore, in the present study, the control surfaces are scheduled to obtain an elliptical spanwise load distribution. It is worth noting that the methodology developed in this work is general and can be used to obtain any desired distribution, not just elliptical. In addition, as demonstrated by Munk[7], induced drag is independent of the distribution of lift in the chordwise direction. For the current study induced drag calculations are carried out considering the “Trefftz-plane”[6] downstream from the trailing edge. The wing is held at a trimmed level flight condition using the angle of attack while the control surfaces are used only to obtain the desired elliptical spanwise pressure distribution.

Induced Drag Reduction Using Aeroelastic Tailoring with Adaptive Control Surfaces

This paper describes the use of full-span deforming control surfaces (including controlled chordwise cambering of the wing) to provide effective subsonic induced drag control with precise control of the spanwise lift distribution. Examples of these proposed controllers are smart, adaptive actuators for advanced unmanned air vehicle concepts. Reshaping the wing spanwise lift distribution with aeroelastic tailoring concepts is shown to reduce induced drag at high airspeeds. Active controllers such as conventional ailerons or leading-edge devices also reduce induced drag if they have a tailored spanwise deflection pattern; the required deflections of these surfaces depend on wing deformation and can be so large that they are not practical. Combining wing aeroelastic stiffness tailoring with active control surface design to create a control-friendly structure reduces induced drag and requires only small controller inputs. An exact solution for the actuator deflections to generate an elliptical lift distribution for the aeroelastic wing, for minimum induced drag, is discussed. A formal optimization problem is posed for cases in which multiple surfaces are used to control drag. This controller optimization solution is complicated because aeroelastic phenomena, such as control reversal, limit the effectiveness of some actuators at high speed.

Wing Tip Vortex in the Near Field: An Experimental Study

Introduction F LOW close to the wing tip is of importance for both low– and high–aspect ratio wings. Indeed, induced drag could be significantly affected by spanwise lift distribution, because there is an introduction of energy that is concentrated at the wing tip. Furthermore, even small errors in the prediction of flow close to the wing tip can significantly affect the evaluation of the structural loads acting on the wing. Knowledge of the physical behavior of the fully developed tip vortex is well established. However, experimental evidence shows, for the initial roll-up of the wing-tip vortex, complex behavior that is not adequately described by existing empirical models of the near-field flow. In fact, the tip vortex evolves from a complex threedimensional separated flow, and the resultant motion of the trailing vortices is highly unsteady. This implies that the core of the vortex fluctuates in time, and this meandering behavior causes the time-averaged Eulerian point measurement to be an average that is weighted both in time and in space. It is thus evident that the unsteadiness of the flow represents a significant challenge for both numerical and experimental analysis. Current knowledge of tip vortex flows is not adequate, and remains essentially qualitative, especially regarding the details of the mechanism of vorticity transport from the near-surface viscous layers into the trailing concentrated vortex. To achieve a better understanding of the initial phase of the tip vortex development an experimental study was performed, and its main results are described here.

Spanwise Lift Distribution for Blended Wing Body Aircraft.

A study is presented of the effects of spanwise lift distribution on aerodynamic efficiency for a blended wing body (BWB) configuration of a given baseline planform. The baseline geometry is initially assessed by a high-fidelity aerodynamic model based on a multiblock structured grid Reynolds-averaged Navier-Stokes (RANS) solution. The accuracy of the simulation is investigated by a grid sensitivity study regarding total drag and its pressure drag and skin-friction drag components. Excessive outer wing loading with associated shock wave, hence, wave drag, has been revealed to be the major factor degrading the aerodynamic performance of the baseline BWB model. To relieve the outer wing, an efficient low-fidelity panel method aerodynamic model is used for the inverse design to achieve the target lift distributions through the variation of twist distribution along the span, shifting the load inboard. For the given BWB geometry, the baseline model is retwisted to achieve an elliptic, a triangular, and an averaged elliptic/triangular spanwise loading distribution. The designs are then analyzed using the high-fidelity RANS aerodynamic model. The wave drag component of the total drag for different span loadings is extracted from the flowfield solution to gain insight into the drag reduction provided by the new twist designs

Euler computation of the nearfield wake vortex of an aircraft in take-off configuration

Numerical simulations based on the three-dimensional Euler equations are used to investigate the predictive capability of an Euler code for calculations of the nearfield wake of a narrow-body airliner wind tunnel model in take-off configuration up to a half span behind the wing tip trailing edge. Simulation results on both structured and unstructured grids are presented. The results on the block-structured grid were obtained within the scope of the EU-project EUROWAKE. The simulation quality of the vortex formation and spatial development is analysed by comparison to wind tunnel measurements of the spanwise lift distribution available from the EU-project DUPRIN II and to experimental PIV data available from the EU-project EUROWAKE.

Lifting-Line Theory for Arbitrarily Shaped Wings

This paper presents a new method for solving the classical lifting-line equation. The spanwise lift (or circulation) distribution is to be determined when the spanwise chord distribution and spanwise twist distribution are resolved separately and arbitrarily into their Fourier-series representations. An infinite set of equations for the resulting circulation Fourier-series coefficients is obtained in terms of the Fourier coefficients for the wing-chord shape and the twist distribution. When the equations are expressed in matrix form, the contributions of planform shape and twist are readily perceived. The set of equations can be truncated arbitrarily for the first N number of unknown coefficients, and easily solved by various software packages. Convergence properties can then be readily established by examining changes associated with changes in levels of truncation. A five-by-five (N = 5) truncation system is found to be sufficient for a wide variety of problems. Several example problems are considered, and the results are compared. The new method converges faster and is more accurate for the same level of truncation. Moreover, in comparison with other methods, it is comprehensive, rigorous, and lends itself to insight regarding the interactions of lift distribution, planform shape, and twist.

Navier-Stokes Analysis of the Flow About a Flap Edge

The current study computationally examines one of the principal three-dimensional features of the e ow over a high-lift system, the e ow associated with a e ap edge. Structured, overset grids were used in conjunction with an incompressible Navier ‐Stokes solver to compute the e ow over a two-element highlift cone guration. The computations were run in a fully turbulent mode using the one-equation Baldwin‐Barth model. Specie c emphasis was given to the details of the e ow in the vicinity of the e ap edge, and so the geometry was simplie ed to isolate this region. The geometry consisted of an unswept wing, which spanned a wind-tunnel test section, equipped with a single-element e ap. Two e ap cone gurations were computed: a full-span and a half-span Fowler e ap. The chord-based Reynolds number was 3.7 3 10 6 for all cases. The results for the full-span e ap agreed with two-dimensional experimental results and verie ed the method. Grid topologies and related issues for the half-span e ap geometry are discussed. Results of the half-span e ap case are compared with three-dimensional experimental results, with emphasis on the e ow features associated with the e ap edge. The results show the effect of the vortex created by the e ap edge, including the impact on e ow separation and spanwise lift distribution.

Sail optimisation for maximal speed

A variety of approaches to sail optimisation are reviewed. A mathematical model is described which allows the optimisation of the spanwise lift distribution of a high speed craft with a criterion of maximal offwind speed. The model uses a numerical lifting line approach to predict the aerodynamic forces and moments allied to an empirical model of the hydrodynamic forces. A further model is presented which yields the optimal sailplan for a yacht based on a criterion of maximal speed made good to windward; this may be calculated either for a single windspeed, or in an averaged sense for a selection of different windspeeds. The vortex lattice method is used to predict aerodynamic forces and moments, whilst the hydrodynamic forces and moments are estimated using a performance prediction approach based on data from the Delft systematic yacht hull series. Three variations in rig type are considered: a basic sloop rig, a sloop rig with a roached mainsail, and a ketch rig. In both models the optimisation is carried out using a genetic algorithm. Typical results are presented, and scope for future work is discussed.

Computation of induced drag for elliptical and crescent-shaped wings

Recent interest in the induced drag characteristics of crescent-shaped wings has led to a closer look at the methods used for determination of induced drag from computational aerodynamic methods. Induced drag may be computed by integration of surface pressure or by evaluation of a contour integral in the Trefftz plane. A high-order panel method was used to study the induced drag of elliptical and crescent-shaped wings using both techniques. Induced drag computations using surface pressure integration were strongly affected by panel density and angle of attack. Drag computations for the crescent wing were especially sensitive to spanwise panel density because of the complex flowfield near the tip. Trefftz-plane results for the two wing planforms were not sensitive to panel density or angle of attack. The effect of correctly modeling the force-free, rolled-up wake geometry on the predicted span efficiency was demonstrated for both wing planforms. The span efficiency predicted from Trefftz-plane integration was about 0.97 for the elliptical wing and 0.99 for the crescent wing, both somewhat less than the classical theoretical maximum for planar wings. Most of the apparent drag reduction of the crescent wing claimed in previous studies was probably an artifact of the surface-pressure integration. The slightly higher span efficiency for the crescent wing was attributed to a more nearly elliptical spanwise lift distribution.

Effect of viscous drag on optimum spanwise lift distribution

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Extension and validation of an unsteady wake model for rotors

A new three-dimensional, finite-state induced-flow model is extended to treat nonlinearities associated with the mass flow induced through the rotor plane. This new theory is then applied to the correlation of a recent set of unsteady, hover laser Doppler velocimetry inflow measurements conducted in the Aeroelastic Rotor Test Chamber at Georgia Institute of Technology. Although the model is intended primarily as a representation of unsteady aerodynamics for aeroelasticity applications, the results show that it has an excellent capability in predicting the inflow distribution in hover except near the root and tip. In addition, the computed unsteady spanwise lift distribution of a rotor is compared with that from an unsteady vortex lattice method for pitch oscillations at various frequencies. The new model is shown to be capable of prediction of unsteady loads typical of aeroelastic response.

Unsteady transonic flow calculations for interfering lifting surfaceconfigurations

Unsteady transonic flow calculations are presented for aerodynamically interfering lifting surface configurations. Calculations are performed by extending the XTRAN3S (Version 1.5) unsteady transonic small-disturbance code to allow the treatment of an additional lifting surface. The research was conducted as a first-step toward developing the capability to treat a complete flight vehicle. Grid generation procedures for swept tapered interfering lifting surface applications of XTRAN3S are described. Transonic calculations are presented for wing-tail and canard-wing configurations for several values of mean angle of attack. The effects of aerodynamic interference on transonic steady pressure distributions and steady and oscillatory spanwise lift distributions are demonstrated. Results due to wing, tail, or canard pitching motions are presented and discussed in detail.

Spanwise lift distribution of forward-and aft-swept wings in comparison to the optimum distribution form

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