Showing papers on "Lift-induced drag published in 2016"

Nonlinear Folding Wing-Tips for Gust Loads Alleviation

Abstract: A recent consideration in aircraft design is the use of folding wing tips with the aim of enabling higher aspect ratio aircraft with less induced drag, but also meeting airport gate limitations. This study builds on previous work investigating the effect of exploiting folding wing tips in-flight as a device to reduce dynamic gust loads, but now with the introduction of a passive nonlinear hinge spring to allow wing-tip deflections only for larger load cases. A representative civil jet aircraft aeroelastic model is used in a multibody simulation code to explore the effect of introducing such a hinged wing-tip device on the loads behavior. It was found that significant reductions in the dynamic loads were possible.

Hydrodynamics of sediment threshold

Abstract: A novel hydrodynamic model for the threshold of cohesionless sediment particle motion under a steady unidirectional streamflow is presented. The hydrodynamic forces (drag and lift) acting on a solitary sediment particle resting over a closely packed bed formed by the identical sediment particles are the primary motivating forces. The drag force comprises of the form drag and form induced drag. The lift force includes the Saffman lift, Magnus lift, centrifugal lift, and turbulent lift. The points of action of the force system are appropriately obtained, for the first time, from the basics of micro-mechanics. The sediment threshold is envisioned as the rolling mode, which is the plausible mode to initiate a particle motion on the bed. The moment balance of the force system on the solitary particle about the pivoting point of rolling yields the governing equation. The conditions of sediment threshold under the hydraulically smooth, transitional, and rough flow regimes are examined. The effects of velocity fluctuations are addressed by applying the statistical theory of turbulence. This study shows that for a hindrance coefficient of 0.3, the threshold curve (threshold Shields parameter versus shear Reynolds number) has an excellent agreement with the experimental data of uniform sediments. However, most of the experimental data are bounded by the upper and lower limiting threshold curves, corresponding to the hindrance coefficients of 0.2 and 0.4, respectively. The threshold curve of this study is compared with those of previous researchers. The present model also agrees satisfactorily with the experimental data of nonuniform sediments.

Drag optimisation of a wing equipped with a morphing upper surface

Abstract: The drag coefficient and the laminar-to-turbulent transition for the aerofoil component of a wing model are optimised using an adaptive upper surface with two actuation points. The effects of the new shaped aerofoils on the global drag coefficient of the wing model are also studied. The aerofoil was optimised with an ‘in-house’ genetic algorithm program coupled with a cubic spline aerofoil shape reconstruction and XFoil 6.96 open-source aerodynamic solver. The wing model analysis was performed with the open-source solver XFLR5 and the 3D Panel Method was used for the aerodynamic calculation. The results of the aerofoil optimisation indicate improvements of both the drag coefficient and transition delay of 2% to 4%. These improvements in the aerofoil characteristics affect the global drag of the wing model, reducing it by up to 2%. The analyses were conducted for a single Reynolds number and speed over a range of angles of attack. The same cases will also be used in the experimental testing of the manufactured morphing wing model.

Aerodynamic Optimization Trade Study of a Box-Wing Aircraft Configuration

Abstract: This study investigates the aerodynamic tradeoffs of a box-wing aircraft configuration using high-fidelity aerodynamic optimization. A total of five optimization studies are conducted, where each study extends the previous one by progressively adding a combination of design variables and constraints. Examples of design variables include wing twist and sectional shape; examples of constraints include trim and stability requirements. In all cases, the objective is to minimize inviscid drag at a prescribed lift and a Mach number of 0.78. Aerodynamic functionals are evaluated based on the discrete solution of the Euler equations, which are tightly coupled with an adjoint methodology incorporating a gradient-based optimizer. For each study, an equivalent conventional tube-and-wing baseline is similarly optimized in order to enable direct comparisons. It is found that the transonic box-wing aircraft considered here, for which the height-to-span ratio is about 0.2, produces up to 43% less induced drag than its c...

Minimum Induced Drag Theorems for Joined Wings, Closed Systems, and Generic Biwings: Theory

Abstract: An analytical formulation for the induced drag minimization of closed wing systems is presented. The method is based on a variational approach, which leads to the Euler---Lagrange integral equation in the unknown circulation distribution. It is shown for the first time that the augmented Munk's minimum induced drag theorem, formulated in the past for open single-wing systems, is also applicable to closed systems, joined wings and generic biwings. The quasi-closed C-wing minimum induced drag conjecture discussed in the literature is addressed. Using the variational procedure presented in this work, it is also shown that in a general biwing, under optimal conditions, the aerodynamic efficiency of each wing is equal to the aerodynamic efficiency of the entire wing system (biwing's minimum induced drag theorem). This theorem holds even if the two wings are not identical and present different shapes and wingspans; an interesting direct consequence of the theorem is discussed. It is then verified (but yet not demonstrated) that in a closed path, the minimum induced drag of the biwing is identical to the optimal induced drag of the corresponding closed system (closed system's biwing limit theorem). Finally, the nonuniqueness of the optimal circulation for a closed wing system is rigorously addressed, and direct implications in the design of joined wings are discussed.

Influence of aerodynamic trailer devices on drag reduction measured in a wind tunnel

Abstract: The value of aerodynamic drag is the largest, when a vehicle is moving with higher velocity. It seems that drag reduction is the most important step for reducing the fuel consumption of haulage trailer sets. Using aerodynamic trailer devices is one of many ways for reduction of fuel consumption. This paper deals with experimental measuring of the truck set model in a wind tunnel. The scale of the model was 1/24. Resultant values of the drag reduction for chosen aerodynamic devices are discussed at the end of the paper.

On Wings of the Minimum Induced Drag: Spanload Implications for Aircraft and Birds

Abstract: For nearly a century Ludwig Prandtl's lifting-line theory remains a standard tool for understanding and analyzing aircraft wings. The tool, said Prandtl, initially points to the elliptical spanload as the most efficient wing choice, and it, too, has become the standard in aviation. Having no other model, avian researchers have used the elliptical spanload virtually since its introduction. Yet over the last half-century, research in bird flight has generated increasing data incongruous with the elliptical spanload. In 1933 Prandtl published a little-known paper presenting a superior spanload: any other solution produces greater drag. We argue that this second spanload is the correct model for bird flight data. Based on research we present a unifying theory for superior efficiency and coordinated control in a single solution. Specifically, Prandtl's second spanload offers the only solution to three aspects of bird flight: how birds are able to turn and maneuver without a vertical tail; why birds fly in formation with their wingtips overlapped; and why narrow wingtips do not result in wingtip stall. We performed research using two experimental aircraft designed in accordance with the fundamentals of Prandtl's second paper, but applying recent developments, to validate the various potentials of the new spanload, to wit: as an alternative for avian researchers, to demonstrate the concept of proverse yaw, and to offer a new method of aircraft control and efficiency.

Minimum Induced Drag Theorems for Joined Wings, Closed Systems, and Generic Biwings: Applications

Abstract: An invariant procedure for the minimization of induced drag of generic biwings and closed systems (Joined Wings) was presented in the companion paper (minimum induced drag theorems for Joined Wings, closed systems, and generic biwings: theory) and is now adopted to study several theoretical open questions regarding these configurations. It is numerically verified that a quasi-closed C-wing presents the same optimal induced drag and circulation of the corresponding closed system. It is also verified that when the two wings of a biwing are brought close to each other so that the lifting lines identify a closed path, the minimum induced drag of the biwing is identical to the optimal induced drag of the corresponding closed system. The optimal circulation of this case differs from the quasi-closed C-wing one by an additive constant. The non-uniqueness of the optimal circulation for a closed wing system is also addressed, and it is shown that there are an infinite number of equivalent solutions obtained by adding an arbitrary constant to a reference optimal circulation. This property has direct positive impact in the design of Joined Wings as far as the wing load repartition is concerned: The percentage of aerodynamic lift supported by each wing can be modified to satisfy other design constraints, and without induced drag penalty. Finally, the theoretical open question regarding the asymptotic induced drag behavior of Joined Wings, when the vertical aspect ratio approaches infinity, has been resolved. It has been shown that for equally loaded wings indefinitely distant from each other, the boxwing minimum induced drag tends to zero. In that condition, the upper and lower wings present a constant aerodynamic load. Prandtl's approximated formula for the minimum induced drag of a boxwing (Best Wing System) cannot be used to describe the asymptotic behavior. This work also shows that the optimal distribution over the equally loaded horizontal wings of a boxwing is not the superposition of a constant and an elliptical functions. This is an acceptable approximation only for small vertical aspect ratios (of aeronautical interest).

Sharp Transition in the Lift Force of a Fluid Flowing Past Nonsymmetrical Obstacles: Evidence for a Lift Crisis in the Drag Crisis Regime.

Abstract: Bluff bodies moving in a fluid experience a drag force which usually increases with velocity. However in a particular velocity range a drag crisis is observed, i.e., a sharp and strong decrease of the drag force. This counterintuitive result is well characterized for a sphere or a cylinder. Here we show that, for an object breaking the up-down symmetry, a lift crisis is observed simultaneously to the drag crisis. The term lift crisis refers to the fact that at constant incidence the time-averaged transverse force, which remains small or even negative at low velocity, transitions abruptly to large positive values above a critical flow velocity. This transition is characterized from direct force measurements as well as from change in the velocity field around the obstacle.

Wake analysis of aerodynamic components for the glide envelope of a jackdaw (Corvus monedula)

Abstract: Gliding flight is a relatively inexpensive mode of flight used by many larger bird species, where potential energy is used to cover the cost of aerodynamic drag. Birds have great flexibility in their flight configuration, allowing them to control their flight speed and glide angle. However, relatively little is known about how this flexibility affects aerodynamic drag. We measured the wake of a jackdaw (Corvus monedula) gliding in a wind tunnel, and computed the components of aerodynamic drag from the wake. We found that induced drag was mainly affected by wingspan, but also that the use of the tail has a negative influence on span efficiency. Contrary to previous work, we found no support for the separated primaries being used in controlling the induced drag. Profile drag was of similar magnitude to that reported in other studies, and our results suggest that profile drag is affected by variation in wing shape. For a folded tail, the body drag coefficient had a value of 0.2, rising to above 0.4 with the tail fully spread, which we conclude is due to tail profile drag.

Calm water performance of hard-chine vessels in semi-planing and planing regimes

Abstract: In the current paper, a mathematical model is developed for performance prediction of hard-chin boats which can be used in both semi-planing and planing regimes. The proposed model bases on the 2D+T theory and implements pressure distributions over the length of the hull in order to compute the forces. To determine the forces in the semiplaning range, a function is proposed for the non-dimensional length at which the transom effect appears. Three drag components, which are: frictional drag, induced drag, and spray drag, are considered in the computations performed using an iterative method to satisfy two equilibrium equations. The validity of the proposed method is verified by comparing the predicted trim angle and resistance against the available experimental data. Based on this comparison, it is observed that the proposed method reveals satisfying accuracy in both semi-planing and planing regimes. The method is then used to study variation of hydrodynamic and hydrostatic forces as the hull makes a transition from the semi-planing regime to the planing regime. In addition, different components of the resistance are analyzed.

Improved Unsteady Far-Field Drag Breakdown Method and Application to Complex Cases

Abstract: Far-field drag extraction has the advantage over near-field integration of providing a phenomenological breakdown of drag. A decomposition into components linked to shock waves, viscous interactions, and lift-induced vortices is straightforward for steady flows. A formulation based on thermodynamic considerations is used at ONERA–The French Aerospace Lab and by its partners, but it is restricted to steady cases. A generalization to unsteady flows of the Van der Vooren formulation has been developed and tested on three unsteady cases previously. The proposed method allowed the breakdown of drag into the three usual components only; however, the induced drag coefficient remained ill-defined. This unsteady formulation is here modified to better express the induced drag. A new drag component is identified as a propagation and acoustics contribution. The new formulation is then applied to complex cases: two-dimensional and three-dimensional pitching cases, and an OAT15A profile subject to buffet simulated by z...

Comparison of Power Requirements: Flapping vs. Fixed Wing Vehicles

Abstract: The power required by flapping and fixed wing vehicles in level flight is determined and compared. Based on a new modelling approach, the effects of flapping on the induced drag in flapping wing vehicles are mathematically described. It is shown that flapping causes a significant increase in the induced drag when compared with a non-flapping, fixed wing vehicle. There are two effects for that induced drag increase; one is due to tilting of the lift vector caused by flapping the wings and the other results from changes in the amount of the lift vector during flapping. The induced drag increase yields a significant contribution to the power required by flapping wing vehicles. Furthermore, the power characteristics of fixed wing vehicles are dealt with. It is shown that, for this vehicle type, the propeller efficiency plays a major role. This is because there are considerable differences in the propeller efficiency when taking the size of vehicles into account. Comparing flapping and fixed wing vehicles, the conditions are shown where flapping wing vehicles have a lower power demand and where fixed wing vehicles are superior regarding the required power. There is a tendency such that fixed wing vehicles have an advantage in the case of larger size vehicles and flapping wing vehicles have an advantage in the case of smaller size ones.

Aircraft High-Lift Aerodynamic Analysis Using a Surface-Vorticity Solver

Abstract: This study extends an existing semi-empirical approach to high-lift analysis by examining its effectiveness for use with a three-dimensional aerodynamic analysis method. The aircraft high-lift geometry is modeled in Vehicle Sketch Pad (OpenVSP) using a newly-developed set of techniques for building a three-dimensional model of the high-lift geometry, and for controlling flap deflections using scripted parameter linking. Analysis of the low-speed aerodynamics is performed in FlightStream, a novel surface-vorticity solver that is expected to be substantially more robust and stable compared to pressure-based potential-flow solvers and less sensitive to surface perturbations. The calculated lift curve and drag polar are modified by an empirical lift-effectiveness factor that takes into account the effects of viscosity that are not captured in the potential-flow solution. Analysis results are validated against wind-tunnel data for The Energy-Efficient Transport AR12 low-speed wind-tunnel model, a 12-foot, full-span aircraft configuration with a supercritical wing, full-span slats, and part-span double-slotted flaps.

The Determination Of Aerodynamic Drag Coefficient Of Truck and Trailer Model By Wind Tunnel Tests

Abstract: In this study, surface pressure and drag measurements were conducted for a heavy vehicle model consisted of 1/32 scaled truck and trailer which was placed in a wind tunnel. The wind tunnel tests of truck trailer combination were carried out in the range of 117 000 - 844 000 Reynolds numbers. The pressure coefficient (CP) distribution and aerodynamic drag coefficient (CD) on truck and trailer were experimentally determined. The regions forming aerodynamic drag on the truck trailer was determined at the result of the flow visualization. The average drag coefficient (CD) was determined as 0.608 for truck. The drag coefficiens was obtained as 0,704 for truc trailer combination. The drag coefficient (CD ) increased 15.8%, when the trailer was attached to the truck.

Plasma actuator for vehicle aerodynamic drag reduction

Abstract: A plasma actuator includes a first electrode disposed on a substrate, covered by a dielectric layer, and a second electrode disposed on the dielectric layer. In operation, the plasma actuator creates a plasma region, altering air flowing over the actuator. The plasma actuator in various embodiments: has no moving parts, helps to improve fuel economy by reducing aerodynamic drag, improves vehicle stability control under severe unsteady flow environments, reduces wind noise around a vehicle on which the actuator is used, and reduces emission and CO2 foot print through the fuel economy improvement.

Parametric Study of the Effects of a Tubercle's Geometry on Wing Performance Through the Use of the Lifting-Line Theory

Abstract: Prandtl’s lifting-line theory has been implemented to determine the effects of a tubercle’s amplitude and wavelength on the lift coefficient, induced drag coefficient, and the lift-toinduced-drag ratio of a NACA 0021 wing at an angle of attack of 3°, and a Reynolds number of 120,000. In addition, a new tubercle parameter has been introduced; the point along a tubercle that a wing terminates. This parameter has been termed the phase of the tubercles. The phase of the tubercles tended to have the greatest effect on the lift coefficient, induced drag coefficient, and the lift-to-induced-drag ratio, while the wavelength had the least. However, the effects of the tubercle amplitude, wavelength, and phase on the wing performance parameters considered were inter-dependent, whereby increasing the amplitude or wavelength not only resulted in these individual parameters to be more effectual, but for the phase to be more effectual as well. Typically, a particular tubercle geometry that reduced the lift coefficient also reduced the induced drag coefficient, and the lift-to-induced-drag ratio would increase. The lift-to-induced-drag ratio was increase by as much as 7.7% for the considered tubercle geometries.

The influence of flight style on the aerodynamic properties of avian wings as fixed lifting surfaces.

Abstract: The diversity of wing morphologies in birds reflects their variety of flight styles and the associated aerodynamic and inertial requirements. Although the aerodynamics underlying wing morphology can be informed by aeronautical research, important differences exist between planes and birds. In particular, birds operate at lower, transitional Reynolds numbers than do most aircraft. To date, few quantitative studies have investigated the aerodynamic performance of avian wings as fixed lifting surfaces and none have focused upon the differences between wings from different flight style groups. Dried wings from 10 bird species representing three distinct flight style groups were mounted on a force/torque sensor within a wind tunnel in order to test the hypothesis that wing morphologies associated with different flight styles exhibit different aerodynamic properties. Morphological differences manifested primarily as differences in drag rather than lift. Maximum lift coefficients did not differ between groups, whereas minimum drag coefficients were lowest in undulating flyers (Corvids). The lift to drag ratios were lower than in conventional aerofoils and data from free-flying soaring species; particularly in high frequency, flapping flyers (Anseriformes), which do not rely heavily on glide performance. The results illustrate important aerodynamic differences between the wings of different flight style groups that cannot be explained solely by simple wing-shape measures. Taken at face value, the results also suggest that wing-shape is linked principally to changes in aerodynamic drag, but, of course, it is aerodynamics during flapping and not gliding that is likely to be the primary driver.

Riblets Induced Drag Reduction on a Spatially Developing Turbulent Boundary Layer

Abstract: Large eddy simulations have been conducted to gain further insight into the drag-reducing mechanisms of riblets in zero-pressure gradient turbulent boundary layer. The retained groove geometry achieves 9.8 % drag reduction on the controlled zone developing from \(Re_{\theta } = 670\) to 975. It is shown that the turbulent contribution to the drag—as defined by Fukagata et al. Phys. Fluids, 14(11):L73, 2002 [7]—is the most affected. In the light of the obtained results, energy and enstrophy budgets will finally conduct to isolate a key mechanism involved in the riblets drag reduction.

Wake and Loss Analysis for a Double Bladed Swept Propeller

Abstract: Inspired by Prandtl’s theory on aircraft wings with minimum induced drag, the authors introduced a double-bladed propeller, the Boxprop, intended for high-speed flight. The basic idea is to join the propeller blades pair-wise at the tip to improve aerodynamics and mechanical properties compared to the conventional propeller. The rather complex geometry of the double blades gives rise to new questions, particularly regarding the aerodynamics. This paper presents a propeller wake energy analysis method which gives a better understanding of the potential performance benefits of the Boxprop and a means to improve its design. CFD analysis of a five bladed Boxprop demonstrated its ability to generate typical levels of cruise thrust at a flight speed of Mach 0.75. The present work shows that the near tip velocity variations in the wake are weaker for this propeller than a conventional one, which is an indication that a counter rotating propeller designed with a Boxprop employed at the front may exhibit lower interaction noise.