Aircraft Drag Research Articles

Drag Reduction by Laminar Flow Control

The Energy System Transition in Aviation research project of the Aeronautics Research Center Niedersachsen (NFL) searches for potentially game-changing technologies to reduce the carbon footprint of aviation by promoting and enabling new propulsion and drag reduction technologies. The greatest potential for aerodynamic drag reduction is seen in laminar flow control by boundary layer suction. While most of the research so far has been on partial laminarization by application of Natural Laminar Flow (NLF) and Hybrid Laminar Flow Control (HLFC) to wings, complete laminarization of wings, tails and fuselages promises much higher gains. The potential drag reduction and suction requirements, including the necessary compressor power, are calculated on component level using a flow solver with viscid/inviscid coupling and a 3D Reynolds-Averaged Navier-Stokes (RANS) solver. The effect on total aircraft drag is estimated for a state-of-the-art mid-range aircraft configuration using preliminary aircraft design methods, showing that total cruise drag can be halved compared to today’s turbulent aircraft.

THE INFLUENCE OF WINGLETS ON THE AERODYNAMIC CHARACTERISTICS AIRCRAFT WITH THE TANDEM SCHEME

«Tandem» aerodynamic aircraft scheme with comparable areas of front and rear wings is well-known in aviation.«Tandem» scheme allows increasing in lift and lifting capacity with limited wingspan. So, it has found a new application in modern unmanned aerial vehicles.However, an increase in the number of lifting surfaces also leads to aircraft drag increasing. To solve this problem on classical aerodynamic schemes winglets, small additional elements at the wingtips which reduce the induced drag and increase the lift, are used. Such an effect can be achieved by selecting the optimal geometric characteristics of the winglets.Winglets are almost not used for tandem-scheme aircraft. Theoretically, winglets installing on the front wing of the tandem aircraft allows reducing the induced drag of both the front wing and the rear wing. But the open sources do not contain the experimental confirmation. Thus, the research task of the winglets effect on the tandem aircraft aerodynamics remains unsolved.Presented investigation allows estimation of the winglets effect on lift-drag ratio, pitch, roll and yaw static stability coefficients for tandem-scheme aircraft.

Wind-Tunnel and CFD Investigations Focused on Transition and Performance Predictions of Laminar Wings

This paper is a continuation of the paper recently published by Hue et al. (“Experimental and Numerical Methods for Transition and Drag Predictions of Laminar Airfoils,” AIAA Journal, Vol. 53, No. 9, Sept. 2015, pp. 2694–2712) that focused on transition and drag predictions of laminar airfoils. The extension of such studies to three-dimensional configurations representative of modern civil aircraft is a further step toward the implementation of natural laminar-flow technologies. The present work, therefore, focuses on validating the laminar design of a low-swept wing for business jet applications. In 2015, an experimental campaign was carried out in the European Transonic Windtunnel, and included laminar/turbulent transition measurements with temperature-sensitive paint at Mach and Reynolds numbers typical of cruise flight conditions. Subsequently, fluid dynamics computations were performed on this aircraft geometry either with a Reynolds-averaged Navier–Stokes solver using both Tollmien–Schlichting and crossflow transition criteria, or with a boundary-layer code using either database methods for transition location or exact stability analyses. In this paper, experimental and numerical transition predictions are compared for three representative cases corresponding to different angles of attack. The agreement that is achieved is satisfactory, and extended regions of laminar flow are observed on the wing at cruise lift levels. In these conditions, the drag reduction can account for 10–15% of the aircraft drag.

Intelligent Support for Aircraft Flight Test Data Processing in Problem of Engine Thrust Estimation

The report deals with the design of the intelligent support for flight test data processing in the problem of separate estimation of aircraft thrust and drag forces in flight experiment. The thrust and drag identification belongs to the class of incorrect inverse problems, for which reason it is not to be solved by general system identification methods and their applications to aerodynamic parameter estimation from the flight data. The paper considers the specific aspects of the intelligent support for thrust and drag identification from the automatic estimation viewpoint of identification accuracy. The proposed algorithms are tested at the set of data generated using a modern aircraft simulation facility.

Response of the shock wave/boundary layer interaction to the plasma synthetic jet

Control of shock wave/boundary layer interaction (SWBLI) is of high practical importance for supersonic aircraft drag reducing. Lots of flow control strategies including passive and active control techniques have been put forward to minimize negative effect of SWBLI.Plasma aerodynamic control technique is considered as a potential one due to its flexibility in manipulating the supersonic flow. The goal of this research is to investigate the control effect of the novel actuator called plasma synthetic jet on the SWBLI.The effect of counter-flow plasma synthetic jet actuator on the SWBLI is investigated experimentally in this paper. The experiments are conducted in a supersonic wind tunnel at Mach number Ma=3.1. The test model is a blunt body with a plasma synthetic jet actuator installed inside its head which is used to create aerodynamic perturbations, and with a conical compression ramp in the rear, enabling the creation of SWBLI flow configuration. The plasma synthetic jet actuator is designed to inject pulsed hot gas by arc discharge into a small cavity in the direction perpendicular to the normal shock wave induced by the blunt body. The schlieren method is used for flow measurement and the flow characteristics are studied according to a sequence of schlieren images (1024512 pixel resolution) captured by a high speed charge-couple device camera with a framing rate of 58 kHz, triggered externally, and an exposure time of 1 s. Additionally, the mechanism of this control strategy on the SWBLI induced by the ramp is revealed by using the numerical method.The characteristics of the plasma synthetic jet in quiescent air are firstly studied. The results show a sudden reduction of averaged jet velocity under the resistance of the air. In addition, some small-scale flow structures in the jet are observed which may enhance the turbulence in the upstream boundary layer. The flow topology of interaction modified by actuation with frequencies of f=1 kHz and f=3 kHz are respectively analyzed. It is shown that by using this type of control strategy, the attached shock is locally degraded with the attachment point moving upward. The separation bubble is suppressed, hence making the separation shock move downstream. In addition, an extensive impact effect is exerted to the interaction region by actuation at f=1 kHz because more hot gas is produced by the actuator. Therefore, the actuator is found to be capable of significantly mitigating the negative effects induced by the SWBLI. The numerical work focuses on the interaction between the jet and the flow after the normal shock. The results show that large-scale vortex is induced by the interaction which increases turbulence and accelerates the flow near the wall during its moving downstream and dissipation, demonstrating turbulence enhancement in the boundary layer and a variation of upstream flow characteristics are the key factors for separation reduction and shock wave mitigation.

Design and Optimisation of Fuel Tanks for BWB Configurations

Abstract This paper describes assumptions, goals, methods, results and conclusions related to fuel tank arrangement of a flying wing passenger airplane configuration. A short overview of various fuel tank systems in use today of different types of aircraft is treated as a starting point for designing a fuel tank system to be used on very large passenger airplanes. These systems may be used to move fuel around the aircraft to keep the centre of gravity within acceptable limits, to maintain pitch and lateral balance and stability. With increasing aircraft speed, the centre of lift moves aft, and for trimming the elevator or trimmer must be used thereby increasing aircraft drag. To avoid this, the centre of gravity can be shifted by pumping fuel from forward to aft tanks. The lesson learnt from this is applied to minimise trim drag by moving the fuel along the airplane. Such a task can be done within coming days if we know the minimum drag versus CG position and weight value. The main part of the paper is devoted to wing bending moment distribution. A number of arrangements of fuel in airplane tanks are investigated and a scenario of refuelling - minimising the root bending moments - is presented. These results were obtained under the assumption that aircraft is in long range flight (14 hours), CL is constant and equal to 0.279, Specific Fuel Consumption is also constant and that overall fuel consumption is equal to 20 tons per 1 hour. It was found that the average stress level in wing structure is lower if refuelling starts from fuel tanks located closer to longitudinal plane of symmetry. It can influence the rate of fatigue.

AIRPLANE FRICTION DRAG REDUCTION BY MEANS OF MICROBLOWING THROUGH PERMEABLE WING SURFACE SECTIONS

The last 45 years have been characterized by escalation of the problem of energy saving and, accordingly, the search and implementation of various resource-saving technologies in various spheres of human activity and, first of all, in the energy and transport sectors. In particular, this problem became urgent for aviation transport and it is not difficult to assume that in the future this process will only increase. Gradual optimization of power plants and aerodynamic design of aircraft in the process of steady improvement of aircraft characteristics and development of the appropriate technological base led to the fact that the traditional methods of fuel consumption reducing are almost exhausted. Aircraft drag consists of several components: inductive, wave, pressure and frictional. The first three components are studied well enough and methods of their reduction are widely used on airplanes. Nevertheless, friction drag can be effectively reduced by boundary layer control methods. Among them there are wellknown and deeply studied ones but for some reasons they were not applied earlier because of the inability or ineffectiveness of their technological implementation. There are methods that are still awaiting for their detailed studies. Determining the optimal, in terms of the frictional drag reduction and the technological considerations, placement and length of the blowing section (or several sections) along the airplane wing chord are practically unexplored. This set of problems is the subject of authors consideration and research presented in this article. On the basis of An-124 aircraft wing the advantages and phenomena of the influence of micro-blowing through the porous section of streamlined surface have been investigated. Parametric analysis of the effect of blowing speed, location and length of the blowing section along the wing has been performed and the conditions and ratios of the parameters, which provide the best effect for the case of maintaining flow without separation, have been determined.

Implicit CFD methods for transitional shock wave – boundary layer interaction

Purpose This paper aims to explore the potential of transition prediction methods for modelling transitional shock wave/boundary layer interactions. The study is fuelled by the strong interest of researchers and airframe manufacturers in reducing the drag of vehicles flying at transonic speeds. The principle of drag reduction via flow laminarity is valid, provided there is no need for the flow to sustain large pressure gradients or shocks. This is true, as laminar boundary layers are less resistant to flow separation. Design/methodology/approach It is, therefore, worthwhile to assess the performance of CFD methods in modelling laminar boundary layers that can be tripped to turbulent just before an interaction with a shock. In this work, the CFD solver of Liverpool University is used. The method is strongly implicit, and, for this reason, the implementation of intermittency-based models requires special attention. The Navier–Stokes equations, the transport equations of the kinetic energy of turbulence and the turbulent frequency are inverted at the same time as the transport equations for the flow intermittency and the momentum thickness Reynolds number. Findings The result is stable and robust convergence even for complex three-dimensional flow cases. The method is demonstrated for the flow around the V2C section of the TFAST EU, F7 project. The results suggest that the intermittency-based model captures the fundamental physics of the interaction, but verification and validation are needed to ensure that accurate results can be obtained. For this reason, comparisons with the TFAST experiments is put forward as a means of establishing confidence in the transition prediction tools used for shock/boundary layer interaction simulation. Research limitations/implications At the moment, experimental data for transonic transitional buffet are not yet available, although this will change in the near future. Practical implications The required CPU time is neither insignificant not prohibitive for routine computations. Social implications Reducing aircraft drag without compromising on stall characteristics will result in lower fuel consumption and contribute to a greener and more economic flight for passengers. Originality/value To the authors’ knowledge, this is the first time that transitional buffet has been addressed.

Morphing wing optimization for steady level flight

This paper presents the basic results of the morphing wing planform optimization of an experimental unmanned air vehicle for minimum drag at steady level flight. The aerodynamic design tool that consists of the three-dimensional panel method, two-dimensional boundary layer solution and generalized reduced gradient method-based optimization is appropriate for fixed wing and morphing wing conceptual and preliminary design. The morphing concept is implemented into the solution with the geometric constraints of the wing planform and the airfoil shape design variables. The drag that is created by other components of the aircraft is calculated according to empirical formulas. Wing drag and aircraft drag comparisons between baseline wing (BASE), optimum fixed wing and morphing wing are discussed with the obtained planform and airfoil shapes.

Formation flight investigation for highly efficient future civil transport aircraft

ABSTRACTFormation flight could greatly assist the air transport industry in tackling the challenges of environmental impact, excessive reliance on fuel and overcapacity. Previous studies have shown drag reductions leading to significant fuel savings for aircraft in formation relative to their solo flight. Safety is guaranteed with the use of extended formation distances, and practical implementation issues could be solved in the near future. Since studies so far have focused on existing aircraft configurations and technology, a case study using a strut-braced wing airliner was carried out to ascertain its applicability to less conventional craft. The present results did not indicate such clear-cut benefits. If formation flight is to be successful and beneficial for the next generations of aircraft, it will be vital to consider its interaction with new technologies developed for highly efficient operation, in particular those aimed at reduction of aircraft drag such as laminar flow, and to do so early in the design of aerospace vehicles and wider systems.

NUMERICAL INVESTIGATION OF WINGLET ANGLES INFLUENCE ON VORTEX SHEDDING

Wingtip devices are usually intended to improve the efficiency of fixed-wing aircraft. There are several types of wing tip devices, and although they function in different manners, the intended effect is always to reduce the aircraft's drag by partial recovery of the tip vortex energy. Wingtip devices can also improve aircraft handling characteristics and enhance safety for following aircraft. Such devices increase the effective aspect ratio of a wing without materially increasing the wingspan. The purpose of this project is to analyse different types of winglets using Auto CAD, GAMBIT and FLUENT and then, to find out what happens when winglets are linked to wingtips and to study about the aerodynamic properties of spiroid winglets which are still under research. One wing model without winglet and three wing models with winglets were created and drawn in Auto CAD and they were meshed in GAMBIT using geometry data gathered from research papers. Those models were read into Fluent where flow boundary conditions were applied. The wing without winglet, the wings with spiroid winglets and the wings with other kinds of winglets performance were analyzed in several angles of attack and the drag coefficient was compared when the aircraft is taking off. Wingtip vortices from models were checked, studied and compared. The best winglet model reduces drag coefficient and wingtip vortices from wing without winglet model and it will be pointed out at the end of this project. --------------------------------------------------------------------***----------------------------------------------------------------------

The impact of control allocation on trim drag of blended wing body aircraft

The trim drag of conventional aircraft configurations in cruise flight is in the order of 0–1% of the total aircraft drag. Blended wing body aircraft typically have multiple redundant flight control surfaces at the trailing edge with only a limited moment arm with respect to the center of gravity. This aircraft configuration therefore potentially has much more trim drag than conventional aircraft. The redundant set of control surfaces should therefore be deflected in an optimal way to minimize trim drag. A study is conducted to investigate the impact of three control allocation methods on the trim drag of a blended wing body aircraft configuration at relatively low airspeeds. The trim drag is measured by a wind tunnel test campaign. Results indicate that the type of control allocation method has a large influence on the trim drag. This effect must be taken into account in the early design stages of blended wing body aircraft since it will significantly affect the overall aircraft drag. Most control allocation schemes are based on a linear representation of the aerodynamics. Results for the test case indicate that a nonlinear aerodynamic database is required to assess the trim drag with sufficient accuracy. The two main nonlinear effects that must be included are the variations of control derivatives with (1) angle of attack and (2) control deflection. Control surface interaction effects have only a minor influence. The methodology developed to assess trim drag and to select a control allocation algorithm can be directly applied in the early design stages when numerical aerodynamic analysis tools are used instead of wind tunnel models.

Parametric Exploration of Wing–Body Junction Flow Using Computational Fluid Dynamics

The wing–body junction flow is investigated parametrically using computational fluid dynamics in an attempt to understand the effects of junction flow on aircraft drag, with a focus on application to large business jet or commercial transport aircraft. A Reynolds-averaged Navier–Stokes computational fluid dynamics methodology is validated against detailed experimental data for a junction flow. Computational fluid dynamics results for a wing with a leading-edge strake (an aerodynamic surface designed to reduce flow separation, thereby reducing aircraft drag) are presented, and the effects of scaling this strake are explored. The effectiveness of the strake on a swept wing is compared to the same for a straight wing. Finally, the results from this parametric study are successfully applied to sizing a leading-edge strake for a commercial transport aircraft. It is demonstrated that a systematic approach, starting with a simple, validated model and building up to a realistic aircraft application, can build confidence in computational fluid dynamics results.

Experimental and Numerical Methods for Transition and Drag Predictions of Laminar Airfoils

The friction component is responsible for more than 40% of typical civil aircraft drag. As a consequence, the issue of laminar flow has been of prime importance in aeronautics for many years now. This article is focused on Tollmien–Schlichting-induced transition and drag predictions of two-dimensional laminar airfoils obtained with experimental and numerical methods. In 2012, a test campaign in the ONERA-S2MA wind tunnel, including infrared acquisitions, pressure sensors, and wake analyses, allowed substantial data to be obtained on such airfoils in transonic conditions. To complete this study, two-dimensional fluid dynamics computations have been performed, either with a Reynolds-averaged Navier–Stokes solver using transition criteria or with a boundary-layer code combined with direct stability analysis. Furthermore, experimental (wake survey) and numerical (far-field theory) techniques allowing airfoil drag breakdown have been employed. Wind-tunnel and computational fluid dynamics transition predictions have been compared. Good agreement has been observed but the transition criteria may show some limitations in particular situations, such as long separation bubble development. The gains in lift and drag due to laminar flow have been quantified (natural vs triggered transition). Concerning drag reduction, the importance of the viscous pressure component has been highlighted. Finally, the effects of parameters such as angle of attack, Mach number, and Reynolds number on transition location and drag have been investigated.

Design of Pneumatic Parachute-Packing Machine

As ground support equipment, drag parachute packing machine is used for packaging aircraft drag parachute. It is driven by high-pressure inert gas. Drag parachute will be compacted in the cylinder loading institution by high pressure. The overall structure of drag parachute packing machine is designed. The finite element analysis and structure optimization of drag parachute packing manipulator is completed. The pneumatic control system is designed. It provides an approach for drag parachute packing technology.

Experimental and numerical investigation of the power-on effect for a propeller-driven UAV

The power-on effect contributes to the performance and the stability of a propeller-driven aircraft. In the present study, the power-on effect for a propeller-driven unmanned aerial vehicle with a pusher propeller has been investigated through the wind tunnel test and the computational fluid dynamics simulation. The positive power-on effect is to increase the maximum lift and delay the stall of the aircraft. However, the most common power-on effect is to produce the slipstream drag and the nose-up pitching moment of the aircraft. The slipstream interaction with the wing and the fuselage increases the fuselage pressure drag leading to an overall increase in aircraft drag. Moreover, the slipstream interaction increases the downwash angle and dynamic pressure in the vicinity of the horizontal tail leads to an overall increase in aircraft nose-up pitching moment. Therefore, the power-on effects tend to reduce the performance and longitudinal stability of the propeller-driven aircraft. The power-on effect is pronounced at slower speeds and higher thrust levels. It is expected that the analysis results presented and discussed in this paper will contribute to the development of the propeller-driven aircraft with the pusher propeller.

Towards Automatic Flight Control for Commercial Airliners in Formation Flight

This paper presents research done towards the goal of achieving automatic flight control for commercial airliners in formation flight. The motivation for this research is to ultimately reduce fuel-consumption through a reduction in the drag of the follower aircraft, which is a result of the formation flight. Traditional aerodynamic equations for conventional flight of fixed-wing aircraft are expanded to include formation flight interactions. A trim analysis uncovers risks, challenges and feasible trim regions for the formation follower to maintain. These regions include a potentially risky region which is sandwiched between two untrimmable regions, with respect to a maximum aileron setting, and an outside region which has only one untrimmable bound, making it less risky but with lower fuel-consumption benefit. Next, a state space representation is constructed, allowing for a linear dynamics analysis. The poles and their movement as a function of the lateral and vertical separation of the follower aircraft relative to the leader aircraft are shown, and indicate greater changes in flight dynamics due to vertical separation than to lateral separation. The results of the trim analysis and linear dynamics analysis form the basis for the design of a formation flight control system.

Shock control bump optimization for a low sweep supercritical wing

Shock control bumps are a promising technique in reducing wave drag of civil transport aircraft flying at transonic speeds. This paper investigates the optimization of 3D shock control bumps on a supercritical wing with a sweep angle of 16° at the 1/4 chord. A similar supercritical wing with a higher sweep angle of 24.5° at the 1/4 chord has been adopted as a baseline for the study. Numerical results show that the drag coefficient of the low sweep wing with the optimized 3D shock control bumps is reduced below that for the high sweep wing, indicating shock control bumps can be used as an effective means to reduce the wave drag caused by reducing the wing sweep angle. From the point of view of the wing structure design, lower sweep angle will also bring the benefits of weight reduction, resulting in further fuel reduction.

Aerodynamic Characteristics of Canard-Forward Swept Wing Aircraft Configurations

The aerodynamic characteristics between the canard and wing of the Canard-forward swept wing aircraft configurations have been investigated numerically at low Reynolds number. The variation of the aerodynamic characteristics at different canard positions is the focus of the present investigation. The aerodynamic interference and the mutual coupling effect between the canard and wing will have great influences on the lift, drag, and sideslip characteristics of the whole aircraft. The canard-generated vortex can induce a favorable interference onto the main wing, controlling the onset of the boundary layer separation from the leading edge. At small angles of attack ( the aerodynamic characteristics are sensitive to the relative position of the canard and the wing, but at high angles of attack ( they are not only related to the orientation of the canard (forward or backward), but also the features of the vortices generated above the canard and the wing, including their strength and location.

Computational Study of the Embedded Engine Static Pressure Thrust Propulsion System

Drag reduction is being pursued for next generation aircraft to reduce fuel consumption and pollutant production. Drag reduction of fuselages is now receiving more consideration, since the fuselage represents 30% of the drag of the entire aircraft. This paper presents numerical simulations of earlier experimental studies using aft suction slots and embedded propulsor systems to reduce fuselage drag. Detailed grid refinement studies were conducted, and different turbulence models were evaluated. A simulation of an airship with a boundary layer suction slot and an embedded engine with aft injection of the ingested flow for propulsion was conducted. The simulations require significantly more propulsor power to achieve a self-propelled condition than reported in the experiments, which were subject to large experimental uncertainties. The computational fluid dynamics flow field can be interrogated in detail, leading to improved understanding of the performance and integration of all system components. The particular arrangement studied in the experiments was shown to be limited by significant internal flow losses. This study provides the basis for the much improved integration of an embedded engine and gives the starting point for a wider analysis of more slender bodies operating in a transonic flow field for application to modern transport aircraft