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Integrated Computational/Experimental Approach to Unmanned Combat Air Vehicle Stability and Control Estimation
Abstract and Figures
A comprehensive research program designed to investigate the ability of computational methods to predict
stability and control characteristics of a generic unmanned combat air vehicle has been undertaken. The integrated
approach to simulating static and dynamic stability characteristics was performed by the NATO Research and
Technology Organization Task Group AVT-161. The vehicle named Stability and Control CONfiguration
(SACCON) was the subject of an intensive computational and experimental study. The stability characteristics of the
vehicle were evaluated via a highly integrated approach, where computational fluid dynamics and experimental
results were used in a parallel and collaborative fashion. The results show that computational methods have made
great strides in predicting static and dynamic stability characteristics, but several key issues need to be resolved
before efficient, affordable, and reliable predictions are available.
Figures - uploaded by Andreas Schütte
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... However, the design of UCAV configurations is a complex task because it requires a high level of integration within the subsystems combined with vorticial flow structures. Moreover, the flow around these configurations needs additional experimental and simulation investigations for verification [8,23]. ...
... Table 1 shows examples of AVT activities that contributed to combat aircraft and UCAV design and performance assessment. Generally, AVT research task groups cover core topics [17,25] such as CFD methods and code validation [8,14,23,[26][27][28]; the study of flow physics and aerodynamic characteristics [22,[29][30][31][32]; predictions of stability and control (S&C) [8,28,[33][34][35][36]; design selection, specification, requirements, and control ideas [37,38]; multidisciplinary design [15,16,18,20]; aeroacoustics [39,40]; and infrared and radar signatures [41,42]. ...
... The conceptual SACCON was developed with the help of the NATO STO/AVT-161 task group [14,15,23,27,[43][44][45][46]. It is a highly swept (Λ 53 deg) lambda wing UCAV model with a parallel LE and TE [14,23]. ...
The design of unmanned combat aerial vehicles (UCAVs) is primarily governed by the low-observability requirement for military applications rather than aerodynamic performance. The conceptual design and optimization of UCAV models via size and shape variables for different missions in different flow regimes form a research area for military vehicle design. Flying wing UCAVs experience flow separation during takeoff and landing, and furthermore exhibit stability issues. The aerodynamic performance of these UCAVs can be significantly improved by redesigning their leading-edge sweep angle and wing planform. In the present work, the initial weight determination, aerodynamic sizing, and planform with and without inlet lip integration, and the conceptual design of a nonconstant leading-edge flying wing UCAV configuration are performed. Next, the obtained conceptual design is downscaled 1:20 to be used as a wind-tunnel model and optimized for low-speed conditions using a kriging-based surrogate model with a vortex-lattice method to maximize the lift-to-drag ratio. Later, the optimized design is validated using an open-source computational fluid dynamics code, OpenFOAM 8.0, to verify the accuracy of the surrogate model and to investigate the aerodynamic characteristics. The optimized UCAV design exhibited improved aerodynamic characteristics in terms of the lift-to-drag ratio. Furthermore, the aerodynamic performance and flowfield of the optimized UCAV model with and without inlet lip integration have been evaluated at low and high speeds.
... Oil film visualization was conducted in the lowspeed wind tunnel and water channel, and the surface pressure measurement was made in the wind tunnel. Subsequently, detailed discussion was conducted to compare the results obtained in this study and those reported in the references (Luckring et al., 2016;Cummings and Schütte, 2012;Schütte et al., 2012). ...
... In this study, SACCON model is adopted as the reference model to understand the effect of spanwise various leadingedge contours. This model was proposed by NATO STO, especially by task group AVT-161 (Luckring et al., 2016;Cummings and Schütte, 2012;Schütte et al., 2012). However, the details of the SACCON configuration were not obtainable in public. ...
... However, the details of the SACCON configuration were not obtainable in public. Our model was designed and made according to the cross-section profiles of SACCON described in the paper (Cummings and Schütte, 2012). ...
The vortex system on the SACCON model is studied under low Reynolds number by using the flow visualization technique and the surface pressure testing measurement in a low-speed water channel and a wind tunnel, respectively. A main topic is discussed below. The blunt leading-edge section on this SACCON model induces various flow phenomena under different angles of attack. The attached flow appearing around the upstream leading-edge region at a low angle of attack induces different flow phenomena downstream, so does the branched vortices appearing near the trailing-edge. Although this attached flow is vanished at higher angle of attack, the stability of forming an outboard vortex can be shown in the results of pressure coefficients as a function of an angle of attack. Moreover, how the blunt leading-edge contour on SACCON model and other models affecting flow field can be compared with the findings given in the previous studies. The models with only blunt leading-edge contour can prompt an outboard vortex and an inner vortex, compared to the flow fields on the models with spanwise-varying leading-edge contours. This inner vortex is formed due to the attached flow inboard passing downstream and being affected by the outboard vortex. However, this attached flow is different from the attached flow observed on SACCON model. Therefore, flow phenomena occurring on different blunt leading-edge models can be differentiated.
... The program which started with a research team, namely, AVT-080, which focuses on the aerodynamics properties of a sharp leading-edge profile, followed by another research team, namely, AVT-113, to further investigate the vortex structure of blunt leading-edge profile [4]. Moreover, an additional research team, namely, AVT-161, has been commenced to concentrate on the assessment of stability and control prediction methods for air and sea vehicles including a new configuration concept called the stability and control configuration (SACCON) to carry out numerical and experimental investigations on the stability and control of the SACCON [5]. Furthermore, the SACCON design is a lambda wing with a leading edge sweep angle of 53°and 5°t wist at the wing tip to delay the stall at high angle of attack. ...
... It also comes with three main sections, namely, the fuselage, outer wing section, and wing tip with different leading-edge profiles starting from the apex towards the trailing edge. The current results illustrate that the flow over the SACCON is very complicated due to multiple leading-edge flow separations [5,6]. In the same way, a research group under AVT program, namely, AVT-183, to investigate and predict the onset of the flow separation of the newly introduced configuration named DIAMOND wing which is simplified from SACCON design. ...
This paper highlights the results and comparison of the flow topology investigation above the unmanned combat aerial vehicle (UCAV) configuration, namely, multidisciplinary design configuration (MULDICON), with modified leading-edge profile at the apex region from a sharp to a blunt profile to reduce the complexity of the flow structure above the wing. It was found from the flow visualization results that at a low angle of attack, for instance, 10°, the onset of the flow separation took place near the apex region; the onset of a tip vortex at the wing tip was also detected. At a medium angle of attack, for instance, 15°, the onset of the flow separation moved further upstream with the formation of the apex vortex, and the magnitude of the tip vortex increased due to increasing incoming flow with increasing the angle of attack. At higher angle of attack, for instance, 20°, the apex vortex intensity increased and wing tip vortices shedding is observed. Furthermore, at an angle of attack of 25°, the configuration is partially stalled, while a complete stalled occurred at an angle of attack of 30°. The current results obtained from this study have shown that the configuration has a maximum lift coefficient of 0.8 obtained from the K-Omega-SST turbulence model while it is 0.93 calculated from the Spalart-Allmaras turbulence model, while the maximum drag coefficient is 0.31 and 0.35, respectively, when calculated for the K-Omega-SST turbulence model and the Spalart-Allmaras turbulence model at an AOA of 25°. The flow visualization results revealed that there is a single flow separation due to modified leading edge from sharp to blunt, thus flow complexity is reduced.
... Hence, the plane maintains its basic triangle or lambda shape. The wing construction is delta (triangular) and stands independently without a fuselage (Cummings & Schütte, 2012). ...
Unmanned Combat Aerial Vehicles (UCAVs) are designed to be lightweight and compact, which can impact their overall lift and aerodynamic capabilities. This study focuses on enhancing the Coefficient of Lift (CL) by optimising the Back Sweep Angle in the Lambda wing-UCAV. The model's baseline geometry remains unchanged during the experimental and numerical analysis, while different back sweep angles ranging from δ=0 0 to δ=50 0 are investigated at varied free-stream velocities and angles of attack. This helps to understand the generation of induced lift in the intricate shapes of the Lambda Wing. The results indicate a 5% to 10% increase in the lift for every 10 0 increments of the Back Sweep Angle, and the vortices' strength increases and reaches a maximum at δ=40 0. At greater angles (δ >40 0), the lift drops gradually with the Reynolds number. The stagnation point shifts from 25% to 35% along the chord towards the pressure surface as the angles of attack increase from α=5 0 to α=10 0. The angle of attack α>10 0 .
... At the low k, Fig. 18 shows that the DSV shedding is very serious with more non-linear fluctuations in aerodynamic responses (Fig. 16). One reasonable explanation is that the unsteady flow structures have enough time to undergo transitions at the low reduced frequency [44], and increasing k can also slow the flow field transitions resulting from the DSV movement (Fig. 18). ...
Dynamic stall is the main reason for the low power efficiency of vertical axis wind turbines (VAWTs) at low tip speed ratios, where the utilizable wind kinetic energy is also high. However, the determining factors behind VAWT dynamic stall have not been adequately investigated. Therefore, we carefully investigated VAWT dynamic stall under the different Reynolds numbers (Re) and reduced frequencies (k). The unsteady flow characteristics are identified using transitional URANS simulations. Although the blade undergoes significant vortex movements during each revolution, the output power is primarily determined by the aerodynamic responses close to the onset of dynamic stall. Increasing Re and k can impact dynamic stall behaviors significantly and also improve VAWT performance effectively. As both Re and k increase, the onset of dynamic stall is progressively postponed to a higher angle of attack, effectively suppressing the separated flow. Dynamic stall behaviors are therefore changed from abrupt stall to moderate stall with attenuated laminar separation bubble (LSB) bursting and dynamic stall vortex (DSV) shedding. An increase in Re hardens the LSB and prevents the DSV formation due to strong inertial forces, particularly at low Re. On the other hand, increasing k slows the flow-field transitions and delays the onset of LSB bursting and DSV shedding, particularly at high k.
... Figure 19 shows the X45A, X47A, and SACCON. The NATO-RTO AVT-161 Task Group [66] was a comprehensive research program designed to investigate the ability of computational methods to predict stability and control characteristics of a generic unmanned combat air vehicle, (SACCON). ...
In the early era of aviation, Frederick Lanchester was both an inventor and a theoretician driven by the need for a theory of flight that would reduce the guesswork in designing new aircraft. His book Aerodynamics in 1907 laid down the early foundations of such a theory. The theory with contributions from others, notably Ludwig Prandtl, was refined to become the basis for the sleek designs of WWII aircraft brought about with little guesswork. New technology changed aircraft design radically with the increased speed of jet propulsion reaching into the transonic range with nonlinear aerodynamics. In the late 1940s and early 1950s substantial guesswork returned to aircraft design. The legacy of Lanchester et al., however, lived on with the development of computational fluid dynamics (CFD) that could guide designers through nonlinear transonic effects. This article presents a historical sketch of how CFD developed, illustrated with examples explaining some of the difficulties overcome in the design of the first-generation swept-wing transonic fighters. The historical study is forensic CFD in search for the likely explanation of the designer’s choice for the wing shape that went into production a long time ago. The capability of current CFD applied to the aerodynamics of aircraft with slender wings is surveyed. The cases discussed involve flow patterns with coherent vortices over hybrid wings and wings of moderate sweep. Vortex-flow aerodynamics pertains to understanding the interaction of concentrated vortices with aircraft components. Modern Reynolds-Averaged Navier-Stokes (RANS) technology is useful to predict attached flow. But vortex interaction with other vortices and breakdown lead to unsteady, largely separated flow which has been found out of scope for RANS. Direct simulation of the Navier-Stokes equations is out of computational reach in the foreseeable future, and the need for better physical modeling is evident. Both cruise performance and stalling characteristics are influenced by strong interactions. Two important aspects of wing-flow physics are discussed: separation from a smooth surface that creates a vortex, and vortex bursting, the abrupt breakdown of a vortex with a subsequent loss of lift. Vortex aerodynamics of not-so-slender wings encounter particularly challenging problems, and it is shown how the design of early-generation operational aircraft surmounted these difficulties. Through use of forensic CFD, the article concludes with two case studies of aerodynamic design: how the Saab J29A wing maintains control authority near stall, and how the Saab J32 mitigates pitch-up instability at high incidence.
... The latter one was designed especially for the AVT-161 task group, with the aim to exhibit a highly complex aerodynamic behavior, serving as a challenge for numerical flow prediction using CFD methods. An overview of the AVT-161 Task Group is provided by (Cummings and Schütte, 2012b;Cummings and Schütte, 2012a). ...
This article provides an overview about the activities performed within the NATO STO Research Task Group AVT-251 on "Multi-Disciplinary design and performance assessment of effective, agile NATO Air Vehicles". After a brief introduction to the preceding task groups and the research questions that led to the formation of AVT-251, the selection of design requirements is discussed and the approach for developing the MULDICON UCAV configuration out of its predecessor, the SACCON concept, is described. A short summary presents the work performed by the four design teams, each of which was responsible for one of the major topics on which the design task had been focused (aerodynamic shaping, control concept, engine integration, and structural concept). The task of a fifth team was two-fold: initially, it was responsible for the specification of the design requirements; later in the process, it had to join together the results of the other four teams into an overall aircraft concept and to assess this concept with respect to the initially specified set of requirements. Finally, a concluding summary of the MULDICON concept, as well as of the research questions of the AVT-251 task group are presented.
Bu çalışmada, Silahsız İnsansız Hava Araçları konulu bilimsel yayınların Web of Science Veri Tabanı Çekirdek Koleksiyonu çerçevesinde bibliyometrik bir analizi yapılmıştır. Analiz neticesinde, son 10 yılda özellikle araştırma odaklı yayınlarda bir artış eğilimi olmakla birlikte, başta incelem odaklı olmak üzere hala yeterli sayıda üst düzey endeksli çalışma olmadığı görülmüştür. Ayrıca, yapılan çalışmalarda uluslararası yazarlık oranı çok düşük olup, genellikle çalışmalar çok yazarlı ve ulusal düzeyde yapılmıştır. Çalışmaların neredeyse tamamı teknik ve mühendislik disiplinlerine odaklanmış olup, sosyal bilimlerle disiplinler arası çalışma düzeyi henüz çok yetersiz seviyededir. Gerek genel performans analizi gerekse de bibliyometrik analiz neticesinde elde edilen bulgular, yazar, yayın, atıf, kurum ve ülke bağlamında önce Çin’in ardından da ABD’nin bu sahadaki çalışmalarda diğer ülkelere kıyasla açık ara öncül pozisyonda olduklarını göstermiştir.
The leading-edge sweep angle significantly influences the flight performance of swept wings. The vortex flow structures affect the flow behavior of the moderate to highly swept wings, which form at low angles of attack because of leading-edge flow separation. This topic of investigation gained momentum due to its application to unmanned combat aerial vehicle (UCAV) configurations that are significantly affected by flow separation at high angles of attack. The present work investigates the flowfield and low-speed aerodynamic characteristics of the generic UCAV models with a constant and nonconstant leading-edge sweep for a wide range of angles of attack. The open-source computational fluid dynamics code OpenFOAM 8.0 is employed for the numerical simulations. The present numerical simulation results agree well with the experimental data in the literature. It is demonstrated that the lift and drag characteristics of the UCAV are sensitive to the leading-edge sweep angle, and a nonconstant leading-edge sweep angle variant exhibits delayed stall and enhanced aerodynamic performance.
A comprehensive research program designed to investigate the ability of computational methods to predict stability and control characteristics of realistic flight vehicles has been undertaken. The integrated approach to simulating static and dynamic stability characteristics for a generic UCAV named SACCON (Stability and Control Configuration), was performed by NATO RTO Task Group AVT-161. The static and dynamics stability characteristics of the vehicles were evaluated by a group of researchers using the Cobalt Navier-Stokes flow solver. Comparisons with experimental results show that fine grids coupled with appropriate RANS and hybrid turbulence models yield good predictions for both the static and dynamic aerodynamics of the vehicle, including the estimation of stability derivatives.
The paper describes a method for viscous flow prediction which includes the prediction of transition onset location as an integral part of the calculation. The method is similar to others in that it is based on stability analysis of the boundary layers but acknowledges that solutions of the Reynolds-averaged Navier-Stokes (RANS) equations yield boundary layers of inadequate accuracy when conventional mesh sizes are used. The boundary layers are therefore recomputed to high accuracy by solving the laminar boundary layer equations. Transition onset locations determined by the stability analysis are passed back to the RANS solver for further iterations. The method described in this paper differs from others in that the laminar boundary layer calculation is fully three-dimensional and the stability analysis is through the fully three-dimensional parabolised stability equations for compressible flow. Results are presented to illustrate the capability of the laminar boundary layer method and the effectiveness of the method for solving the parabolised stability equations. In addition, preliminary validation results are presented from calculations of the viscous flow over the AFRL 1303 UCAV concept to demonstrate that the fully three-dimensional laminar boundary layer and stability analysis components can be combined successfully with a RANS method for calculating flows with natural transition. Copyright © 2009 by QinetiQ Ltd., Imperial College, London, and Horton Aerodynamics Ltd.
The numerical simulation of the flow for the VFE-2 delta wing configuration with rounded leading edges is presented using the Cobalt Navier–Stokes solver. Cobalt uses a cell-centered unstructured hybrid mesh approach, and several numerical results are presented for the steady RANS equations as well as for the unsteady DES and DDES hybrid approaches. Within this paper the focus is related to the dual primary vortex flow topology, especially the sensitivity of the flow to angle of attack and Reynolds number effects. Reasonable results are obtained with both steady RANS and SA-DDES simulations. The results are compared and verified by experimental data, including surface pressure and pressure sensitive paint results, and recommendations for improving future simulations are made.
The numerical simulation of the flow around a 65° delta wing configuration with rounded leading edges is presented. For the numerical simulation the DLR TAU-Code is used which is based on an unstructured hybrid mesh approach. Within this paper several numerical results are shown, solving the steady RANS equations by different turbulence models. The simulations are carried out within the RTO/AVT 113 working group focusing on experimental and numerical research on delta wing configurations with rounded leading edges. Within this paper the focus is related to the flow topology depending on the angle of attack as well as on Reynolds number effects. Finally the results will be compared and verified by experimental data.
Nine organizations participated in the Cranked-Arrow Wing Aerodynamics Project International study and have contributed steady and unsteady viscous simulations of a full-scale semispan model of the F-16XL aircraft. Three different categories of flight Reynolds/Mach number combinations are computed and compared with flight-test measurements for the purpose of code validation and improved understanding of the flight physics. Steady-state simulations are done with several turbulence models (of different complexity, with no topology information required) that overcome Boussinesq-assumption problems in vortical flows. Detached-eddy simulation and its successor, delayed detached-eddy simulation, are used to compute the time-accurate flow development. Common structured and unstructured grids as well as individually adapted unstructured grids were used. Although discrepancies are observed in the comparisons, overall reasonable agreement is demonstrated for surface pressure distribution, local skin friction, and boundary velocity profiles at subsonic speeds. The physical modeling, be it steady or unsteady flow, and the grid resolution both contribute to the discrepancies observed in the comparisons with flight data, but at this time, how much each part contributes to the whole cannot be determined. Overall, it can be said that the technology readiness of computational fluid dynamics simulation technology for the study of vehicle performance has matured since 2001, such that it can be used today with a reasonable level of confidence for complex configurations.
The flow-field around a "1303" UCAV configuration is computationally simulated using a high-order overset-grid algorithm for a variety of Reynolds numbers and angles of attack. The high-order spatial scheme used here is based on a compact, sixth-order finite-differencing with a tenth-order, adjustable, Padé-type filter. Turbulence modeling is provided by an implicit large-eddy simulation technique whereby the discriminating filter is used to regularize unresolved scales in the computation. Overset-grid techniques, including high-order interpolation and one-sided operators at hole and computational boundaries, are employed to simplify grid generation and provide a domain decomposition capability for parallel processing. Results are obtained at Reynolds numbers based on the mean aerodynamic chord of Remac = 17927, 29879, and 59758 and at angles of attack of 6°, 12°, and 15°. The computations are compared to experimental PIV-data recently obtained at these conditions. Particular focus is placed on the validation of the computational approach by comparing mean stream-wise velocity and span-wise normal vorticity between computation and experiment at a variety of span-wise and stream-wise locations.
The Second International Vortex Flow Experiment provided a variety of experimental data for a 65° delta wing with sharp and blunt/rounded leading edges. Flow measurements including forces and moments, surface pressures, Pressure Sensitive Paint measurements, and off-surface flow variables from Particle Image Velocimetry were made available for comparisons with computational simulations. A number of test cases were chosen for simulation by seven numerical groups, and a summary of their results is presented here. The ability of computational fluid dynamics to predict such flow features as the dual primary vortex system found on the blunt leading edge configuration and a shock/vortex interaction for the sharp leading edge are assessed. While computational simulation has made great strides in recent years, there are still areas where further improvement can be made, including in turbulence modeling, transition modeling, and the ability to accurately compute unsteady flows. Copyright © 2008 by the American Institute of Aeronautics and Astronautics, Inc.