Showing papers in "Journal of Aircraft in 2012"

Realization of Morphing Wings: A Multidisciplinary Challenge

Abstract: Morphing-wing research is growing in significance, as it is driven by the need to improve aircraft performance. There are many aspects to consider when designing a morphing wing, making the task a multidisciplinary challenge. This has led to a multitude of approaches to morphing-wing research. This paper provides an overview of the field, drawing together these different approaches. Morphing wings can be classified in terms of shape parameters (what to morph), performance benefits (why morph), and enabling technologies (how to morph). Regarding the structural system, the majority of morphing-wing concepts have consisted of distinguishable substructure, skin, and actuator components. However, these components need to be integrated to such a level that all share the functions of carrying loads and changing shape, thus blurring the distinction between these components. The trends include shifts from using conventional mechanisms and actuators to smart-material-based systems to topology-optimized compliant-mechanism designs. Furthermore, concepts found in nature may offer potential morphing solutions, and the working principles of muscles and plants may be emulated in a morphing wing. The focus of this paper is on morphing for traditionally fixed-wing aircraft and on the structural system in particular.

Aeroservoelastic Design Optimization of a Flexible Wing

Abstract: In this paper a multidisciplinary design optimization framework is developed that integrates control system design with aerostructural design. The equations of motion are derived for a flexible aircraft and used to perform aeroservoelastic analysis. The objective of this framework is to go beyond the current limits of aircraft performance through simultaneous design optimization of aerodynamic shape, structural sizing and control system. The control system uses load alleviation to reduce the critical structural loads. Time-domain analysis of the aircraft performing an altitude change maneuver and encountering an atmospheric gust is included in the design process. The optimal trade-off between aerodynamics, structures and control system is found by maximizing the endurance subjected to stress and maneuverability constraints. Two cases — with and without load alleviation system — are considered. Due to the proposed MDO framework, the inclusion of load alleviation system in design leads to a significant increase in endurance performance.

Flight Test Maneuvers for Efficient Aerodynamic Modeling

Abstract: Novel flight test maneuvers for efficient aerodynamic modeling were developed and demonstrated in flight. Orthogonal optimized multi-sine inputs were applied to aircraft control surfaces to excite aircraft dynamic response in all six degrees of freedom simultaneously while keeping the aircraft close to chosen reference flight conditions. Each maneuver was designed for a specific modeling task that cannot be adequately or efficiently accomplished using conventional flight test maneuvers. All of the new maneuvers were first described and explained, then demonstrated on a subscale jet transport aircraft in flight. Real-time and post-flight modeling results obtained using equation-error parameter estimation in the frequency domain were used to show the effectiveness and efficiency of the new maneuvers, as well as the quality of the aerodynamic models that can be identified from the resultant flight data.

Reduced-Order Modeling of Flutter and Limit-Cycle Oscillations Using the Sparse Volterra Series

Abstract: For the past two decades, the Volterra series reduced-order modeling approach has been successfully used for the purpose of flutter prediction, aeroelastic control design, and aeroelastic design optimization. The approach has been less successful, however, when applied to other important aeroelastic phenomena, such as aerodynamically induced limit-cycle oscillations. Similar to the Taylor series, the Volterra series is a polynomial-based approach capable of progressively approximating nonlinear behavior using quadratic, cubic, and higher-order functional expansions. Unlike the Taylor series, however, kernels of the Volterra series are multidimensional convolution integrals that are computationally expensive to identify. Thus, even though it is well known that aerodynamic nonlinearities are poorly approximated by quadratic Volterra series models, cubic and higher-order Volterra series truncations cannot be identified because their identification costs are too high. In this paper, a novel, sparse representation of the Volterra series is explored for which the identification costs are significantly lower than the identification costs of the full Volterra series. It is demonstrated that sparse Volterra reduced-order models are capable of efficiently modeling aerodynamically induced limit-cycle oscillations of the prototypical NACA 0012 benchmark model. These results demonstrate for the first time that Volterra series models are capable of modeling aerodynamically induced limitcycle oscillations.

Application of Value-Driven Design to Commercial Aero-Engine Systems

Abstract: Value-Driven Design provides a framework to enhance the systems engineering processes for the design of large systems. By employing economics in decision making, Value-Driven Design enables rational decision making in terms of the optimum business and technical solution at every level of engineering design. This paper explains the application of ValueDriven Design to the aero-engine system through two case studies, which were conducted through workshops under the Rolls-Royce plc Advanced Cost Modeling Methodologies project. The Surplus Value Theory was utilized to provide a metric that can trade-off component designs with changes in continuous and discrete design variables. Illustrative results are presented to demonstrate how the methodology and modeling approach can be used to evaluate designs and select the value-enhancing solution.

Integrated Computational/Experimental Approach to Unmanned Combat Air Vehicle Stability and Control Estimation

Abstract: 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.

Effectiveness of Wing Twist Morphing in Roll Control

Abstract: This paper is focused on numerical investigations that analyze the advantages obtained from high-aspect-ratio wingswithunconventionalrollcontrolstrategiesbasedonwingtwistmorphing.ThesailplaneG103-B,produced by the GROB Werke company, was chosen as the reference aircraft for the analyses. For confidentiality reasons, the datadisclosedbythe buildercovered only general properties suchas themain dimensions, the liftingsurface airfoils and attitudes, the characteristic speeds, and a rough mass budget. As a consequence of this, “reverse-engineering” wasconsiderednecessarytodefineareasonablewingstructurallayoutthatenabledtheanalysisoftheelastic-aircraft rolldynamics.ApreliminarysizingofthewingstructurewasaddressedusingCS-22airworthinessrequirementsand by adopting fast elementary approaches. The estimated structural arrangement, which was verified using a finite element analysis, was then used to generate the aircraft aeroelastic model. The conventional (aileron-based) and the unconventional (wing twist morphing) roll control strategies were compared from the aerodynamic and the aeroelastic standpoints, and the benefits achieved with the unconventional strategy are summarized.

Real-Time Aerodynamic Parameter Estimation without Air Flow Angle Measurements

Abstract: A technique for estimating aerodynamic parameters in real time from flight data without air flow angle measurements is described and demonstrated. The method is applied to simulated F-16 data, and to flight data from a subscale jet transport aircraft. Modeling results obtained with the new approach using flight data without air flow angle measurements were compared to modeling results computed conventionally using flight data that included air flow angle measurements. Comparisons demonstrated that the new technique can provide accurate aerodynamic modeling results without air flow angle measurements, which are often difficult and expensive to obtain. Implications for efficient flight testing and flight safety are discussed.

Flow Physics Analyses of a Generic Unmanned Combat Aerial Vehicle Configuration

Abstract: Within the NATO Research and Technology Organisation Applied Vehicle Technology (AVT)-161 task group, titled “Assessment of Stability and Control Predictions for NATO Air and Sea Vehicles,” a 53 swept and twisted lambda wing with rounded leading edges is considered. In a first step, the symmetric flow conditions are analyzed in this paper in order to understand the corresponding flow physics. Experiments by the task group are used to develop proper numerical simulation tools for further applications in the design process of unmanned combat aerial vehicles as a part of future air-combat systems. The philosophy of the configuration under consideration is explained. The vortical flowfield is simulated using the DLR, German Aerospace Center TAU-Code applied with different turbulence models on various computational grids. Finally, a best practice is evaluated for medium and large angles of attack. A combination of these numerical results and experimental data lead to a proper understanding of the complex flow structure. Furthermore, this paper addresses the necessity for the predictability and understanding of controlled and uncontrolled flow separation, together with the interaction of the corresponding vortex systems in order to estimate stability and control issues for the entire flight envelope.

Ship-Helicopter Operating Limits Prediction Using Piloted Flight Simulation and Time-Accurate Airwakes

Abstract: This paper gives an overview of the ship―helicopter dynamic interface simulation facility at the University of Liverpool, with an emphasis on recent improvements made through the inclusion of unsteady computational fluid dynamics (CFD) ship airwake data. A FLIGHTLAB model of an SH-60B Seahawk helicopter has been flown in a full motion base simulator to the deck of a Type 23 frigate and a Wave class auxiliary oiler, under the influence of unsteady airwakes derived from CFD. Pilot workload ratings have been obtained for the deck landing task, using both the Bedford workload rating scale and the deck interface pilot effort scale, from which fully simulated ship―helicopter operating limits have been derived. Analysis of pilot ratings, comments, and control inputs has also enabled both subjective and objective assessments of workloads at various wind-over-deck conditions, highlighting the dominant aerodynamic airwake features which contribute to the difficulty of the landing task. Having access to the underlying CFD data allows the aircraft handling qualities and pilot workload to be correlated with the aerodynamic characteristics of the airwake and identification of the geometric features of the ship that cause them.

Requirements, Issues, and Challenges for Sense and Avoid in Unmanned Aircraft Systems

Abstract: The sense and avoid capability is one of the greatest challenges that has to be addressed to safely integrate unmanned aircraft systems into civil and nonsegregated airspace. This paper gives a review of existing regulations, recommended practices, and standards in sense and avoid for unmanned aircraft systems. Gaps and issues are identified, as are the different factors that are likely to affect actual sense and avoid requirements. It is found that the operational environment (flight altitude, meteorological conditions, and class of airspace) plays an important role when determining the type of flying hazards that the unmanned aircraft system might encounter. In addition, the automation level and the data-link architecture of the unmanned aircraft system are key factors that will definitely determine the sense and avoid system requirements. Tactical unmanned aircraft, performing similar missions to general aviation, are found to be the most challenging systems from an sense and avoid point of view, and further research and development efforts are still needed before their seamless integration into nonsegregated airspace

Multifunctional Unmanned Aerial Vehicle Wing Spar for Low-Power Generation and Storage

Abstract: DOI: 10.2514/1.C031542 This paper presents the investigation of a multifunctional energy harvesting and energy-storage wing spar for unmanned aerial vehicles. Multifunctional material systems combine several functionalities into a single device in order to increase performance while limiting mass and volume. Multifunctional energy harvesting can be used to provide power to remote low-power sensors on unmanned aerial vehicles, where the added weight or volume of conventional harvesting designs can hinder flight performance. In this paper, a prototype self-charging wing spar containing embedded piezoelectric and battery elements is modeled, fabricated, and tested to evaluate its energy harvesting and storage performance. A coupled electromechanical model based on the assumed modes method is developedtopredictthevibrationresponseandvoltageresponseofacantileveredwingsparexcitedunderharmonic base excitation. Experiments are performed on a representative self-charging wing spar, and the results are used to verify the electromechanical model. The power-generation performance of the self-charging wing spar is investigatedindetailforharmonicexcitationinclamped–freeboundaryconditions.Experimentsarealsoconducted to demonstrate the ability of the wing spar to simultaneously harvest and store electrical energy in a multifunctional manner.Itisshownthat,foraninputbaseaccelerationlevelof0:25 gat28.4Hzatthebaseofthestructure,1.5mW of regulated dc power isdelivered from the piezoelectric layers to the thin-film battery, resulting in a stored capacity of 0.362 mAh in 1 h.

Using a Simple Crack Growth Model in Predicting Remaining Useful Life

Abstract: DOI: 10.2514/1.C031808The remaining useful life of a system can be predicted from available data and/or physical models, which iscommonly known as prognosis. In this paper, the remaining useful life (e.g., the number of cycles to failure) of asystemexperiencingfatiguecrackgrowthisestimatedusingasimplephysicalmodel.Thispapershowsthatasimplemodel,suchastheParismodelwithanassumedanalyticalstress-intensityfactor,canbeusedforcomplexgeometriesbycompensatingfortheerrorintheassumedstress-intensityfactorbyadjustingmodelparameters.Theadjustmentis done automatically by a process of Bayesian identification. True damage growth is simulated using the extendedfinite-elementmethodtomodeltheeffectsofcracklocationandgeometryontherelationshipbetweencracksizeandstress-intensityfactor.Thedetectionprocessofcracksizeusingstructuralhealthmonitoringsystemsismodeledbyadding random noise and a deterministic bias to the simulated damage growth. Equivalent model parameters arethenidentifiedusingtheBayesianinference,fromwhichtheremainingusefullifeisestimated.Usingthreeexamplesofdamagegeometries,itisshownthattheremainingusefullifeestimatesareaccurateevenwiththeuseofanassumedanalytical stress-intensity factor.

Design of a Morphing Airfoil with Composite Chiral Structure

Abstract: The paper presents a design study for a morphing structural concept, which could be used to obtain a passively actuated high-lift wing configuration. A composite chiral honeycomb core is used to allow large variations of camber at limited strain levels in the structure of the aerodynamic surface. The design hypothesis is first assessed bymeans of structural analyses, which are performed applying two-dimensional and three-dimensional finite elements schemes. The results confirm the morphing capabilities in the chordwise direction of the structure, which still retains noteworthy axial and torsional stiffness properties. The aeroelastic performances of the morphing airfoil are then optimized, taking into account aeroelastic stability as well as strength constraints. The optimal parameters of chiral network and the required stiffness properties of the covering skin are identified. Overall, the work confirms the promising performances of morphing structures based on chiral topologies and assesses a numerical approach for the design of morphing aerodynamic structures.

En Route Speed Reduction Concept for Absorbing Air Traffic Flow Management Delays

Abstract: This paper proposes an en route speed reduction to complement current ground delay practices in air traffic flow management. Given a nominal cruise speed, there exists a bounded range of speeds that allows aircraft to fly slower with the same or lower fuel consumption than the nominal flight. Therefore, flight times are increased and delay can be partially performed in the air, at no extra fuel cost for the operator. This concept has been analyzed in an initial feasibility study, computing the maximum amount of delay that can be performed in the air in some representative flights. The impact on fuel consumption has been analyzed, and two scenarios are proposed: the flight fuel remains the same as in the nominal flight, and some extra fuel allowance is permitted in order to face uncertainties. Results show significant values of airborne delay that may be useful in many situations, with the exception of short hauls where airborne delay may be too short. If cruise altitude is changed, the amount of airborne delay increases, since changes in cruise speed modify the optimal flight altitudes. From the analyzed flights, a linear dependency is found relating the airborne delay with the amount of extra fuel allowance.

Evaluation of a Correlation-Based Transition Model and Comparison with the eN Method

Abstract: The correlation-based γ-Reθ transition model has been implemented into a hybrid Reynolds-averaged Navier-Stokes solver and evaluated on various test cases. The original model formulation and recently published approaches of different authors for relevant empirical model functions as well as different transition criteria are compared. The prediction of transition with the correlation-based model was applied to a zero pressure gradient flat plate test case and some well known one -element airfoil test cases. Comparison with the standard approach for transition prediction in the flow solver used based on the e N method was accomplished. Simulation results in terms of transition locations and skin friction coefficient distributions , performance and arising difficulties of both models for the various test cases are presented and discussed.

Numerical Analysis of Effects of Leading-Edge Protuberances on Aircraft Wing Performance

Abstract: This thesis investigates the effects of biologically inspired leading-edge protuberances on aircraft wings. The study of humpback whales and their flipper performance was the impetus to modifying the leading edge of an aircraft wing in order to gain an aerodynamic advantage during flight. This study examines the effect of leading-edge modification on wing performance at a low Reynolds number (Re), since low Reynolds number flows have unique features, and the knowledge about this flight regime is extremely important for small aircraft, called unmanned aerial vehicles (UAVs), flying at low speeds. Simulations were executed on wings with leading-edge sinusoidal protuberances, in order to compare the lift and drag characteristics with that of a wing with a smooth leading edge. All wings had the same cross section of National Advisory Committee for Aeronautics (NACA) 2412 and a simulated Reynolds number of 5.7 * 10^5. Results from numerical simulations revealed that a decrease in lift and an increase in drag was observed at low angles of attack (AoA) in all cases of the modified wings. At higher angles (α ≥16o), the lift of the modified wings was up to 48% greater than the baseline wing, with 44% less drag or no drag penalty. The amplitude of protuberances significantly affects wing performance. Although the maximum lift generated by modified wings was lower than baseline, protuberances along the leading edge of the wing proved to have a profound advantage in obtaining higher lift at high angles of attack.

Aerodynamic Loading Characteristics of a Model-Scale Helicopter in a Ship's Airwake

Abstract: Experiments have been conducted in a water tunnel using a specially designed Airwake Dynamometer (AirDyn) to characterize the aerodynamic loading of a helicopter immersed in the airwake of a generic frigate ship. The AirDyn is a 1:54 model-scale helicopter based on a Merlin AW-101, with a six-component force balance mounted inside the fuselage and a simplified spinning main rotor. The AirDyn has been used to measure the unsteady forces and moments imposed by the ship airwake at fixed locations along the flight path of a landing maneuver for a headwind and a 45 wind-angle. A region of ‘thrust-deficit’ in the headwind and a ‘pressure-wall’ in the 45 wind-angle were identified as ‘time-averaged’ loading characteristics caused by spatial velocity gradients in the airwake. The unsteady loading on the AirDyn in the headwind was compared with the 45 case in terms of the severity of the airwake disturbances and the stages of the landing maneuver at which they are significant. The causes of the observed AirDyn loading characteristics have been explained using unsteady Computational Fluid Dynamics analysis of the ship airwakes. The implications of the AirDyn aerodynamic loading characteristics for pilot workload and control input strategies during a real landing are discussed.

Static and Forced-Oscillation Tests of a Generic Unmanned Combat Air Vehicle

Abstract: A series of three wind-tunnel static and forced-oscillation tests were conducted on a model of a generic unmanned combat air vehicle. These tests are part of an international research effort to assess the state of the art of computational fluid dynamics methods to predict the static and dynamic stability and control characteristics. The experimental data set includes not only force and moment time histories, but also surface pressure and offbody particle image velocimetry measurements. The extent of the data precludes a full examination within the scope of this paper. This paper provides a general description and selected examples of the available static and dynamic data, as well as some of the observed trends.

Review of Turbulent Boundary Layer Models for Acoustic Analysis

Abstract: A survey of the available turbulent boundary layer models for acoustic analysis is presented. The empirical mean square pressure models predict a relatively large spread in the values; bounded on the high end by Kraichnan and on the low end by Bull, and Willmarth and Wooldridge. The single point wall pressure spectrum models are evaluated and the two more modern models of Smol’yakov and Goody seem to perform the best. Important features of the normalized wavenumber-frequency spectrum models are presented. Separable models in the Corcos class tend to over-predict the response for a range of cases.

In Situ Velocity Measurements in the Near-Wake of a Ship Superstructure

Abstract: Velocity measurements in a ship airwake are obtained in situ aboard a 108 ft naval training vessel. The measurements and analyses aremotivated by the need for validation data for airwake computational fluid dynamics simulations. Three-component anemometers are placed above the bow of the ship and at numerous locations above a flight deck at the stern of the ship. Data are presented for a direct headwind (nominally 0 deg wind-over-deck). The mean velocity field shows a clear structure to the flow, dominated by a recirculation region in the near-wake of a hangar-like backward-facing step. The location of this primary vortex and the reattachment point on the flight deck are estimated. Reynolds stresses are presented to quantify the turbulent fluctuations, which are required for the prediction of unsteady loading on rotorcraft operating in this environment. Significant anisotropy is measured in the wake, both within the primary vortex and in the far field. The peak Reynolds shear stress is located in the recirculation region, while the streamwise normal stress is found to increase with height throughout the measurement domain. Finally, autoand two-point velocity correlations from the flight deck provide an estimate of flow scales, showing the potential influence of turbulence on piloted helicopter operations.

Adaptive Reduced-Order-Model-Based Control-Law Design for Active Flutter Suppression

Abstract: The design of classic active flutter controllers has often been based on low-fidelity and low-accuracy linear aerodynamic models. Most of these models were usually treated as a linear time-invariant system, without considering time-varying parameters, such as the Mach number, the angle of attack, the Reynolds numbers, etc. A high-fidelity reduced-order model based on the proper orthogonal decomposition adaptation algorithm is used to develop a new general linear parameter-varying aeroservoelastic model with aerodynamic nonlinearity. A robust gain-scheduling control-law design method for active flutter suppression based on the proposed linear parametervarying model is investigated. The proposed design method is demonstrated with the Goland wing aeroelastic model. The simulation results show that the linear parameter-varying gain-scheduled controller can effectively suppress flutter over a range of airspeeds, and the flutter boundary in the transonic regime is simultaneously increased by nearly 20% to 30%.

Structural and Aeroelastic Characteristics of Truss-Braced Wings: A Parametric Study

Abstract: flexible in nature. This paper presents a set of parametric studies of truss-braced-wing configurations to understand the influence of the wing geometry parameters on the wing structural and aeroelastic characteristics. The primary parametersconsideredherearethewinghalf-span,strutsweep,spanwiselocationofwing-strutjoint,andnumberof truss members in the wing configuration. Each truss-braced-wing parametric configuration is sized based on strength considerations and studied for aeroelastic behavior. The results indicate strong influence of all the parameters considered here. For most cases, increasing the half-span monotonically increases the wing weight and reduces both the natural frequencies and the flutter speed. A larger difference between the wing- and strut sweep angles is seen to increase wing weight, but with a positive influence on flutter speed for various truss-braced-wing configurations. The spanwise intersection location has distinct optima for wing weight and flutter speed, which typically lie in between 55 and 70%. These results are expected to provide guidance for future multidisciplinary design optimization studies for truss-braced-wing configurations.

Shape, Structure, and Kinematic Parameterization of a Power-Optimal Hovering Wing

Abstract: In this paper, we investigate the aeroelastic hovering motions of a highly-flexible flapping wing. It is desired to parameterize the wing shape, structural composition, and kinematic hovering motions, and then minimize the peak power required during the stroke, subject to trim and mechanical failure constraints. The aeroelastic model couples a nonlinear threedimensional beam model to a quasi-steady blade element aerodynamic model, which is then solved in an implicit time-marching manner (with sub-iterations within each time step to accommodate various nonlinearities) until the response becomes time-periodic. Gradients of the response with respect to the disparate design variables are computed analytically for optimization. Power-optimal flapping configurations are found to exploit interdependencies among the three types of design variables to effectively tailor the aeroelastic response.