Gust Load Research Articles

Robust Gust Load Alleviation of Flexible Aircraft Equipped with Lidar

This work concerns the problem of gust load alleviation of a flexible aircraft by focusing on Airbus’s XRF1 aircraft concept, with a fully actuated wing. The aircraft is equipped with a lidar sensor, which measures the wind velocity ahead of it, together with standard sensors. Based on the available measurements, controllers are then designed by and syntheses, with emphasis put on the multiple-input multiple-output robustness in order to reduce a selected set of loads due to the wind, hence potentially saving mass in the aircraft design. A state-space model of the flexible aircraft is obtained by means of an aeroelastic computation and system identification from frequency data. The controllers’ performance is evaluated through their capability to reduce shear force, bending, and torsion moments on different locations of the aircraft in response to different types of discrete gusts and continuous turbulence; the constraints of the sensors and the actuators are taken into account. The gain in performance due to the use of lidar is assessed, and a tradeoff is discussed concerning the optimal measurement distance. Finally, the closed-loop robustness is assessed by simulations where different types of uncertainties are applied to the system.

Nonlinear Incremental Control for Flexible Aircraft Trajectory Tracking and Load Alleviation

This paper proposes a nonlinear control architecture for flexible aircraft simultaneous trajectory tracking and load alleviation. By exploiting the control redundancy, the gust and maneuver loads are alleviated without degrading the rigid-body command tracking performance. The proposed control architecture contains four cascaded loops: position control, flight path control, attitude control, and optimal multi-objective wing control. Because the position kinematics are not influenced by model uncertainties, the nonlinear dynamic inversion control is applied. On the contrary, the flight path dynamics are perturbed by both model uncertainties and atmospheric disturbances; thus the incremental sliding mode control is adopted. Lyapunov-based analyses show that this method can simultaneously reduce the model dependency and the minimum possible gains of conventional sliding mode control methods. Moreover, the attitude dynamics are in the strict-feedback form; thus the incremental backstepping sliding mode control is implemented. Furthermore, a novel load reference generator is designed to distinguish the necessary loads for performing maneuvers from the excessive loads. The load references are realized by the inner-loop optimal wing controller, whereas the excessive loads are naturalized by flaps without influencing the outer-loop tracking performance. The merits of the proposed control architecture are verified by trajectory tracking tasks in spatial von Kármán turbulence fields.

Structural damping models for passive aeroelastic control

Aeroelastic qualification requirements are typically met by sizing aircraft to achieve adequate stability margins and keep peak gust responses below specified thresholds. A possible alternative approach is delaying flutter and alleviating gust response by embedding dissipative materials into structural components. This approach requires accurate damping models applicable to the analysis of complex configurations. In this paper, the effect of damping models in the evaluation of flutter boundaries and gust response of an aeroelastic test-bed is studied. In particular, three damping models completely different in frequency are considered to model the presence of skin patches for passive aeroelastic control: viscous damping, hysteretic damping and Biot damping models. In order to make them comparable the three models are tuned in order to dissipate the same amount of power at the flutter frequency by means of the introduction and definition of a generalized loss factor. The damping models are compared by evaluating their effect on flutter suppression and gust load alleviation. Finally, the sensitiveness of the stability and response aeroelastic analyses to the considered damping models are outlined and the obtained results are compared to provide modeling recommendations for passive flutter suppression and gust alleviation studies. The proposed methodology is applicable to any linear material model for which the related complex stiffness can be expressed in the frequency domain meeting the Hilbert restriction.

Design and testing of aeroelastically tailored composite wing under fatigue and gust loading including effect of fatigue on aeroelastic performance

The aim of the paper is to experimentally validate a numerical design methodology for optimizing composite wings subject to gust and fatigue loading requirements and to assess the effect of fatigue on the aeroelastic performance of the wing. Traditionally, to account for fatigue in composite design, a knockdown factor on the maximum stress allowable is applied, resulting in a conservative design. In the current design methodology, an analytical fatigue model is used to reduce the conservativeness and exploit the potential of composite materials. To validate the proposed analytical model, a rectangular composite wing is designed and manufactured to be critical in strength, buckling and fatigue. An experimental campaign comprising wind tunnel and fatigue tests is performed. In the wind tunnel, both static and dynamic aeroelastic experiments are conducted to validate the numerical dynamic aeroelastic model. The fatigue test is used to validate the analytical fatigue model and to understand the effect of fatigue on aeroelastic properties of the wings. The results from experimental campaign validated both the aeroelastic predictions as well as fatigue predictions of the numerical design methodology. However the fatigue process resulted in degradation of the wing stiffness leading to change in the aeroelastic response of the wing.

Gust load alleviation by normal microjet

This paper presents an investigation on the capability of gust load alleviation by normal microjet. The numerical method integrates the unsteady Reynolds averaged Navier-Stokes (URANS) solutions, structural dynamic equations of motion and the Field Velocity Method. The method is verified for gust responses of rigid and elastic models. The numerical results of microjet are validated against experimental and previous numerical data. Load control capabilities of normal microjet are evaluated on the 2D NACA0012 airfoil and the 3D BAH wing with constant and dynamic momentum coefficients under steady subsonic and transonic incoming flow conditions. Thereafter, gust load alleviation effects using microjet are tested on the airfoil and the BAH wing with and without the consideration of aeroelasticity. The results show that normal microjet has a strong capability for load control for transonic incoming flow. This is due to the jet effect on the shock strength on the airfoil upper surface. For the 3D BAH wing, significant load control effects can be shown for jet deployment on the span near the wing tip. Load reduction has also been observed near the wing root away from the jet region. The test cases show that normal microjet is a promising approach for gust load alleviation with a fast frequency response characteristic. A near constant lift response under gust condition can be achieved by adaptively adjusting the blowing momentum coefficients.

Integrated fault estimation and fault-tolerant control for a flexible regional aircraft

The article focuses on the design and application of an active reconfigurable controller that mitigates the effects of gust load and actuator faults on a flexible aircraft. A novel integrated adaptive output feedback scheme is investigated to address the actuator faults. The real-time fault values provided by the fault estimation module are considered in the reconfigurable control law to improve the fault-tolerant capability. The estimate values of faults and control gains are calculated by analyzing the stability of the overall system. The proposed controller is simulated using a flexible aircraft model with a discrete ‘1-cosine’ gust, and the results show that it can effectively mitigate the wing root moments and recover the flight maneuver stability after the aircraft suffered from gusts.

Numerical Simulation and Validation of Aerodynamics of Static and Dynamic Spoilers

Spoilers play a vital role in the flight control systems of modern transport aircraft. Because of their efficiency and fast deflection rates, they are widely used to assist in roll control or for gust load alleviation purposes. Simulating the aerodynamic behavior of spoilers still is a challenging task, as deflecting a spoiler always induces flow separation. The work presented in this paper therefore focused on extending the application range of DLR, German Aerospace Center’s in-house flow solver TAU by verifying and validating it for spoiler applications. In a first step, the work focused on steady and unsteady simulations of static and dynamic spoiler deflections in the low-speed regime. It was shown that the chosen numerical approach is well suited to reproduce the surface pressure distribution for static spoiler deflections of up to 50 deg. Above that, the simulated transient aerodynamic response was found to be in good agreement with experimental data for a variety of deflection times and deflection angles.

Assessing the Risks of Vessel Icing and Aviation Hazards during Downslope Windstorms in the Russian Arctic

Downslope windstorms are known sources of hazardous weather, such as severe gusty winds, rapid temperature changes, ship icing, strong turbulence and others, posing a great danger to people and infrastructure. This paper investigates the risks of ship icing and aviation hazards (rapid changes in the angle of attack and gust load factor) during downslope windstorms in five regions in the Russian Arctic based on observational data, reanalysis, and mesoscale numerical modeling. The highest frequency of ship icing was found during downslope windstorms downstream from Svalbard and Novaya Zemlya. Icing is rare during Tiksi and Wrangel Island downslope windstorms due to almost permanent sea ice cover during the cold season, while icing is absent during very warm Pevek downslope windstorm even in the ice-free conditions. Conditions for heavy icing are rather frequent (up to 5% of cases in winter) during Novaya Zemlya downslope windstorms and less frequent (up to 0.5% in spring) during Svalbard windstorm. The presence of downslope windstorms in those regions causes an increase in the maximum icing rate by about 2 times. Strong aviation turbulence hazardous for light aircraft is typical for all considered regions with downslope windstorms; it is observed mainly at an altitude of 1 to 4 km above the surface. Hazardous turbulence for jet aircraft like Boeing 737 on its cruising flight levels was found on Svalbard and in Tiksi region in 10–15% of cases during strong downslope windstorms.

Seamless Active Morphing Wing Simultaneous Gust and Maneuver Load Alleviation

This paper deals with the simultaneous gust and maneuver load alleviation problem of a seamless active morphing wing. The incremental nonlinear dynamic inversion with quadratic programming control allocation and virtual shape functions (denoted as INDI-QP-V) is proposed to fulfill this goal. The designed control allocator provides an optimal solution while satisfying actuator position constraints, rate constraints, and relative position constraints. Virtual shape functions ensure the smoothness of the morphing wing at every moment. In the presence of model uncertainties, external disturbances, and control allocation errors, the closed-loop stability is guaranteed in the Lyapunov sense. Wind tunnel tests demonstrate that INDI-QP-V can make the seamless wing morph actively to resist “1-cos” gusts and modify the spanwise lift distribution at the same time. The wing root shear force and bending moment have been alleviated by more than 44% despite unexpected actuator fault and nonlinear backlash. Moreover, during the experiment, all the input constraints were satisfied, the wing shape was smooth all the time, and the control law was executed in real time. Furthermore, as compared with the linear quadratic Gaussian control, the hardware implementation of INDI-QP-V is easier; the robust performance of INDI-QP-V is also superior.

The effect of folding wingtips on the worst-case gust loads of a simplified aircraft model

In recent years, the development of lighter and more efficient transport aircraft has led to an increased focus on gust load alleviation. A recent strategy is based on the use of folding wingtip devices that increase the aspect ratio and therefore improve the aircraft performance. Moreover, numerical studies have suggested such a folding wingtip solution may incorporate spring devices in order to provide additional gust load alleviation ability in flight. It has been shown that wingtip mass, stiffness connection and hinge orientation are key parameters to avoid flutter and achieve load alleviation during gusts. The objective of this work is to show the effects of aeroelastic hinged wingtip on the problem of worst-case gust prediction and the parameterization and optimization of such a model for this particular problem, that is, worst-case gust load prediction. In this article, a simplified aeroelastic model of full symmetric aircraft with rigid movable wingtips is developed. The effects of hinge position, orientation and spring stiffness are considered in order to evaluate the performance of this technique for gust load alleviation. In addition, the longitudinal flight dynamics of a rigid aircraft with an elastic wing and folding wingtips is studied. Multi-objective optimizations are performed using a genetic algorithm to exploit the optimal combinations of the wingtip parameters that minimize the gust response for the whole flight envelope while keeping flutter speed within the safety margin. Two strategies to increase flutter speed based on the modification of the wingtip parameters are presented.

Wind-induced responses and equivalent static design method of oval-shaped arch-supported membrane structure

This paper presents the wind-induced responses and wind resistant design method of arch-supported membrane structure. Series of rigid model wind tunnel tests are carried out to record wind pressure data on the roof surface of experimental models. Random vibration time history analysis is employed to get wind-induced responses. Geometric nonlinearity is taken into account in dynamic analysis. Effects of different factors such as wind velocity, wind direction, arch span, rise-span ratio, and membrane prestress on peak wind-induced responses are presented meticulously. Displacement, membrane stress, internal force of cable and abutment reaction are analyzed to study the peak responses of structure. This study considers the influence of boundary wall by conducting wind tunnel tests on both enclosed and open models. Results show that peak responses increase significantly as wind velocity and/or arch span increase. The performance of membrane structures can be enhanced by increasing membrane prestress, but the effect is quite small. Similarly, responses increase with rise-span ratio, but the rate of increment is not constant. Effect of the boundary wall depends significantly on wind direction and rise-span ratio. Additionally, this paper proposes the concept of equivalent static design method based on gust loading factor and nonlinear adjustment factor.

Identification and assessment of a nonlinear dynamic actuator model for controlling an experimental flexible wing

In this paper, the effect of nonlinear actuator dynamics on the performance of an active load alleviation system for an experimental flexible wing is studied. Common nonlinearities such as backlash or rate limits are considered for the control surface actuator. An aeroelastic simulation model of a flexible wing with control surface is being used. With this, a parameter study is carried out to quantify the impact of the individual nonlinearities on the overall closed-loop performance by means of describing functions. Finally, the nonlinear actuator model with parameters identified from dedicated tests is experimentally validated allowing for an accurate prediction of the expected gust load alleviation performance.

A multi-channel varvec{H}_infty preview control approach to load alleviation design for flexible aircraft

Gust load alleviation functions are mainly designed for two objectives: first, alleviating the structural loads resulting from turbulence or gust encounter, and hence reducing the structural fatigue and/or weight; and second, enhancing the ride qualities, and hence the passengers’ comfort. Whilst load alleviation functions can improve both aspects, the designer will still need to make design trade-offs between these two objectives and also between various types and locations of the structural loads. The possible emergence of affordable and reliable remote wind sensor techniques (e.g., Doppler LIDAR) in the future leads to considering new types of load alleviation functions as these sensors would permit anticipating the near future gusts and other types of turbulence. In this paper, we propose a preview control design methodology for the design of a load alleviation function with such anticipation capabilities, based on recent advancements on discrete-time reduced-order multi-channel H_infty techniques. The methodology is illustrated on the DLR Discus-2c flexible sailplane model.

Nonlinear aeroelastic analysis of airfoil section under stall flutter oscillations and gust loads

As the desire for more flexible aircraft structures increases, so does the need to incorporate the intrinsic nonlinearities in their aeroelastic analyses. In that manner, it becomes essential to fully characterize the aeroelastic behavior and to investigate the effects that common external disturbances, such as atmospheric gust turbulence, might induce. In this work, an augmentation of the semi-empirical Beddoes–Leishman dynamic stall model to include gust loads is presented. This augmented dynamic stall model is coupled with the typical section equations of motion to yield a complete aeroelastic framework for gust-induced stall flutter analyses. The stability boundary of a classic 2-DOF typical section is investigated, and a stochastic gust response study reveals that depending on the turbulence intensity, the probabilities of reaching aeroelastic instability can be quite high even for airspeeds well below the one for linear flutter.

Flow Behaviour and Aerodynamic Loading on a Stand-Alone Heliostat: Wind Incidence Effect

Heliostats represent a significant proportion of the cost of central tower concentrating solar power systems; hence, there is a need to reduce the cost of these. One of the challenges faced in doing this is to ensure that any new design is still able to cope with the loads imposed upon it, particularly the aerodynamic loads. Numerous studies have reported the aerodynamic loadings encountered by heliostats for multiple operational conditions. However, these studies only extracted and reported these values without relating their findings of the variation in wind loads with heliostat orientation to the airflow characteristics around the structure. Besides, there is a marked absence of studies that rigorously explore in any detail the wind incidence effect and the impact that has on the aerodynamic behaviour of a heliostat. In this study, computational fluid dynamics was utilized to investigate the effect of wind incidence angles on a heliostat operating at varying tilt angles, to better characterize the aerodynamic loading of these structures and to relate these loads to the wind flow field around the heliostat. The results, validated against wind tunnel test data, showed that the tilt and wind incidence angle had a significant influence on the aerodynamic coefficients that varied strongly across the multiple operational conditions investigated. Moreover, the airflow field around the heliostat structure showed markedly different behaviour characteristics with the change in wind incidence angle. Considering a full range of tilt angles between 90° and − 90°, and incidence angles ranging from 0° to 90° with an angular resolution of 11.25°, the work delivers a fuller characterization of the lift, drag, base overturning moment and hinge moment coefficients than previously available. In achieving this, it delivers a generalized correlation for each of the coefficients based on the heliostat’s orientation with respect to the wind. When paired with existing gust loading relationships, these formulations provide a useful analytical tool to assess structural loads on a heliostat.

The interaction between active aeroelastic control and structural tailoring in aeroservoelastic wing design

This paper presents an analysis of the interaction and trade-off between active aeroelastic control and passive structural tailoring on a free-flying fully flexible aircraft model. Both technologies are included in the preliminary design of a typical transport aircraft configuration with a conventional control surface layout containing trailing edge control surfaces and spoilers. The passive structural tailoring is facilitated by exploiting the anisotropic properties of composite materials to steer the static and dynamic aeroelastic behaviour. Active aeroelastic control is implemented by scheduled control surface deflections redistributing the aerodynamic loads during manoeuvres to achieve manoeuvre load alleviation and a feed-forward control law for gust load alleviation. The panel-based aerodynamic modelling of spoiler deflections is improved by a correction of the spatial distribution of the boundary condition derived from higher fidelity simulation data. The optimisation of active control laws requires the consideration of constraints of the actuation system, namely rate and deflection saturation, in a nonlinear manner. The interaction of manoeuvre load alleviation, gust load alleviation and passive structural tailoring is investigated on the basis of results of different aeroservoelastic optimisations. Therefore the primary wing structure is simultaneously optimised with the individual technologies being activated or deactivated, resulting in eight different wing structures. The results of the individual and combined optimisations reveal significant design differences. The potentials of the different technologies can only be optimally exploited by simultaneous optimisation. The paper concludes with a study of the sensitivity of the major findings with respect to the knockdown factor for failure applied to the material properties. A substantial shift of effectiveness from active aeroelastic control to passive structural tailoring is observed with increased allowables resulting in more flexible and hence less stiff wing designs.

Comparative assessment of ASCE 7-16 and KBC 2016 for determination of design wind loads for tall buildings

Wind load is typically considered as one of the governing design loads acting on a structure. Understanding its nature is essential in evaluation of its action on the structure. Many codes and standards are founded on state of the art knowledge and include step by step procedures to calculate wind loads for various types of structures. One of the most accepted means for calculating wind load is using Gust Load Factor or base bending Moment Gust Load Factor (MGLF), where codes are adjusted based on local data available. Although local data may differ, the general procedure is the same. In this paper, ASCE 7-16 (2017), which is used as the main reference in the U.S., and Korean Building Code (KBC 2016) are compared in evaluation of wind loads. The primary purpose of this paper is to provide insight on each code from a structural engineering perspective. Herein, discussion focuses on where the two codes are compatible and differ. In evaluating the action of wind loads on a building, knowledge of the dynamic properties of the structure is critical. For this study, the design of four figurative highrise buildings with dual systems was analyzed.

Aeroelastic optimisation of manufacturable tow-steered composite wings with cruise shape constraint and gust loads

In the structural design of aircraft wings, aeroelastic tailoring is used to control the aeroelastic deformation to improve the aerostructural performance by making use of directional stiffness. Recently, tow-steered composites, where the fibre angles continuously vary within each ply, have been proven to have the potential to further expand the advantages of aeroelastic tailoring. This work extends TU Delft aeroelastic tailoring framework PROTEUS by introducing a lay-up retrieval step, so that it can be used for the conceptual design of tow-steered composite wing structures. In the extended framework, aeroelastic tailoring and lay-up retrieval are sequentially and iteratively performed to take static and dynamic loads, manufacturing and cruise shape constraints into consideration. The first step is carried out using PROTEUS, in which the lamination parameters and thickness of the wing sections are optimised under manoeuvre and gust load conditions. Further, for ensuring optimal aircraft performance in cruise flight conditions, the jig twist distribution is allowed to be optimised to maintain a desired prescribed cruise shape. In the second step, the stacking sequence, including minimum steering radius constraint, is retrieved. Since the lamination parameters cannot be matched exactly during the retrieval step, the constraints are checked, and tightened to take the performance loss during retrieval into account. The first step is repeated until all constraints are satisfied after fibre angle retrieval. To demonstrate the usefulness of the proposed optimisation framework, it is applied to the design of the NASA Common Research Model (CRM) wing, of which the objective is minimizing wing mass subjected to aerostructural design constraints, such as aeroelastic stability, aileron effectiveness, material strength and buckling load.

Interpolatory Methods for Generic BizJet Gust Load Alleviation Function

The paper's main contribution concerns the use of interpolatory methods to solve end to end industrial control problems involving complex linear dynamical systems. More in details, contributions show how the rational data and function interpolation framework is a pivotal tool (i) to construct (frequency-limited) reduced order dynamical models appropriate for model-based control design and (ii) to accurately discretise controllers in view of on-board computer-limited implementation. These contributions are illustrated along the paper through the design of an active feedback gust load alleviation function, applied on an industrial generic business jet aircraft use-case. The closed-loop validation and performances evaluation are assessed through the use of an industrial dedicated simulator and considering certification objectives. Although application is centred on aircraft applications, the method is not restrictive and can be extended to any linear dynamical systems.

Numerical Simulations on Unsteady Nonlinear Transonic Airfoil Flow

Large-amplitude excitations need to be considered for gust load analyses of transport aircraft in cruise flight conditions. Nonlinear amplitude effects in transonic flow are, however, only marginally taken into account. The present work aims at closing this gap by means of systematic unsteady Reynolds-averaged Navier-Stokes simulations. The RAE2822 airfoil is analyzed for a variety of sinusoidal gust excitations at different transonic Mach numbers. Responses are evaluated with respect to lift and moment coefficients, their derivatives and the unsteady shock motion. A strong dependency on inflow Mach number and excitation frequency is observed. Generally, amplitude effects decrease with lower Mach numbers or higher excitation frequencies. The unsteady nonlinear simulations predict lower maximum lift values and lower lift and moment derivatives compared to their linear counterparts for lower frequencies in combination with large-amplitude excitations. For the mid-frequency range, trends are not as clear. Additionally, it is shown that the variables of harmonic distortion and maximum shock motion might not be reasonable indicators to predict a nonlinear response.