Load Alleviation Research Articles

A Comparative Analysis of Active Control vs. Folding Wing Tip Technologies for Gust Load Alleviation

As part of the Ultra High Aspect Ratio Wing Advanced Research and Designs (U-HARWARD) project, funded by CS2JU, various gust load alleviation (GLA) technologies have been developed and studied. GLA plays a crucial role in the development of new generation ultra-high aspect ratio wings (UHARWs), as it reduces gust loads, thereby decreasing the structural weight of the wing and, consequently, the entire aircraft. This weight reduction enhances overall aircraft efficiency, enabling a higher aspect ratio. GLA technologies are categorized into passive systems, which require no active intervention, and active systems, where control surfaces redistribute the aerodynamic loads. In this study, passive GLA was implemented using a folding wing tip (FWT) developed by the University of Bristol, while active GLA employed a Static Output Feedback controller developed by Politecnico di Milano. Both approaches were compared against a baseline aircraft configuration. A flutter assessment confirmed that FWT does not introduce aeroelastic instabilities, ensuring the aircraft remains flutter-free across its flight envelope. A thorough comparison of load envelopes, based on nearly 2000 load cases across different flight points and mass configurations, was conducted in compliance with CS25 regulations, examining both positive and negative gust conditions. The results show a possible 15% reduction in the dynamic load envelope for both passive and active solutions. Using NeOPT, a hybrid finite element (FE) model was developed, with a detailed global FEM (GFEM) for the wingbox and stick elements for other components. Linear gust analyses in Nastran, with the hinge locked and released, provided high-fidelity results, comparing wing failure indexes and demonstrating the effectiveness of the FWT solution.

Gust load alleviation of a flexible flying wing with linear parameter-varying modeling and model predictive control

This paper presents a practical model predictive control (MPC) framework for gust load alleviation of a flexible flying wing. Both the controller solving and state estimation are based on a reduced-order model, which features a linear parameter-varying (LPV) form, avoiding online linearization and reducing the scale of the corresponding quadratic programming problem. An improved modeling and model reduction process is used to enhance modeling efficiency and ensure that the reduced-order model can accurately capture the rigid-flexible coupled characteristics of the flexible flying wing under arbitrary gusts. By reconstructing the output of the control-oriented model to include both rigid-body motion and flexible vibrations, the rigid-flexible coupled multi-objective control is established as an MPC problem for reference tracking. The online optimization is formulated in a sparse fashion and combined with an iterative algorithm based on predicted trajectories, describing the variation of model dynamics within the prediction horizon more accurately. With a time-varying Kalman estimator for state updating, the closed-loop simulations are performed for gust alleviation performance validation. Additionally, the real-time potential of the proposed MPC framework is demonstrated through Monte Carlo simulations.

Active Maneuver Load Alleviation for a Pitching Wing via Spanwise-Distributed Camber Morphing

This paper presents the design and verification of a nonlinear model inversion (NMI) controller for the maneuver load alleviation of a pitching oscillating wing based on spanwise-distributed active camber morphing. Recurrent neural networks (RNNs) are used to predict nonlinear and unsteady aerodynamic forces due to wing's large amplitude pitching maneuver, and a fully connected neural network is introduced to build the dynamic inversion of the aeroelastic system for control law design. The inversed system is concatenated with a PI controller to assemble a nonlinear active controller. The controller is first utilized in an offline environment for a 1DoF pitching finite-span wing with spanwise-distributed active camber morphing and then verified in CFD-based fluid-structure-control coupling simulation. The results show that the offline controller could eliminate the maneuver load. In the online CFD-based fluid-structure-control simulation, the bending moment can be alleviated by 38%.

Adaptive Wind Field Estimation Using an Empirical Bayesian Approach

In lidar-based gust load alleviation, the wind profile ahead of the aircraft cannot be measured directly but has to be reconstructed (estimated) based on the acquired line-of-sight measurements. Such wind reconstruction algorithms typically include regularization in order to adequately handle the noise within the data. This paper presents an empirical Bayesian approach to choosing optimal regularization parameters for any given set of measurements. Using simulations of flight through turbulence, the Bayesian approach is compared with a former approach (based on engineering guess) and an omniscient optimizer, which yields the best achievable results for a constant set of parameters by using the full knowledge of the wind field. The Bayesian approach is shown to outperform the engineering guess and performs close to the omniscient optimizer while purely relying on the lidar measurement data.

Linear Modeling of Doppler Wind Lidar Systems for Gust Load Alleviation Design

Preview control using wind estimates derived from Doppler wind lidar measurements is a promising technique for designing active gust load alleviation functions. Due to the relatively high noise levels in the lidar measurements, the associated estimator must use some type of smoothing to obtain a usable estimate for gust load alleviation. In the resulting wind estimate, this results in the loss of some of the information as well as a filtered and reduced component of the measurement noise. Taking the losses and noise into account during control synthesis should help ease the tuning procedure and improve the performance and robustness of the resulting controller. This paper proposes a method based on a maximum a posteriori interpretation of the wind estimation problem to consistently produce linear filters that closely approximate the behavior of the complete measurement chain (sensor and wind estimator) and that can be integrated into a linear control synthesis framework. Its characteristics are shown to closely match those of the real estimator over a range of types of turbulence using a standard set of system parameters.

Loads-based optimal fuel-usage strategy by using a neural-network-based reduced-order model for vertical sloshing

This paper explores methods to reduce aircraft design loads through an optimal fuel usage strategy, which maximises the beneficial effects of wing-tank sloshing-induced damping. A reduced-order neural network models the nonlinear sloshing behaviour in various tank fill level scenarios. Integrated into an aeroelastic framework, this model allows for the assessment of incremental sloshing-induced damping in gust response of wings. The optimised fuel usage strategy enables control of fuel consumption from each individual tank to provide maximisation of load alleviation.

Wind Tunnel Testing of Tethered Inflatable Wings

A wind tunnel testing approach for tethered inflatable wings is presented. Use cases for such wings range from airborne wind energy systems to high-altitude communication platforms. The tests were conducted for two tethered inflatable wings, one made out of nylon fabric and the other an ultra-high-molecular-weight polyethylene fabric. They were tested within a speed range of 15–32.5 m/s for three tether attachment configurations. Stereo photogrammetry data, force and moment measurements, and wake pressure measurements were recorded for each speed and test configuration. These measurements provide an experimental database for aeroelastic model validation and comparison with high-fidelity computational fluid dynamics studies. The effects of wing fabric material and tether attachment configurations on aerodynamic performance were explored and found to have a profound impact. These tests also highlight the possibility of passive aeroelastic tailoring of the wing configuration to achieve desired aerodynamic performance in the form of high lift and load alleviation. Some testing challenges and possible sources of measurement uncertainty are also discussed.

Unsteady load alleviation on highly flexible bio-inspired wings in longitudinally oscillating freestreams

This study delves into the aerodynamic behaviour of a highly flexible NACA 0012 aerofoil, drawing inspiration from avian feathers to handle a gust. We first examined unsteady flow on a rigid wing both experimentally and numerically and then explored the implications of introducing wing flexibility purely numerically. Our findings underscore the potential of composite materials in alleviating the oscillating aerodynamic forces on a wing under a streamwise gust. This behaviour is attributed to its capacity to destabilize the laminar separation bubble, fostering a more stable turbulent boundary layer. While direct avian evidence remains limited, it is postulated that in nature, such mechanisms could mitigate undesired wing flapping, optimizing energy consumption in perturbed environments.

Gust Load Alleviation Control Strategies for Large Civil Aircraft through Wing Camber Technology

Gust Load Alleviation Control Strategies for Large Civil Aircraft through Wing Camber Technology

A sensitivity-based estimation method for investigating control co-design relevance

Abstract. Control co-design is a promising approach for wind turbine design due to the importance of the controller in power production, stability, load alleviation, and the resulting coupled effects on the sizing of the turbine components. However, the high computational effort required to solve optimization problems with added control design variables is a major obstacle to quantifying the benefit of this approach. In this work, we propose a methodology to identify if a design problem can benefit from control co-design. The estimation method, based on post-optimum sensitivity analysis, quantifies how the optimal objective value varies with a change in control tuning. The performance of the method is evaluated on a tower design optimization problem, where fatigue load constraints are a major driver, and using a linear quadratic regulator targeting fatigue load alleviation. We use the gradient-based multi-disciplinary optimization framework Cp-max. Fatigue damage is evaluated with time-domain simulations corresponding to the certification standards. The estimation method applied to the optimal tower mass and optimal cost of energy show good agreement with the results of the control co-design optimization while using only a fraction of the computational effort. Our results additionally show that there may be little benefit to using control co-design in the presence of an active frequency constraint. However, for a soft–soft tower configuration where the resonance can be avoided with active control, using control co-design results in a taller tower with reduced mass.

Experimental and numerical gust identification using deep learning models

Identifying gusts and turbulence events is of primary importance for designing future gust load alleviation systems, calculating airframe load, and analysing incidents. Due to the impossibility of their direct measurement, indirect methods are used and ad hoc experiments are necessary to validate the methodology. This paper employs Convolutional Neural Network and Long Short Term Memory (CNN-LSTM) as well as CNN models for in-flight gust identification. Two aeroelastic models, with different levels of fidelity, representative of a civil and commercial aircraft, are used to generate gust responses to train and test the Deep Learning (DL) models. The results highlight the capability of both LSTM-CNN and CNN models in reconstructing gusts across the entire flight envelope of a civil commercial aircraft. The CNN model demonstrated its ability to identify gusts and turbulence when they occur concurrently, similar to real-world scenarios, in a significantly shorter amount of time. Furthermore, its application to wind tunnel gust response measurements, where the inflow has previously been characterised, demonstrated the effectiveness of the proposed methodology for experimental measurements.

Aeroelastic Scaling of a High-Aspect-Ratio Wing Incorporating a Semi-Aeroelastic Hinge

There has been a growing interest in utilizing flared folding wingtips as an in-flight load alleviation device to enable increased wing spans that meet airport gate limits but with little increase in wing weight. The semi-aeroelastic hinge (SAH) concept is implemented in high-aspect-ratio wings to enable wingtips to be released during severe load cases such as maneuvers and gusts to alleviate the bending moments while maintaining optimum aerodynamic shape for the rest of the flight. In this paper, scaling methods for wings incorporating the SAH are explored, allowing for the development of equivalent scaled unmanned aerial vehicles or wind tunnel models with similar aeroelastic behavior as full-size aircraft. Three scaling approaches are considered in this study, namely, Iso-Froude, Iso-Frequency, and Iso-Strain, where a set of governing nondimensional quantities and scaling factors are determined. Despite the significant nonlinearities resulting from large wingtip fold angles, it is shown that a linear scaling approach can be appropriate for such a wing configuration. Furthermore, the aeroelastic properties of each scaled model are compared to those of the full-scale model, where the best match was obtained from the Iso-Strain model, although it is challenging to meet the required operational conditions.

Boosted Incremental Nonlinear Dynamic Inversion for Flexible Airplane Gust Load Alleviation

Active gust load alleviation is an important technology for reducing loads on future commercial airplanes so that the structure can be designed to be lighter in order to save fuel. For the intense reduction of gust loads, a highly reactive control system of sensors, control algorithms, and actuators is required, which is not yet available. In this paper, a control algorithm based on incremental nonlinear dynamic inversion is extended so that actuators respond faster. The control algorithm is applied to a future medium-range aircraft with distributed trailing-edge flaps. It is assumed that the vertical acceleration of the center of gravity and the displacement, velocity, and acceleration of the first wing bending mode are available as feedback variables. To allow comparability with other actuator and sensor setups, the authors abstract the actuator dynamics and sensor filter as control system time delay. The feedback gains are adjusted automatically to the control system time delay. In simulation, the authors investigate the influence of the control system time delay on the peak gust loads. They show that gust loads can indeed be intensely reduced with sufficiently small control system time delay in simulation.

Fluid–Structure Coupled Analysis of Maneuver Load Alleviation on a Large Transport Aircraft

Fluid–structure coupled simulations are performed for quasi-steady pitching maneuvers of a large transport aircraft with a load alleviation system consisting of trailing edge flaps and droop noses at the leading edge. For the fluid part of the simulation, a RANS approach is used, whereas the structural simulation is based on a linear modal model. The selected maneuvers are located on the lift boundary of the maneuver envelope at maximum load factor, as not only are the structural loads high here, but also redistributing lift for load alleviation is especially challenging as the wing is close to stall. For a target value of 33% reduction in wing bending loads, which is derived from the CS-25, the effects of applying load alleviation on wing loads, deformations, and aircraft maneuverability are investigated. It is found that the desired reduction of wing bending loads can be achieved for all maneuvers considered, while controlling the torsional moment turns out to be more difficult. A loss of maneuverability due to load alleviation is observed for only one of the maneuvers, while for the other maneuvers, maneuverability could even be increased. To assess the influence of the elasticity of the wing, additional simulations of the rigid wing are carried out and compared with the elastic results. Finally, other potential applications of the considered flap system are also explored. Here, for example, a cruise drag reduction of up to 1.8% is possible by adjusting the spanwise lift distribution.

Integrity Ratio: A Damage Mitigation Control Metric for Component Life Extension

Component life extension using control schemes involves a trade-off between vehicle maneuver performance and maneuver’s impact on component life usage. In many cases, effective schemes are predicated on the availability of effective metrics in their evaluations. In this paper, a new metric, called the integrity ratio, is introduced to improve existing control schemes and guide the development of future ones. It contains information pertaining to component damage growth and maneuver performance and is evaluated using a surrogate model for damage prediction and a harmonic load limiting control scheme. Important findings indicate that limiting harmonic load with a higher integrity ratio helps to reduce damage growth while preserving vehicle maneuverability. A demonstration showcased that a reduction of 10% in damage growth can be achieved with less than a 1% change in maneuver. The utilization of the integrity ratio metric enables a holistic and less conservative damage-driven control strategy for extending the operational life of a component, compared to traditional methods such as load alleviation and load limiting.

SCAMORSA-1: a camber-morphing wind turbine blade with sliding composite skin

Wind turbines form an increasingly important source of renewable and sustainable energy. Traditional rigid wind turbine blades are unable to control the flow of air over their surfaces, resulting in higher loads and lower aerodynamic efficiency at non-optimal wind speeds and angles of attack. This paper presents the design of SCAMORSA-1 (“Sliding CAmber- MORphing Skin Action”), a camber-morphing turbine blade in a Horizontal Axis Wind Turbine (HAWT) for increased aerodynamic efficiency and improved extreme load alleviation. The blade is linearly tapered and comprises three sections: a conventional rigid section, a morphing section, and a fixed blade tip, all covered by functionally graded composite skin. Measured from the root, the rigid section spans 0%-60% of the total blade length and its profile transitions at 20% from SD7062 thick airfoil to the thinner SD7037 airfoil. The rigid section includes carbon fiber composite spars that resist flap-wise bending. The morphing section occupies the next 30% of the span with SD2030 airfoil profile and can seamlessly change camber angle up to 10°. This section is composed of three hybrid ribs connected via two leading-edge composite spars and a trailing-edge synchronizing rod. Each hybrid rib has a solid leading-edge segment connected to a flexible trailing-edge segment via T-slots. The trailing-edge segment, where morphing occurs, is an enhanced version of the corrugated FishBAC design, with hexagonal honeycomb infill that increases the out-of-plane stiffness and allows for morphing deformation without internal buckling. Two of these hybrid ribs have servomotors housed in the leading-edge segment. These integrated actuators in the hybrid ribs actuate flexible carbon fiber ribbons that run through slits in the trailing-edge segment to morph it. At the trailing-edge portion of the morphing section, the composite skin transitions to a thin flexible layer that can slide over the deforming trailing-edge segment via skin sliders. Computational simulations were performed to quantify the performance gains and ensure safe operation of all components. A proof-of-concept model of SCAMORSA-1’s morphing section was manufactured and tested to demonstrate the effectiveness of the design.

Flight dynamics of aircraft incorporating the semi-aeroelastic hinge

Aircraft like the Boeing 777-X use on-ground folding wingtips to meet the airport gate size restriction while increasing the aspect ratio during flight to reduce induced drag. A recent concept of aircraft design is to utilise in-flight floating wingtips as a means of load alleviation, which is known as semi-aeroelastic hinge (SAH). This device allows wingtips to be released during manoeuvre and severe gusts to alleviate wing loads, while locking the wingtips during cruise to maintain an optimum aerodynamic shape. This paper develops a flight mechanics model incorporating flexible wings to study the influence of the SAH device on multiple aspects of aircraft flight dynamics. The dynamic responses of the aircraft to gusts and control surface inputs are computed and compared for various hinge and wingtip configurations and release times. It was found that the gust load measured from the wing with free hinge configuration was approximately 40% lower compared to that of the fixed hinge case. It is also shown that when the SAH is released, it can significantly reduce an aircraft's roll damping and the frequency of the short-period mode, leading to higher roll and pitch rates. Furthermore, it shows that the transient responses following the wingtip release will exacerbate the responses induced by gusts, and greater load alleviation can be achieved by manipulating the hinge release time for each gust length. Finally, a step input of the elevator during the hinge release was found to be beneficial for enhancing the gust load alleviation.

Enhancing gust load alleviation performance in an optimized composite wing using passive wingtip devices: Folding and Twist approaches

This paper introduces an innovative numerical method for the design and optimization of high-aspect-ratio composite wings equipped with passive control systems, specifically, Folding WingTip (FWT) and Twist WingTip (TWT) devices. The aim is to enhance Gust Load Alleviation (GLA) performance in the baseline wing. Recent numerical studies have indicated that the inclusion of spring devices and wingtip modifications can offer additional benefits in alleviating gust loads during flight. The baseline wing is designed using a comprehensive multi-disciplinary optimization framework, taking into account aerostructural constraints and exploiting the anisotropic properties of composite materials. The proposed methodology integrates Finite Element (FE) software, an in-house Reduced Order Model (ROM) framework for nonlinear aeroelastic analyses, and Particle Swarm Optimization (PSO). This method, implemented in the Nonlinear Aeroelastic Simulation Software (NAS2) package, facilitated the streamlined design of composite wings with optimized aeroelastic and structural performance. The paper is divided into two main parts. Part 1 introduces a Multidisciplinary Design Optimization (MDO) approach for high-aspect-ratio composite wings, leading to the development of a baseline wing model. Part 2 evaluates the effectiveness of the FWT and TWT devices in alleviating gust loads on the baseline wing, with a focus on the Root Bending Moment (RBM) as a critical criterion for comparison. In wingtip modeling, geometrical nonlinearity is incorporated, and elastic trim is adjusted in each iteration to accommodate shape changes under load and aerodynamic panel movement is synchronized with structural adjustments.

Fluidic Flow Control Devices for Gust Load Alleviation

A reduction in the structural weight and climate-relevant emissions of future transport aircraft can be realized through active gust load alleviation, limiting the peak aerodynamic loads experienced in flight. Two fluidic concepts, surface and Coandă jets, offer promising solutions. The surface jet induces a separated flow region on the wing’s suction side near the trailing edge, while the Coandă jet utilizes tangential blowing over a rounded trailing edge to make use of the Coandă effect for direct circulation control. This study investigates these fluidic actuation concepts using two-dimensional unsteady Reynolds-averaged Navier–Stokes simulations on a supercritical airfoil to alleviate gust-induced lift increases. Results reveal that fluidic concepts can surpass the load reduction capability of a trailing edge flap. Notably, the Coandă-type design, while more efficient, exhibits limited control authority compared to surface jet, achieving maximum gust load reductions of 40% versus 70% for surface jet. Optimizing the temporal deployment of the surface jet leads to increased peak lift reduction. Even under challenging conditions with reduced gust anticipation time and actuator placement near the wing tip with a reduced chord length, the surface jet maintains consistent performance and therefore offers a promising solution for active gust load alleviation on future aircraft.

Kalman state estimator for aircraft structural load alleviation using eigenpair assignment

Process of finding the “best estimate” from noisy signals amounts to “filtering out” the noise. Many methods exist to filter the unwanted noise. A tried and tested method is to use a Kalman Filter which not only cleans up the signals but can also be used to provide signal estimates, for use in reduced order feedback control. Typically, Kalman filter gains are computed using the Linear Quadratic Regulator theory. Reduced order feedback control has been applied in aircraft control problems. One such application area is in synthetic load alleviation, structural loads that arise due to gusts and or control surface deflections. Aircraft structural load alleviation necessitates the use of robust feedback control. The controllers are required to provide load alleviation in cases where the feedback signals are contaminated with noise or missing due to sensor failures. In this paper a method of computing the Kalman gains for the purposes of reduced order feedback is presented, which completely specifies the error dynamics of the estimator in terms of its eigenvalues and associated eigenvectors. The state estimator synthesized using the proposed approach has excellent noise rejection properties and shown to be robust and can be successfully used in reduced order state feedback control, specifically in Aircraft Structural Load Alleviation control schemes. The proposed scheme demonstrates reduction in structural loads.