Load Alleviation Control Research Articles

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

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.

A Review of Flow Control for Gust Load Alleviation

Effective control of aerodynamic loads, such as maneuvering load and gust load, allows for reduced structural weight and therefore greater aerodynamic efficiency. After a basic introduction in the types of gusts and the current gust load control strategies for aircraft, we outline the conventional gust load alleviation techniques using trailing-edge flaps and spoilers. As these devices also function as high-lift devices or inflight speed brakes, they are often too heavy for high-frequency activations such as control surfaces. Non-conventional active control devices via fluidic actuators have attracted some attention recently from researchers to explore more effective gust load alleviation techniques against traditional flaps for future aircraft design. Research progress of flow control using fluidic actuators, including surface jet blowing and circulation control (CC) for gust load alleviation, is reviewed in detail here. Their load control capabilities in terms of lift force modulations are outlined and compared. Also reviewed are the flow control performances of these fluidic actuators under gust conditions. Experiments and numerical efforts indicated that both CC and surface jet blowing demonstrate fast response characteristics, capable for timely adaptive gust load controls.

Individual pitch control by convex economic model predictive control for wind turbine side-side tower load alleviation

The wind turbine side-side tower motion is known to be lightly damped. One viable active damping solution is realized by deploying individual pitch control (IPC) such that counteracting blade forces are created to alleviate the tower fatigue loading caused by this motion. Existing IPC methods for side-side tower damping in the literature, such as linear quadratic regulator and lead-lag controller, cannot accommodate direct optimization and tradeoff tunings of the wind turbine economic performance. In this work, a novel side-side tower damping IPC strategy under a convex economic model predictive control (CEMPC) framework is therefore developed to address these challenges. The main idea of the framework lies in the variable transformation in power and energy terms to obtain linear dynamics and convex constraints, over which the economic performance of the wind turbine is maximized with a globally optimal solution in a receding horizon manner. The effectiveness of the proposed method is showcased in a high-fidelity simulation environment under both steady and turbulent wind cases. Lower fatigue damage on the side-side tower bending moment is attained with an acceptable level of pitch activities, negligible impact on the blade loads, and minor improvement on the power production.

Repetitive model predictive control for load alleviation on a research wind turbine using trailing edge flaps

SummaryThis paper presents the results of an advanced control strategy that employs active trailing edge flaps to reduce the fatigue loads of an experimental wind turbine. The strategy, called repetitive model predictive control, is a multiple‐input multiple‐output controller that aims at the alleviation of out‐of‐plane blade root bending moments. The strategy incorporates the control commands, output errors, and state deviation from the previous rotation. This way, the time lag in the strain sensor input due to the blade inertia is compensated. Additionally, a strategy to limit the computational costs is presented. The load alleviation performance is evaluated at different yaw cases and compared with different individual flap control strategies. The repetitive model predictive control is able to reduce the fatigue loads by up to 23% compared with the better performing individual flap control strategy. This improvement in load reduction is accompanied by an increase in flap travel of up to 7% compared with the individual flap control strategies.

Design of Gust Load Alleviation Control Based on UD-PSO for Civil Aircraft

Abstract Civil aircraft will produce undesirable normal overload under the disturbance of vertical turbulence or gusts, which will lead to deterioration of the ride quality of passenger. To solve this problem, a design scheme of gust load alleviation (GLA) control based on UD-PSO optimization is proposed. Firstly, design a classical gust load alleviation control strategy in the control system. The GLA controller will automatically generate the control command, which is calculated by the sensor feedback signal of turbulence or gust. That is, the aileron and spoiler are deflected in the same direction to counteract the undesired normal overload; at the same time, the elevators are deflected to ensured the longitudinal stability of the aircraft. Then, selects a suitable reference model and a reasonable objective function to achieve the desired ride quality performance index. Finally, the UD-PSO algorithm is used to automatically optimize the controller parameters. The simulation results show that the load alleviation control strategy optimized based on UD-PSO can effectively reduce the gust interference, improve the ride comfort, and achieve a good control performance, which shows the effectiveness of the proposed method.

Observer-based gust load alleviation via reduced-order models*

In this paper, we investigate the design of linear active output feedback control for aircraft gust load alleviation, involving standard sensors and control surfaces. The control architecture is composed of (i) a robust observer, allowing for estimating some wing bending moments despite the wind perturbation, and (ii) a structured H∞-norm oriented control law exploiting this additional information. To cope with the complexity of the aeroelastic model and make the control and observer synthesis computationally tractable, a model reduction step, based on Petrov-Galerkin projections, is first applied. One specificity of this work is that the obtained projector is exploited in the observer design, linking the reduction and the estimation. The effectiveness of the methodology is illustrated through numerical simulation involving an open access high complexity aeroelastic model.

Active Flow Control Devices for Wing Load Alleviation

Active flow control has not found wide application in aircraft due to a combination of technical challenges, costs, and small impact on overall aircraft performance. Future applications, like alleviating dynamic loads, could be made possible through continuous improvement of flow control methods and increasingly restrictive environmental regulations. To date, few studies deal with active flow control for load alleviation on transport aircraft wings at high Reynolds and Mach numbers. A systematic assessment of flow control methods for load alleviation is therefore carried out. Multiple flow actuation schemes for gust load alleviation through dynamic lift reduction are investigated over a range of flight conditions using two-dimensional Reynolds-averaged Navier–Stokes simulations. Unsteady gust interactions with a clean airfoil produce target loads for load alleviation. Actuator performance is consequently evaluated based on an engineering model. The results show that select fluidic actuators outperform micro-mechanical actuators at low flight speeds and feature similar steady load response as trailing edge flaps for the entire flight envelope. The Coandă jet features relatively low mass flow requirements, but necessitates minimal dual-slot blowing for baseline performance recovery. The results of this study highlight the potential of fluidic flow control schemes for implementation into a dedicated gust load alleviation system.

Gust load alleviation on an aircraft wing by trailing edge Circulation Control

This paper reports an investigation of gust load alleviation using Circulation Control for a three-dimensional wing including aerodynamic and structure interaction. The Field Velocity Method is introduced to the unsteady Reynolds averaged Navier–Stokes solutions for gust response simulations. Structural dynamic equations of motion are coupled with the unsteady Reynolds averaged Navier–Stokes solutions to consider the aeroelastic effects due to the gust response. The effects of gust load alleviation on the BAH wing using Circulation Control are tested under typical ‘one-minus-cosine’ gust profiles from certification specification defined by European Aviation Safety Agency. The results show a promising capability of Circulation Control for gust load alleviation as significant gust load alleviation effects have been achieved for both the rigid and elastic BAH wings in the test cases. For the BAH wing considering aeroelasticity, the displacement oscillations induced by gusts have been effectively suppressed by Circulation Control. The results also show that Circulation Control has the fast frequency response characteristics, where more than 50% of the total change in lift coefficient can be obtained within the non-dimensional time s =U∞tc¯=1.

Biomimetic individual pitch control for load alleviation

Individual pitch control has shown great capability of alleviating the oscillating loads experienced by wind turbine blades due to wind shear, atmospheric turbulence, yaw misalignement or wake impingement. This work presents a bio-inspired structure for individual pitch control where neural oscillators produce basic rhythmic patterns of the pitch angles, while a deep neural network modulates them according to the environmental conditions. This mimics, respectively, the central patterns generators present in the spinal chord of animals and their cortex. The mimicry further applies to the neural network as it is trained with reinforcement learning, a method inspired by the trial and error way of animal learning. Large eddy simulations of the reference NREL 5MW wind turbine using this biomimetic controller show that the neural network learns how to reduce fatigue loads by producing smooth pitching commands.

Repetitive Individual Pitch Control for Load Alleviation at Variable Rotor Speed

In recent years, wind turbines have been growing in size and became more lightweight and thus more flexible. Spatial variation in the wind speed results in asymmetrical blade loads, which include a periodic component increasing with growing wind turbine size. Asymmetrical blade loads can be reduced by individual blade pitch control in general and repetitive control can reduce especially the periodical parts of the loads. We investigate, how a repetitive control based individual blade pitch controller as extension to an existing collective pitch controller can reduce periodical loads resulting from unsteady non-uniform wind conditions under consideration of variable rotor speeds. As plant, we use a simulation model of a 3 MW wind turbine, developed by W2E Wind to Energy GmbH, and control it with a model predictive collective pitch controller. This controller is extended with the proposed repetitive individual pitch control scheme. This study shows, that the presented repetitive controller reduces especially the tower yaw moments by up to 65% and higher harmonics of the blade root moments by up to 30% at the cost of increased pitch activity. Hence, for the use of this controller one has to balance the load reduction of blades and tower with increased loads of the pitch actuators.

Research on Command Allocation Method for Flying Wing Aircraft

Flying wing has nice aerodynamic efficiency, good loading and stealth characteristics, it is an ideal aerodynamic configuration for reconnaissance, transportation and bombing type aircraft. However, flying wing aircraft suffers from poor stability and control characteristics, which makes it difficult to design and control. Due to elimination of horizontal and vertical tails, flying wing aircraft has poor longitudinal and directional dynamic characteristics. Unlike most of the conventional aircraft, flying wing aircraft usually have a large number of control effectors, the redundant effectors are designed and required to provide adequate lift, pitch and yawing force or moments. There-fore, flying wing aircraft usually need to adopt reasonable control allocation algorithms to exploit the control capability of redundant effectors. Especially for large-aspect ratio flying wing with very high level of effector redundancy, the pursuit of performances, e.g., gust load alleviation control and maximum lift mode, makes control allocation extremely important and indispensable. This paper summarizes the typical characteristics of command allocation methods, and its applicability in different flight missions, providing a reference for the design of command allocation method for flying wing aircraft.

Handling-Qualities Perspective on Rotorcraft Load Alleviation Control

The objective of this research effort is to assess the impact of load alleviation control on the quantitative handling-qualities specifications (also known as predicted handling qualities) of both conventional helicopters and compound rotorcraft in forward flight. First, an overview on how the harmonic decomposition methodology is used toward load alleviation control is presented. Next, flight control laws are developed based on an explicit model-following architecture. Parametric studies are performed to provide insights on how both the feedforward and feedback paths of the model-following control laws can be used to alleviate the rotor loads. The impact of load alleviation on predicted handling qualities is studied. It is shown that, for the standard helicopter configuration considered, load alleviation comes at the cost of a degradation in handling qualities. However, for the compound rotorcraft considered, allocation of the control signal to the redundant control surfaces provides load alleviation without degradation in predicted handling qualities. The flight control laws are subsequently optimized using the Control Designer’s Unified Interface to meet a comprehensive set of stability, handling-qualities, and performance specifications for specific mission task elements while minimizing the unsteady rotor loads.

Design and Optimization of an Aeroservoelastic Wind Tunnel Model

Through the combination of passive and active load alleviation techniques, this paper presents the design, optimization, manufacturing, and update of a flexible composite wind tunnel model. In a first step, starting from the specification of an adequate wing and trailing edge flap geometry, passive, static aeroelastic stiffness optimizations for various objective functions have been performed. The second optimization step comprised a discretization of the continuous stiffness distributions, resulting in manufacturable stacking sequences. In order to determine which of the objective functions investigated in the passive structural optimization most efficiently complemented the projected active control schemes, the condensed modal finite element models were integrated in an aeroelastic model, involving a dedicated gust load alleviation controller. The most promising design was selected for manufacturing. The finite element representation could be updated to conform to the measured eigenfrequencies, based on the dynamic identification of the model. Eventually, a wind tunnel test campaign was conducted in November 2018 and results have been examined in separate reports.

Fault Tolerant Control of an Experimental Flexible Wing

Active control techniques are a key factor in today’s aircraft developments to reduce structural loads and thereby enable highly efficient aircraft designs. Likewise, increasing the autonomy of aircraft systems aims to maintain the highest degree of operational performance also in fault scenarios. Motivated by these two aspects, this article describes the design and validation of a fault tolerant gust load alleviation control system on a flexible wing in a wind tunnel. The baseline gust load alleviation controller isolates and damps the weakly damped first wing bending mode. The mode isolation is performed via an H 2 -optimal blending of control inputs and measurement outputs, which allows for the design of a simple single-input single-output controller to actively damp the mode. To handle actuator faults, a control allocation scheme based on quadratic programming is implemented, which distributes the required control energy to the remaining available control surfaces. The control allocation is triggered in fault scenarios by a fault detection scheme developed to monitor the actuators using nullspace based filter design techniques. Finally, the fault tolerant control scheme is verified by wind tunnel experiments.

Dielectric Barrier Discharge Plasma Actuator for Load Alleviation and Instability Control in a Compressor Cascade

The aim of this study is to investigate the performance of dielectric barrier discharge plasma actuators (DBD-PAs) applied to control the aeroelastic response of a compressor cascade in subsonic flow conditions. Simulations involve a cascade composed by 7-blade, 3 of which show a not-synchronous pitching behaviour. Two different inter blade phase angle (IBPA) have been tested and compared. Actuators capabilities in terms of load alleviation and instability control have been evaluated through the measurement of mean value, standard deviation and hysteresis area of the airfoil response in terms of lift and moment coefficients.

LQG based model predictive control for gust load alleviation

A linear quadratic Gaussian (LQG) based model predictive control (MPC) method is proposed to alleviate the dynamic gust loads of flexible aircrafts flying through turbulence, utilizing look-ahead information of the turbulence via light detection and ranging (LIDAR) systems or on board alpha probe. The new method features both infinite prediction horizon and infinite control horizon. The forepart of the infinite control sequence consists of a few online optimized variables, and the rest are outputs of an LQG controller, designed offline using an improved LQG method. The advantages of the proposed method are twofold. Firstly, the stability property of the controlled system is improved due to application of the infinite prediction horizon and the LQG controller. Secondly, adoption of an infinite control horizon not only improves the control performance, but also greatly reduces the number of online optimized control variables whilst retaining control performance. Furthermore, a technique to tackle the effects of control delay is also designed. The effectiveness and advantages of the proposed approach are demonstrated through numerical results using a general transport aircraft model.

Gust load alleviation wind tunnel tests of a large-aspect-ratio flexible wing with piezoelectric control

An active control technique utilizing piezoelectric actuators to alleviate gust-response loads of a large-aspect-ratio flexible wing is investigated. Piezoelectric materials have been extensively used for active vibration control of engineering structures. In this paper, piezoelectric materials further attempt to suppress the vibration of the aeroelastic wing caused by gust. The motion equation of the flexible wing with piezoelectric patches is obtained by Hamilton’s principle with the modal approach, and then numerical gust responses are analyzed, based on which a gust load alleviation (GLA) control system is proposed. The gust load alleviation system employs classic proportional-integral-derivative (PID) controllers which treat piezoelectric patches as control actuators and acceleration as the feedback signal. By a numerical method, the control mechanism that piezoelectric actuators can be used to alleviate gust-response loads is also analyzed qualitatively. Furthermore, through low-speed wind tunnel tests, the effectiveness of the gust load alleviation active control technology is validated. The test results agree well with the numerical results. Test results show that at a certain frequency range, the control scheme can effectively alleviate the z and x wingtip accelerations and the root bending moment of the wing to a certain extent. The control system gives satisfying gust load alleviation efficacy with the reduction rate being generally over 20%.

Reducing Induced Drag and Maneuver Loads by Active Aeroelastic Alteration

Induced-drag reduction and maneuver load relief are two recent benefits of active aeroelastic wing technology. Judiciously deflecting wing-edge flaps can alter an unfavorable force distribution aeroelastically. Active distribution matching control to reduce induced drag and active load alleviation control by reducing wing bending moments are such active aeroelastic wing applications. Kolonay and Eastep (“Optimal Scheduling of Control Surfaces on Flexible Wings to Reduce Induced Drag,” Journal of Aircraft, Vol. 43, No. 6, Nov.–Dec. 2006, pp. 1655–1661) posed a least-squares problem to match the calculated distribution to an elliptic distribution. Zink et al. (“Maneuver Trim Optimization Techniques for Active Aeroelastic Wings,” Journal of Aircraft, Vol. 38, No. 6, 2001, pp. 1139–1146) posed aircraft trimming as bending-moment minimization to provide external-load relief at the wing root. Analytic gradients, exact aeroelastic analyses, and closed-form trim solutions are unique features of the Automated Structural Optimization System. Both teams employed the Automated Structural Optimization System to solve their multidisciplinary optimization problems but still encountered inconveniences in obtaining required sensitivity data and in performing required trim optimization. This paper treats two forms of active aeroelastic alteration, with each being a generalization of these active distribution matching control and active load alleviation control formulations, and it develops analytic solution techniques that render the sensitivity evaluations and optimization calculations natural and convenient. Two strategies combine to resolve the inconveniences and enable in-process noniterative trim optimization and analytic sensitivity evaluation. The active aeroelastic alteration solution techniques are demonstrated by results of initial test and verification on a classical forward-swept wing example.

Aeroelastic scaling laws for gust load alleviation control system

Gust load alleviation (GLA) tests are widely conducted to study the effectiveness of the control laws and methods. The physical parameters of models in these tests are aeroelastic scaled, while the scaling of GLA control system is always unreached. This paper concentrates on studying the scaling laws of GLA control system. Through theoretical demonstration, the scaling criterion of a classical PID control system has been come up and a scaling methodology is provided and verified. By adopting the scaling laws in this paper, gust response of the scaled model could be directly related to the full-scale aircraft theoretically under both open-loop and closed-loop conditions. Also, the influences of different scaling choices of an important non-dimensional parameter, the Froude number, have been studied in this paper. Furthermore for practical application, a compensating method is given when the theoretical scaled actuators or sensors cannot be obtained. Also, the scaling laws of some non-linear elements in control system such as the rate and amplitude saturations in actuator have been studied and examined by a numerical simulation.