Flexible Aircraft Research Articles

CONTROLLER GAINS ADJUSTMENT FOR CLOSED-LOOP MODAL REQUIREMENTS

This paper deals with a post-synthesis problem : how can a nominal controller be adjusted, in order to correct the assignment of some closed-loop poles? A parametric sensitivity tool, called the PRABI, is here proposed to mathematically solve this controller adjustment problem. It finds the most relevant combination of controller gains, which masters a defined modal variation. This solution is more simple if the nominal controller has a readable structure, like an observer-based structure. All these theoretical results are applied successfully to the adjustment of flight control laws of a flexible aircraft.

Stability analysis of the nonlinear dynamics of flexible aircraft

A method for nonlinear analysis of dynamic systems and its application to aircraft is presented. In extension of existing averaging methods this procedure follows the linearizing eigenvalue analysis, but defines the eigenvalues and eigenvectors as functions of the deflection from the examined equilibrium. This approach allows the determination of the systems stability properties beyond the close vicinity of the equilibrium. Its application to the model of a large aircraft with flexible degrees-of-freedom is shown. The analysis shows very good agreement with the results of bifurcational analyses and allows a quantitative prediction of the systems stability behavior even between the branches of the bifurcation diagram. The unstable spiral mode of the flexible aircraft shows the existence of additional stable equilibria in the vicinity of the examined equilibrium due to system nonlinearities.

Time Simulations of the Response of Maneuvering Flexible Aircraft

This work is concerned with time simulations of the response of flying flexible aircraft to initial conditions and external excitations, such as gusts. The simulations are based on a unified formulation of the problem of dynamics and control of maneuvering flexible aircraft developed recently by the authors. The formulation represents a seamless integration of pertinent material from the areas of analytical dynamics, structural dynamics, aerodynamics, and controls. It incorporates both the aircraft rigid-body motions and elastic deformations and it includes the aerodynamic, propulsion, gravity, and control forces, all in a rigorous manner. The formulation lends itself to efficient computer processing of the aircraft response. The formulation contains improvements over the earlier formulation by the authors in the form of a reduced-order structural model and newly designed, easier-to-implement controller and state observer. The corresponding time simulations are entirely new, and no similar results have been obtained by other investigators. The developments should help with the design of the new generation of unmanned aerial vehicles (UAVs) and in particular with the design of autonomous UAVs controlled by autopilots.

Tuning of Observer-Based Controllers

A particular postsynthesis problem is examined. Assume a first complex controller running on a given system. It turns out that a closed-loop mode is not sufficiently damped. Is it easy to know which adjustable combinations of controller parameters are the most relevant to master this problem? A Bayesian identification procedure is proposed to analyze the relevance of such a given parameter combination with respect to a given modal specification. One application, for which such a controller tuning is very interesting, is the adjustment of flight control laws during flight tests. In this practical context, the observer-based realization of the controller is used to derive an architecture suitable for its implementation, that is, an architecture in which physical tuning parameters can be easily highlighted. The results presented concern the lateral flight control law of a highly flexible aircraft that has been synthesized by a modern robust control approach.

Adaptive control of an aeroelastic flight vehicle using piezoelectric actuators

The paper studies the feasibility of applying piezoelectric actuators to suppress aeroelastic vibrations in a flexible aircraft so that the aeroelastic response can be tailored to comply with specified dynamic performance characteristics. To that end, an adaptive aeroelastic flight vehicle demonstrator concept was designed, developed, manufactured and tested. Closed-loop buffet attenuation, gust response alleviation and flutter suppression results are presented. The performance of the active aeroelastic control approach using piezoelectric actuators is compared to the performance of the aerodynamic control surfaces. Issues related to instrumentation for flight testing are presented as well.

Effects of Flexibility on the Stability of Flying Aircraft

Traditionally, flying aircraft have been treated within the confines of flight dynamics, which is concerned, for the most part, with rigid aircraft. On the other hand, flexible aircraft fall in the domain of aeroelasticity. In reality all aircraft possess some measure of flexibility and carry out rigid body maneuvers, so that the question arises as to whether rigid body motions and flexibility in combination can affect adversely the stability of flying aircraft. This paper addresses this question by solving the eigenvalue problem for the following three cases: (i) the flight dynamics of a flexible aircraft regarded as rigid and whose perturbations about the flight path are controlled by feedback control, (ii) the aeroelasticity of a corresponding flexible aircraft prevented from undergoing rigid body translations and rotations, and (iii) the control of the actual flexible aircraft using the control gains derived in the first case by regarding the aircraft as rigid. This investigation demonstrates that it is not always safe to treat separately rigid body and flexibility effects in a flying flexible aircraft.

The Generalized (μ,MU)-Iteration Illustrated by Flexible Aircraft Robust Control

Thanks to real-μ-analysis, the (μ,Mu)-iteration generalizes the discrete multi-model approach to a continuum of models, represented by LFT. The present paper proposes and explains how to generalize the (μ,Mu)-iteration to take into account robust performance requirements rather than stability. Moreover, it illustrates the advantages of this approach by applying it to reduction, structuration and performance robustness improvement of a high-order controller designed for flexible aircraft robust control.

Unified Theory for the Dynamics and Control of Maneuvering Flexible Aircraft

This work represents a new paradigm for the dynamics and control of maneuvering flexible aircraft. Using the system concept, the theory integrates seamlessly all the necessary material from the areas of analytical dynamics, structural dynamics, aerodynamics, and controls. It includes automatically all six rigid-body degrees of freedom and elastic deformations, as well as the gravity, propulsion, aerodynamic, and control forces, in addition to forces of an external nature, such as gusts. The seamless integration is achieved by using the same reference frame and the same variables to describe the aircraft motions and the forces acting on it, including the aerodynamic forces. The formulation is modular in nature, in the the sense that the structural model, the aerodynamic theory, and the controls method can be replaced by any other ones to better suit different types of aircraft, provided certain criteria are satisfied. A perturbation approach permits the separation of the equations of motion into a flight dynamics problem for the maneuvering aircraft rigid-body translations and rotations and an extended perturbation problem for the elastic deformations and perturbations in the rigid-body variables, where the second problem is subject to inputs from the maneuvering aircraft

Applying the principle of integrated navigation systems to estimating the motion of large vehicles

Combining inertial sensors and GPS has led to integrated navigation systems of a performance surpassing classical navigation requirements. A utilisation of this technology suggests itself which considers not only the classical movement of a small, rigid body but expands the application range also to large aircraft, space stations and suchlike. The signals generated on this basis are usable both for vehicle guidance and for structural control. The key element of such a generalisation is the kinematical model describing the vehicle motion: Until now, the mentioned restriction to rigid bodies is regular and considers only inertial sensors referring to a single vehicle point. A more general navigation system design considers, however, a specific model comprising additional degrees of freedom from structural distortions as well as spatially distributed sensors. This model is employed within an extended Kalman filter estimating the vehicle motion state of high resolution regarding time. In addition, the filter uses the signals of gyros, accelerometers, satellite navigation receivers, and structural sensors. For this purpose, the paper discusses firstly different variants of integrated navigation systems followed by a presentation of the advantage of spatially distributing satellite navigation antennas over the vehicle structure. Finally, the integrated system for a simple flexible aircraft structure is introduced.

Simulation of asymmetric landing and typical ground maneuvers for large transport aircraft

It is important to determine static and dynamic ground loads for the aircraft ground maneuvers accurately in the design phase. Simulation of the important operational cases is a better way compared to the tests on real aircraft to investigate these loads at a reasonable cost. In this paper, the simulation of an asymmetric landing case and typical ground maneuvers for large transport aircraft is presented. The aircraft models are built in the multibody simulation tool SIMPACK. The tire model implemented in SIMPACK includes lateral dynamics of the tire. Cornering, braked runs and pushback are simulated as typical ground maneuvers. For simulation of braking, a simple but effective Antilock Braking System (ABS) algorithm is implemented. The article is based on the work done in cooperation between DLR and Airbus in the Flexible Aircraft 2 project.

Robust 2-DOF H-infinity Controller for Highly Flexible Aircraft : Design Methodology and Numerical Results

However, the potential proximity of the aircraft's first flexible modes with the controller bandwidth may generate a spillover phenomenon causing aircraft instability if the flexible modes are not ...

Comparison of static and dynamic fluid-structure interaction solutions in the case of a highly flexible modern transport aircraft wing

This paper describes the computational work performed by five of the fifteen partners in the Brite-Euram project UNSI, for the prediction of static aeroelastic configurations and dynamic flutter response at transonic conditions, using advanced time-domain fluid-structure coupling methods. The aerodynamic models, mechanical models, and coupling strategies implemented in the different solvers are presented. A code to code validation of the fluid-structure coupled solvers has been achieved in the case of the highly flexible MDO wing. The coupled codes have each been used first for the computation of steady state flow and static deformations at different flight conditions. The investigation of flutter has been carried out for off-design, heavy-cruise flight conditions, using time-consistent numerical simulations. Dynamic responses have been recorded and compared for stable and flutter conditions.

Flexible aircraft model identification for control law design

The large commercial aircraft, developed today by manufacturers, are characterized by a high flexibility which results in a stronger interaction between the flight control system and the structural modes. The active control of the first elastic modes is needed to meet the performance requirements. This paper proposes an identification methodology of a flexible aircraft from flight test data, which is appropriate for control law design with modern control techniques (LQG, H2/H∞). In a first step a procedure based on Eigensystem Realization Algorithm (ERA) is used to determine an initial aeroelastic model which is subsequently combined with a linearized rigid-body model and optimized by an output-error minimization method. Two application examples show the good performances of the approach.

Robust H Design for Lateral Flight Control of Highly Flexible Aircraft

A lateral flight control design for a flexible aircraft is presented. It is based on a standard two-input two-output problem formalism and H2 synthesis. The proposed approach consists of two steps. The first step deals with the synthesis of a dynamic output feedback satisfying modal specifications on the rigid modes, stability requirements on the structure and performance robustness with respect to the various loading cases. The originality in the synthesis setup we propose lies in the fact that it combines eigenstructure assignment,H2 frequency domain synthesis and parametric robustness design. The second step uses a recent theoretical result to transform a given compensator into an linear quadratic gaussian (LQG) compensator involving a judiciously selected on-board model. The two degree of freedom compensator thus obtained allows time domain specifications in response to the pilot’s commands to be met satisfactorily.We provide a detailed discussion of the approach together with the aircraft models used and corresponding closed-loop analysis.

Model and controller reduction for flexible aircraft preserving robust performance

This paper presents a systematic model/controller order reduction method applied to flexible aircraft. The method, based on mixed /spl mu/-synthesis, determines which flexible modes can be truncated from the full-order model of the aircraft and finds a corresponding reduced order controller preserving robust closed-loop performance. This method is of interest for practical model and controller reduction for flexible aircraft because in this context it is important to keep the physical interpretation of the truncated and remaining modes. Numerical examples are given for a flexible model of a B-52 bomber and a three-mass flexible system.

Load and Control Effects on Crack Growth in Flexible Aircraft

The fundamental leverage generated by load tailoring and flight control system feedback loops for reducing fatigue damage and enhancing long-term integrity of aircraft structural components is investigated. Nonlinear fatigue behavior indicates two significant factors that influence crack growth are overload magnitude ratio and application period. This behavior implies existence of nonintuitive optimal overload strength and frequency values that minimize crack growth. Such results have important implications for design of new control logic that exploits this nonlinear phenomenon to suppress crack growth and enhance structural life. A state-space crack growth model is briefly reviewed and linked to a flexible aircraft model. Long-term structural integrity predictions, and the influence feedback control and loading variations have on such predictions, are made with a set of simulations covering the operational life of the vehicle. A mix of open-loop and closed-loop cases with nominal and variable feedback gains, vehicle motions including nominal maneuvers with superimposed gusts, deterministic and stochastic overloads, and mean stress levels are considered. For an existing control architecture, individual feedback gains have substantial influence on crack growth. This result implies significant extension of structural life may be possible by control gain adjustment within an existing architecture. Maneuver overload strength and frequency also have significant influence. By tailoring the overload maneuver with additional control logic, this result also implies significant structural life enhancement may be possible.

Reliable computation of the robustness margin for a flexible aircraft

Abstract In this paper, an efficient method for directly computing a reliable estimate of the maximal structured singular value μ is proposed. A method, which directly computes a μ upper bound over a frequency interval, is first extracted from Ferreres and Biannic (Proc. ACC 4 (1998) 2294–2298). The problem is then to evaluate the conservatism of this upper bound by computing an accurate μ lower bound which is the central issue of the paper. The proposed algorithm is successfully applied—with a reasonable computational effort—to a challenging aeronautical application, namely the evaluation of the robustness properties of the flight control system of a flexible transport aircraft.

Multidisciplinary Design of Vehicle Structures with Improved Roll Maneuverability-Transonic Regime

The objective of this research is the preliminary design of aircraft structures for improved steady-state roll performance in the transonic regime. The control surface effectiveness (a parameter that indicates the relative rolling power produced by an aileron of a flexible aircraft with respect to the rigid aircraft) is employed for quantifying the roll performance. Control surface effectiveness is evaluated by iteratively solving the antisymmetric trim equation on roll rate. Each iteration of the trim equation necessitates the computation of nonlinear airloads. To make the multidisciplinary design optimization computationally practical, the transonic small disturbance (TSD) theory is used for the aerodynamic analysis. Transonic loads including shock effects are captured by employing the TSD theory for which the CAP-TSD program is used. For design optimization, a nongradient-based method is adopted. Control surface effectiveness variation with respect to the statistically significant structural variables (skins, spars, ribs, and posts) is approximated using the response surface method. Multidisciplinary optimization for minimum weight, with control effectiveness, stress, displacement, and frequency constraints, is performed.

Multidisciplinary Simulation of the Maneuvering of an Aircraft

A computational methodology for the simulation of the transient aeroelastic response of an unrestrained and flexible aircraft during high-G maneuvers is presented. The key components of this methodology are: (a) a three-field formulation for coupled fluid/structure interaction problems; (b) a second-order time-accurate and geometrically conserva- tive flow solver for CFD computations on unstructured dynamic meshes; (c) a corotational finite element method for the solution of geometrically nonlinear and unrestrained structural dynamics problems; (d) a robust method for updat- ing an unrestrained and unstructured moving fluid mesh; and (e) a second-order time-accurate staggered algorithm for time-integrating the coupled fluid/structure semi-discrete equations of motion. This computational methodology is illus- trated with the simulation on a parallel processor of several three-dimensional high-G pullup maneuvers of the Langley Fighter in the transonic regime, using a detailed finite element aeroelastic model.

Nonlinear Aeroelastic Analysis of Complete Aircraft in Subsonic Flow

Aeroelastic instabilities are among the factors that may constrain the flight envelope of aircraft and, thus, must be considered during design. As future aircraft designs reduce weight and raise performance levels using directional material, thus leading to an increasingly flexible aircraft, there is a need for reliable analysis that models all of the important characteristics of the fluid-structure interaction problem. Such a model would be used in preliminary design and control synthesis. A theoretical basis has been established for a consistent analysis that takes into account 1) material anisotropy, 2) geometrical nonlinearities of the structure, 3) unsteady flow behavior, and 4) dynamic stall for the complete aircraft. Such a formulation for aeroelastic analysis of a complete aircraft in subsonic flow is described. Linear results are presented and validated for the Goland wing (Goland, M., The Flutter of a Uniform Cantilever Wing, Journal of Applied Mechanics, Vol. 12, No. 4, 1945, pp. A197-A208). Further results have been obtained that highlight the effects of structural and aerodynamic nonlinearities on the trim solution, flutter speed, and amplitude of limit-cycle oscillations. These results give insight into various nonlinear aeroelastic phenomena of interest: 1) the effect of steady-state lift and accompanying deformation on the speed at which instabilities occur, 2) the effect on nonlinearities in limiting the amplitude of oscillations once an instability is encountered, and 3) the destabilizing effects of nonlinearities for finite disturbances at stable conditions.