108 results found
Palacios R, Wang Y, Wynn A, et al., 2013, Condensation of large finite-element models for wing load analysis with geometrically-nonlinear effects
This paper describes a procedure to construct 1-D geometrically-nonlinear structural dynamics models from built-up 3-D linear finite-element solutions. The nonlinear 1-D model is based on an intrinsic form of the equations of motion, which uses sectional inertial velocities and stress resultants as primary degrees of freedom. It is further written in modal coordinates, which yields an finite-dimensional approximation of the geometrically-exact beam dynamics through ordinary differential equations with quadratic nonlinearities. We show that the evaluation of the coefficients in the resulting equations of motion does not actually require the generation of a finite-element model with beam elements. Instead, they are directly identified in the 3-D model through a process of static condensation on nodes defined along spanwise stations, as it is typically done in aircraft dynamic load analysis. In fact, the method exploits the multi-point constraints of linear load models that are normally used to obtain sectional loads. We illustrate the approach on simple aircraft-type structures modelled using shell elements.
Simpson RJS, Palacios R, 2013, Numerical aspects of nonlinear flexible aircraft flight dynamics modeling, ISSN: 0273-4508
A critical review of the numerical approximations made in flexible aircraft dynamics modeling is presented. The baseline model is a geometrically-exact. composite beam model describing the flexible-body dynamics which are subject to aerodynamic forces predicted using the unsteady vortex-lattice method (UVLM). The objectivity of the beam formulation is first investigated for static problems with large nodal rotations. It is found that errors associated with non-objectivity of the formulation are minimized to negligible levels using quadratic (3-noded) elements. In addition to this, two force calculation methods are presented and compared for the UVLM. They show subtle but important differences when applied to unsteady aerodynamic problems with large displacements. Nonlinear static aeroelastic analysis of a very flexible high-altitude long-endurance (HALE) wing is also carried out. and time-marching analysis is applied to the Goland wing in order to predict to the response at, and around, the flutter velocity. Conclusions drawn from the studies in this work work are directly applicable in the identification of appropriate modeling strategies in nonlinear flexible aircraft flight dynamics simulations. © 2013 by Robert J. S. Simpson and Rafael Palacios.
Simpson RJS, Palacios R, Murua J, 2013, Induced-Drag Calculations in the Unsteady Vortex Lattice Method, AIAA JOURNAL, Vol: 51, Pages: 1775-1779, ISSN: 0001-1452
Wang Y, Wynn A, Palacios R, 2013, Robust aeroelastic control of very flexible wings using intrinsic models, ISSN: 0273-4508
This paper explores the robust control of large exible wings when their dynamics are written in terms of intrinsic variables, that is, velocities and stress resultants. Assuming 2-D strip theory for the aerodynamics, the resulting nonlinear aeroelastic equations of motion are written in modal coordinates. It is seen that a system which experiences large displacements can nonetheless be accurately described by a system with only weak nonlinear couplings in this description of the wing dynamics. As result, a linear robust controller acting on a control surface is able to effectively provide gust load alleviation and flutter suppression even when the wing structure undergoes large deformations. This is numerically demonstrated on various representative test cases. © 2013 by Yinan Wang, Andrew Wynn and Rafael Palacios.
Wynn A, Wang Y, Palacios R, et al., 2013, An energy-preserving description of nonlinear beam vibrations in modal coordinates, JOURNAL OF SOUND AND VIBRATION, Vol: 332, Pages: 5543-5558, ISSN: 0022-460X
Hesse H, Murua J, Palacios R, 2012, Consistent structural linearization in flexible aircraft dynamics with large rigid-body motion, ISSN: 0273-4508
This paper investigates the linearization, using perturbation methods, of the struc- tural deformations in the nonlinear flight dynamic response of aircraft with slender, flex- ible wings. This has been achieved by first coupling a displacement-based, geometrically- nonlinear flexible-body dynamics formulation with the 3D Unsteady Vortex-Lattice Method, followed by a consistent linearization of the structural degrees of freedom, which are as- sumed to be small in a body-fixed reference frame. The translations and rotations of that reference frame can be arbitrarily large, however. The resulting system preserves all cou- plings between rigid and elastic motions and can be projected onto a few vibration modes of the unconstrained aircraft with arbitrarily-large, geometrically-nonlinear deformation at a trim condition. The dynamics of the system are then written in tensor form, with up to cubic terms due to the nonlinear rigid-body terms, and with a limited number of coefficients that can be pre-computed prior to the time-marching simulation. Numerical studies on a representative HALE UAV are presented to illustrate the approach and results are compared to the mean-axes solution. © 2012 AIAA.
Hesse H, Murua J, Palacios R, 2012, Consistent structural linearization in flexible aircraft dynamics with large rigid-body motion
This paper investigates the linearization, using perturbation methods, of the struc-tural deformations in the nonlinear ight dynamic response of aircraft with slender, flex- ible wings. This has been achieved by first coupling a displacement-based, geometrically- nonlinear flexible-body dynamics formulation with the 3D Unsteady Vortex-Lattice Method, followed by a consistent linearization of the structural degrees of freedom, which are as- sumed to be small in a body-flxed reference frame. The translations and rotations of that reference frame can be arbitrarily large, however. The resulting system preserves all cou- plings between rigid and elastic motions and can be projected onto a few vibration modes of the unconstrained aircraft with arbitrarily-large, geometrically-nonlinear deformation at a trim condition. The dynamics of the system are then written in tensor form, with up to cubic terms due to the nonlinear rigid-body terms, and with a limited number of coeficients that can be pre-computed prior to the time-marching simulation. Numerical studies on a representative HALE UAV are presented to illustrate the approach and results are compared to the mean-axes solution. © 2012 by Henrik Hesse, Joseba Murua, Rafael Palacios.
Hesse H, Palacios R, 2012, Consistent structural linearisation in flexible-body dynamics with large rigid-body motion, Publisher: PERGAMON-ELSEVIER SCIENCE LTD, Pages: 1-14, ISSN: 0045-7949
Murua J, Palacios R, Graham JMR, 2012, Assessment of Wake-Tail Interference Effects on the Dynamics of Flexible Aircraft, AIAA JOURNAL, Vol: 50, Pages: 1575-1585, ISSN: 0001-1452
Murua J, Palacios R, Graham JMR, 2012, Applications of the unsteady vortex-lattice method in aircraft aeroelasticity and flight dynamics, PROGRESS IN AEROSPACE SCIENCES, Vol: 55, Pages: 46-72, ISSN: 0376-0421
Murua J, Palacios R, Graham MRJ, 2012, Open-loop stability and closed-loop gust alleviation on flexible aircraft including wake modeling, ISSN: 0273-4508
This paper numerically investigates the dynamics of a flexible, lightweight, unmanned aircraft, evaluating its stability boundaries and focusing on the response of the aircraft under atmospheric disturbances. This is achieved by integrating a time-domain 3-D un- steady vortex-lattice aerodynamics method with a geometrically-exact composite beam model encompassing elastic and rigid-body degrees of freedom. The resulting framework is a medium-fidelity tool for the analysis of vehicles that exhibit substantial couplings between their aeroelastic and flight dynamics responses. In its general nonlinear form, the unified model captures the instantaneous shape of the lifting surfaces and the free wake, including large geometrically-nonlinear displacements, in-plane motions, and aerodynamic interfer- ence effects. The linearization of the equations leads to a monolithic state-space assembly, ideally suited for stability analysis and control synthesis. The numerical studies illustrate these capabilities, designing linear PID controllers in order to alleviate gust-induced loads and trajectory deviations. © 2012 AIAA.
Ng BF, Palacios R, Graham JMR, et al., 2012, Robust control synthesis for gust load alleviation from large aeroelastic models with relaxation of spatial discretisation, Pages: 1157-1166
This paper introduces a methodology for the design of gust load control systems directly from large aeroelastic models with relaxation of spatial discretisation. A convenient state-space representation of the vortex-panel unsteady aerodynamics suitable for control synthesis is presented. This allows a full understanding of the dynamics of the linearized vortex aeroelastic model and is suitable for control system design. Through the use of robust controllers, large reductions in loading could be achieved. Comparisons are also made between robust and classical control methods. It further demonstrates that controllers synthesized from models of coarse spatial discretizations and of an order of magnitude smaller in size were capable of rejecting disturbances on fully converged models, with performances comparable to expensive higher order controllers developed from full models.
Palacios R, Wang Y, Karpel M, 2012, Intrinsic models for nonlinear flexible-aircraft dynamics using industrial finite-element and loads packages, ISSN: 0273-4508
A procedure is introduced to construct 1-D geometrically-nonlinear structural dynam- ics models from built-up 3-D finite-element solutions. The nonlinear 1-D model is based on an intrinsic form of the equations of motion, that uses beam velocities and internal forces as primary degrees of freedom. It is further written in modal form, which yields a description of the beam dynamics through ordinary differential equations with quadratic non-linearities. We show that the evaluation of the coeficients in these nonlinear equa- tions of motion does not require the generation of a beam finite-element model. Instead, they are directly identified in the 3-D model through a process of static condensation of the dynamics on nodes defined along along spanwise stations, as it is done in aircraft dy- namic load analysis. In fact, the method exploits the multi-point constraints of linear load models, that are normally used to obtain sectional loads, and we show how it can be inte- grated in full-vehicle aeroelastic analysis. Finally we illustrate this approach on an isotropic cantilever box beam modelled using shell elements. © 2012 by Rafael Palacios, Yinan Wang and Moti Karpel.
da Ronch A, Badcock KJ, Wang Y, et al., 2012, Nonlinear model reduction for flexible aircraft control design
The paper describes a systematic approach to the model reduction of large dimen- sion fluid-structure-flight models, and the subsequent flight control design of very flexible aircraft. System nonlinearities may be due to the large wing deformations, the coupling be- tween flexible and rigid body dynamics and/or flow separation at large angles of incidence. A nonlinear reduced order model is used to reduce the computational cost and dimension of the large-order nonlinear system for a practical control law design. The approach uses information on the eigenspectrum of the coupled system Jacobian matrix and projects the system through a series expansion onto a small basis of eigenvectors representative of the full-order dynamics. For a pitch-plunge aerofoil with structural nonlinearities, a controller based on reduced models was designed to alleviate gust loads. The approach to model re- duction was also demonstrated for a two-dimensional problem with aerodynamics modelled using the computational fluid dynamics equations, and a flexible wing modelled using the geometrically-exact nonlinear beam equations. In all cases, the model reduction was found adequate to predict the large order system dynamics at a neglegible cost compared to that incurred by solving the nonlinear full-order system. © 2012 by A. Da Ronch, K.J. Badcock, Y. Wang, A. Wynn, and R. Palacios.
Arbos-Torrent S, Pang ZY, Ganapathisubramani B, et al., 2011, Leading and trailing edge effects on the aerodynamic performance of compliant aerofoils, 49th AIAA Aerospace Sciences Meeting, Orlando, Florida, USA
Cook R, Palacios R, Goulart P, et al., 2011, Robust Manoeuvring and Gust Alleviation of Very Flexible Aircraft using Novel Control Effectors, 15th International Forum of Aeroelasticity and Structural Dynamics, Paris, France
Hesse H, Palacios R, 2011, Consistent structural linearisation in flexible-body dynamics with large rigid-body motion, 15th International Forum of Aeroelasticity and Structural Dynamics, Paris, France
Murua J, Hesse H, Palacios R, et al., 2011, Stability and open-loop dynamics of very flexible aircraft including free-wake effects, ISSN: 0273-4508
The paper investigates the coupled nonlinear aeroelasticity and flight mechanics of very flexible lightweight aircraft. A geometrically-exact composite beam formulation is used to model the nonlinear flexible-body dynamics, including rigid-body motions. The aerodynamics are modeled by a general 3-D unsteady vortex-lattice method, which can capture the instantaneous shape of the lifting surfaces and the free wake, including large displacements and interference effects. The coupled governing equations are solved in a variety of ways, allowing linear and nonlinear time-domain simulations of the full vehicle and frequency-domain linear stability analysis around trimmed configurations. The resulting framework for the Simulation of High-Aspect Ratio Planes (SHARP) provides a medium-fidelity representation of flexible aircraft dynamics, based on an intuitive and easily linearizable structural representation using displacements and the Cartesian rotation vector, time-domain aerodynamics, and at relatively low computational costs. Previous verification studies on the structural dynamics and aerodynamics modules are complemented here with studies on the flexible-body implementation and on the integrated simulation methodology. A numerical investigation is finally presented on a representative high-altitude long-endurance model aircraft, investigating its stability properties and its open-loop dynamic response. Copyright © 2011 by Joseba Murua, Henrik Hesse, RafaelPalacios and Michael Graham.
Murua J, Palacios R, Graham JMR, 2011, A discrete-time state-space model with wake interference for stability analysis of flexible aircraft, 15th International Forum of Aeroelasticity and Structural Dynamics, Paris, France
Palacios R, 2011, Nonlinear normal modes in an intrinsic theory of anisotropic beams, JOURNAL OF SOUND AND VIBRATION, Vol: 330, Pages: 1772-1792, ISSN: 0022-460X
Palacios R, Epureanu BI, 2011, An intrinsic description of the nonlinear aeroelasticity of very flexible wings, ISSN: 0273-4508
A modal solution is presented to the aeroelastic equations of very flexible wings in intrinsic form, that is written using inertial velocities and strains as primary variables. After assuming 2-D thin-airfoil aerodynamics on the wing sections, it is shown that the equations of motion can be written in canonical state-space form on the intrinsic modal coordinates without any matrix inversion and including only quadratic nonlinearities. Flutter characteristics are readily obtained from a linearized description of the dynamics equations, and the approach provides an efficient way of computing the nonlinear response with large wing displacements. Both situations are exemplified numerically on the Golang wing. The use of modal coordinates will serve to highlight some of the particular characteristics of the use of intrinsic beam solutions in aeroelastic problems with geometrical nonlinearities. Copyright © 2011 by Rafael Palacios and Bogdan Epureanu.
Cook RG, Palacios R, Roberts I, 2010, Manoeuvre Efficiency of Unconventional Control Effectors on Very Flexible Aircraft, RAeS Applied Aerodynamics Conference, Bristol, England
This paper presents a numerical framework to evaluate the efficiency of roll control e ectors on very flexible aircraft. Trailing edge control surfaces may be impractical on this class of aircraft, which drives the need to research novel and unconventional control devices. Here, we introduce a coupled aeroelastic and flightdynamics model using a strain-based structural model with indicial response potential flow aerodynamics. This model is used on a representative twin-boom, solar-powered, high-altitude long-endurance unmanned air vehicle (HALE UAV) to compare the roll response obtained from ailerons deployed on the main wings, with that obtained from using spoilers on various wing sti nesses. Results show that for this aircraft, at the designed cruise velocity, the ailerons exhibit full control reversal, while spoilers perform very well, and only show major ine ciencies on the most flexible wing covered. This conclusion suggests that spoilers may bemore suitable control devices than alierons for HALE UAVs. Potential for improvements are suggested which would increase the e ciency of the spoiler, but also a simple control method is presented which exploits the adverse control reversal that was shown by the aileron quite e ectively.
Murua J, Palacios R, Graham JMR, 2010, Modeling of nonlinear flexible aircraft dynamics including free-wake effects
This paper addresses the unified aeroelastic and flight dynamics characterization of low-speed slender-wing aircraft, including free-wake effects and aerodynamic interference. An analysis framework is presented that targets the prediction of stability and handling qualities of high-altitude long-endurance vehicles, which are prone to experience large wing excursions, leading to an inherently nonlinear and coupled problem between aerodynamics, elasticity and flight dynamics. In this work, the structural dynamics are based on a geometrically-exact composite beam model, discretized using displacement-based finite elements, and cast into an extended flexible-body dynamics model. The aerodynamic model is defined by a general unsteady vortex lattice method. The governing equations of motion of the integrated system are formulated in a tightly-coupled state-space form, which allows for the equations to be solved simultaneously. Verification of the model has been carried out for static and dynamic problems, including both rigid and flexible wings. Numerical studies are presented for the particular case of prescribed rigid-body motions, paying special attention to the likely interference between wake and tail. Results show that the current approach represents a suitable alternative for configuration analysis of flexible atmospheric vehicles, offering a good balance between degree of fidelity and computational cost. © 2010 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
Murua J, Palacios R, Peiro J, 2010, Camber effects in the dynamic aeroelasticity of compliant airfoils, Publisher: ACADEMIC PRESS LTD- ELSEVIER SCIENCE LTD, Pages: 527-543, ISSN: 0889-9746
Palacios R, Murua J, Cook R, 2010, Structural and Aerodynamic Models in Nonlinear Flight Dynamics of Very Flexible Aircraft, AIAA JOURNAL, Vol: 48, Pages: 2648-2659, ISSN: 0001-1452
Chimakurthi SK, Tang J, Palacios R, et al., 2009, Computational Aeroelasticity Framework for Analyzing Flapping Wing Micro Air Vehicles, AIAA JOURNAL, Vol: 47, Pages: 1865-1878, ISSN: 0001-1452
Friedmann PP, Glaz B, Palacios R, 2009, A moderate deflection composite helicopter rotor blade model with an improved cross-sectional analysis, INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES, Vol: 46, Pages: 2186-2200, ISSN: 0020-7683
Glaz B, Palacios R, Friedmann PP, 2009, Incorporation of VABS composite beam sectional analysis into a comprehensive rotorcraft analysis code with application to aeroelastic tailoring, Pages: 689-706, ISSN: 1552-2938
The compatibility between a composite beam cross-sectional analysis based on the variational asymptotic approach, and a helicopter rotor blade model which is part of a comprehensive rotorcraft analysis code is examined. It was found that the comprehensive analysis code can be upgraded with the finite element cross-sectional analysis code VABS (Variational Asymptotic Beam Sectional Analysis) without modifying the existing computer code structure. The new rotor blade model accounts for arbitrary cross-sectional warping, in-plane stresses, and moderate deflections. The composite rotor blade model was validated against experimental data and various rotor blade analyses by examining displacements and stresses under static loads, as well as aeroelastic stability of a composite rotor blade in hover, and forward flight vibratory hubloads of a four bladed composite rotor. Furthermore, the upgraded analysis code was used to examine the effectiveness of aeroelastic tailoring for vibration reduction when using aerodynamic models of varying sophistication, and to study the effects of aeroelastic tailoring on vibration and strain levels when employing active vibration reduction. Copyright © 2009 by the American Helicopter Society International, Inc.
Palacios R, Cesnik CES, 2009, Structural models for flight dynamic analysis of very flexible aircraft, ISSN: 0273-4508
Dissimilar analysis models are considered for the large structural deformations of aircraft with high-aspect-ratio composite wings. The different approaches include displacement-based, strain-based, and intrinsic geometrically-nonlinear beam models. Comparisons are made in terms of numerical efficiency and simplicity for integration of full aircraft flexibility in flight dynamics models. An analysis procedure is proposed based on model substructuring with a (linear) modal representation of both fuselage and tail and (nonlinear) intrinsic beam elements for the flexible wings. Copyright © 2009 by Rafael Palacios and Carlos E. S. Cesnik.
Chimakurthi SK, Tang J, Palacios R, et al., 2008, Computational Aeroelasticity Framework for Analyzing Flapping Wing Micro Air Vehicles, 49th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, Schaumburg, Illinois, USA, April 2008.
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