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See detailA time–frequency technique for the stability analysis of impulse responses from nonlinear aeroelastic systems
Dimitriadis, Grigorios ULg; Cooper, Jonathan E

in Journal of Fluids & Structures (2003), 17(8), 11811201

A time–frequency method is proposed for the analysis of response time histories from nonlinear aeroelastic systems. The approach is based on a time-varying curve-fit of the short time Fourier transform of ... [more ▼]

A time–frequency method is proposed for the analysis of response time histories from nonlinear aeroelastic systems. The approach is based on a time-varying curve-fit of the short time Fourier transform of the impulse response. It is shown that the method can be used in order to obtain a clear picture of the sub-critical stability of a number of aeroelastic systems with a variety of structural and aerodynamic nonlinearities. Additionally, frequency and amplitude information can be obtained for both the linear and nonlinear signatures of the response signals in the sub- and postcritical regions. Finally, it is shown that, given certain types of nonlinear functions, sub-critical damping trends can be extrapolated to predict bifurcation airspeeds. [less ▲]

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See detailDevelopment of a Multiple Modes Simulator of Rotating Bladed Assemblies for Blade Tip-Timing Data Analysis
Gallego-Garrido, Jon; Dimitriadis, Grigorios ULg; Wright, Jan Robert

in Van Hal, B.; Sas, P. (Eds.) Proceedings of the 2002 International Conference on Noise and Vibration Engineering (2002, September)

Blade Tip Timing (BTT) is a promising technique for the measurement of blade-tip vibrations in bladed assemblies. After an engine test is performed, the vibration’s Engine Order (EO) and amplitude ... [more ▼]

Blade Tip Timing (BTT) is a promising technique for the measurement of blade-tip vibrations in bladed assemblies. After an engine test is performed, the vibration’s Engine Order (EO) and amplitude information is recovered from the collected BTT data using suitable analysis methods. The validation of such analysis methods can be carried out more conveniently on simulated BTT data. In this paper, a BTT data simulator is developed to produce controlled BTT data for bladed assemblies in which multiple modes of vibration are excited. The mathematical model used allows for multiple blade modes and multiple assembly modes to be simulated. Additionally, single or multiple, integral and non-integral EO excitations can be selected and the rotational speed of the assembly can be varied linearly. Results from several test cases are shown, demonstrating that the simulator successfully emulates assembly and blade mode combinations. A number of BTT analysis methods were able to recover with accuracy the correct EOs from the simulated BTT data. [less ▲]

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See detailIdentification of the nonlinear function in a vibrating nonlinear beam
Dimitriadis, Grigorios ULg; Cooper, Jonathan E

in Lees, A. W.; Prells, U.; Norton, J. (Eds.) Proceedings of the Third International Conference on Identification in Engineering Systems (2002, April)

The Constant Level Identification (CLI) method for nonlinear dynamical systems has been shown to identify accurately a wide variety of systems with discrete degrees of freedom. In this paper, the method ... [more ▼]

The Constant Level Identification (CLI) method for nonlinear dynamical systems has been shown to identify accurately a wide variety of systems with discrete degrees of freedom. In this paper, the method is applied to a continuous nonlinear system, namely a simulated model of a vibrating beam attached to a nonlinear spring. A Rayleigh-Ritz series representation of the beam is assumed in order to extract the desired number of modal responses. Then, a combination of modal and physical coordinates are used in the identification process. It is shown that the CLI method can accurately capture the shape of the nonlinearity in the system. [less ▲]

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See detailGust Loads
Dimitriadis, Grigorios ULg

in Multiple (Ed.) Design Loads for Future Aircraft (2002)

The selection of design loads and requirements is defining the structural weight of airplanes and their safety. Therefore the definition of requirements should be performed very critically by the customer ... [more ▼]

The selection of design loads and requirements is defining the structural weight of airplanes and their safety. Therefore the definition of requirements should be performed very critically by the customer and structural weight should be assessed based on sensitivity analysis of the total aircraft which includes flight manoeuvre simulation, flight control system, aerodynamics and elastic effects introduced by finite elements. To produce these analyses is the job of the aircraft companies. After selection of most load critical flight manoeuvres (pull up manoeuvres, initiation of roll manoeuvres etc.) the calculation of airloads and inertia loads must include the flight control system and its failure cases because it affects the motion of the control surfaces and therefore the aircraft. With the advent of carbon fibre composite structures discrete loads are the predominant limiting design conditions but it should be emphasised that most structures are of a hybrid nature with metal frame which are still susceptible to fatigue loads. For airplanes designed to civil requirements such as transport airplanes, tankers etc. the definition of continuous turbulence and inclusion of FCS failure cases and nonlinearities such as control surface angles is extremely important. There was a long way from load assumptions used by the Wright Brothers who designed their Flyer to a 5g limit to the load limiting capabilities of the care free handling flight control system of the Eurofighter. Also the US-Airforce Mil-Specifications which were used to design NATO airplanes such as Tornado, F16 and F18 in the 1970’s are obsolete today and the MIL-A-87221 (USAF) is only a frame without the essential quantitative material. All these issues are addressed in this manual including comparisons of regulations and descriptions of new specifications. Complete procedures how to establish design loads are presented which should help for the design of new airplanes. The importance of dynamic phenomena which produce design loads for various aircraft parts such as intakes, leading edges etc. is also highlighted. Loads monitoring systems are necessary to prove calculated loads and monitor fatigue loads to establish the remaining structural life. There is a description of a modern system. For transport type aircraft gust load cases are the most critical for strength design and they are also the main fatigue loading source for the major part of the structure. Methods for discrete and continuous gust loading cases are presented together with nonlinear example calculations. In the appendix there is a description of failure cases and their effect on loads for transport aircraft and a specification of a landing gear which could be used as an example how to specify the whole structure as a system. The military use of this manual is to establish procedures to build the lightest structure for the military requirements. Agreement on requirements and design loads within the NATO countries could standardise pilot training, aircraft usage, increase aircraft life and reduce maintenance. Since the search of the best usage of the aircraft for its military purpose will continue to integrate structure and avionics such as fire and flight control systems as an example there will be a continuous need for future work. [less ▲]

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See detailBlade-Tip Timing Measurement Of Synchronous Vibrations Of Rotating Bladed
Dimitriadis, Grigorios ULg; Carrington, Ian B.; Wright, Jan Robert et al

in Mechanical Systems & Signal Processing (2002), 16(4), 599-622

Blade-tip timing (BTT) is a promising method for the detection, measurement and analysis of blade vibrations in rotating bladed assemblies. However, the intricacies of the method when applied to real ... [more ▼]

Blade-tip timing (BTT) is a promising method for the detection, measurement and analysis of blade vibrations in rotating bladed assemblies. However, the intricacies of the method when applied to real rotating structures undergoing synchronous (Engine Ordered) vibrations are not yet fully understood. In this paper, a mathematical model is developed to simulate data from typical BTT tests of rotating assemblies. The simulator is then used in order to provide a qualitative analysis of several phenomena that can be associated with the synchronous vibrations of rotating assemblies, including mistuning, coupling, excitation at multiple Engine Orders and simultaneous synchronous and asynchronous responses. It is concluded that none of these phenomena on its own will render the identification of the frequency and amplitude of blade vibrations impossible. However, there is no single BTT data analysis method that is able to deal with all of these phenomena. [less ▲]

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See detailExperimental Validation Of The Constant Level Method For Identification Of Non-Linear Multi-Degree-Of-Freedom Systems
Dimitriadis, Grigorios ULg

in Journal of Sound & Vibration (2002), 258(5), 829-845

System identification for non-linear dynamical systems could find use in many applications such as condition monitoring, finite element model validation and determination of stability. The effectiveness ... [more ▼]

System identification for non-linear dynamical systems could find use in many applications such as condition monitoring, finite element model validation and determination of stability. The effectiveness of existing non-linear system identification techniques is limited by various factors such as the complexity of the system under investigation and the type of non-linearities present. In this work, the constant level identification approach, which can identify multi-degree-of-freedom systems featuring any type of non-linear function, including discontinuous functions, is validated experimentally. The method is shown to identify accurately an experimental dynamical system featuring two types of stiffness non-linearity. The full equations of motion are also extracted accurately, even in the presence of a discontinuous non-linearity. [less ▲]

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See detailOnline Flight Flutter Testing
Dimitriadis, Grigorios ULg; Cooper, Jonathan E

in Link, M. (Ed.) Proceedings of the International Conference on Structural System Identification (2001, September)

A number of different identification methods are considered as candidate approaches to undertake online flight flutter testing. Rather than flying the test aircraft to a new flight condition and then ... [more ▼]

A number of different identification methods are considered as candidate approaches to undertake online flight flutter testing. Rather than flying the test aircraft to a new flight condition and then performing a data analysis at a constant flight condition, it is shown how it is feasible to track changes in frequencies and damping ratios as the flight speed is increased. It is also possible to provide an updated estimate of the predicted flutter speed, giving the flight engineers added confidence in the margin of safety. Both frequency and time domain methods are evaluated on a simulated 3 DOF aeroelastic system. These preliminary results show that such an approach is feasible although further work is required to determine the best testing and analysis approach. [less ▲]

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See detailExperimental Validation of the Constant Level method for Identification of Nonlinear Multi Degree of Freedom Systems
Dimitriadis, Grigorios ULg

in Starr, A. G.; Rao, R. B. K. N. (Eds.) Proceedings of the 14th International Congress on Condition Monitoring and Diagnostic Engineering Management (2001, September)

System identification methods for nonlinear dynamical systems could find uses in many applications such as condition monitoring, finite element model validation and stability determination. The ... [more ▼]

System identification methods for nonlinear dynamical systems could find uses in many applications such as condition monitoring, finite element model validation and stability determination. The effectiveness of existing nonlinear system identification techniques is limited by various factors such as the complexity of the system under investigation and the type of nonlinearities present. In this work, the Constant Level Identification approach, which can identify multi degree-of-freedom systems featuring any type of nonlinear function, including discontinuous functions, is validated experimentally. The method is shown to yield accurate identifications of an experimental dynamical system featuring two types of stiffness nonlinearity. The full equations of motion are accurately extracted, even in the presence of a discontinuous nonlinearity. [less ▲]

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See detailA time-frequency technique for the stability analysis of impulse responses from nonlinear aeroelastic systems
Dimitriadis, Grigorios ULg; Cooper, J. E.

in Proceedings of the 2001 International Forum on Aeroelasticity and Structural Dynamics (2001, June)

A time-frequency method is proposed for the analysis of response time histories from nonlinear aeroelastic systems. The approach is based on a time-varying curve-fit of the Short Time Fourier Transform of ... [more ▼]

A time-frequency method is proposed for the analysis of response time histories from nonlinear aeroelastic systems. The approach is based on a time-varying curve-fit of the Short Time Fourier Transform of the impulse response. It is shown that the method can be used in order to obtain a clear picture of the sub-critical stability of a number of aeroelastic systems with a variety of structural and aerodynamic nonlinearities. Additionally, frequency and amplitude information can be obtained for both the linear and nonlinear signatures of the response signals in the sub and post-critical regions. Finally, it is shown that, given certain types of nonlinear functions, subcritical damping trends can be extrapolated to predict bifurcation airspeeds. [less ▲]

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See detailAeroelastic and Structural Design of an All-Moveable Aircraft Fin
Cooper, J. E.; Bureerat, S.; Sensberg, O. et al

in Hönlinger, Heinz (Ed.) Proceedings of the CEAS Conference on Multidisciplinary Aircraft Design and Optimization (2001, June)

The Lagrange Multi-Disciplinary Design Optimisation (MDO) code is used for the preliminary study into design aspects of a generic all-movable aircraft fin. It is shown how the control effectiveness at ... [more ▼]

The Lagrange Multi-Disciplinary Design Optimisation (MDO) code is used for the preliminary study into design aspects of a generic all-movable aircraft fin. It is shown how the control effectiveness at high speeds can be increased through variation of the attachment position and stiffness, whilst still meeting strength and aeroelastic stability constraints. [less ▲]

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See detailA comparison of blade tip timing data analysis methods
Carrington, Ian B; Wright, Jan Robert; Cooper, Jonathan E et al

in Proceedings of the Institution of Mechanical Engineers - Part G - Journal of Aerospace Engineering (2001), 215(6), 301-312

The experimental determination of the vibration characteristics of rotating engine blades is very important for fatigue failure considerations. One of the most promising techniques for measuring the ... [more ▼]

The experimental determination of the vibration characteristics of rotating engine blades is very important for fatigue failure considerations. One of the most promising techniques for measuring the frequency of blade vibrations is blade tip timing. In this paper, three vibration analysis methods were specifically formulated and applied to the tip timing problem for the first time, using data obtained from a simple mathematical blade tip timing simulation. The results from the methods were compared statistically in order to determine which of the techniques is more suitable. One of the methods, the global autoregressive instrumental variables approach, produced satisfactory results at realistic noise levels. However, all of the techniques produced biased results under certain circumstances. [less ▲]

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See detailFlutter Prediction from Flight Flutter Test Data
Dimitriadis, Grigorios ULg; Cooper, Jonathan E

in Journal of Aircraft (2001), 38(2), 355-367

The most common approach to flight flutter testing is to track estimated modal damping ratios of an aircraft over a number of flight conditions. These damping trends are then extrapolated to predict ... [more ▼]

The most common approach to flight flutter testing is to track estimated modal damping ratios of an aircraft over a number of flight conditions. These damping trends are then extrapolated to predict whether it is safe to move to the next test point and also to determine the utter speed. In the quest for more reliable and efficient flight flutter testing procedures, a number of alternative data analysis methods have been proposed. Five of these approaches are compared on two simulated aeroelastic models. The comparison is based on both the accuracy of prediction and the efficiency of each method. It is found that, for simple aeroelastic systems, the Nissim and Gilyard method (Nissim, E., and Gilyard, G. B., “Method for Experimental Determination of Flutter Speed by Parameter Identification,” AIAA Paper 89-1324, 1989) yields the best flutter predictions and is also the least computationally expensive approach.However, for larger systems, simpler approaches such as the damping fit and envelope function methods are found to be most reliable. [less ▲]

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See detailLimit Cycle Oscillation Control
Dimitriadis, Grigorios ULg

Poster (2000, December)

Over the last fifteen years the Aerospace industry has started to show a continuously increasing interest in the investigation of the vibrations of aircraft subjected to nonlinear structural and ... [more ▼]

Over the last fifteen years the Aerospace industry has started to show a continuously increasing interest in the investigation of the vibrations of aircraft subjected to nonlinear structural and aerodynamic forces. Here, a control scheme designed to alleviate or even stop some types of nonlinear aircraft vibrations is presented. [less ▲]

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See detailCharacterization of the Behaviour of a Simple Aeroservoelastic System with Control Nonlinearities
Dimitriadis, Grigorios ULg; Cooper, Jonathan E

in Journal of Fluids & Structures (2000), 14(8), 1173-1193

The characterization of the behaviour of nonlinear aeroelastic systems has become a very important research topic in the Aerospace Industry. However, most work carried to-date has concentrated upon ... [more ▼]

The characterization of the behaviour of nonlinear aeroelastic systems has become a very important research topic in the Aerospace Industry. However, most work carried to-date has concentrated upon systems containing structural or aerodynamic nonlinearities. The purpose of this paper is to study the stability of a simple aeroservoelastic system with nonlinearities in the control system and power control unit. The work considers both structural and control law nonlinearities and assesses the stability of the system response using bifurcation diagrams. It is shown that simple feedback systems designed to increase the stability of the linearized system also stabilize the nonlinear system, although their effects can be less pronounced. Additionally, a nonlinear control law designed to limit the control surface pitch response was found to increase the flutter speed considerably by forcing the system to undergo limit cycle oscillations instead of fluttering. Finally, friction was found to affect the damping of the system but not its stability, as long as the amplitude of the frictional force is low enough not to cause stoppages in the motion. [less ▲]

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See detailCharacterization of nonlinear aeroservoelastic behaviour
Dimitriadis, Grigorios ULg; Cooper, Jonathan Edware

in RTO-MP-036 Structural Aspects of Flexible Aircraft Control (1999, October)

The characterisation of the behaviour of nonlinear aeroelastic systems has become a very important re- search topic. Nevertheless, most of the work carried out to date concerns the development of unsteady ... [more ▼]

The characterisation of the behaviour of nonlinear aeroelastic systems has become a very important re- search topic. Nevertheless, most of the work carried out to date concerns the development of unsteady CFD solutions in the transonic region. Important though this work is, there is also a need for research which aims at understanding the behaviour of non- linear systems, particularly the occurance of Limit Cycle Oscillations (LCOs). The purpose of this pa- per is to study the stability of a simple aeroservoelas- tic system with nonlinearities in the control system. The work considers both structural and control law nonlinearities and assesses the stability of the system response by use of bifurcation diagrams. It is shown that simple feedback systems designed to increase the stability of the linearised system also stabilise the nonlinear system, although their effects can be less pronounced. Additionally, a nonlinear control law designed to limit the control surface pitch response was found to increase the flutter speed considerably by forcing the system to undergo limit cycle oscilla- tions instead of fluttering. Finally, friction was found to affect the damping of the system but not its sta- bility, as long as the amplitude of the frictional force is low enough not to cause stoppages in the motion. [less ▲]

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See detailA Comparison of Blade Tip-Timing Data Analysis Methods
Carrington, I. B.; Wright, J. R.; Cooper, J. E. et al

in Leung, A. Y. T.; Wright, J. R.; Sandoz, D. J. (Eds.) et al Proceedings of the First International Conference on the Integration of Dynamics, Monitoring and Control (1999, September)

The determination of the vibration characteristics of rotating engine blades is very important for fatigue failure considerations. One of the most promising techniques for measuring blade vibrations is ... [more ▼]

The determination of the vibration characteristics of rotating engine blades is very important for fatigue failure considerations. One of the most promising techniques for measuring blade vibrations is blade tip-timing. Three vibration analysis methods were used on data obtained from a simple mathematical blade tip-timing simulator. The results from the methods were compared statistically. One of the methods, the Global Autoregressive Instrumental Variables approach produced satisfactory results at realistic noise levels. However, all of the techniques produced biased results under certain circumstances. [less ▲]

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See detailInvestigation of Nonlinear Aeroelasticity
Dimitriadis, Grigorios ULg

Doctoral thesis (1999)

The investigation of nonlinear aeroelastic phenomena is becoming increasingly important to the aerospace community. The existence of structural and aerodynamic nonlinearities in aircraft has always been ... [more ▼]

The investigation of nonlinear aeroelastic phenomena is becoming increasingly important to the aerospace community. The existence of structural and aerodynamic nonlinearities in aircraft has always been acknowledged but, it is only mainly with the advent of modern digital computers that their investigation has become possible. Additionally, aircraft control systems are becoming increasingly nonlinear with the introduction of Active Control Technology. The effects of these nonlinearities on the dynamic response of aircraft have created the need for further research into the modelling, identification and prediction nonlinear aeroelastic systems. This thesis deals with four aspects of nonlinear aeroelasticity. Firstly, the effect of the common industrial approach to nonlinearity, i.e. that of linearisation, is investigated. Six flutter prediction methods for linear aircraft are tested and compared on linear and nonlinear mathematical models of aeroelastic systems. The performances of the methods on linear systems are evaluated and compared. Subsequently, their predictions predictions when applied to nonlinear systems are assessed. Secondly, the dynamic response of nonlinear aircraft is investigated by means of the Harmonic Balance method and the direct integration of the nonlinear mathematical model. Emphasis is given to the explanation of the appearance of Limit Cycle Oscillations as Hopf bifurcations and on the control and suppression of these oscillations by means of a feedback control system. The chaotic vibration of nonlinear aeroelastic systems is also investigated by means of Poincare diagrams and Lyapunoff exponents. Thirdly, the identification of nonlinear aeroelastic systems is considered. Identification of aeroelastic systems is important since, especially in the case of structural nonlinearities, it is often not known whether an aircraft is linear or not and what nonlinearities it may contain until it is tested, either on the ground (Ground Vibration Testing) or in the air (Flight Flutter Testing). An existing nonlinear system identification method is compared to an approach developed during the course of the present project. The two techniques are applied to a nonlinear mathematical aeroelastic system and to a set of nonlinear input-output data obtained from an experimental system. Both methods were found to be able to deal with both systems with varying degrees of success. Finally, the gust response of nonlinear aircraft is investigated with particular emphasis on the calculation of gust design loads. Turbulent gust clearance is a very important part of any airworthiness testing procedure. Until recently, the linear assumption was considered adequate by the requirements however, there is a current shift towards setting new requirements that take into account nonlinear phenomena. Eight gust load prediction methods for nonlinear aircraft(both stochastic and deterministic) are applied to a simple and a more complex nonlinear mathematical aircraft model. The performance of the methods is assessed with respect to both accuracy and computational efficiency. [less ▲]

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See detailLimit Cycle Oscillation Control and Suppression
Dimitriadis, Grigorios ULg; Cooper, Jonathan E

in Aeronautical Journal (1999), 103(1023), 257-263

The prediction and characterization of the Limit Cycle Oscillation (LCO) behaviour of nonlinear aeroelastic systems has become of great interest recently. However, much of this work has concentrated on ... [more ▼]

The prediction and characterization of the Limit Cycle Oscillation (LCO) behaviour of nonlinear aeroelastic systems has become of great interest recently. However, much of this work has concentrated on determining the existence of LCOs. This paper concentrates on LCO stability. By considering the energy present in di®erent limit cycles, and also using the Harmonic Balance Method, it is shown how the stability of limit cycles can be determined. The analysis is then extended to show that limit cycles can be controlled, or even suppressed, by the use of suitable excitation signals. A basic control scheme is developed to achieve this, and is demonstrated on a simple simulated nonlinear aeroelastic system. [less ▲]

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See detailIdentification of Nonlinear Aeroelastic Systems
Dimitriadis, Grigorios ULg; Cooper, J. E.

in Proceedings of the 1998 International Conference on Noise and Vibration Engineering (1998, September)

The successful identification of nonlinear systems could find uses in many aerospace applications including the validation of finite element models and tracking the stability of aircraft during flight ... [more ▼]

The successful identification of nonlinear systems could find uses in many aerospace applications including the validation of finite element models and tracking the stability of aircraft during flight flutter testing. The effectiveness of existing nonlinear system identification techniques is limited by various factors such as the complexity of the system under investigation and the type of nonlinearities present. In this work, a new approach is introduced that is able to identify multi degree-of-freedom systems featuring any type of nonlinear function, including discontinuous functions. The method is shown to yield accurate identifications of three different simulated aeroelastic systems containing a wide range of structural nonlinearities. [less ▲]

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See detailAnalysis and Prediction of Instability Behaviour in Non-Linear Aeroelastic Systems
Dimitriadis, Grigorios ULg; Cooper, J. E.

in Tagungsband Aeroelastik-Tagung der DGLR (1998, June)

The effect of non-linearities, be they structural, aerodynamic or control, on the aeroelastic behaviour of aircraft is becoming of increasing importance in the Aerospace industry. As well as the ... [more ▼]

The effect of non-linearities, be they structural, aerodynamic or control, on the aeroelastic behaviour of aircraft is becoming of increasing importance in the Aerospace industry. As well as the prediction of °utter instabilities, there is a need for the prediction and modelling of the onset of Limit Cycle Oscillations (LCO). A thorough understanding of the possible types of aeroelastic phenomena that can occur due to non-linearities in the system is required before attempting to predict the behaviour. In this paper, the onset and suppression of LCOs is investigated by means of an examination of the stability and energy content of limit cycles. [less ▲]

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