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See detailIdentification and model Updating of a non-stationary vibrating system
Dimitriadis, Grigorios ULg; Fassois, Spilios; Poulimenos, Aggelos et al

in Akay, A.; Arnas, O.; Cooper, J. E. (Eds.) et al Proceedings of 7th Biennial ASME Conference on Engineering Systems Design and Analysis (2004, July)

Non-stationary systems, which are commonly encountered in many fields of science, are characterized by time-varying features and require time-frequency methods for their analysis. This study considers the ... [more ▼]

Non-stationary systems, which are commonly encountered in many fields of science, are characterized by time-varying features and require time-frequency methods for their analysis. This study considers the problem of identification and model updating of a non-stationary vibrating system. In particular, a number of identification methods and a model updating procedure are evaluated and compared through application to a time-varying “bridge-like” laboratory structure. The identification approaches include Frequency Response Function based parameter estimation techniques, Subspace Identification and Functional Series modelling. All methods are applied to both output-only and input-out-put data. Model updating is based upon a theoretical model of the structure obtained using a Rayleigh-Ritz methodology, which is updated to account for time-dependence and nonlinearity via the identification results. Interesting comparisons, among both identification and model updating results, are performed. The results of the study demonstrate high modelling accuracy, illustrating the effectiveness of model updating techniques in non-stationary vibration modelling. [less ▲]

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See detailMultiple Frequency Analysis Methods for Blade Tip-Timing Data Analysis
Gallego-Garrido, Jon; Dimitriadis, Grigorios ULg

in Akay, A.; Arnas, O.; Cooper, J. E. (Eds.) et al Proceedings of 7th Biennial ASME Conference on Engineering Systems Design and Analysis (2004, July)

Blade Tip-Timing (BTT) is a method for the measurement of blade vibration in rotating bladed assemblies such as those found in turbomachinery. The system aims to replace strain gauge technology. However ... [more ▼]

Blade Tip-Timing (BTT) is a method for the measurement of blade vibration in rotating bladed assemblies such as those found in turbomachinery. The system aims to replace strain gauge technology. However all current BTT analysis methods fail to recover the correct frequencies when two blade modes are excited simultaneously by a synchronous vibration. In this paper, five new methods that can recover simultaneous frequencies from BTT data are presented. The methods are based on the auto-regressive approach. The approaches make use of data either from a single blade and single revolution or from multiple revolutions. Furthermore, some of the methods are designed to allow for the presence of measurement errors. The techniques are validated on three test cases in which simulated data was used. It is shown that most of the methods produce accurate estimates for the vibration frequency, even in the presence of significant noise levels, provided that a suitable amount of the response waveform is measured. The most consistent estimates are obtained from the methods that make use of data from multiple revolutions. [less ▲]

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See detailStability and Limit Cycle Oscillation Amplitude Prediction for Simple Nonlinear Aeroelastic Systems
Dimitriadis, Grigorios ULg; Vio, Gareth Arthur; Cooper, Jonathan Edward

in Proceedings of the 45th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference (2004, April)

The prediction of the bifurcation and post-bifurcation behaviour of nonlinear aeroelastic systems is becoming a major area of research in the aeroelastic community due to the need for improved transonic ... [more ▼]

The prediction of the bifurcation and post-bifurcation behaviour of nonlinear aeroelastic systems is becoming a major area of research in the aeroelastic community due to the need for improved transonic aeroelastic prediction, the use of non-linear control systems, and new construction techniques that reduce the amount of inherent damping. In this paper, a novel application of the Centre Manifold Theorem is used to accurately predict bifurcation conditions and Limit Cycle Oscillation amplitudes for simple aeroelastic systems with various types of nonlinearity. A simple aeroelastic system with hardening cubic stiffness nonlinearity is considered and is demonstrated to display a wide variety of bifurcation phenomena. These make it dif cult for some of the standard existing methods, such as Normal Form, Cell Mapping and Tangential Linearisation, to quantify the Limit Cycle Oscillation amplitudes through the entire speed range of the system. Then, the proposed approach is introduced and applied to the same system. It is shown that it can accurately predict the limit cycle amplitudes of the system undergoing all types of bifurcation. Finally, the new technique is applied to the same system but with softening cubic stiffness nonlinearity. It is shown that the method can accurately predict both the static and dynamic divergence boundaries and that it can be used to draw a worst-case stability boundary, inside which the solution is always stable. [less ▲]

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See detailLimit Cycle Prediction For Subsonic Aeroelastic Systems Using Nonlinear System Identification
Dimitriadis, Grigorios ULg; Vio, Gareth Arthur; Cooper, Jonathan Edward

in Ferman, M. A.; Petersson, B. A. T.; Rizzi, S. A. (Eds.) et al Proceedings of the VIII International Conference on Recent Advances in Structural Dynamics (2003, July)

The prediction of aeroelastic instabilities caused by nonlinear unsteady aerodynamic forces acting on aircraft has recently become an important area of research. Emphasis is placed on the capability to ... [more ▼]

The prediction of aeroelastic instabilities caused by nonlinear unsteady aerodynamic forces acting on aircraft has recently become an important area of research. Emphasis is placed on the capability to predict the occurrence of Limit Cycle Oscillations (LCOs) at both the design and prototype testing stages. In this paper, the prediction of LCOs is attempted for a simulated aeroelastic system subjected to nonlinear subsonic unsteady aerodynamic forces, using system identification. Response data from the simulated system are curve-fitted by means of a series of polynomial basis functions. This approach yields very accurate identified models of the actual system at individual flight conditions. These identified models are extrapolated to a global aeroelastic identified model. Using this model, the flight conditions at which LCOs occur is accurately predicted but the amplitude of the oscillations is underestimated. [less ▲]

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See detailNonlinear Aeroelastic Research at the Dynamics and Aeroelasticity Research Group
Dimitriadis, Grigorios ULg

Conference (2003, June)

This work is a presentation of the recent research in nonlinearity carried out at the Dynamics and Aeroelasticity Research Group (DARG) of the University of Manchester, School of Engineering

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See detailImprovements of some analysis methods for flight flutter test data using unsteady aerodynamic considerations
Dimitriadis, Grigorios ULg

in Proceedings of the 2003 International Forum on Aeroelasticity and Structural Dynamics (2003, June)

To date the analysis of data from ight utter tests is routinely carried out under the assumption of steady or, at best, quasi-steady aerodynamics. This approach is favored mainly because of its simplicity ... [more ▼]

To date the analysis of data from ight utter tests is routinely carried out under the assumption of steady or, at best, quasi-steady aerodynamics. This approach is favored mainly because of its simplicity. In this paper it is shown that, even for some very simple simulated aeroelastic systems, utter predictions that ignore the unsteadiness of aerodynamic loads can be signi cantly inaccurate. Then, two simple system identi cation techniques are proposed that can model both the steady and unsteady aerodynamic loads acting on an aeroelastic system in incompressible ow. These methods can be used in order to obtain a complete description of an aeroelastic system subject to unsteady aerodynamic forces, including its dependence on ight conditions. Finally, the new methods are applied to a simple simulated unsteady aeroelastic model. The resulting utter predictions are compared to those obtained from two standard ight utter test data analysis approaches and are shown to be signi cantly more accurate. [less ▲]

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See detailOn the use of control surface excitation in flutter testing
Wright, Jan R; Wong, Jerry; Cooper, Jonathan Edward et al

in Proceedings of the 2003 International Forum on Aeroelasticity and Structural Dynamics (2003, June)

Flutter testing is used to demonstrate that the aircraft flight envelope is flutter free. Response measurements from deliberate excitation of the structure are used to identify and track frequency and ... [more ▼]

Flutter testing is used to demonstrate that the aircraft flight envelope is flutter free. Response measurements from deliberate excitation of the structure are used to identify and track frequency and damping values against velocity. In this paper, the common approach of using a flight control surface to provide the excitation is examined us ing a mathematical model of a wing and control surface whose rotation is restrained by a simple actuator. In particular, it is shown that it is essential to use the demand signal to the actuator as a reference signal for data processing. Use of the actuator force (or strain) or control angle (or actuator displacement) as a reference signal is bad practice because these signals contain response information. It may also be dangerous in that the onset of flutter may not be seen in the test results. Control sur face flutter is of particular concern. [less ▲]

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See detailOn the use of control surface excitation in flutter testing
Wright, Jan R; Wong, Jerry; Cooper, Jonathan E et al

in Proceedings of the Institution of Mechanical Engineers - Part G - Journal of Aerospace Engineering (2003), 217(6), 317-332

Flutter testing is aimed at demonstrating that the aircraft flight envelope is flutter free. Response measurements from deliberate excitation of the structure are used to identify and track frequency and ... [more ▼]

Flutter testing is aimed at demonstrating that the aircraft flight envelope is flutter free. Response measurements from deliberate excitation of the structure are used to identify and track frequency and damping values against velocity. In this paper, the common approach of using a flight control surface to provide the excitation is examined using a mathematical model of a wing and control surface whose rotation is restrained by a simple actuator. In particular, it is shown that it is essential to use the demand signal to the actuator as a reference signal for data processing. Use of the actuator force (or strain) or control angle (or actuator displacement) as a reference signal is bad practice because these signals contain response information. It may also be dangerous in that the onset of flutter may not be seen in the test results. [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, 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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