Operational Modal Analysis for Simulated Flight Flutter Test of an Unconventional Aircraft (original) (raw)
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Modal parameter estimation and monitoring for on-line flight flutter analysis
Mechanical Systems and Signal Processing, 2004
The clearance of the flight envelope of a new airplane by means of flight flutter testing is time consuming and expensive. Most common approach is to track the modal damping ratios during a number of flight conditions, and hence the accuracy of the damping estimates plays a crucial role. However, aircraft manufacturers desire to decrease the flight flutter testing time for practical, safety and economical reasons by evolving from discrete flight test points to a more continuous flight test pattern. Therefore, this paper presents an approach that provides modal parameter estimation and monitoring for an aircraft with a slowly time-varying structural behaviour that will be observed during a faster and more continuous exploration of the flight envelope. The proposed identification approach estimates the modal parameters directly from input/output Fourier data. This avoids the need for an averaging-based pre-processing of the data, which becomes inapplicable in the case that only short data records are measured. Instead of using a Hanning window to reduce effects of leakage, these transient effects are modelled simultaneously with the dynamical behaviour of the airplane. The method is validated for the monitoring of the system poles during flight flutter testing. r
Flight flutter testing and aeroelastic stability of aircraft
Aircraft Engineering and Aerospace Technology, 2007
Purpose-This paper sets out to provide a general review of the flight flutter test techniques utilized in aeroelastic stability flight testing of aircraft, and to highlight the key items involved in flight flutter testing of aircraft, by emphasizing all the main information processed during the flutter stability verification based on flight test data. Design/methodology/approach-Flight flutter test requirements are first reviewed by referencing the relevant civil and military specifications. Excitation systems utilized in flight flutter testing are overviewed by stating the relative advantages and disadvantages of each technique. Flight test procedures followed in a typical flutter flight testing are described for different air speed regimes. Modal estimation methods, both in frequency and time domain, used in flutter prediction are surveyed. Most common flight flutter prediction methods are reviewed. Finally, key considerations for successful flight flutter testing are noted by referencing the related literature. Findings-Online, real time monitoring of flutter stability during flight testing is very crucial, if the flutter character is not known a priori. Techniques such as modal filtering can be used to uncouple response measurements to produce simplified single degree of freedom responses, which could then be analyzed with less-sophisticated algorithms that are more able to run in real time. Frequency domain subspace identification methods combined with time-frequency multiscale wavelet techniques are considered as the most promising modal estimation algorithms to be used in flight flutter testing. Practical implications-This study gives concise but relevant information on the flight flutter stability verification of aircraft to practising engineers. The three important steps used in flight flutter testing-structural excitation, structural response measurement, and stability prediction-are introduced by presenting different techniques for each of the three important steps. Emphasis has been given to the practical advantages and disadvantages of each technique. Originality/value-This paper offers a brief practical guide to all key items involved in flight flutter stability verification of aircraft.
How to perform Flight Vibration Testing based on Operational Modal Analysis
2018
Flutter is a dynamic instability caused by the interaction of the structural dynamics of the aircraft and unsteady aerodynamic forces. It occurs when one of the elastic modes of the airframe tends to negative damping above a critical speed. Prediction and flutter clearance are important problems in the design, testing and typecertification of sailplanes. From a practical point of view, Flight Vibration Testing (FVT) needs to be performed whenever a new sailplane is built or an existing type is modified. The application of Operational Modal Analysis (OMA) methods may provide improvements to identify modal parameters of multiple modes in one step without knowing the type of external excitation. If the identification is repeated for increasing flight velocities, it is possible to find the aeroelastic damping trends and to extrapolate to the stability boundary. The application of OMA needs a broadband excitation spectrum, which result from impulsive control kicks or continuing random ex...
Automatic Operational Modal Analysis for Aeroelastic Applications
The development of new aircraft requires the evaluation of the aeroelastic stability to avoid the phenomenon of flutter, a self-excited oscillation of the airframe. Since the rational analysis of the flutter stability comprises coupled simulations using numerical structural models and unsteady aerodynamic loads, the accomplishment is complex and the implementations must be checked for their validity by comparison of analytical and experimental results. In the so-called Ground Vibration Test (GVT) the natural modes, eigenfrequencies and damping ratios of the prototype aircraft are identified using classical Experimental Modal Analysis (EMA) methods. Depending on the complexity of the new design, conducting such a test requires a time slot of several days shortly before the first flight. Consequently, there is an ongoing need to reduce the testing time to improve the availability of the aircraft prototype. This paper addresses the application of Operational Modal Analysis (OMA) methods during the GVT of an aircraft, which might cut down the efforts in time and labour. An automatically running fast implementation of the Stochastic Subspace Identification method (SSI) is introduced, which analyses the output acceleration response of the airframe randomly excited by modal shakers. The identification process is specified in detail for a glider aircraft, where acceleration time series must be evaluated to generate the stabilization diagram. To isolate the physical mode shapes from the mathematical poles, a pole-weighted Modal Assurance Criteria (MAC) is evaluated for several model orders to clean the stabilization diagram. Since the process needs no further operator interaction, it is suitable for monitoring airframe vibrations of the aircraft in flight, provided that the changes in flight conditions are significantly slower than the duration of the vibration periods considered. For the sucess of the methods, the OMA requirements should be fulfilled, i.e. the excitation of aircraft should be non-deterministic with broad-band spectra. Such conditions are provided by atmospheric turbulence excitation and/or pilot control inputs. The presented autonomous process is applied to a simulated Flight Vibration Test (FVT) of an research aircraft with real-time modal identification where changes of eigenfrequencies and damping ratios are tracked with changes in flight conditions.
Journal of Aerospace Technology and Management, 2016
The Operational Modal Analysis technique is a methodology very often applied for the identification of dynamic systems when the input signal is unknown. The applied methodology is based on a technique to estimate the Frequency Response Functions and extract the modal parameters using only the structural dynamic response data, without assuming the knowledge of the excitation forces. Such approach is an adequate way for measuring the aircraft aeroelastic response due to random input, like atmospheric turbulence. The in-flight structural response has been measured by accelerometers distributed along the aircraft wings, fuselage and empennages. The Enhanced Frequency Domain Decomposition technique was chosen to identify the airframe dynamic parameters. This technique is based on the hypothesis that the system is randomly excited with a broadband spectrum with almost constant power spectral density. The system identification procedure is based on the Single Value Decomposition of the power spectral densities of system output signals, estimated by the usual Fast Fourier Transform method. This procedure has been applied to different flight conditions to evaluate the modal parameters and the aeroelastic stability trends of the airframe under investigation. The experimental results obtained by this methodology were compared with the predicted results supplied by aeroelastic numerical models in order to check the consistency of the proposed output-only methodology. The objective of this paper is to compare in-flight measured aeroelastic damping against the corresponding parameters computed from numerical aeroelastic models. Different aerodynamic modeling approaches should be investigated such as the use of source panel body models, cruciform and flat plate projection. As a result of this investigation it is expected the choice of the better aeroelastic modeling and Operational Modal Analysis techniques to be included in a standard aeroelastic certification process.
Identification of flutter parameters for a wing model
Journal of The Brazilian Society of Mechanical Sciences and Engineering, 2006
A flexible mounting system has been developed for flutter tests with rigid wings in wind tunnel. The two-degree-of-freedom flutter obtained with this experimental system can be described as the combination of structural bending and torsion vibration modes. Active control schemes for flutter suppression, using a trailing edge flap as actuator, can be tested using this experimental setup. Previously to the development of the control scheme, dynamic and aeroelastic characteristics of the system must be investigated. Experimental modal analysis is performed and modes shape and frequencies are determined. Then, wind tunnel tests are performed to characterize the flutter phenomenon, determining critical flutter speed and frequency. Frequency response functions are also obtained for the range of velocities below the critical one showing the evolution of pitch and plunge modes and the coupling tendency with increasing velocity. Pitch and plunge data obtained in the time domain during these tests are used to evaluate the ability of the Extended Eigensystem Realization Algorithm to identify flutter parameter with increasing velocity. The results of the identification process are demonstrated in terms of the evolution of frequency and damping of the modes involved in flutter.
Active Flutter Mitigation Testing on the FLEXOP Demonstrator Aircraft
AIAA Scitech 2020 Forum, 2020
The paper details the research and corresponding implementation and testing steps of the FLEXOP demonstrator aircraft. Within the EU funded project an unmanned demonstrator aircraft is built to validate the mathematical modelling, flight control design and implementation side of active flutter mitigation. In order to validate the different methods and tools developed in this project, a flight test campaign is planned, in which the design and manufacturing of stiff wings (-0), are compared with very flexible wings (-1) with active flutter control, to see the overall benefit vs. risk of such technology. The mathematical models of the aircraft are first developed using FEM and CFD tools, what are later reduced by model order reduction techniques. The high-fidelity models are updated using Ground Vibration Test results. Manufacturing tolerances and variations in aircraft parameters are captured by systematic modelling of parametric and dynamic uncertainties. Both the simulation environment and the control design framework use different modelling fidelity, what are described within the paper. Reduced models are developed using two distinctive methods, respecting the control design needs: top-down balanced LPV reduction and bottom-up structure preserving methods. Based on the reduced order models various control design techniques have been elaborated by the consortium partners. In particular DLR developed and implemented a modal control method using H2 optimal blends for inputs and outputs. University of Bristol developed structured H-infinity optimal control methods, while SZTAKI proposed a worst-case gain optimal method structured controller synthesis method handling parametric and complex uncertainties. After the brief introduction of hardware-in-the-loop test setup and the description of mission scenarios the implementation issues of the baseline and flutter controllers are discussed. DLR and SZTAKI flutter controllers are evaluated in a hybrid software-/ hardware-in-the-loop test setup as at this stage of development the latter can not tolerate the estimated delay of the hardware system but their comparison is advantageous before future developments. Recommendations on active flutter mitigation methods are given based on the experience of synthesis and implementation of these controllers. Flight test results will follow these experiments, once the flight testing of the flutter wing commences.
Mechanical Systems and Signal Processing, 2014
In this article a different approach to wind tunnel flutter testing is presented. This procedure can now be performed as one continuous test, resulting in a major time saving. Both analysis of the current behaviour of the structure, and prediction towards higher velocities, are important for flight flutter testing, and are dealt with in this article. The recently developed time-varying weighted non-linear least-squares estimator (TV-WNLS) [1] is applied to the aeroelastic flutter problem. Smooth variation of the transfer function coefficients is forced through the TV-WNLS estimator, and the obtained polynomials are used as basis for predicting the damping ratio towards higher velocities. Selection of the model order is based on linear variation of the airspeed and the evaluation of Theodorsen's unsteady aerodynamics for the frozen time-varying aeroelastic system at a certain constant velocity. Therefore, providing a physical justification for the extrapolation of the damping ratio towards higher velocities. The method is applied to wind-tunnel measurements on a cantilevered wing. It is shown that the proposed method outperforms flutter speed prediction by classic damping ratio extrapolation and a non-parametric analysis of the time-varying signal.
Journal of Aircraft, 2005
Accurate prediction of flutter speeds is essential to efficient and safe flight testing for envelope expansion. Such accuracy is particularly difficult to obtain when analyzing flight data from speeds well below the critical speed at which flutter occurs. The flutterometer was introduced as a tool that could predict the onset of flutter even at lowspeed conditions; however, the conservatism in those predictions reduced testing efficiency. A method to augment the flutterometer to decrease the conservatism, and consequently increase the accuracy, of the predicted flutter speeds is presented. The method incorporates a scheme for model updating that ensures consistency between the analytical dynamics and the flight data. The tool is used to compute flutter speeds for the aerostructures test wing. The speeds predicted with the model updating scheme are very close to the actual flutter speed and demonstrate the benefits for improving efficiency of envelope expansion.