Online Flight Flutter Testing

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 ▲]

A time-frequency technique for the stability analysis of impulse responses from nonlinear aeroelastic systems

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 ▲]

A comparison of blade tip timing data analysis methods

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 ▲]

Limit Cycle Oscillation Control

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 ▲]

Characterization of nonlinear aeroservoelastic behaviour

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 ▲]

Investigation of Nonlinear Aeroelasticity

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 ▲]

Identification of Nonlinear Aeroelastic Systems

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 ▲]

A method for identification of non-linear multi-degree-of-freedom systems

in Proceedings of the Institution of Mechanical Engineers - Part G - Journal of Aerospace Engineering (1998), 212(4), 287-298

System identification methods for non-linear aeroelastic systems could find uses in many aeroelastic applications such as validating finite element models and tracking the stability of aircraft during ... [more ▼]

System identification methods for non-linear aeroelastic systems could find uses in many aeroelastic applications such as validating finite element models and tracking the stability of aircraft during flight flutter testing. 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, a new approach is introduced which can identify multi-degree-of-freedom systems featuring any type of non-linear function, including discontinuous functions. The method is shown to yield accurate identification of three mathematical models of aeroelastic systems containing a wide range of structural non-linearities. [less ▲]