13:30
Aeroelastic testing 5
Chair: Jack Hagelin
13:30
30 mins
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Experimental investigation of a highly flexible wing
Giuliano Coppotelli, Roberto Sbarra, Ludovic Onofri, Marcello Righi
Abstract: High-aspect ratio, flexible wing configurations are increasingly relevant to aircraft design. Large bending deflections may induce geometric non-linearity, aerodynamic derivatives depend on static aeroelastic solution: existing modelling methodologies must be fine-tuned for accuracy yet limited CPU costs. For these reasons, one of the test cases proposed in the third Aeroelastic Prediction Workshop consisted in a slender and flexible PAZY wind tunnel model. This model is designed to exhibit a flutter mechanism – second bending / first torsion – which appears as a hump mode with on and offset speed values essentially depending on the static aeroelastic displacement. Further, it exhibits LCO - which have shown up in virtually all wind tunnel experiments and are (relatively) easy to capture.
Many participants in the workshop exploited low-order models and showed a remarkably good agreement with experimental results in terms of flutter speed. However, the characterization of the LCO has so far proven to be a harder challenge and in particular its presumed subcritical nature. In this study, we wish to obtain additional data from wind tunnel campaigns and improve the experimental database available to researchers. We have been working with two wind tunnels, at Sapienza in Rome and at ZHAW in Zurich. Two PAZY models were installed in the two wind tunnels and measurements were carried out with different instrumentation and approaches. Overall, the PAZY models have being tested over a large range of wind speed and angles of attack, with a maximum tip deflection close to 50% span. At time of submission, the available data shows a good agreement with numerical predictions (in the literature) in terms of flutter speed, flutter mechanism and LCO mode. Unlike the linear instability data (flutter speed and mode), LCO amplitudes have being showing a more elusive behaviour and in practice an unpleasant dependency on boundary conditions, model structure’s details and instrumentation. Operational Modal analysis (OMA) has allowed the identification of the LCO and its mechanisms at different speeds. Providing also damping data, it is an invaluable support for validation of numerical approaches. Ongoing efforts investigate the LCO nature, with current data pointing to a sub-critical behaviour.
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14:00
30 mins
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Flight shape determination of the highly flexible up wing wind tunnel model using high and low fidelity aerodynamic methods in conjunction with nonlinear structural methods
Jos Aalbers, Peter Blom, Huub Timmermans
Abstract: In today’s search for lighter and more efficient aircraft significant research is spent on optimizing the structural and aerodynamic aircraft properties. One of the ways to reduce aerodynamic drag due to lift is by increasing the aspect ratio of the wing. However, this raises the structural flexibility resulting in significant challenges on aero-elastic stability. Disturbances such as gusts in the flow potentially could disturb the flow leading to instable behaviour resulting in loss of structural integrity. Control surfaces can be used to alleviate the structural loads and stabilize the wing due to a gust. One of the research goals of the Clean Aviation UP WING project is to test a wind tunnel model in transonic conditions and disturb the incoming flow using a ‘gust-rig’.
In this paper an aero-elastic analysis of the UP WING wind tunnel model will be presented. Specifically, as a result of the high flexibility of the wing, the flight shape during nominal test conditions may be significantly different from the jig shape. This results in a ‘pre-stressed’ structure, which may have different dynamic structural properties such as mode shapes and eigen frequencies. Due to the high deformations, this may be beyond the validity of linear structural methods. In the first part of the paper a method will be presented how to obtain the flight shape using a coupling between the aerodynamics and the structure solvers. Regarding computation of the aerodynamic forces, a low-fidelity method based on potential theory (ZAERO) will be presented and a high-fidelity method will be presented based on NLR’s in-house tool (ENSOLV). On the structural side a MSC NASTRAN non-linear solution will be used allowing for large deflections. In the second part of the paper a one-minus-cosine gust will be applied to the newly obtained flight shape to see the effect of the pre-stressed structure.
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14:30
30 mins
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Design and experimental tests of a flexible wing of high-aspect ratio for investigating flutter mechanisms
Cyrille Stephan, Xavier Amandolese
Abstract: Increasing the aspect ratio of wings may have beneficial effects in terms of aerodynamics, such as a higher lift on drag ratio.
However, High-Aspect-Ratio Wings (HARW) also have a natural flexibility that can make them prone to aeroelastic instabilities for specific flight conditions.
Unfortunately, computing the flutter-free domains of these HARWs is tricky due to the onset of nonlinear phenomena present for high amplitudes of wing deflections.
In that context, this paper presents the study of a taut-strip flexible wing model, particularly designed to experience flutter in a wind tunnel at low-to-moderate Reynolds numbers.
The objective was to keep the structural complexity as low as possible, while exhibiting fluid-structure interactions typically observed for these types of wings.
The choice of structural design was based on the numerical prediction coming from a low-order aeroelastic model combining beam theory and simplified aerodynamics.
The choice of structural design was based on the numerical prediction coming from a linear low-order aeroelastic model combining beam theory and simplified aerodynamics.
Thanks to dynamical tests in laboratory, structural parameters (inertia, stiffness, damping coefficients) were used to update the numerical model.
Using this linear model, a flutter involving a coupling between the 2nd bending mode and the 1st torsion mode was expected in the velocity range of the wind tunnel.
Wind tunnel tests however show an earlier flutter bifurcation involving flapwise, chordwise and torsion motions, for which the route to flutter and post-critical limit-cycle oscillations have been measured by non-contact techniques.
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15:00
30 mins
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Experimental Comparison of Real-Time Shape Estimation Methods on Very Flexible Wings
Francisco P. Reis, Bilal Sharqi, Leandro Lustosa, Charles Poussot-Vassal, Carlos E. S. Cesnik
Abstract: As aircraft become more flexible as a consequence of design choices to improve fuel efficiency and operating costs, they experience larger deflections during normal operation. The ability to dynamically estimate or measure shape is useful for such flexible structures for path planning, advanced control and handling qualities. This work proposes an investigation of three model-free, real-time shape estimation methods for control applications focused on very flexible wings. Model-free techniques do not rely on precise dynamic aeroelastic models, resulting in estimators that do not require high computing power or in-depth system knowledge. Consequently, these methods are more easily adaptable to new applications. The methods rely on, respectively, fiber optic strain sensing (FOSS), a combination of inertial measurement units (IMUs) with sighting devices, and a combination of IMUs with magnetometer measurements. This paper compares the performance of the aforementioned methods regarding shape estimation and analyzes their data handling and processing requirements.
Data from various static and dynamic tests conducted on the enhanced aeroservoelastic (EASE) model, a very flexible wing developed at the University of Michigan, are used to recover the shapes of the structure. The model consists of a very flexible wing attached to a rigid fuselage and tail, and is equipped with IMUs containing magnetometers along its wing. In addition, fiber optic cables were installed on the wing to acquire continuous strain measurements, and visual markers were attached for use with motion capture devices. The shape estimation methods include displacement recovery from the FOSS, complementary filters based on magnetometer and inertial data, and extended Kalman filters based on inertial and visual data to minimize error covariance and increase the methods’ precision.
Results from the suite of tests performed on the EASE model were chosen to highlight differences between the estimators’ performances. Expected differences include deviations caused by the sensors’ bandwidth and precision, and estimator tuning. Since the control application dictates this tuning, these differences will be further discussed in the final paper.
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