Experimental investigation of a highly flexible wing

ifasd2024 Tracking Number 65

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.