AEROELASTIC MODELING OF A FLEXIBLE WING FOR WIND TUNNEL FLUTTER TEST

1. AEROELASTIC MODELING OF A FLEXIBLE WING FOR WIND TUNNEL FLUTTER TEST WESTIN, Michelle Fernandino; GÓES, Luiz Carlos Sandoval; SILVA, Roberto Gil Annes da AERONAUTICS INSTITUTE OF TECHNOLOGY - ITA BACKGROUND RESULTS The V-g and V-f curves obtained from ZAERO are, for first and second models respectively: Figure 3: First model computational results (flutter velocity: 22,5m/s; flutter frequency: 17,4Hz) Figure 4: Second model computational result for each CG position The flutter velocity for the second model is 8,1m/s for first CG position, 12,3m/s for the second and 14,6m/s for the third. Wind tunnel measurements are: Figure 5: PSD data acquisition for the first model (flutter velocity 19m/s) Figure 6: PSD acquisition for second model, CG at 0,5cm, 1cm and 1,5cm forward center chord, respectively These PSD curves in Figure 6 correspond to 9,28m/s, 12,36m/s and 14,88m/s , respectively for first, second and third CG positions. • The aeroelastic phenomena study is multidisciplinary and this is the reason each subject must be treated carefully to best understand aeroelastic problems. In order to achieve flutter comprehension, it is necessary to review the following issues: • Finite Element Method (FEM): NASTRAN solver is used to structural dynamics analysis • Modal Analysis • Unsteady Aerodynamics: ZONA 6 method is applied, which consider mutual interference between singularities located over a lifting surface • Aeroelasticity: ZAERO solver is applied, using g method for flutter solution PURPOSE The scope of the present investigation is specially the design of flexible wings which flutter occurs within the wind tunnel speed range, and how to perform flutter tests and its implications. The design of flexible wings is very important to provide models that can be used in further researches using smart structures and other related phenomena observed during experimental tests. The first wing model designed is similar to Dowell and Tang’s (2002) wing and the second was developed in order to study flutter mechanism with CG position variation (Figure 1). Figure 1: Wing models designed for flutter tests MATERIALS AND METHODS This research can be divided into basically four steps: Structural model solution using FEM method, wing models aeroelastic computation, wing models construction and wind tunnel flutter tests (Westin, et. al., 2009). For the first wing model, its main dimensions will be the same as Dowell and Tang’s model (2002), that is 45,7cm in span, 5cm in chord and 1,27cm in plate width. With geometry defined, using both first and second steps, a new slender body was designed in order to achieve 30m/s in flutter velocity, at most. The material to be used is aluminum 2024-T3 and it is modeled with plate elements. The new slender body dimensions, which is a squared section aluminum bar, are: 0,635cm in width and height, 13,2cm in length and mass of 14,4g. In order to find a model easy to build and with good flutter characteristics, another case of study will be a flat plate made of aluminum. It will be analyzed the influence of the slender body CG position on flutter velocity. Three different positions will be tested. The model dimensions are: 35cm in span and 4cm in chord. The slender body is a brass cylindrical bar with mass of 32,75g, 7,93mm in diameter and 89,9mm in length. Like the first model case, those dimensions are in order to achieve 30m/s in flutter velocity. The wings in Figure 1 are tested in different wind tunnels: the first model is tested in a closed test section, but opened circuit wind tunnel and the second is tested in a blower wind tunnel with opened test section. Theses wind tunnels are presented in Figure 2: Figure 2: Wind tunnels used for first and second wing models, respectively (ITA/IEA, 2010) The wing models are positioned for the test, so the boundary condition will be similar to a clamp. This is the best way to find experimental and computational results close from each other. The power spectral density approach is employed as a way to identify flutter (Sheta, et.al., 2002). The output signal from an accelerometer placed in the wing structure allows, through its power spectral density computation, identify flutter onset condition and the corresponding undisturbed flow speed, which is calculated from Betz’s manometer dynamic pressure. The PSD function increase means flow energy extraction, condition to have flutter. CONCLUSIONS The difference in results can be attributed to undesirable support flexibility and to the link between the plate and the slender body. Connect them with screws, especially in flat plate case, was important, so the CG could be varied, and also because it is simple to build. However, constructions with a more rigid connection are important to compare, in the future. The flutter mode shapes observed during the tests were similar from those presented in Figure 7: Figure 7: First and second wing models flutter mode shape, respectively The importance of this work is to present a very simple aeroelastic model. With the wing models presented here an entire study in aeroelasticity can be developed like nonlinear effects, new support development for aeroelastic testing, flutter control study, smart material application in flutter control and other studies. BIBLIOGRAPHY [1] Dowell, E. H. and Tang, D.; Experimental and Theoretical Study of Gust Response for High-Aspect-Ratio Wing, AIAA Journal, number 40, March, 2002, pp. 419-429 [2] ITA/IEA (February, 5th 2010) http://www.aer.ita.br/conteudo/laborat-rio-prof-feng-bancos-ensaios. São José dos Campos, SP. [3] Sheta, E. F., Harrand, V. J., Thompson, D. E., and Strganac, T. W. (January-February de 2002). Computational and Experimental Investigation of Limit Cycle Oscillations of Nonlinear Aeroelastic Systems. AIAA Journal , pp. 133-141. [4] Westin, M. F., Góes, L. C., Ramos, R. L., and Silva, R. G. (2009). Aeroservoelastic Modeling of a Flexible Wing for Wind Tunnel Flutter Test. Proceedings of 20th International Congress of Mechanical Engineering, (p. 9). Gramado, RS.