Identification of Modal Properties of Composite Thin Plate using OMA in Wind Tunnel Environment Z. A. C. Saffry 1, a , D. L. Majid 2, b , F. I. Romli 3, c , F. Mustapha 4, d and E. J. Abdullah 5, e 1, 2, 3, 4, 5 Department of Aerospace, Faculty of Engineering, University of Putra Malaysia (UPM), 43400 Serdang, Selangor, Malaysia. a zetleen44@yahoo.com, b dlaila@eng.upm.edu.my, c firomli@eng.upm.edu.my, d faizal@eng.upm.edu.my, e ermira@eng.upm.edu.my Keywords: Operational Modal Analysis, Hybrid Kevlar Composite, laser vibrometer, modal parameters, wind tunnel. Abstract. Identification of modal parameters is crucial especially in aerospace applications whereby the interactions of airflow with aircraft structures can result in undesirable structural deformations. This structural deformation can be predicted with knowledge of the modal parameters. This can be achieved through conventional modal testing that requires a known excitation force in order to extract these dynamic properties. This technique can be experimentally complex because of the need for artificial excitation and it also does not represent actual operational condition. The current work presents part of research work that address the practical implementation of operational modal analysis (OMA) applied to a cantilevered hybrid composite plate exposed to low speed airflow in a wind tunnel. A single contactless sensing system via a laser vibrometer is employed to measure the response. OMA technique applied in a wind-on condition succeeded in extracting the modal parameters of the hybrid composite plate which correlate well with modal testing using impact hammer excitation. Introduction Aerospace structures especially on lifting surfaces are continually exposed to fluctuating airloads and these aero-structural interactions lead to structural deformations that can be destructive in nature or compromise its structural stability. The science that studies these interactions is known as aeroelasticity. Aeroelastic instabilities can be static or dynamic in nature. Flutter is dynamic aeroelastic instability predominant on aircraft’s lifting surfaces. To predict flutter occurrence, knowledge of the natural modes of a structural system is necessary. Technique that looks into the assessment of these modes is known as modal analysis. Modal analysis identifies the modal properties of a structural system which are the natural frequency, modal damping and mode shapes and also known as dynamic properties. By artificially exciting the structure, measuring its operating deflection shapes (motion at two or more DOFs) and post-processing the vibration data, the modal properties can be obtained. Determination of the excitation input force is needed for conventional or experimental modal analysis (EMA) where the frequency response functions (FRF) are determined and represented as the ratio of input over output response. Presently, impact testing, developed during the late 1970’s is the most popular, fast and economical modal testing technique. On-site modal testing can be complicated if the structure is difficult to access and excite as well as to mount the numerous wired sensors that are placed at strategic points on the structure that within the sensing system. Response sensing system is usually composed of accelerometers that introduce mass-loading effects into the dynamics of the structure. The effect of the mass-loading can be reduced if laser scanning vibrometer is used.