Fluid Structure Interaction (FSI) can be easily observed in nature and explains the underlying mechanism of bird and insect flight, fluttering of leaves, and the flapping of flags. In a uniform axial fluid flow, when a flexible structure undergoes continuous deformation, it alters the flow field around it, thereby affecting the forces acting on the structure in return. Such passive interaction exhibits self-excited vibrations, which further leads to flapping once the instability threshold is exceeded. Further, energy is continuously pumped into the thin flexible structure from the engulfing fluid flow, which aids in sustaining the flapping motion. Thus, in the present study, we perform wind tunnel experimentation to observe the flapping motion and investigate the flow-induced instability of a thin flexible plate-like structure in a uniform axial flow. The objective of this study is to understand the relationship between critical velocity, which marks the onset of flapping, and the critical aspect ratio of the specimen, below which the flexible plate remains in a stationary state and does not experience any flapping motion. Further, the study investigates the effect of varying plate length, plate width, aspect ratio, and flow velocity on the onset of flapping, flapping frequency, and flapping amplitude.