Fluid-structure interaction (FSI) simulations have been widely conducted on thin plates. However, the conditions in some studies do not adequately replicate those relevant to airplane wings and rocket fins, which are typically attached to the sides of the main bodies and can be subjects for flutter phenomena. The simulations used Cartesian orthogonal meshes and employed the FSI method, transferring force and displacement data between fluid and structure solvers. The results are compared with experimental data obtained from wind tunnel tests, which were documented in two separate studies. The first case involved a plexiglass plate with a dimension of 5 × 131 × 562 mm, while the second case used a polycarbonate model with a dimension of 5 × 125 × 1000 mm. The results demonstrate that the flutter speed obtained from the simulations for the first case was 66.6 m/s, which closely aligns with the experimental result of 72.5 m/s. The simulations for the second case yielded a flutter speed of 29.8 m/s, compared to the experimental result of 24.89 m/s. The study suggests that the FSI method using Cartesian orthogonal meshes has the potential for approximating the flutter speed of a simple rectangular cantilever plate.