The vibration behavior of a three-layer smart rotary blade with an electrorheological (ER) core is investigated in this study. The blade is modeled as a sandwich structure .This type of smart structure combines elastic layers for stiffness with an ER core that can change its mechanical properties under an electric field, allowing semi-active control of vibration and damping. The elastic layers are described by the Euler–Bernoulli beam theory, while the ER core is represented through viscoelastic material theory to accurately capture its field-dependent properties and adaptive behavior. Using the energy method, potential and kinetic energy expressions of the layers are formulated considering longitudinal and transverse deformations and their temporal derivatives. The equations of motion are then systematically derived by applying Lagrange’s method. To obtain a practical modal form of the governing equations, the Ritz method is carefully employed. The predicted natural frequencies and corresponding loss factors of the blade with a specified ER core are validated by comparison with existing references, confirming the reliability and accuracy of the model. Finally, a detailed parametric study is performed to assess the influence of key factors such as electric field intensity, rotational speed, hub radius, thickness ratio, setting angle, and the selected ER fluid. The results clearly show that these parameters significantly affect both the natural frequencies and the overall damping characteristics of the proposed adaptive smart sandwich blade structure. Therefore, it can be concluded that controllable stiffness and damping can be achieved through the ER layer.