In this paper a simple formulation of dynamic model of a turboprop installations has been presented incorporating aerodynamic forces. A four-bladed propeller-engine is simplified to have six rigid-body degrees of freedom. The engine is connected to the nacelle through resilient elements in order to reduce vibration transmission from the engine to the airplane structure. The aerodynamic forces acting on the propeller are modeled utilizing quasi-steady airfoil theory by considering their effect as reaction forces. Using the formulated dynamic model, an analysis of the stability of the engine-mount is conducted by deriving a second order eigenvalue problem. The system natural frequencies are found to be dependent on the rotational speed. At lower speeds, the aerodynamic forces have a stabilizing effect on the system. At higher rotational frequencies, the engine reaches a speed after which the system becomes unstable. The use of soft resilient mounts maximizes the vibration isolation but lowers instability threshold speed.