This paper presents a mathematical model for the dynamic behavior of a electric propulsion system for an aircraft. The system is composed of a Li-Po battery, an electronic speed control (ESC), a brushless DC motor (BLDC), and a propeller. Models are obtained for each component of the system and then they are implemented and integrated for numerical simulation in m language in Octave. The model takes into account the hybrid nature of the system, given that the switching dynamics of the ESC is considered as a discrete event model, while the dynamics of the other components is continuous. The parameters used in the simulation have realistic values associated to the model of a small unmanned aerial vehicle (UAV) propulsion system, but with adecuate parameters it could represent the dynamics of a larger scale electric manned aircraft. Simulation results show a reasonable behavior and further work will be done to estimate the parameters for a real UAV prototype, based on the proposed model.