This paper presents a stochastic thrust model for small propeller driven UAVs whose power plant is a small electric motor. Such a stochastic model is indispensable in the design of state estimation and control systems for UAVs where the performance requirements of the systems are specified in stochastic terms. A model which relates thrust generated by a small propeller driven electric motor as a function of throttle setting and commanded engine RPM is developed. A Monte Carlo simulation and a perturbation analysis are used to characterize the uncertainty in throttle setting; RPM commanded; and propeller geometry and aerodynamic characterization to the error in the predicted thrust. Validation results from a series of wind tunnel tests are presented.