The overarching goal of this work was to investigate hybrid powertrains for propulsion of small unmanned aircraft systems. The specific task objectives were: 1.) Development of computational tools for hybrid powertrain sizing and design; 2.) Performance measurement of powertrain components at steady-state and transient conditions; 3.) Evaluation of hybrid configurations using metrics most relevant to small unmanned aircraft systems; 4.) Development and bench testing of a hybrid powertrain with associated controllers; 5.) Integration of the powertrain and controller with a suitable airframe. The demand on unmanned aerial vehicles (UAVs) for execution of combat, surveillance, and reconnaissance missions has increased greatly across the world, henceforth modeling and simulation of aircraft systems are necessary to eliminate unnecessary costs. In this project, a longitudinal three degree of freedom aircraft model was developed and its performance monitored via two basic control inputs the throttle position and elevator deflection. The model utilizes the nonlinear longitudinal equation of motion considered for aircraft designs. The nonlinear aircraft model is further trimmed to obtain the operating points necessary for the linearization. Trimming and linearization of the model were achieved by using a linear analysis tool available in MATLAB/Simulink. Using the Ziegler Nichol control scheme the corresponding gains necessary for stabilizing the system are developed. To further validate the model, an open source aircraft data (Aerosonde) was used. The performance of the aircraft was evaluated by comparing the non-linearized and linearized model results.