This thesis presents the design, modelling, and preliminary sizing of a passive hydrogen recirculation ejector for a PEM fuel cell system intended for airborne applications. A comprehensive methodology was developed, combining a one-dimensional (1D) analytical model, high-fidelity Computational Fluid Dynamics (CFD) simulations, and experimental validation. The analytical model was first validated against CFD and experimental results, achieving a significant reduction in prediction error (from 41% to below 5%) through coefficient tuning of mixing efficiency and nozzle efficiency. CFD simulations further provided insights into operational phenomena such as subsonic nozzle behavior, humidity influence, and performance deviations. Finally, the validated design tool was applied to size an ejector for a light trainer aircraft powered by hydrogen fuel cell.