The study of hybrid-electric aircraft in recent years is motivated by efforts for diminishing the emissions on aerial transportation. Alternative powertrain design developments for replacing actual devices employ several approaches and methodologies. However, to set the basic specifications of these new devices is a complex challenge. Present work shows the detailed design and model of the electrical subsystem of a hybrid-electric engine in serial topology, that must replace actual powertrain on a fixed-wing drone from a Brazilian company. The basic specifications were defined by a previous work that analyzes the complete aircraft powertrain and its subsystems. The application that includes electrical generator, power electronic converters, battery pack, and EM, was developed in MATLAB/Simulink. Several power conditions were tested from zero to nominal power of the set, and electrical variables were sampled in steady state conditions. The results yielded polynomial equations that represent the steady-state relations of power and efficiency at the input and output of each device, and will be applied in a more in-depth analysis for the aircraft studied.