This work presents the performance prediction activities carried out by the research team of Pisa University within the Euopean project “HYPSTAIR”, concerning the development and validation of hybrid propulsion system components and sub-systems for electrical aircraft. The first part of the paper discusses the performance analysis of a serial hybrid general aviation airplane for a reference mission profile. In particular, the best flight performance is evaluated varying the relevant mission parameters (e.g. range, cruise altitude, and cruise speed) and the amount of available energy, in terms batteries and fuel. In the second part, a hybrid plane simulator, conceived to implement different mission profiles and to include pilot effects on power management by adopting a human-in-the-loop approach, is presented. Such simulator consists of three main software modules linked to each other in real time: a flight simulator, used to compute the aerodynamic forces and to visualize the airplane in flight, a flight planner, in which the mission profile can be defined, and a performance module, which calculates the instantaneous consumption of energy and provides the endurance prediction.