Trade off studies of hybrid-electric aircraft by Fuzzy logic methodology

In this paper, an innovative methodology for the conceptual design of hybrid-electric powered aircraft is proposed. The main idea is the optimization of the designed airplane through the negotiation and settling down of the high-level requirements. This optimization problem is multiobjective, as the goal is the minimization of the fuel and maximum takeoff weight, the time reduction of the climb phase and the enhancement of the safety level during takeoff. The safety level is evaluated as the minimum altitude reached during the climb soon after the takeoff at which the aircraft is able to succeed in an emergency landing in case an engine failure occurs. In order to fulfil these objectives, the three Top Level Aircraft Requirements (TLARs) are relaxed. In more details, the payload and the range – i.e. the maximum distance flown by the airplane – are reduced, while the takeoff run distance (namely the takeoff field length) is increased. A one more design variable is considered, the hybridization degree, meant as the ratio between the electric power and the total propulsive power. The proposed design and optimization methodology is based on the Fuzzy Logic approach. For each objective function, a fuzzy set is defined. The fuzzy sets fix the boundaries of acceptable resulting objectives included between a minimum and a maximum value. The consideration of all the fuzzy sets identifies the optimal solution, which is characterized by the maximum customer’s satisfaction degree. An Overall Aircraft Design (OAD) environment is set up, in which a deterministic optimizer and fuzzy rules are implemented. The design and optimization problem is analysed and the optimal solution is obtained. The results demonstrate that the optimal solution is achieved by a hybrid airplane. Furthermore, the negotiation of the three requirements entails an improvement of the customer’s satisfaction.