The aircraft design is a complex multidisciplinary and collaborative process. Thousands of disciplinary experts with different design competences are involved within the whole development process. The design disciplines are often in contrast with each other, as their objectives might be not coincident, entailing compromises for the determination of the global optimal solution. Therefore, Multidisciplinary Design and Optimization (MDO) algorithms are being developed to mathematically overcome the divergences among the design disciplines. However, a MDO formulation might identify an optimal solution, but it could be not sufficient to ensure the success of a project. The success of a new project depends on two factors. The first one is relative to the aeronautical product, which has to be compliant with all the capabilities actually demanded by the stakeholders. Furthermore, a “better” airplane may be developed in accordance with customer expectations concerning better performance, lower operating costs and fewer emissions. The second important factor refers to the competitiveness among the new designed product and all the other competitors. The Time-To-Market should be reduced to introduce in the market an innovative product earlier than the other aeronautical industries. Furthermore, development costs should be decreased to maximize profits or to sell the product at a lower price. Finally, the development process must reduce all the risks due to wrong design choices.
These two main motivations entail two main objectives of the current dissertation. The first main objective regards the assessment and development of design techniques for the integration of the aircraft subsystems conceptual design discipline within a collaborative and multidisciplinary development methodology. This methodology shall meet all the necessities required to design an optimal and competitive product. The second goal is relative to the employment of the proposed design methodology for the initial development of innovative solutions. As the design process is multidisciplinary, this thesis is focused on the on-board systems discipline, without neglecting the interactions among this discipline with all the other design disciplines. Thus, two kinds of subsystems are treated in the current dissertation. The former deals with hybrid-electric propulsion systems installed aboard Remotely Piloted Aerial Systems (RPASs) and general aviation airplanes. The second case study is centered on More and All Electric on-board system architectures, which are characterized by the removal of the hydraulic and/or pneumatic power generation systems in favor of an enhancement of the electrical system.
The proposed design methodology is based on a Systems Engineering approach, according to which all the customer needs and required system functionalities are defined since the earliest phase of the design. The methodology is a five-step process in which several techniques are implemented for the development of a successful product. In Step 1, the design case and the requirements are defined. A Model Based Systems Engineering (MBSE) approach is adopted for the derivation and development of all the functionalities effectively required by all the involved stakeholders. All the design disciplines required in the MDO problem are then collected in Step 2. In particular, all the relations among these disciplines – in terms of inputs/outputs – are outlined, in order to facilitate their connection and the setup of the design workflow. As the present thesis is mainly focused on the on-board system design discipline, several algorithms for the preliminary sizing of conventional and innovative subsystems (included the hybrid propulsion system) are presented. In the third step, an MDO problem is outlined, determining objectives, constraints and design variables. Some design problems are analyzed in the present thesis: un-converged and converged Multidisciplinary Design Analysis (MDA), Design Of Experiments (DOE), optimization. In this regard, a new multi-objective optimization method based on the Fuzzy Logic has been developed during the doctoral research. This proposed process would define the “best” aircraft solution negotiating and relaxing some constraints and requirements characterized by a little worth from the user perspective. In Step 4, the formulation of the MDO problem is then transposed into a MDO framework. Two kinds of design frameworks are here considered. The first one is centered on the subsystems design, with the aim of preliminarily highlighting the impacts of this discipline on the entire Overall Aircraft Design (OAD) process and vice-versa. The second framework is distributed, as many disciplinary experts are involved within the design process. In this case, the level of fidelity of the several disciplinary modules is higher than the first framework, but the effort needed to setup the entire workflow is much higher. The proposed methodology ends with the investigation of the design space through the implemented framework, eventually selecting the solution of the design problem (Step 5).
The capability of the proposed methodology and design techniques is demonstrated by means of four application cases. The first case study refers to the initial definition of the physical architecture of a hybrid propulsion system based on a set of needs and capabilities demanded by the customer. The second application study is focused on the preliminary sizing of a hybrid-electric propulsion system to be installed on a retrofit version of a well-known general aviation aircraft. In the third case study, the two kinds of MDO framework previously introduced are employed to design conventional, More Electric and All Electric subsystem architectures for a 90-passenger regional jet. The last case study aims at minimizing the aircraft development costs. A Design-To-Cost approach is adopted for the design of a hybrid propulsion system
