**2. Hybrid-electric and universally-electric propulsion system architecture**

An overview of the topological variety of propulsion systems featuring an electric drive approach is presented in this section. Electric approaches to aircraft propulsion system are known as hybrid-electric and universally-electric [4]. By distinguishing between the generation and the transmission to the drive-shaft, the components and their possible combination for the layout of hybrid-electric and universally-electric propulsion system architecture can be illustrated as in Figure 1 [5]. Focusing on the propulsion system, the consumer of electric power is the propulsive device. Ducted and unducted propulsors are typically considered to provide the thrust required to propel transport aircraft. The commonly known-term ducted-fan is understood to be covered by the category ducted propulsor while unducted propulsor includes propeller and open-rotor arrangements. In a conventional propulsion system, shaft power is generated by burning fuel in a gas-turbine. A gas-turbine converts chemical energy into mechanical power through an aero-thermodynamical process. In an electrical propulsion system, shaft power is produced through a process that can be subdivided into the generation, the distribution and the conversion of electrical energy into mechanical power.

emissions and external noise reduction targets declared by Europe in the Flightpath 2050 program and the corresponding Strategic Research and Innovation Agenda (SRIA) [1] and by the United-States of America in the NASA Environmentally Responsible Aviation N+ series [2]. It is then motivated by the growing evidence that evolutionary improvement of the technologies might not be sufficient to fulfill these targets. Investigations [3] have shown that the reduction potentials might fall short of the 2035 targets and that the deficit becomes even more substantial towards the 2050 goals. This trend can be best explained by the very high maturity reached by contemporary technologies, in particular the propulsion system technology implemented in transport aircraft. Prospects for technological improvements have consequently reached an asymptote leaving not enough potentials for achieving the aggressive targets. As a result, the introduction of disruptive technologies turns out to be essential in view of meeting the future aviation goals. Finally, the progress and perspective in electrical technology development trigger the initiatives and the extensive research activities deployed by the aeronautical community to investigate the feasibility and the potentials of

A current application of electrical technology for transport aircraft is the so-called more-electric aircraft [67-71]. The objective of the more-electric aircraft initiative, which targets the aircraft power systems, is basically to replace pneumatic and hydraulic systems by electrical systems. The Boeing B787 is the first aircraft utilizing a more electric power system architecture. A logical future conceivable step is the electrification of the propulsion system of the aircraft which is the topic of this chapter. While aiming for the ultimate goal of an universally-electric aircraft, hybrid-electric approach will be first necessary to match the requirement of aircraft propulsion system and the development pace of the electrical components technology. Hybrid-electric aircraft feature typically a combined conventional and electrical propulsion system. The combinatorial variety of hybrid-electric and universally-electric propulsion system topology considered for transport aircraft application as well as enabling technologies are first discussed in Section 2. A compendium of hybrid and universally-electric advanced aircraft concepts is then proposed in Section 3 to obtain a notion of the cloud of aircraft configurations and electric drive options investigated up to this point in time. The feasibility of hybrid-electric aircraft needs to be established for future market segments. On the basis of selected concepts, the integrated

**2. Hybrid-electric and universally-electric propulsion system architecture** An overview of the topological variety of propulsion systems featuring an electric drive approach is presented in this section. Electric approaches to aircraft propulsion system are known as hybrid-electric and universally-electric [4]. By distinguishing between the generation and the transmission to the drive-shaft, the components and their possible combination for the layout of hybrid-electric and universally-electric propulsion system architecture can be illustrated as in Figure 1 [5]. Focusing on the propulsion system, the consumer of electric power is the propulsive device. Ducted and unducted propulsors are typically considered to provide the thrust required to propel transport aircraft. The commonly known-term ducted-fan is understood to be covered by the category ducted propulsor while unducted propulsor includes propeller and open-rotor arrangements.

prospects of hybrid-electric aircraft are finally investigated in Section 4.

electric technology application to transport aircraft.

116 New Applications of Electric Drives

For transport aircraft application, batteries and fuel cells are considered for the generation of electrical energy. Even if not explicitly shown in Figure 1, supercapacitors or flywheels could be also considered in a layout of a hybrid-electric propulsion system [6]. Another means of producing electrical power is through the utilization of a generator driven by a conventional gas-turbine. Known under the terminology of turboelectric, it is discussed in more detail in Section 2.1.1. The electric energy is monitored and distributed from the sources to the consumers by a Power Management and Distribution system (PMAD). The PMAD is typically composed of controllers, converters, inverters, cables, electrical buses and circuit breakers. The layout of the PMAD system is a rather complex task. Optimum layout of the PMAD system results from the analysis of system efficiency, mass, bill-of-material, reliability and maintenance under constraints of abnormal mode of operation. Another important aspect related to the electrical system is thermal regulation to provide for every operating condition an appropriate thermal environment to the electrical and power electronics components. The consideration of thermal management is essential in the early design phase of the integrated electric drive due to its highly-interlaced interactions with the propulsion and power systems. Presenting the development of a framework for concurrent sizing of the powertrain and the thermal management system, Freeman et al. [7] highlighted the design options and implications of thermal management. Three main options for thermal regulation were discussed [7] including air cooling, liquid cooling and cryogenic cooling system providing the advantages and drawbacks of each options in terms of system performance, integration implications, complexity and bill-of-material. The critical importance of adequate flow rate for heat transfer ensuring a proper thermal regulation of the electrical components was highlighted. Freeman et al. [7] predicted the revival of radiator technology due to the introduction of electric drive. The design considerations of radiators were described as well as their interactions with the propulsion system and the implications at aircraft level in terms of weight and parasitic drag. The potential synergistic use of the excess heat produced by the electrical components for anti-icing system, cabin Environmental Control System and galleys were also indicated [7]. The thermal management system needs to be integrated globally. The necessity of a transverse approach is argued by Liscouet-Hanke [8] in order to avoid developing localized dedicated engineered thermal management solutions for each of the electrical components.

In order to drive the propulsor device, the electric energy is converted into shaft power by an electric motor (see Section 4.2). It can drive by itself the propulsor device or it can be mounted on the shaft of a combustion engine to support its operations. This latest arrangement known as a parallel system is discussed in Section 2.1.2. Due to the nature of the electric energy which can be easily partitioned, the field of distributed propulsion which aims to achieve highly coupled structural-aero-propulsive configurations, is often combined to hybrid-electric and universally-electric approaches (see Section 2.2.3).

**Figure 1.** Conventional, hybrid and universally-electric propulsion system topology [5]
