**3. Electrification of Aviation**

In essence, there are two main types of aircraft—turbo-propeller aircraft (or turboprops) and jets. The turboprops were the first aircraft type to be electrified due to the technical simplicity of such a modification: directly substituting an electric motor for the engine and batteries for the fuel tank [28]. Regional aviation, where electrification provides a feasible option, operates mostly in the realm of "business aviation", as opposed to the major airlines' scheduled business models. In Europe, turboprops constitute 34% of the business aviation fleet and are responsible for 26% of the flights [29], see Table 1.


**Table 1.** Characteristics of aircraft segments in European business aviation.

The European turboprop fleet of over 1000 aircraft is represented by ten main brands of airplane from seven manufacturers in the US and Europe, Table 2.

The specific energy of batteries, defined as how much energy a battery contains per its mass, is currently considered to be the main constraining factor for electrical aviation [30]. In a nutshell, batteries are simply too heavy. The existing specific energy levels for batteries range between 0.1 to 0.25 kWh/kg, which is ten times lower than the energy density of kerosene in combination with the specific power of combustion engine [16]. Nevertheless, by extrapolating along major historic technology improvement trends, the airline industry forecasts storage solutions to easily increase to 0.8 kWh/kg within the coming decades [17].

A second major factor frequently under-explored in current aviation research is the actual specific power of an electric motor [31]. Table 3 highlights the specific power progression of Siemens motors over since 2015 [32], which clearly demonstrates the recent rapid technological progression of electric motors.


**Table 2.** Major characteristics of business aviation turboprops operating in the EU in 2020, based on [29], with data added for seats and maximum takeoff weight from each aircraft description.

**Table 3.** Characteristics of Siemens electric motors developed for aviation, based on [32].


Since the power of the motor is a function of its torque and rotational speed, highspeed solutions can substantially increase specific power as illustrated in the last motor in Table 3. Unfortunately, such high-speed motors subsequently require the addition of a gearbox that is connected to the propeller (all the other motors from Table 3 operate via direct drive) [32]. Consequently, any additional gearbox mass must be accounted for when comparing the specific power of different electric motors. For example, when accounting for (say) a standard 50 kg aircraft gearbox, the overall specific power of the powertrain of the Siemens SP2000D would be reduced to 6.4 kWh/kg.
