Several different eVTOL aircraft designs have emerged across the globe to serve Advanced Air Mobility; however, there is yet to be an international consensus on the taxonomy for such designs. This short article reviews some methods for eVTOL aircraft categorization and presents how some of the existing and emerging eVTOL aircraft designs could be distinguished according to a specific method of categorization.
By Leo Jeoh
The fundamental design feature of an electric vertical takeoff and landing (eVTOL) aircraft, by virtue of its name, is its ability to take off and land vertically with electrically powered propulsion systems in its design. Several eVTOL aircraft designs have emerged to address this design feature; however, there has yet to be an international consensus on a taxonomy for the design configurations. This article aims to describe a method for categorization and how some of the eVTOL aircraft design configurations that have emerged would be categorized according to such a method.
Lawless and Newman have proposed a taxonomy for eVTOL aircraft that uses the function of the aircraft’s main propulsor as the central distinguishing feature (Lawless and Newman, 2024). In this approach, a propulsor is defined as any device that generates thrust for lift, propulsion, or control applications.
The taxonomy proposed by Lawless and Newman aims to comprehensively encapsulate all known and future VTOL designs and includes multiple tiers of features. It is posited that their method can be simplified and adapted to focus on encapsulating eVTOL aircraft configurations.
Based on a propulsor-function-based method, eVTOL aircraft designs could be distinguished at the top tier by the function that the aircraft’s main propulsors serve – either purely to generate lift (lift), purely to generate thrust for forward flight (cruise), or to provide both lift and cruise across the phases of flight as required (lift/cruise).
eVTOL aircraft may have a combination of any number of these three propulsor functions, except for a cruise-only configuration, which would not have any vertical flight capability. The potential eVTOL aircraft design configurations, as categorized according to propulsor functions, are shown in Table 1.
Lift/cruise propulsion involves the articulation of thrust from the electric propulsion systems from vertical to horizontal thrust between low and high-speed forward flight, respectively. Emergent eVTOL aircraft designs accomplish this by turning either the propulsion system, wing, or airframe. A slowed/stopped rotor configuration involves the conversion of the rotors providing vertical lift into a non-rotating airfoil for forward flight.
eVTOL aircraft where the propulsors mainly function to generate lift may be directly referred to as a multicopter or an electric rotorcraft. An electric rotorcraft typically comprises a standard rotorcraft design with the main rotor (or rotors, in the case of a tandem helicopter) directly driven by electric propulsion systems, while a multicopter involves the distribution of thrust amongst multiple smaller electric propulsion systems. Both configurations involve tilting the effective tip-path plane of the rotor system to provide thrust for both lift and forward flight. The tip-path plane can be changed by either tilting the entire aircraft (more commonly with multicopters) or through pitch and/or cyclic control of the rotor(s).
Unlike conventional helicopters, some eVTOL aircraft designs use fixed airfoils (i.e., wings) as the primary means to generate lift while cruising. It could thus be possible to differentiate eVTOL aircraft designs according to the means of lift generation during cruise.
Distinguishing eVTOL aircraft as designs that generate lift using wings (i.e., non-rotating airfoils) and designs that use propulsion system thrust (wingless) during cruise flight would more readily enable eVTOL aircraft designs to be identified by an international aircraft classification. Specifically, the International Civil Aviation Organization (ICAO) officially classifies heavier-than-air power-driven aircraft as being either aeroplanes, rotorcrafts, or ornithopters in ICAO International Standards and Recommended Practices (SARPs) Annex 7 (ICAO, 2012), but a powered-lift aircraft classification is defined in ICAO SARPs Annex 1 as follows:
Heavier-than-air aircraft capable of vertical take-off, vertical landing, and low-speed flight, which depends principally on engine-driven lift devices or engine thrust for lift during these flight regimes and on non-rotating aerofoil(s) for lift during horizontal flight (ICAO, 2022).
The US Federal Aviation Administration (FAA) also adopts the powered-lift aircraft classification, which is defined in 14 CFR Part 1 (FAA, 2024). In this classification, eVTOL aircraft that generate lift using non-rotating airfoils would be classified as powered-lift aircraft, while most wingless eVTOL aircraft would be classified as rotorcraft.
It is possible to combine the propulsor-function-based method of categorization with the powered-lift/wingless method of categorization by applying the propulsor-function-based method onto the powered-lift eVTOL aircraft category as a further means of subdivision. The categorization of eVTOL design configurations based on this combined method is as shown in Table 2.
This method is slightly different from the adapted and simplified method of Lawless and Newman in that it adds a tier above the propulsor configurations and directly places eVTOL aircraft with only lift-type propulsion systems into a Rotorcraft aircraft classification. The taxonomy that is proposed in Table 2 has the potential to be expanded beyond eVTOLs and to all VTOL aircraft, which would mean adding other propulsors such as jets and vectored efflux into the list of design types. Further studies will be required to determine how well the method of category would apply to a more generic group of aircraft designs.
FAA (2024), Title 14 / Chapter 1 / Subchapter A / Part 1 – Definitions and abbreviations, https://www.ecfr.gov/current/title-14/chapter-I/subchapter-A/part-1.
Lawless, A., Newman, D., (2023) Reinventing the Wheel – VTOL Aircraft Taxonomy and Lexicon.