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Coming to Terms: Taxonomy of VTOL Aircraft Configuration Types — Part 2
  • 11 Jan 2023 09:06 AM
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Coming to Terms: Taxonomy of VTOL Aircraft Configuration Types — Part 2

By Daniel I. Newman
Vertiflite, Jan/Feb 2023

This series addresses terminology that are misleading or erroneous, and proffers definitions to be used as canonical. Here the basic terms of propulsion architectures are discussed. See www.eVTOL.news/terms for past columns and links to these graphics.

Part 1 of this discussion in the Nov/Dec 2022 issue of Vertiflite described several taxonomies of vertical flight aircraft configurations, contrasting the differing approaches to categorizing aircraft and category labels, and outlining the issues with each approach. These prior approaches to classifying vertical flight aircraft are reviewed briefly here, their issues are recapped, and a new approach is offered that addresses each issue. The objective is to be sufficiently robust for use going forward for all concepts.

This new taxonomy focuses specifically on the mechanisms for vertical and low-speed flight, as these are what distinguish vertical flight aircraft from conventional/horizontal takeoff and landing (CTOL/HTOL) airplanes, and differentiate them from each other. Five major categories are identified, with sub-categories for each. This taxonomy is independent of energy and power storage, conversion or transfer approach (i.e., mechanical, electric or other). Wings are treated as a feature, as they can be added to any vertical lift configuration, and so are not addressed in the primary taxonomy categories. It is addressed in second-tier sub-categories.

To calibrate the discussion, please note the definitions of terminology discussed in previous columns, as summarized in the sidebar.

Past Classifications

The earliest known approach, by NASA’s John P. Campbell (1962), identified 16 aircraft configuration types as the product of four thruster types (rotor, propeller, fan and jet) and four thruster employment methods (aircraft tilting, thrust tilting, thrust deflection and dual propulsion). Sixteen different aircraft configurations make for an unwieldy taxonomy, made large by distinguishing between the four thruster types. Also, the Campbell taxonomy did not consider combinations of propulsors, as is now common with many of the vertical takeoff and landing (eVTOL) concepts proposed, and considering them would expand the number of types yet further. So, the proposed taxonomy considers thruster employment combinations in the primary sorting, while thruster types are considered in the sub-categories.

McDonnell Aircraft’s “V/STOL Aircraft Summary” in the 1960s and 1970s — and the Vertical Flight Society’s version in the 1990s and 2000s (discussed in Part 1) — were specifically intended to address the combination of vertical flight and high-speed, called vertical and/or short takeoff and landing, V/STOL (see “Rolling Along — The VSTOL Wheel,” Vertiflite, March/April 1997). The later versions of the Wheel did not include conventional helicopters.

Unlike Campbell, the “V/STOL Wheel” variations from McDonnell and VFS did not consider the type of thruster. Like Campbell, though, the Wheel identifies four major categories of lift/thrust approaches. They are similar, but not identical, as the Wheel relies heavily on the method of powering the thrust devices. The Wheel labels this as “propulsion,” but this discussion refers to it as “power” or “powerplant” (see sidebar and “Coming to Terms: Propulsion,” Vertiflite, March/April 2022). Emerging power options such as battery-electric and hybrid-electric would further complicate the Wheel. The new taxonomy excludes the method of providing power and its distribution.

A New Taxonomy

The taxonomy used by VFS on World eVTOL Aircraft Directory (www.eVTOL.news/aircraft) currently has five major categories, but as discussed in Part 1, it could collapse to three: Vectored Thrust, Lift + Cruise and Lift Only (including Rotorcraft). This taxonomy is currently limited to aircraft using electrical power distribution, so is not useful for the full portfolio of VTOL and V/STOL aircraft.

The new taxonomy features five primary categories: two focusing on different ways that individual propulsors for vertical and low-speed flight can be installed on the airframe, and three “compound” combinations of these two installations. These five first-tier categories do not consider the propulsor types, the powerplant, means of power distribution, or the presence of a wing. They nevertheless cover the bulk of approaches to vertical and low-speed flight. Within each primary category, multiple sub-categories are identified that do consider the propulsor used to differentiate among the many possible approaches.

The first major category of the new taxonomy is “Lift,” encompassing aircraft with propulsors fixed to the airframe for low-speed lift and control, that continue to use them in the same configuration in all flight phases. The Lift system is generally a compromise to accommodate all flight phases. This covers conventional helicopters (sub-category “Rotorcraft”)

and the abundant eVTOL configurations (sub-category “Multicopters”). This category is included in Campbell and in the VFS World eVTOL Aircraft Directory, but not in the V/STOL Wheel.

The second category is Lift/Cruise, covering propulsor installations that re-orient the thrust relative to the aircraft when transitioning from low-speed flight to cruise. Variation is done by moving the propulsor relative to the airframe (subcategory “Tilting”) or by re-directing the thrust with a nozzle or with vanes (“Vectoring”).

Many vertical flight aircraft have only one type of propulsor installation, so they fall into one of these first two categories. Other configurations employ more than one installation approach — a situation not addressed at all by Campbell. The remaining three top-tier categories cover the most common combinations, and are considered Compound configurations.

Proposed V/STOL configuration nomenclature and application to prior taxonomies. (Author)

The first Compound category, “Lift + Cruise” encompasses Lift systems in the first category that are compounded by a cruise-only propulsor dedicated to high-speed flight. This includes thrust-compounded helicopters, multicopters with auxiliary horizontal thrust, fixed-wing aircraft with lift-kits added, and fan-in-wing concepts. Leveraging two different types of propulsors generally avoids the compromise noted above in Lift; it allows each system to be better optimized for just one flight phase and also can achieve higher speed than possible with some Lift-only systems (e.g. rotors). Moreover, any propulsor not in use can be altered to minimize its impact when not required, such as slowing/stopping/retracting rotors or propellers.

The second Compound category, “Lift/Cruise + Cruise” encompasses Lift/Cruise systems that are compounded by a propulsor dedicated to cruising flight. This is typically done to allow the Lift/Cruise system to be biased toward vertical flight performance. As above, propulsors not in use can be altered to reduce impact.

Finally, the third Compound category, “Lift + Lift/Cruise” encompasses aircraft combining the first two categories. This allows the Lift/Cruise system to be biased toward cruise performance and the lift system to be suppressed in cruise. This is seen on the F-35B Lightning II, which converts to cruise by stopping and covering its dedicated thrust-only lift fan and redirecting its turbofan thrust from vertical to horizontal. Some emerging eVTOL concepts feather lift propellers in cruise to a low drag orientation.

Table 1 presents the five categories of the new taxonomy, and their sub-categories, with specific aircraft examples from the previous century and from this one. The table also maps each category/sub-category to the three legacy taxonomies discussed. A more expansive graphic can be found at www.eVTOL.news/terms that expands on these categories. Additional historical vertical flight examples are mapped to this new taxonomy, identifying specific features including propulsor type(s), powerplant and whether winged.

These five new categories cover most approaches to powering vertical and low-speed flight. It is intended that this taxonomy will require few exceptions. Of course, there can always be configurations proposed with far more complicated combinations of propulsor installations, but they are beyond this simple taxonomy. When a simple combination of propulsors not covered in these categories and sub-categories emerges as dominant, this taxonomy should be revised to account for it.

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