Vertical Flight Society explores VTOL aircraft configuration taxonomies

Electric VTOL News by the Vertical Flight Society has developed a series addressing the use of vertical takeoff and landing (VTOL) aircraft terminology that are misleading or erroneous, and proffers definitions to be used as canonical. Basic terms of propulsion architectures are discussed.

Since the early days of powered vertical flight, there have been a wide range of labels placed on approaches to aircraft lift and cruise propulsion, also called the “configuration.” An early taxonomy (see Figure 1 from NASA’s John P. Campbell, 1964) identified a robust set of 16 approaches to vertical and/or short takeoff and landing (V/STOL) aircraft as combinations of four different types of propulsor devices (rotor, propeller, ducted fan and turbojet) and four different lift/thrust installations (aircraft tilting, thrust tilting, thrust deflection and dual propulsion). The taxonomy did not offer specific labels for each of the combinations. Note that there wasn’t an example graphic provided for the combination of dual propulsion with separate lifting propellers and cruising propellers, perhaps due to the maturity of powerplants at the time and the complexity of mechanical shafting.

This contrasts to the widespread use of distributed propellers today enabled by electric power. Also note that the Campbell’s methods of installation (rows) are exclusive, and do not consider combinations such as lifting systems with tilting systems as used on the F-35B supersonic V/STOL jet or many recent advanced air mobility (AAM) approaches, historically called “lift + lift/cruise.” Subsequently, other organizations have published compendia of the major known V/STOL aircraft and organized them into categories.

Beginning in the early 1960s, the McDonnell Aircraft Co. published the “V/STOL Aircraft Summary,” and updated periodically with the latest concepts (built and unbuilt) through at least 1978. The 1973 “V/STOL Wheel” (Figure 2) identified four major categories of lift/thrust approaches somewhat similar to Campbell’s rows:

• Same Propulsion System for Hover and Forward Flight.
• Separate Power Plant for Hover.
• Combined Power Plant for Hover.
• Augmented Power Plant for Hover.

A fifth category, “Special Types,” included flying platforms, ground effect machines (GEMs), flying jeeps, personal flying devices, etc.

At first glance, the four V/STOL Wheel categories are somewhat similar to the rows in Campbell’s table, but do not align directly. The Wheel’s “Separate Powerplant for Hover” is a direct match for “dual propulsion,” but the Wheel’s “Same Propulsion System for Hover and Forward Flight” aligns with both the second and third rows of Figure 1, thrust tilting and thrust deflection. Each of the Wheel’s four major categories include a wide range of propulsors, similar to the columns of Figure 1. As in Figure 1, the Wheel doesn’t label specific aircraft types.

The major difference between Campbell and the Wheel is that these V/STOL Wheel categories are heavily focused on powerplants, and not just the thrust devices/propulsors. With such a wide range of lift and cruise approaches, and so many methods to provide power, just four “Wheel” categories were inadequate to characterize the wide range of configurations available at that time, let alone to those that would emerge in the future.

In the April/May 1997 issue of Vertiflite, Mike Hirschberg (now the VFS executive director) published an updated version of the Wheel (Figure 3). The four original major categories endure to this day with the latest Wheel, current through the end of 20th century, posted at www.vtol.org/wheel.

This kept the major categories but tailored the names of the subcategories:

• Same Propulsion System for Hover and Forward Flight — with subcategories for Deflected Slipstream, Tail Sitters, Tilt Duct, Tilt Jet, Tilt Prop, Tilt Rotor, Tilt Shaft/Rotor, Tilt Wing and Vectored Thrust.
• Separate Power Plant for Hover — with a single subcategory of Lift + Cruise.
• Combined Power Plant for Hover — with subcategories of Lift + Lift/Cruise and Tip Jets.
• Augmented Power Plant for Hover — with subcategories of Ejector, Fans and Rotors.

Since 2017, VFS has compiled a continuously updated compendium of electric vertical takeoff and landing (eVTOL) configuration concepts (both crewed and uncrewed) as the World eVTOL Aircraft Directory. The qualifications for inclusion are (1) human-carrying, or a gross weight of 300 pounds (135 kilograms) or more, and (2) battery-electric, hybrid-electric or hydrogen-electric power. The five major categories in the directory are (with “Hover Bikes/Personal Flying Devices” being analogous to the “Special Types” of the McDonnell Wheel):

• Vectored Thrust: An eVTOL aircraft that uses any of its thrusters for lift and cruise.
• Lift + Cruise: Completely independent thrusters used for cruise versus for lift without any thrust vectoring.
• Wingless (Multicopter): No thruster for cruise — only for lift.
• Hover Bikes/Personal Flying Devices: These single-person eVTOL aircraft are considered to be in the general class of hover bikes or personal flying devices with the primary differentiation being that the pilot sits on a saddle or is standing, or something similar. Nearly all are multicoptertype wingless configurations.
• Electric Rotorcraft: An eVTOL aircraft that uses a rotor, such as an electric helicopter or electric autogiro.

The eVTOL’s “Vectored Thrust” category is a match for the Wheel’s “Same Propulsion System for Hover and Forward Flight,” and Campbell’s second and third rows. And the eVTOL “Lift + Cruise” aligns directly with the Wheel’s “Separate Power Plant for Hover” and Campbell’s “dual propulsion.” Similar to Campbell, this taxonomy excludes breaking-out combinations (e.g., lift + lift/cruise).

But the next three eVTOL categories are redundant as all have only lifting propulsors and no dedicated cruise thrust. The “Hover Bikes/Personal Flying Devices” admits in its definition that “nearly all are multicopter-type wingless configurations.” And the only distinction between “Wingless (Multicopter)” and “Electric Rotorcraft” is the quantity, size and the lifting propulsors and the control approach (e.g., rotor cyclic — see “Coming to Terms: Rotor,” Vertiflite, Nov/Dec 2021). So, there really are only three eVTOL categories here. Perhaps these three similar categories should be combined as “Lifting Only” or something similar. Note that this original eVTOL listing does not have a category for tilting aircraft, as in Campbell, perhaps as there are very few tilting crewed aircraft envisioned (NASA’s Puffin) or existing (Opener BlackFly), which are both covered under “Vectored Thrust.”

The International Civil Aviation Organization (ICAO) and the U.S. Federal Aviation Administration (FAA) define a powered-lift aircraft as a “heavier-than-air aircraft capable of vertical takeoff, vertical landing, and low-speed flight, which depends principally on engine-driven lift devices or engine thrust for the lift during these flight regimes and on non-rotating aerofoil(s) for lift during horizontal flight.” There’s an issue with a definition based on fixed wing for lift during high speed and cruising flight, as it is entirely independent of the approach to lift, thrust and control at low speeds. The fixed wing only becomes relevant at and above moderate forward speeds, so it’s irrelevant to lift and control at low speed. Effectively any VTOL and V/STOL configuration could employ a wing. So, the wing as a distinction at the highest level is an unnecessary complication to a VTOL platform taxonomy. It is an interesting feature but not truly relevant to low-speed flight.

A major issue with the introduction of new VTOL and V/STOL concepts is the host of new terms introduced by the developers — often newcomers to aviation, unaware of the long history of vertical flight developments — such as “lift plus push.” It is admirable that an entrepreneurial supplier would aspire to establish a brand name that becomes iconic to represent a specific product, such as Band-Aid or Kleenex, but a more objective taxonomy (i.e., bandages or facial tissue) is much preferred as we work together to mature such complex technology. Brand names and trademarks do not add clarity, and so tend to confuse and obfuscate. Creating new terms should be avoided in our technical discussions, unless a new configuration approach really is different than existing categories.

The objective of this discussion is to identify a taxonomy of configuration types that is robust and appropriate to describe all or most new VTOL or V/STOL configurations. Campbells’ categorization of lift/thrust (rows in the table) are robust, but lacks specific configuration names and 16 is far too itemized. The four configuration categories of the Wheel were useful at the time during the boom of V/STOL research, but are made complex by the insertion of powerplant architectures. The categories that have been used in the VFS World eVTOL Aircraft Directory — Vectored Thrust, Lift + Cruise, Wingless (Multicopter) and Hover Bikes/Personal Flying Devices — may have been adequate when the directory was first created in 2016 with only a half dozen designs, but is too coarse and inconsistent for today’s overwhelming 700+ designs. And each of these taxonomies lack the combinations of approaches that are now common in emerging configurations.

See www.eVTOL.news/terms for past columns and links to these graphics.