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The V/STOL V: Fine-Tuning the Triangular V/STOL Wheel
  • 06 Jan 2025 05:30 AM
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The V/STOL V: Fine-Tuning the Triangular V/STOL Wheel

By Dan Newman, with Al Lawless
Vertiflite, Jan/Feb 2025

Figure 1. New taxonomy proposed in 2023 to characterize V/STOL aircraft configurations, consisting of three primary propulsor categories and three combinations.

The “Coming to Terms” column in the March/April 2023 issue of Vertiflite, “A Taxonomy of VTOL Aircraft Configuration Types — Part 3: Reinventing the Wheel,” was the last in a series of three successive installments that proposed a new taxonomy for characterizing vertical and/or short takeoff and landing (V/STOL) aircraft. The approach centers on the thrust devices used to provide vertical lift and how they are installed on the aircraft.

This “Coming to Terms” trilogy chronicled the history of cataloguing V/STOL aircraft, reviewed the strengths and weaknesses of each of the various taxonomy approaches, and offered a new approach to classification purported to cure all ills. This capstone column offered the V/STOL Triangle (Figure 1) as a drop-in replacement for the wheel of V/STOL of Aircraft and Propulsion Concepts. The V/STOL Wheel was originally developed in the 1960s for the vertical flight aircraft beyond helicopters — advanced rotorcraft, propellercraft and jetborne aircraft — being studied at the time. This was updated for Vertiflite (Figure 2) several times over the years, including descriptions of each aircraft (www.vtol.org/wheel). The configurations that could be accommodated were limited to mechanical and jet-powered propulsion, thus was not easily adapted for the now-ubiquitous electrical-power-distribution concepts, e.g. electric vertical takeoff and landing (eVTOL) aircraft.

Figure 2. The 2018 update to the V/STOL wheel of 20th century demonstrators.

The V/STOL Triangle actually consists of six smaller triangles. The three sides in primary colors represent three distinct types of thrusting propulsor devices used for vertical lift — pure vertical lift (L), pure horizontal cruise (C), and those capable of both lift and cruise (L/C). The three intervening corner triangles in secondary colors represent compound aircraft applications using pairwise combinations of the primary propulsors (L+C, L+L/C, L/C+C).

The more detailed triangle in Figure 3 presents how the proposed taxonomy goes a level down to account for different approaches to each propulsor type and allow differentiation between the many combinations. For each propulsor type, lines separate different varieties of devices and different installations on the airframe (refer to the trilogy of “Coming to Terms” articles for details, posted at www.eVTOL.news/terms).

In fact, the trilogy was originally to be in only two parts, but the approach matured in parallel with the writing as a result of extensive discussion and healthy debate that followed publication. The initial categories did not stand up to scrutiny. After consideration of the prior V/STOL aircraft-naming precedents and consideration of the entire design space available, the six-lobe V/STOL Triangle emerged and has proven quite robust, able to accommodate both mechanically and electrically powered concepts.

Figure 3. Detailed V/STOL Triangle with propulsor types shown to categorize all different types and combinations.

After subsequent presentations of the new taxonomy at VFS conferences and meetings with the vertical flight technical community, the authors offer here a further tweak to that V/STOL Triangle. The two changes made are rather subtle, but together yield what is considered a more accurate, appealing and memorable representation.

First, the blue “cruise-only” thrust sector of the triangle is removed, as aircraft here have no powered thrust in the vertical direction. The five remaining sectors then correctly represent the full design space of powered vertical flight aircraft.

The second change is to invert the figure to yield a “V” shape, signifying the word “vertical” that heralds the essential advantage of runway-independent aircraft (Figure 4). In contrast to the original V/STOL Wheel that was created using only the aircraft configurations being developed at the time, this taxonomy was specifically developed to accommodate all possible corners of the design space for pure and pairwise compounded concepts. It should also be future-proof with its capability to represent any combination of vertical thrust.

Another valuable feature of this approach is that it naturally highlights similarities between configurations that use the same type(s) of propulsor devices, even if the two aircraft look dramatically different. This offers the opportunity to easily repurpose analytical methods and tools, test fixtures and data, design techniques, lessons learned and perhaps even certification means of compliance. This last offers the opportunity for model-based certification — for major simplifications to review, test and subsequent approval by regulatory agencies — with associated reductions in workload and timeline.

Figure 4. Refined taxonomy into the V/STOL V, now with only powered vertical fight aircraft and emphasizing the word “vertical.”

The V/STOL V is offered as an improved means to characterize different vertical thrust aircraft configurations. Comments are always welcome for further improvements.

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