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Air taxis: largely about the batteries, but also much more


 Small air taxis – generally called vertical take-off and landing aircraft (VTOLs) – have been on dream and wish lists for decades. They were popularized in the 1960’s animated TV series The Jetsons, which ran for only a few years yet developed a loyal following and has been shown in reruns ever since. The Jetson family could hop from place to place using their personal air taxi called the Space Car, Figure 1; its fuel and powerplant specifics were never made clear, but that didn’t matter.

Figure 1 The concept of the personal air taxi was popularized in the 1960s by the short-lived but beloved animated series The Jetsons. (Image source: Nostalgia Central)

Fast forward to the 21st century and the VTOL dream is very much alive and well with a clearly defined means of propulsion, namely rechargeable batteries, and electric motors. About a dozen companies are working on designs, and they vary in stages from rough prototypes to units ready for test and certification (that’s a very large consideration). The source and use of power is a major factor in their design challenge.

Conventional gas-powered small helicopters use a single main rotor, a small tail rotor, and usually some sort of forward-thrust engine. For these VTOLs, the use of batteries and rotors (fans) powered by electric motors gives these designs considerable freedom in arrangement of these elements, and this is reflected in the units under development. Some designs use fewer larger fans, other use a great number of smaller ones, Figure 2.

Figure 2 VTOLs come in many different configurations beginning with variations on fan size and number as well as aircraft size, capacity, range, and more. (Image source: IEEE Spectrum)

Which approach is “better?” It’s the classic engineering tradeoff scenario, every each arrangement has pros and cons with respect to factors such as efficiency, lifting power, capacity, and more. There is no best answer, as is the case in most complicated and simple designs. These tradeoffs have to be explored in the context of the VTOL objective with respect to range, number of passengers (2? 4? 6?) and cruising speed.

I’ve been following these electric VTOLs (EVTOLs) as an example of a multifaceted complicated design challenge, but most of the material I found was not helpful. There were the usual “breathless” articles saying these are coming soon and our lives swill be changed forever; there were some decent overviews but lacking in numbers and analysis; and there were some analyses by authors who were skeptical and provided good reasons to be so.

Fortunately, I came across a technically focused, three-page article with a two-page supplement (References 1 and 2) in a credible publication which gave substantial insight into key factors and how they interact with the various EVTOL possibilities. First, it outlined the three basic designs: 1) multiple rotors distributed over the aircraft; 2) lift plus cruise, with one set of rotors are used for vertical flight and another set are used for cruising; and 3) vectored thrust, where the motor subsystem of the aircraft is used in both vertical and horizontal flight.

The article also defined and analyzed the critical parameters and figures of merit for lift off and flight and explained how these play into the needed battery energy and power. Among the factors are “disk loading” which is the ratio of maximum takeoff mass to total rotor disk area (kilograms per square meter), while horizontal flight power requirements are affected by the “lift-to-drag ratio.” There’s also the specific energy (W-hr/kg) and specific power (kW/kg) requirements which characterize the energy burst needed to take off vertically, Figure 3. Another obvious factor is energy efficiency, defined as the energy consumption per unit distance per unit payload carried.

Figure 3 The PMAS paper looked at the key metric of specific energy, specific power, and various battery technologies for different announced EVTOLs. (Image source: PNAS)

To make their project “real” the researchers looked at five specific EVTOLs under develop with different arrangements and capacities, to provide a good sense of the spread of potential performance and needed resources. It’s a complicated situation and with many interactions among the many factors and degrees of freedom.

The good and bad news is that the researchers conclude that the needed battery technology is not quite here yet, but it’s close for some EVTOL designs with carefully constrained performance requirements. It’s much further away for others. I suspect the start-ups cited in this study would have a different assessment.

Even if the purely engineering issues of the EVTOLs are worked out – and I‘m not making any predictions here one way or the other – their real problem may be on the ground. An insightful article in The Wall Street Journal (Reference 3, behind a paywall) laid out the issues for siting and operating so-called “vertiports” which must provide recharging, basic maintenance, ground handling, passenger on/off paths, and more.

It’s easy but simplistic to say you’ll just take off/land these in a large back yard, office park, or even a rooftop, but there are many practical “ground” issues which must be acknowledged, addressed, and resolved, in addition to regulatory approvals. Building the infrastructure to deal with VTOLs will not be easy, especially if they each model has different arrangements and on-ground requirements.

What’s your view on EVTOLs? Are the coming soon, or will they always “coming soon?” Will their success or failure be due to battery technology or “mundane” vertiport considerations?

References

  1. Proceedings of the National Academy of Sciences, “The promise of energy-efficient battery-powered urban aircraft
  2. Proceedings of the National Academy of Sciences, “Supplementary Information: The promise of energy-efficient battery-powered urban aircraft
  3. The Wall Street Journal, “The Biggest Problem With Flying Cars Is on the Ground
  4. IEEE Spectrum, “EVTOL Companies Are Worth Billions—Who Are the Key Players?
  5. IEEE Spectrum, “Are eVTOLs the ‘mother of all aerospace bubbles’?

Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.

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