Peter Garrison Flying Magazine
Weighing in on Electric Air Taxis
August 9, 2021
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  • I never thought I’d see the day, but General Motors—among a number of other auto manufacturers, not to mention nations—has pronounced a death sentence upon the internal-combustion engine. Stays are possible, but permanent commutation is unlikely. It appears that electricity, in one form or another, may triumph in the end.

    The powerplants of cars and reciprocating-engine airplanes have a lot in common, and while aviation benefits from a lot of special pleading—look at the ever-lingering matter of leaded avgas—the intense research and development going into electric cars and their batteries is bound to spill over into airplanes.

    Electric point-to-point flight is currently possible—marginally. It is practical for small, light and clean fixed-wing airplanes, such as the Pipistrel Alpha Electro, flying short distances at moderate speed. High speeds over long distances are out of the question. At present—even taking into account the superior ability of electric motors to convert available energy into useful work—batteries are roughly 15 times heavier, per thrust-horsepower-hour, than fossil fuels. For a mode of transport that prizes lightness above all else, this is not a promising situation, and it takes a pathological optimist to foresee a 15-fold improvement in the performance of batteries.

    Three years ago, I wrote skeptically about the prospects for electric airliners. At the time, one of the most grandiloquent aspirants was the Eviation Alice, a three-motor, nine-passenger all-electric air taxi. The Alice had not flown, but Eviation was publishing impressive numbers for speed, range and battery capacity. They did not add up, as I noted at the time. Early in 2020, the Alice prototype, which had mysteriously transformed itself from a tricycle to a taildragger design (I think they must have discovered belatedly that the wing was too far forward), was destroyed in a fire in Prescott, Arizona, where it was to be tested. Eviation has been quiet lately, but a recently published rendering purporting to be the “new” Alice showed an entirely different design.

    It’s natural that the process of development and testing leads to design changes, but one expects them to be incremental, not wholesale. Wholesale changes reveal an underlying uncertainty about how to proceed. In electric aviation, there exists nothing like the design consensus to which fixed-wing aircraft manufacturers converged long ago. If the preferences of investors mean anything, however, the preferred orientation of the herd is not currently toward medium-size fixed-wing airplanes but instead toward small VTOL flying taxis.

    Because the term “air taxi” is already in use for something else and “ee-vee-tol” sounds like a medicine, I will call these “air cabs.” Of would-be air cabs, there is no shortage. It is said that more than 250 firms are developing products to satisfy this purported demand.

    Shortly after the Alice fire, one of two prototypes of a German air-cab project called Lilium also went up in flames. Lithium-ion batteries, whose comparatively high energy density makes electric flight possible at all, are notoriously pyrotechnic. Ask Samsung. This is but one of the hurdles that a firm aspiring to certificate an electric airplane must vault. Luckily for Lilium, they had a second prototype, and it can be seen on the company’s website doing a graceful vertical takeoff, an around-the-patch flight and a vertical landing. Flight video is the gold standard for air-cab credibility, even if the aircraft is empty and flies for only a few minutes.

    Early this year, a startup called Archer Aviation announced a deal with United Airlines—such deals are highly contingent and barely deserve the name—for its air-cab design, which earns high marks for aspirational computer-graphic renderings. Surprisingly, the name of the vaporware airplane is not “Arrow” but “Maker.” A relative latecomer, Archer has assembled a lot of money and talent, and has so far avoided barefaced lies about speed and range, confining itself to the claim that it will be carrying paying passengers in 2024.

    China’s EHang was one of the first off the blocks, starting in 2015 with a four-rotor, single-seat design that was basically a scaled-up hobby drone. Its rotors have since multiplied to 16, but winglessness has gone out of fashion. Most recent proposals use some variation on a tilt-rotor or tilt-wing arrangement. The small size and low weight of electric motors encourage distributed propulsion and variable geometry. A wing provides superior efficiency and speed in the cruising segment of the flight, and whatever difficulties of piloting the transition from hover to cruise entail can be handled by digital electronics. Air cabs are expected to be fully autonomous, like flying carpets, demanding of their passengers only the ability—which is not always available late at night—to state clearly where they want to go.

    Read More from Peter Garrison: Technicalities

    Any aircraft that takes off and lands vertically must contend with the extremely high-power requirement of hovering flight. The drive toward compactness exacerbates the problem. A vision of urban “vertiports” analogous to rooftop taxi stands assumes air cabs will be more similar in footprint to automobiles than to helicopters, but small rotors are invariably less efficient than large ones and, for many of the same reasons, noisier. The Lilium concept takes the small-rotor philosophy to an extreme, with 36 8-inch multiblade, ducted-fan thrusters arrayed in rows along the trailing edges of its wing and foreplane. Each must produce 100 pounds of thrust to lift the 3,300-pound five-seater. It is perhaps not accidental that the website’s flight video is unaccompanied by sound.

    Joby Aviation’s design is more in the mainstream, with six tilting rotors that appear to be 7 feet or so in diameter, while Archer’s Maker uses 12 rotors mounted on the ends of six hideous sausages attached to the underside of an otherwise graceful wing. The ingenuity of the Archer design is that the multiblade front rotors tilt for cruise, while the two-blade rear ones stop turning and align themselves with the direction of flight for minimum drag.

    For some reason, both the Lilium and Archer style themselves “jets.” Neither is a jet in any meaningful sense. I suppose the Lilium, at least, can get away with it because its propellers are hidden inside tubes. Or maybe, among the uninformed, “jet” is synonymous with “airplane,” only sexier.

    The air traffic control aspects of swarms of air cabs delivering people from, say, central London to Heathrow in a dense fog seem pretty daunting, but not a day goes by without a new PDF in my email inbox proposing algorithms for autonomous collision avoidance in swarms. Fish do it, birds and fruit flies do it, so why shouldn’t we? There may be some difficulty, however, sequencing the arrivals and departures, within a limited surface area, of air cabs whose ability to loiter in the air might be limited to few minutes and whose battery will take much longer to recharge than its cabin takes to refill.

    One of the least plausible claims made for this class of aircraft is that a ride in one will be comparable in cost to the same trip in a Lyft. Fat chance. The promoters of novel technologies are not under oath, but one look at a Joby should disabuse you of any notion that its acquisition and maintenance costs could resemble those of a Prius.

    Are electric air cabs the next big thing, or are they just giant pumps for sucking up venture capital? Are they iPhones or Segways? Time will tell, but don’t give up on fossil-fuel-powered helicopters just yet.