According to a company called Joby Aviation, in a few years you’ll be able to summon up an air taxi on your Uber phone app for trips of 25 miles or so. And you won’t have to feel guilty about the environmental impact of your trip because the plane will actually be a Joby eVTOL, an electric vertical takeoff and landing, aircraft. Joby envisions a network of “vertiports” first in the U.S. and eventually worldwide, with perhaps 10,000 such six-engined aircraft in service at 20 cities within 10 years.
As you might suspect, these plans have attracted a lot of skepticism. Among the reasons: Joby’s eVTOL, dubbed the S4, has yet to be certified for carrying passengers by the FAA. And though the firm has a big investment from Toyota for factory automation expertise, it has never produced a single flying airplane before the S4, let alone 10,000 of them.
Skeptics also have problems with the technical details of the S4, though these are hard to discern because Joby is secretive about the 
design. The eVTOL’s battery is said to be comprised of lithium-nickel-cobalt-manganese-oxide cells, the same type used in EVs such as the BMW i8, Chevy Bolt, and Nissan Leaf S Plus. Company officials have said the cells have “almost” 300 W-h/kg of specific energy, high for that type of battery chemistry.
One analysis by Carnegie Mellon University engineering Prof. Venkat Viswanathan used this figure to show what it implies for the plane’s design. Joby says the S4 will have a top cruising speed of 200 mph and a range of 150 miles. The FAA requires passenger planes to have a reserve of 45 minutes of flight time over and above their advertised range. To do this, Viswanathan calculates the S4 would need a battery pack with 200 kW of energy.
Of course, battery packs need insulation and packaging to ensure a thermal runaway in any one cell won’t propagate, as well as a battery management system and some capacity for cooling. Consequently, Viswanathan estimated that the assembled battery pack would have an energy density of 200 Wh/kg. That implies an S4 battery pack would weigh 2,200 lb. (In interviews, Joby officials have claimed an even higher energy density, 235 Wh/kg.) For comparison, the Tesla Model X currently carries a battery pack sporting 186.21 Wh/kg.
Joby has put the S4’s max gross weight—it’s empty weight plus passengers and baggage–at 4,800 lb. Subtracting the weight of the battery, as well as 1,000 lb for the pilot and four passengers, leaves only 1,600 lb for the airframe, avionics and everything else onboard. This relatively small fraction of gross weight devoted to the airplane’s structure is also noteworthy. For comparison with a conventional plane, consider the Cirrus SR22 which also carries four passengers and a pilot. It’s empty weight is 2,272 lb and gross weight (which in the case of the Cirrus would include fuel) is 3,600 lb.
Recall that Joby foresees running these planes seven hours a day, seven days a week. That works out to roughly 12,000 takeoffs and landings annually, more than what airliners rack up. So it’s fair to wonder if the 1,600 lb-worth of light composite airframe in the S4 will be up to that kind of beating.
Nevertheless, the kind of distributed electric propulsion that Joby envisions has advantages such as motors with a low part count and a design conceived with the help of sophisticated computer software. This approach will probably have a place somewhere in aviation even if the S4 isn’t it.
Filed Under: Batteries