Every eVTOL needs motors that are light, redundant, and fail-safe -- but Joby and Archer have landed on different architectures to get there. Here's what radial and axial flux actually mean, and why the choice isn't just an engineering preference.
Electric propulsion sounds simple until you look at how differently two leading eVTOL programs have built it. Joby Aviation uses six large radial-flux, outer-rotor motors, each with dual independent windings. Archer Aviation uses twelve smaller radial-flux motors distributed across the wing, split between tilting and fixed lift roles. Both are trying to solve the same problem -- enough power, enough redundancy, low enough mass -- and they've arrived at different answers.
In a radial-flux motor, the magnetic gap between rotor and stator runs radially, like the wall of a cylinder -- this is the layout used in most EV and industrial motors today. In an axial-flux motor, that gap runs parallel to the shaft, more like a disc. The geometry matters: torque in an axial-flux design scales with the cube of rotor diameter, versus a square relationship in radial-flux, which is why axial-flux motors can pack more torque into a shorter, wider package for a given volume. It's also why axial flux is drawing interest from newer eVTOL motor suppliers such as Evolito, even though it hasn't displaced radial flux at Joby or Archer.
Jon Wagner, who leads power-train engineering at Joby and previously ran battery engineering at Tesla, put the difference between automotive and aviation motor design plainly in an interview with IEEE Spectrum: on the ground, a motor failure means pulling over. In the air, it can't mean that -- so redundancy has to be designed in from the start, not bolted on. That's the logic behind Joby's dual-winding approach: each motor is effectively built as two motors sharing one housing, so a single winding failure doesn't take down the whole unit. Archer's twelve-motor layout achieves a similar goal a different way -- distributing lift and control authority across more, smaller units, so no single motor failure is catastrophic on its own.
That same aviation-specific cost-versus-mass tradeoff shows up in materials, too. Wagner noted that Joby uses Permendur, a cobalt-iron alloy roughly ten times the cost of standard motor steel, purely because the small performance gain is worth it when every kilogram has to be lifted and every failure mode has to be engineered out. In automotive manufacturing, that tradeoff almost never clears the cost bar.
Neither radial nor axial flux is objectively better in the abstract -- the right choice depends on how a manufacturer weighs redundancy, manufacturability, and integration against raw power density. What's notable is that Joby and Archer, two of the best-funded eVTOL programs, made different bets on motor count and layout while converging on the same radial-flux base topology. Axial flux remains a technology to watch rather than one that's proven out in a certified eVTOL yet.
IEEE Spectrum -- What Makes eVTOL Motors Different Than EV Motors?: https://spectrum.ieee.org/evtol-joby-jon-wagner-motors
Evolito's Breakthrough Axial Motors -- eVTOL.news: https://evtol.news/news/evolitos-breakthrough-axial-motors
Axial flux motor -- Wikipedia: https://en.wikipedia.org/wiki/Axial_flux_motor
