Joby Publishes Direct Drive; Archer’s Gearbox Claim Needs Evidence

A good comparison has to preserve an asymmetry in the evidence: Joby has disclosed its transmission choice, while Archer has not publicly supplied the equivalent detail.

What Joby has actually disclosed

Joby’s direct-drive claim is unusually specific for an aircraft still moving through certification. Its published material describes a motor and inverter assembly weighing 28 kilograms, with 236 kW peak power, 1,800 Nm peak torque and 1,380 Nm continuous torque. The company says it tested a seven-gear reduction system in 2016, rejected the inspection and upkeep burden, and designed a high-torque motor that turns the propeller without a gearbox.

That is not merely an efficiency claim. Removing gears also removes teeth, shafts, seals and several bearings whose condition would have to be tracked. Joby says its resulting architecture has one moving bearing. The broader propulsion unit is still not maintenance-free: bearings, blades, windings, insulation, inverter electronics, cooling, sensors and tilt actuators all retain inspection and life requirements.

What Archer has not established publicly

The supplied comparison says Midnight uses gearboxes to provide higher torque at each rotor. Archer’s current public aircraft and certification pages describe twelve electric engines, six packs, simplified propulsion and fewer moving parts than a turbine or piston engine. They do not specify a reduction ratio, gearbox type, lubricant system or inspection interval for the production Midnight engines.

An old biography noting that an Archer motor engineer once worked as a motor-and-gearbox engineer elsewhere is not evidence that Midnight contains gearboxes. Nor does the word “engine” settle the issue; Archer uses it for an electric propulsion assembly. Until the company publishes a cutaway, maintenance basis, certification document or direct statement, the responsible entry in a comparison table is “not publicly specified.”

Why a gearbox might still be chosen

A reduction gearbox lets a motor spin at a speed favorable to power density while a propeller turns more slowly. That can shrink motor mass or move the electromagnetic design toward a different efficiency region. The price is mechanical complexity, losses, lubrication, backlash, vibration paths and scheduled inspection. Direct drive instead asks the motor to produce high torque at propeller speed, which can increase diameter or active-material demands even as it eliminates transmission hardware.

There is no universal winner. The design problem is installed mass and reliability at the required thrust, noise and duty cycle—not whether a product brochure can use the word gearless. An excellent geared system can outperform a poor direct-drive unit. A direct-drive motor can be attractive when low RPM, low noise and high torque are central, provided its thermal and structural design closes.

Motor count also changes the comparison

Joby has six propellers, each with dual electrical windings and independent drive paths. Archer has twelve engines and six battery packs arranged so a pack feeds diagonally opposed forward and aft motors. The resulting maintenance burden depends on line-replaceable-unit boundaries. Twelve small assemblies may be quicker to swap individually; six larger ones may mean fewer removals. Public rotor counts do not reveal shop hours per flight hour.

Continuous rating matters more than a headline peak. Hover near maximum mass on a hot day, repeated short sectors and a diversion after a fault ask different things of copper, magnets, bearings and inverters. Certification testing must demonstrate temperatures and controllability through those cases. Neither manufacturer publishes enough comparable thermal maps to declare a powertrain winner.

The questions that would make this comparison fair

For each aircraft, ask for propeller-shaft speed, peak and continuous torque, motor and inverter mass, cooling method, dispatch allowance after a channel fault, scheduled inspection tasks and time to replace a propulsion unit. If a gearbox exists, add ratio, efficiency, lubrication, chip detection and life limits. If it does not, ask about bearing replacement and motor air-gap inspection.

These values convert architecture into operating economics. A gearbox debate without them is mostly aesthetics. Joby deserves credit for publishing a concrete direct-drive design; that disclosure does not prove lower lifetime cost. Archer’s simplified-electric-propulsion language is relevant, but it cannot be expanded into a gearbox specification the company has not made.

Why unsupported detail is especially risky here

A propulsion claim propagates into maintenance, reliability, noise and cost conclusions. If the assumed gearbox is wrong, every downstream comparison built on its inspection burden is wrong too. Procurement teams should distinguish three columns in their evidence ledger: confirmed architecture, company performance target and unknown detail. Archer’s transmission belongs in the third column. That is not a criticism of the aircraft; it is a limit on what an external comparison can responsibly say.

Bottom line

The defensible difference is disclosure, not an invented mechanical contrast. Joby says direct drive and supplies useful numbers. Archer says twelve redundant electric engines but leaves the transmission topology unpublished. Any stronger conclusion should wait for evidence.