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Joby S4: Coming to your airport in 2025?

The leader of the eVTOL pack

It can be a challenge to sort out aviation’s burgeoning eVTOL segment. With hundreds of competitors laying down a dense PR fog it can be difficult to discern which project is real, which is hype, and which stands a chance of making it to market.

Joby S4

Photography by Chris Rose Grabbing a handful of inceptors  at the Marina, California, facilities’ simulator. A Garmin display screen shows representations of the S4’s powerplants, motors, and batteries. Green symbology means “A-OK.” A pilotless Joby prototype S4 flies over its flight test area. Conventional wings with ailerons and flaps augment the tiltrotors’ thrust—and can even let the aircraft glide in an emergency. Joby’s flight operations hangar at Marina Municipal Airport. The simulator in Joby’s Washington, D.C., offices. Carbon-fiber tape placement machine adds layers to a wing surface.

Making a website is easy; building a new aircraft type is not. These are speculative times, and even a spot on the stock market doesn’t automatically confer legitimacy—or survival.

But a recent visit to electric vertical takeoff and landing aircraft manufacturer Joby Aviation gave a tangible sense of that company’s viability and commitment. Joby founder and CEO JoeBen Bevirt is a mechanical engineer and serial entrepreneur and inventor of robotic bioscience equipment, wind turbines, and the Gorillapod flexible camera tripod. But from an early age his heart was in aviation. He grew up among redwoods and faced long uphill walks coming home from school. “I thought, if I could only fly home—but how could a plane take off and land with all these trees around?” he recalled. “That’s when I began thinking about a personal VTOL.” By 2009, that concept began to take shape.

Bevirt, together with Edward Stilson, Alex Stoll, and former Pipistrel chief aerodynamicist Gregor Veble Mikic, came up with the design that would become today’s Joby S4. The aircraft combines traditional, carbon-fiber wing and control surfaces with six electric tiltrotors—two on each wing and two on its V-tail. The motors not only drive the propellers, they also control the engines’ tilt and propeller pitch and run the coolant pumps. Power for the motors—each of which has dual windings for redundancy—comes from four liquid-cooled battery packs, two in each wing, each with seven individual battery modules. Joby claims to be using standard automotive lithium-ion battery cells, each with an energy density of “almost 300 Watt-hours per kilogram” (Wh/kg), somewhat greater than the 260 Wh/kg from spec sheets from typical suppliers. However, we don’t know the number of individual cells. For battery fire prevention, Joby uses what it calls “active cooling plates” mounted between each battery’s individual cells. These maximize coolant flow over the cells’ external surfaces.

Fly-by-wire flight computers take control inputs and convert them into digital signals to manage the S4’s multiple flight and thrust modes, and handle asymmetric thrust in the event of a power failure. For redundancy, there are three fly-by-wire computers. Fly-by-wire flight control also fulfills one of Bevirt’s prime mandates: that the aircraft be easy to fly. There are only two flight control sticks, called inceptors. They can work in two fly-by-wire control law modes: translational rate, or unified control. Pressing a button on the right inceptor lets you switch between the modes.

Transitional rate is used most often when hovering. Pull pack on the right control stick—er, inceptor—and the Joby smoothly lifts off, or climbs, vertically. Release pressure on the inceptor and the ship stops its movement and remains motionless. Twist the inceptor and the aircraft rotates about the yaw axis. Hovering couldn’t be simpler, or more intuitive. Best of all, there’s none of the constant stick-jockeying required in conventional helicopters. Instead of taking gobs of training to master hovering, the Joby’s fly-by-wire makes you an expert in very short order.

In unified control mode, fly-by-wire gives you more conventional control modes. For example, applying left or right control pressure to the inceptor will result in a bank; in transitional rate control, sideways control pressure produces level motion. In unified control mode, the left control stick provides a throttle-like mode—push forward to go forward, pull back to move backwards. The more you push or pull, the faster you’ll travel.


Coming Soon

In flight, rotors vertical. An S4 in hover mode. Joby founder and CEO JoeBen Bevirt. A human pilot in the DCS’ “cockpit” serves as the real pilot during autonomous test flights. Assembling one of Joby’s batteries. The digital control station (DCS). When telemetry from test aircraft are beamed to a nearby antenna, data is displayed on large screens and monitored by staffers.

Fly-by-wire automatically handles engine-out conditions by reducing thrust on the engine or engines that are operating. This happens automatically through the system’s logic, and the result is a balance of power that will allow the flight to continue, or let the pilot use the S4’s wing to glide to an emergency landing site.

I flew two iterations of the Joby flight simulator. One was in Washington, D.C., and the other was on the second floor of Joby’s flight operations hangar at its compound at the Marina, California, Airport (OAR). The simulator at Marina was more sophisticated, and geared to the flight test activities there. On the cockpit display screen appears a virtual duplicate of the Joby you’re “flying.” This allows a pilot to better examine the effects of flight control inputs on attitude and control surface responses. One floor below this installation is an “iron bird” that duplicates the actions and interactions of the actual Joby’s control feedback, avionics, hardware, and software integrations. It’s like a ground-bound version of the actual test aircraft designed, among other tasks, to answer the question: Is this the way the prototype will actually behave in flight? It can also be used to duplicate any squawks or anomalies discovered during the prototype’s flight testing.

When it’s time to do a test flight, a thorough preflight briefing of all participants goes over the mission’s objectives, along with parameters such as the altitudes, airspeeds, directions of flight, and safety guidelines. Safety in flight test operations is always important, maybe more so at Joby. One of its prototypes, flying autonomously (without pilot or passengers), crashed in 2022. The NTSB has yet to rule on any causes or contributing factors.

Joby’s Marina operation is currently focused on building and supporting prototype S4s. Apart from the flight operations hangar, there’s a building that concentrates on composite components. Some are carbon fiber and made using state-of-the-art robotic tape-placement tooling, some are made using old-school hand layup. There are two autoclaves for curing the components under specific high-temperature conditions. From there the parts are sent to the “tent,” a building that’s built like a huge fabric Quonset hut. This is where final assembly happens. Battery and propulsion component fabrication, assembly, and testing are the prime functions at a separate facility in San Carlos, California. Yet another site in Santa Cruz has what Joby calls the “Whirly,” a circular track. Engines and propellers are mounted on a test rig, then they propel the rig around the Whirly’s track while instrumentation takes down data intended for use in extending the propulsion unit’s service life.

The Whirly is just one of the tests Joby performs. Lithium-ion batteries can catch fire or experience thermal runaways under the right conditions, usually when they’re being charged, if they’re damaged, or experience failures in their internal electronics. Joby says it minimizes fire hazards and protects airframe integrity by enclosing each cell within in their titanium cases, then tests them by subjecting them to 50-foot drops and extremes in temperature. In addition to the cooling plates mentioned earlier, Joby also provides extendable panels that can channel flames away from the aircraft, and a ground cooling unit that can be connected to the aircraft while charging.

Right now, flight test is a prime concern; so far more than 1,000 hours of flight testing have been logged. Joby is aiming for an entry into service by 2025, and it must complete two more FAA systems reviews. Two previous reviews have been completed, and there’s also an ongoing means of compliance review that examines certification plans for each of the aircraft’s systems. So far, Joby has announced that 94 percent of its means of compliance plans have been accepted by the FAA. Joby has already received its Part 135 air carrier certificate from the FAA, which will enable it to operate air taxi services as soon as the S4 is certified. Joby has two placeholders in its infant Part 135 fleet—a Cirrus SR22.

Like other eVTOL manufacturers, Joby is waiting to learn about details of the FAA’s latest change of mind regarding eVTOL certification. Originally, they were to be certified under Part 23 fixed-wing rules, with certain exceptions. Late in 2022, the FAA determined that eVTOLs, with their transitions from vertical to horizontal flight, needed to be put under new rules and categorized as “powered lift” aircraft which will be certified under FAR 21.17(b)—rules intended to apply to aircraft for which standards don’t already exist. The new rules let companies choose from among portions of other regulations, such as Part 33 (engines), Part 35 (propellers), and Parts 27 and 29 (rotorcraft)—with changes to allow safe flight below the wing’s stall speed of a powered lift aircraft. Joby expects approval by 2024, followed by entry of the S4 into service in 2025.

It has taken a lot of investment to pursue this target. Joby seems to have done fairly well in this area. In 2017 Toyota put $400 million into the S4 program, plus technical personnel. Delta Airlines has made an initial investment of $60 million, with an eye to using the S4 as a shuttle fleet to and from the airports it serves, with an option for $200 million more as milestones are met. A special purpose acquisition company (SPAC) took Joby to a spot on the New York Stock Exchange, where trading is at $4 per share at this writing. Additionally, a contract worth potentially $75 million has been signed with the U.S. Air Force and Marine Corps to provide S4s to the Agility Prime program. Industry watchdog SMG Consulting says Joby’s total funding has reached the $1.9 billion mark, and rates Joby as “highly likely” to succeed in the long run. Joby’s business model is vertically integrated; it fabricates and manufactures all its components. As for the finished products, in effect, Joby will only sell S4s to itself. In operation, it plans to follow the Uber Elevate plan, which isn’t surprising as Uber invested $75 million in Joby back in 2020, forming an expanded partnership. Passengers would use an Uber Air app to reserve a seat, and an S4 would show up at a designated pick-up spot—the company has contracted with Skyports to provide terminals—then fly them between airports and central locations in and around such major cities as New York, Washington, D.C., Miami, Los Angeles, and San Francisco. Knowing that quiet operation will be a critical issue to public acceptance, the S4 is fitted with long, specially shaped propellers that turn slowly (250-800 rpm) to create barely perceptible noise levels (an average of 45.2 dBA at 1,640 feet) in an urban environment.

The S4 isn’t Joby’s only interest. In April 2022, when no one was looking, Joby bought German hydrogen aircraft startup H2FLY, a company with a vision to produce a range of hydrogen-powered airplanes, from four- to six-seat air taxis to 40-seat regional airplanes with 1,000-nautical-mile ranges. A four-seat prototype H2FLY powered by 20 pounds of pressurized hydrogen, with a massive wingspan and a claimed 380-nm range, was on display at last year’s Aero Friedrichshafen, and flew to 7,230 feet. So, it seems reasonable to suspect that Joby will extend itself into hydrogen-powered flight. “I am committed to decarbonizing aviation,” Bevirt said.

Joby, despite its reputation for secrecy, looks like it’s in for a lot more attention.

[email protected]


Thomas A. Horne

Thomas A. Horne

AOPA Pilot Editor at Large
AOPA Pilot Editor at Large Tom Horne has worked at AOPA since the early 1980s. He began flying in 1975 and has an airline transport pilot and flight instructor certificates. He’s flown everything from ultralights to Gulfstreams and ferried numerous piston airplanes across the Atlantic.

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