Company offers more details on last month's first piloted transition
For years, Joby Aviation chief test pilot James “Buddy” Denham has remotely flown the company’s S4 electric air taxi prototypes, watching the unoccupied aircraft soar overhead and grow quieter as they made a full transition from vertical takeoff to forward flight, in which lift is provided entirely by the aerodynamic forces on the wings.
On April 22, Denham finally got the chance to experience this wingborne flight firsthand — also becoming the first person to pilot an S4 through the transition. He took the white tilt-rotor aircraft up to 1,200 feet above Marina, California, flying one brief pattern downwind to transition to forward flight before returning for a vertical landing at the Marina Municipal Airport, home to Joby’s production and flight test center.
“It’s always been a whisper when it flies over in forward flight,” Denham told me after the flight, “but I’ve always wondered what it would be like to be sitting inside of it. So this first was a thrill. I heard the noise of the propulsion system come down, and it got quiet. I watched the propellers spin down to 280 rpms [revolutions per minute]. To hear how quiet the airplane got was the culmination of all the things that we have been working hard for within the company, in terms of that quiet acoustic signature.”
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Of the 60-some aircraft types he’s flown, Denham says he’s not aware of any that are able to fly under power with propellers spinning that slowly.
“You hardly hear the propulsion system working. You just hear the wind rush by the fuselage. And the closest analogy I have to that is my sailplane, where there is no propulsion system,” he says. “Most of the noise from the propulsion disappeared and all you hear is the wind rush at 100 knots [185 kph] around the canopy or the cockpit.”
The reason the propellers can spin so slowly in forward flight, he says, is that the S4’s wings are bearing the weight of the plane. A successful transition means that the propellers have tilted forward and that air is attaching smoothly to the curved top of the wing — meaning there isn’t enough turbulence in the flow that would jeopardize the aerodynamic lift. Due to the shape of the wing, air is flowing faster over the top surface and slower underneath.
Joby has been testing this full transition from vertical to forward flight with remotely piloted full-size prototypes since 2017. Since Denham’s flight in late April, the company said in a press release that it’s “completed multiple transition flights with three different pilots at the controls.” Vermont air taxi developer BETA Technologies was the first to achieve a piloted transition last year in its vertical-lift ALIA.
Asked why Joby’s first piloted transition took so long to occur, Denham has a blunt answer: “The F-35 had an ejection seat. So if we had gotten it wrong, the pilot could simply eject. We don’t have that in the S4, so we have to be really certain before we put a life at risk that we have identified all of our safety risks. We understand them now, and we’ve retired those risks now.”
That risk-retirement testing included landing an S4 with two of its six propellers not working, he says.
It’s no coincidence that Denham was chosen for the landmark flight. The company hired him partly because of his experience at U.S. Naval Air Systems Command, where he helped develop what is now known as unified control for the F-35B Joint Strike Fighter, another aircraft with vertical takeoff and landing capability.
Joby’s cockpit controls also use the unified control method, in which a pilot manipulates a single set of controls to send commands to a computer that tilts the rotors. Denham says that this configuration allowed him to focus on setting the aircraft’s flight path and speed without needing to determine how or when the propellers would tilt or to what degree to achieve wingborne flight. After that transition, the propellers rotate just fast enough to help offset aerodynamic drag.
“Basically, the pilot has direct command and control of glide slope, speed and direction,” Denham says, and the aircraft software determines vector angle, thrust and attitude of the aircraft.
Joby said in a press release that the ability to transition from vertical takeoff to forward flight and back again is a “key design attribute” of the S4 that will enable point-to-point passenger service, particularly in crowded urban areas. The company continues to manufacture and assemble production prototype versions of its aircraft in Marina as it awaits FAA type certification.
Editor’s note: The headline on this article was updated.
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