- 22 Dec 2020 02:35 PM
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Inside Joby’s Unicorn: Flight Tests and Patents Reveal New Details
By Kenneth I. Swartz
Vertiflite, Jan/Feb 2021
Founded in 2009, Joby has long been recognized by experts as the leading pioneer in the development of electric vertical takeoff and landing (eVTOL) technology, but the company stopped sharing details of its activities after 2015 as other companies began catching up.
Joby Founder and CEO JoeBen Bevirt (the 2018 recipient of the VFS Paul E. Haueter Award) and engineer Alex Stoll shared details of the two-seat S2 eVTOL at the Vertical Flight Society’s 1st Annual Transformative Vertical Flight (TVF) Workshop — the forerunner of today’s annual Electric VTOL Symposiums — in 2014 and the four- seat S4 at the 2015 workshop (both linked from www.vtol.org/TVF).
Although Joby’s thinking has progressed a long way since then, the original aircraft concepts highlight just how advanced Joby’s development was before most of the aviation community even knew there was an emerging eVTOL industry!
Early Reveals
The design of the two-seat Joby S2 with 12 tilting propellers benefitted from two years of collaborative study with NASA that resulted in the NASA GL-10, Joby Lotus and joint NASA/Joby LEAPTech concepts, wrote NASA’s Mark Moore in his article, “The Joby S2 VTOL Concept” published in the Nov/Dec 2014 issue of Vertiflite (also available on www.eVTOL.news).
The article noted that, compared to the two-seat Robinson R22 helicopter, the S2 had a higher disk loading of 16.3 lb/ft² (80 kg/ m²) compared to 2.6 lb/ft² (13 kg/m²) for the R22, with a resulting cruise speed of 200 mph (320 km/h) for the S2 and 110 mph (177km/h) for the R22.
While the S2 concept required more power for a minute or two of hover and transition time, it would cruise at essentially the same power as the R22, but at nearly twice the speed. Both aircraft could carry a 400-lb (180-kg) payload about 200 miles (320 km), but the S2 would weigh about 2,000 lb (900 kg) with 2014-era batteries, compared to only 1,370 lb (620 kg) for the R22.
The principle benefits of the S2 were higher speed coupled with much lower operating costs and noise.
By 2015, Joby’s focus had shifted to the four-seat, six-propeller Joby S4, which was the subject of a presentation Joby’s Stoll made at the 2nd Annual TVF Workshop in 2015.
Stoll’s presentation showed that the Joby S4 would carry a similar payload to the four-seat Robinson R44 — 840 lb (381 kg) vs. 828 lb (375 kg) — but at a much higher speed — 200 mph (322 km/hr) vs. 135 mph (217 km/hr) — and much lower noise due to significantly lower tip speeds.
Joby’s modeling predicted it could significantly reduce the price of a flight to a level that it could make money charging fares similar to those charged by Uber for on-demand ground transportation, but cover the same distance at 200 mph.
One of Joby’s innovations was to increase safety by having a “distributed propulsion architecture, where we have no single points of failure or very few single points of failure compared to a rotorcraft,” said Stoll.
The concept was that if one propeller failed, the corresponding motor on the other side of the aircraft would be shut down, while increasing the power on the remaining four motors to 150% thrust to maintain level flight.
Joby’s S4 design also reduced the interdependency of the propeller and diameter and the wing span, which reduced the disk loading and the span loading. The shift from 12 to six propellers gave Joby a much more manageable aspect ratio and disk loading.
The design parameters for the propellers included the tip speed, the torque (which had a strong effect on motor weigh) and the ability of the propeller to withstand a bird strike. In fact, Joby set up a “chicken gun” at its lab in Bonny Doon near Santa Cruz and shot test birds into an instrumented propeller to correlate their finite element model (FEM) for designing blade survivability, which is a US Federal Aviation Administration (FAA) Part 35 certification requirement for propellers.
Originally, Joby was also looking at folding blade designs for the S4, where the propellers would stop, and the blades would pivot and fold back, nesting into the nacelle to reduce drag, when only two of the six propellers were needed to produce the necessary thrust for forward flight during cruise. The concept was similar to the electric propeller blades on the Schempp-Hirth Ventus-2cxa sailplane, which folds back into indents in the aircraft’s nose when not required.
The four-seat S4 design, now referred to as the Generation 1.0 aircraft, began flight testing in early 2017 (see “Joby Transitions,”). As the design evolved and mission analysis indicated that a pilot plus four passengers — as promoted by Uber Elevate — would lead to a much more economically compelling product, Joby stretched the fuselage for its second generation (2.0) prototype with five seats. In fact, the work on folding blades was abandoned and never flight tested following the shift to the higher fineness- ratio fuselage and a greater emphasis on maturing a less complex product for the market.
For the 2.0 prototype, Joby chief aerodynamicist and flight physics lead Dr. Gregor Veble Miki — who had previously been head of research at Pipistrel Aircraft in Solvenia — developed a blade design that was thin enough for low drag but robust enough to exceed the FAA blade impact requirements.
Patent Filings
By 2016, Joby and other start-ups recognized that the eVTOL research community was transitioning into a competitive business and the company declined to present further details on the S4 at the Society’s 72nd Annual Forum that May or in subsequent meetings for many years.
The last interview Bevirt gave anyone was probably for Vertiflite’s “The Demand for On-Demand Mobility” (Jan/Feb 2017). The follow-on article, “Air Mobility Bonanza Beckons Electric VTOL Developers,” (March/April 2017) said, “The S4 is expected to make its first flight this year. Bevirt declined to provide an update for this article, saying in an email, ‘We are heads down and not taking media requests at the moment.’”
Like many other start-up tech companies, these pioneers of the eVTOL industry decided to begin keeping many details of their aircraft development programs very close, in order to maintain tight control of their intellectual property (IP).
However, there comes a point in the life of every tech start-up when it needs to start filing patents to protect its proprietary technology. Recent patent application publications provide significantly more details regarding Joby’s air taxi aircraft, control systems and other technology that have only been hinted at since 2015.
With the unveiling of Joby’s 2.0 air taxi on Jan. 15, 2020 — and the exclusive keynote given at the VFS Electric VTOL Symposium the next week — Bevirt gave the public the first glimpses into their progress, but again, details were rare.
However, in October, the US Patent and Trademark Office (USPTO) published several Joby patents, part of more than 18 patents filed since October 2019 (see the full listing, with links, on eVTOL. news). Some of these patents update earlier applications dating back to 2015, while others are entirely new.
Aircraft Control System and Method
The patent application for an “Aircraft Control System and Method” (application 2020/0092052) is probably the most revealing, since it describes Joby’s unified command system in detail and contained the first publicly available drawings of the Joby 2.0 tilt- propeller eVTOL (and can be compared to drawings released at the 2015 TVF Workshop of the 1.0 configuration), showing it in VTOL, transition, airplane and conventional takeoff and landing (CTOL) configurations, as well as drawings of a sample control inceptor.
Led by former British Royal Air Force (RAF) test pilot Justin Paines for the past two years, Joby representatives have been praising unified control system installed in the short takeoff and vertical landing (STOVL) Lockheed Martin F-35B Joint Strike Fighter to reduce pilot workload and increase safety.
Joby’s unified command system includes an input mechanism, a flight processor, a control output and effectors (e.g. control surfaces and motor power levels). The patent says the system can also accommodate optional sensors that would determine the vehicle state and or/flight regime and flightpath.
Electric Tiltrotor Aircraft
Patent application 2020/0148347, “Electric Tiltrotor Aircraft,” filed May 10, 2019 and published May 14, 2020, highlights that the six propellers of the Generation 2.0 air taxi are arranged on three different planes. The cantilevering tilt mechanisms extend the propeller disk away from the leading edge of the wing to achieve a desired hover arrangement and disk position relative to the other disks.
The propellers can also be designed to be “articulated into a negative angle of attack, which can function to produce reverse thrust without changing the direction of rotation of the propeller,” which is a technique used by short takeoff and landing (STOL) aircraft to do a steep landing approach without gaining air speed.
Aircraft Noise Mitigation
At the 7th Annual VFS eVTOL Symposium in San Jose in January 2020, Bevirt stated that “we were quite pleased [with] the acoustic profile” of the Joby 1.0 prototype and showed the audience a slide indicating that the aircraft had been measured with a noise signature of 60 dB during an overflight test on Sept. 7, 2017. But, he said, “there’s still work to do and we’ve been continuing to improve it,” presumably with the 2.0 blade design.
Joby has long recognized that a low noise signature is a key design objective for eVTOL aircraft since reducing the psychoacoustic penalty can increase community access.
Beyond operating at low tip speeds, Joby was granted a patent on Nov. 24, 2020 (Patent 10,843,807) for an aircraft noise mitigation system that has four operating modes that can be used separately or in a combination with one another.
According to the patent, “The method can additionally or alternatively function to: spread out the acoustic power of the emitted sound across the acoustics frequency spectrum; shift the acoustic frequency spectrum of the emitted sound; reduce the total acoustic power of the emitted sound; dynamically adjust the frequency spectrum of the emitted sound,” all of which will use a network of acoustic sensors on the aircraft and or at ground positions to “reduce the tonality of the acoustic spectrum emitted by the aircraft in flight.”
In the position-controlled mode, the azimuthal position of individual propeller blades is actively controlled by a computer. In the phase-controlled mode, the position of the blades are controlled relative to the position of the blades on the other propellers. In the variable RPM-mode, the RPM of each propeller is actively controlled. And in spread RPM mode, the variation in RPM of different propellers is simultaneously controlled.
One of the novel concepts is the ability to automatically shift the angles of attack of individual blades on a single propeller hub by 1–3° based on the acoustic emissions of the propeller. Another mode allows the six propellers on the aircraft to operate at different RPM speeds (and blade pitches) to shift the noise spectrum, which would be extremely challenging to achieve with fuel-burning engines.
Battery Thermal Management System
Joby filed patent application 2020/0339010 for a battery thermal management system that includes a battery pack, circulation subsystem and heat exchanger, and can optionally include a cooling system, reservoir, de-ionization filter, battery charger and controller. This system is designed to set the temperature of a battery pack, determine the heat and redistribute the heat within the pack.
In a typical trip plan, for example, the system would calculate the expected power consumption based on the distance, calculate the expected heat that would be generated from the power consumption, and redistribute the heat in the battery pack with circulating fluid to equalize the temperature to cool or heat the pack.
A variation of the system could include a ground-based cooling system that would establish the ideal battery temperature for a specific trip and provide the necessary heating or cooling, greatly reducing the weight of the onboard system.
The system is also designed to automatically plan for contingencies, such as the loss of motor, which would lead to an automatic power increase on the operating motor and the need to re-distribute the resulting heat. The system would also automatically respond to a change in flight profile or aircraft routing by adjusting the flow of the circulation in the battery pack to meet the required temperature profile.
Airfoil and Design Method
Joby filed patent application 2020/0331602 for “a new and useful rotary airfoil and design method” that addressed both aerodynamics and acoustic requirements.
Joby found that the conventional way of optimizing airfoils for aerodynamic efficiency can have adverse and counterintuitive effects on the acoustic performance of the airfoil and propeller.
/S2-S4-comparison.jpg "Joby S4 eVTOL aircraft, Jan. 15, 2020")
.jpg "Lateral view of the Joby S4 eVTOL aircraft")
This could mean that to improve the acoustics signature, an airfoil might incur a performance penalty of up to 3%.
Compounding positive effects on blade acoustics include tapering the blade along the length, twisting the blade, and setting a different angle at the blade tip.
Aircraft Drag Reduction System & Internally Cooled Electric Motor System
Patent application 2019/0329858 is for a system that minimizes drag behind the propellers by ingesting air into the nacelles to encourage forming and maintaining the boundary layer to prevent separated flow. The airflow through the nacelle could also be used to drive a fan coupled to a liquid pump that drives coolant through the motor, reducing the need for electrical power for thermal management of the motor.
Electric Power System Architecture and Fault Tolerant VTOL Aircraft
Designing an eVTOL power system for high reliability requires the system architecture to instantaneously accommodate battery, motor or winding failure.
Patent application 2020/0010187 describes an architecture for a six-motor aircraft, where each motor may be powered by two or more batteries, and each motor has two or more windings, with each winding powered by a different battery. In the event of a failed winding, failed battery or failed motor, the power routing would automatically be altered to provide proper attitude control and to provide sufficient thrust.
The power requirements vary during five phases of the flight profile: hover out of ground effect, vertical ascent, vertical descent, cruise climb and cruise. In each scenario, there is a normal operating power requirement and an emergency power requirement, which could vary from 50 kW under normal circumstances in a hover to 130 kW for hover in an emergency situation, such as a motor failure.
The system is designed such that if a motor failure occurs, to maintain pitch and roll control, the flight computer reduces power on the opposing motor as needed, as the remaining four motors increase to emergency power to compensate for the two motors that are shut down.
The batteries are distributed throughout the aircraft to enhance reliability and fault tolerance. The major concern is loss of control of the aircraft as a result of a sudden shift in attitude resulting from the failure of one of the propellers in hover. By linking a battery that powers the furthest outboard motor on the other side of the centerline of the aircraft, a battery failure then has its effect more spread out across the aircraft, reducing the amount of impact a battery failure would have on the aircraft’s attitude.
Other Patents
Joby also filed patent applications in 2020 for a vehicle navigation system that was granted in November (10,845,823) that fuses sensors and uses a voting scheme to continually determine the vehicle state.
Another patent application (2020/0324910) is for a system to determine the airspeed of an aircraft based on a set of propeller operating parameters and models. The system could reduce weight and drag of other airspeed systems and reduce system complexity.
As of Nov. 1, a total of 10 granted patents and 18 patent applications — all of which include Bevirt as a patent holder — have been posted on the VFS site www.eVTOL.news, with links to both the PDF and html versions. In addition, another 30 granted patents that name Bevirt but are not related to Joby Aviation — including patents related to his prior work on medical robotics and his Joby photography accessories inventions — are also linked.
These patents, both individually and collectively, give significant new insights into the evolution of the Joby aircraft as well as many of the key innovations, and the creative minds of the inventor himself and his team.
About the Author
Ken Swartz runs the agency Aeromedia Communications in Toronto, Canada. He specializes in aerospace market analysis and corporate communications. He’s worked in the regional airline, commercial helicopter and commercial aircraft manufacturing industries for 25+ years and has reported on vertical flight since 1978. In 2010, he received the Helicopter Association International’s “Communicator of the Year” award. He can be reached at kennethswartz@me.com.
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