By David Latimer
Those who are not close to the industry consider urban air mobility (UAM) to be science fiction. The idea of an autonomous drone transporting you from London Heathrow to central London is seen as some sort of scary fantasy.
In fact this is very close to becoming a reality with many of the UAM trailblazers targeting to start commercial operations in 2024. Though many of these early aircraft are likely to have pilots. For many of these new start-ups the proposition is mobility direct to consumer rather than the current business models of aircraft manufacturer and airlines. Rather like Uber, whose UAM division Uber Elevate recently merged with Joby and are about to deliver a journey, that is not simply about the vehicle itself.
For doubters this market is attracting serious investment. Joby Aviation recently listed on the New York Stock Exchange with a valuation of US$4.5 bn. The California based company developing all-electric aircraft for aerial ridesharing now considers it has sufficient cash (~US$1.6 bn) to fund it to initial commercial operations. It has completed over 1,000 test flights and has agreed a path to G1 certification of the aircraft with the FAA.
The company has also taken the first steps toward becoming the first eVTOL airline with application for Part 135 Air Carrier Certificate issued by the FAA. Joby expects to start operations of their quiet, all electric aircraft with pilot and four passengers in 2024, creating possibly the first EVTOL airline.
Palo Alto based Archer Aviation (value ~US$1.5bn) un-veiled their Maker aircraft in June 2021. They too plan to be in service by 2024. Another notable player in the USA is Wisk, who were born out of Google co-founder Larry Page’s Kitty Hawk and Boeing.
In Europe, Munich based Lilium are working towards joint EASA and FAA certification of their 7-seater aircraft also by 2024. They have announced their intention to list on NASDAQ. Their German rivals Volocopter are also targeting first commercial operations in two to three years. UK based Vertical Aerospace announced it to list in the US with a US$2.2 bn SPAC deal on the basis of pre-orders of up to 1,000 aircraft with a total value of up to $4 bn. Like Volocopter, Vertical Aerospace have their sights on the 2024 Paris Olympics as a high-profile backdrop for initial commercial operations.
No doubt more conventional companies have more clandestine programs on their books. However, there are some very interesting parallels with the burgeoning electric vehicle market back in 2008. New entrants like Tesla, Coda and Fisker were emerging whilst at the same time the mainstream automotive industry was in crisis, just as now the aerospace industry is hit by COVID19 and severe travel restrictions.
One common feature is that all these aircraft use multiple electric motors. And whilst this requires the development of lightweight batteries, the performance density of the motors is also critical and a key to a performance breakthrough. For longer range UAMs and even regional electric aircraft companies like AMSL, Zero Avia and Universal Hydrogen are developing hydrogen fuel cell power trains.
Historically the route to power density in electric machines has been to operate permanent magnet motors (PM) at higher speeds, often in excess of 10,000 rpm. Effective propellers or rotors operate at around one seventh of these speeds, to be efficient and perhaps more importantly for aircraft in urban environments, quiet. This introduces the requirement for a mechanical gear box, raising the issues of reliability, wear, maintenance and noise.
Tallerico, Cameron, Schiedler and Hasseeb have pointed out that the reliability of mechanically gear driven propulsion motors would have to increase four-fold to meet the required safety standard for UAM aircraft.
Meanwhile NASA has introduced the idea of magnetic gears and suggested torque densities of 49Nm/kg at a scale of 100Nm. This is close to the torque density of a mechanical gear. NASA has also considered how a magnetic gear can be integrated with a conventional stator to form an Outer Stator Magnetically Geared Motor (OSMGM).
By integrating a magnetic gear with a motor the inner high-speed magnetic rotor fulfills two functions. It is both the high-speed rotor of a PM motor and the high-speed rotor of the magnetic gear. This reduces the overall mass of the Tallerico et al went on to develop a high-fidelity design of an OSMGM with a torque density of >30Nm/kg and > 95% efficiency. They also concluded that this was roughly double what could be achieved using a direct drive electric motor for UAM.
Magnomatics, a spin-out from the University of Sheffield in 2006, has been commercializing this innovative technology based on magnetic gears for nearly 20 years. For Magnomatics an OSMGM is a Pseudo Direct Drive (PDD®), one of a number of patented products it has developed. Whilst being aware of work at NASA the company’s early application priorities were developing the technology for offshore wind, marine propulsion and light rail. In 2019 a 500kW demonstration direct drive generator for offshore wind was built and tested. It has a torque of 200,000 Nm and efficiency in excess of 97%. The company has licensed its PDD
technology in these sectors to blue chip international organizations and it is now turning its attention to the UAM market.
Magnomatics has researched the performance density of most of the motors being promoted for the UAM market with expectations at least 15% lighter, to avoid all the penalties associated with mechanical gearboxes and liquid cooling systems.
Magnetic gears are incredibly reliable. The company has had a magnetic gear running continuously in a harsh application for more than two years. Even after this period of time it required no maintenance. In aviation terms this represents a time between maintenance schedules of at least 17,000 hours.
 Outer Stator Magnetically Geared Motors for Urban Air Mobility Vehicle, Tallerico et al, NASA Glenn Research Centre, Cleveland OH, USA
David Latimer, an industry veteran, has lead various specialist engineering firms including: Weir Valves & Controls Ltd, Brush HMA b.v. (electric machines and turbo generators), Bedford Pumps and Howden Wade Ltd (aircraft air-conditioning and hydraulic systems). Most of these roles involved large elements of change management. He also worked at Bluebird Vehicles, a manufacturer of low floor accessible buses as the Executive Chairman. Under his leadership output increased twofold. In 2008 he became Chief Executive Officer of EVO Electric; an Imperial College spin out. Under Dave’s leadership, EVO’s strength in the electric and hybrid vehicle sector grew and they formed a joint venture with GKN Driveline in June 2011. In 2013, Dave joined Magnomatics as business development manager, before being appointed CEO in 2014.