Magnetic Gears For Urban Air Mobility

David Latimer explains why there is so much interest in this new type of motor

The Vertical Flight Society eVTOL (electric vertical takeoff and landing) aircraft directory currently lists over 450 aircraft in development. These range from electric rotorcraft similar to today’s helicopters, to multi-rotor aircraft where the rotors are used for both lift and thrust. While many of these are desk-based studies unlikely to make it into service, some are serious undertakings with expectations of entering service by 2025. The creators of the latter designs can be broadly divided into two camps – new entrants who have raised substantial funding and more traditional aircraft manufacturers.

Among the former, Joby Aviation, EHang, Archer Aviation, Lilium, Vertical Aerospace, Astro Aerospace and BLADE have all raised substantial funding. Leading the way, perhaps, is Joby, which has completed over 1,000 flights and has agreed the path to G1 certification with the Federal Aviation Administration (FAA). The company suffered what it claims to be a minor setback with the crash of its first (unmanned) prototype aircraft in February while operating at the extreme end of the flight envelope. So far, the root cause of the crash has not been revealed, although there were reports of a component failure. In March, the company announced the testing of its second prototype – with the aim to start services in 2024.

The incumbent manufacturers also have their programmes. In September 2021, Airbus announced the CityAirbus NextGen. Initially pilot operated, this four-seater features eight electric motors powering four-bladed propellers. Airbus claims to have already conducted extensive subscale testing and virtual modelling and is now into the detailed design of the aircraft.

The first prototype flight is planned for 2023, with certification following in 2025.

Bell, which already has experience of tilt-rotor aircraft with the V-22 Osprey, is working with Safran on the Nexus 6 aircraft. Boeing appears to have taken a slightly different route by joining forces with Kitty Hawk to form Wisk. In February this year, Boeing invested a further US$450m into the Californian start-up.

The business model for most of these eVTOL urban air mobility (UAM) is to provide a fast, zero-emission commuting service from major airports into city centres, replacing limousine services which are subject to the vagaries of congestion. To achieve this, these aircraft need to be highly reliable as they will be operating over densely populated areas. They also need to be very quiet. Joby claims noise levels as low as 55dB, compared with around 90dB for a conventional helicopter.

All these aircraft use multiple electric motors that need to be both highly reliable and power dense. The normal route to achieving power density with electric motors is to increase their operating speed – often to as much as 25,000RPM. However, propellers must be operated at relatively slow speeds (1,500 - 3,000RPM) in order to keep propeller tip speeds subsonic and to reduce noise – a critical factor for eVTOL UAM. Gearing a high-speed electric motor down from 25,000RPM to 2,500RPM clearly requires a 10:1 gear, which typically means two stages. This raises new issues in terms of losses, extra mass and, most importantly, maintenance and reliability.

NASA has identified magnetic gears as being a potential technology for electric aircraft. In 2018 the agency embarked on a programme described in the paper Magnetic Gearing Research for Electrified Aircraft Propulsion. Its original focus was on pure magnetic gears, but in the 2020 paper Outer Stator Magnetically Geared Motors for Urban Air Mobility Vehicles, it concluded that the concentric combination of a magnetic gear and a permanent magnet motor would be ideal for
an eVTOL UAM aircraft.

Magnomatics was formed in 2006 to commercialise magnetic gear technology developed at The University of Sheffield. This technology includes an outer stator magnetically geared motor, which Magnomatics calls the Pseudo Direct Drive (PDD). The first commercial priorities for the PDD were for relatively large machines for applications such as wind power, marine propulsion and rail. In 2019, Magnomatics built and tested the largest PDD to date with an incredible 200,000Nm rating. This was subsequently tested at the Offshore Renewable Energy Catapult at Blyth in the UK. Based on the results, a large global engineering company is now evaluating the PDD technology for several sectors.

Having achieved success at a large scale, Magnomatics is now developing smaller motors for eVTOL UAM, partially spurred on by the conclusions reached by NASA. The company has designed an air-cooled 86kW motor with a class-leading torque density of 30Nm/kg. The motor is also rated to IP67 to be tolerant of water and dust ingress. The company recently won a NATEP grant to investigate further lightweighting of this exciting machine. Magnomatics will be partnering with Carbon ThreeSixty to see how composite materials can be used to reduce mass and potentially enhance performance.

This is not the company’s only activity in the aerospace sector. It previously worked with partners Triumph Actuation, Romax and the University of Sheffield to develop a prototype Electromechanical Magnetic Actuator System (EMMAS) suitable for an aircraft control surface. Magnomatics is also coordinating a CleanSky II project to develop a lightweight PDD-based pitch control mechanism for the Safran Open Rotor engine.

David Latimer is CEO at Magnomatics