Magnetically geared machines for electric air mobility propulsion
09 July 2021
Whilst many may feel that urban air mobility (UAM) is the stuff of science fiction the reality is that it is not very far away, and it is not just small drones for the delivery of packages by the likes of Amazon, which is actually already being done in China. It is also UAM for the transport of people.
Established industry leaders, such as Boeing, Airbus, Embraer, Bell, Sikorsky and Rolls-Royce, are all involved in programmes to produce prototypes and demonstrators. Many are also aligning themselves with some of the well-funded start-ups who may well be the real trailblazers in this sector.
In many respects this is like the early days of electric vehicles. Whilst the traditional players do have electric programmes, they also have factories and plants in place to manufacture and support traditional aircraft. This brings inertia and pressure to use low emission fuels such as green hydrogen or biofuels to sweat current assets longer. Meanwhile, there are several very well-funded start-ups that could become the “Tesla” of the air mobility world. These start-ups are much more focused on the delivery of a single objective, getting into service and generating revenue. They are also starting to show “Tesla like” valuations.
A great example of a well-funded start-up is Joby Aviation. It has raised US$820 million from credible investors that include Toyota. Joby are developing a multi-tilt rotor four-seater with a range of up to 240km. It has completed over 1,000 test flights and is now working on certification with the FAA to part 23-64 rules plus a number of special conditions. The company has recently joined forces with Uber, expect certification in 2023 and to be operational by 2024. There is now talk of an IPO with a multibillion-dollar valuation. Another Silicon Valley based start-up, Archer, is treading a similar path and Ehang, the Chinese air taxi company, has a market value of US$1.4 billion. In November 2020 in Seoul, Korea, they demonstrated their two-man aircraft with 3.6km journey using an 80kg bag of rice in place of a person. One of the most promising in Europe is Lilium who have raised US$375 million for its full-size tilt wing five-seater prototype, which first flew on the 4th March 2019. It too is now working on certification and expect to take a public listing very soon. Other credible players include Volocopter (Germany), Dufour (Switzerland), Kitty Hawk (USA and linked with Boeing) and Vertical Aerospace (UK).
For most of these start-ups the business model in similar. The idea is to use these EVTOL aircraft for short to medium journeys between places like Heathrow and central London. This really cuts down journey times for a small price premium. So, the target consumers are those who consider their time is of great value. Joby use the example of flying from Los Angeles Airport (LAX) to Newport Beach, cutting the journey time from 1 ¼ hours to just 15 minutes. Sure, the ticket price maybe a multiple of that for a limousine but for the cash rich, time poor that is not a problem. Further advantages for these tilt-rotor electric aircraft include that they are considerably quieter than a helicopter and most have a specially designed wing which improves safety. They also avoid the use of gearboxes which are a further source of failure. With these aircraft it is also likely that they will use high levels of autonomous operation reducing the chances of pilot error.
It is advantageous to include a high-speed electrical machine and a reduction gear system to reduce the speed down to that required by the rotor blades (similar to Rotax aeroengines) in UAM electrically driven systems. Whilst this gives the lightest electrical machine, and often leads to the lowest system mass, the use of a mechanical gearbox will lead to an increased probability of failure. These failure modes of a mechanical gearbox can often be catastrophic.
The additional safety aspect associated with the inclusion of a gear system is much more difficult to mitigate by design. The failure modes include a completely jammed system potentially caused by debris being drawn into the converging wedge of the gear mesh, for example. This failure is compounded by the difficulties in detecting the onset of failure. Often the safety case is much more readily satisfied without the use of gear systems and direct drive is adopted. These direct drive systems are especially suited to flight critical applications (such as propulsion and flight surface actuation), however, the light weighting advantages that gearing brings are lost.
A magnetic gear solution for this problem has been proposed by NASA. The magnetic gear is analogous to a single stage epicyclic gear, however, instead of mechanical contact using a converging wedge mesh, the torque is transmitted magnetically using plain airgaps, similar to those in electrical machines. Figure 1 shows the analogy between a mechanical and a magnetic gear.
The combined gear and electrical machine (shown schematically in Figure 2) operates in a similar manner to any other brushless DC machine. They also have the same fault tolerant features, specifically for the gear element this gives advantages in terms of:
• The torque is transmitted across a uniform airgap avoiding the jamming failure modes associated with drawing material into a converging gap.
• There are no contacting torque transmitted surfaces which results in a zero-wear condition.
• The system is operated without liquid lubricants and the failure modes associated with ageing, contamination and leakage are avoided.
• The concentric rotating shafts give convenient packaging and do not introduce additional non-torsional loads into the structure.
The electrical machine is operated at high speed and the advantages of light-weighting are captured. The output speed is geared down to the required speed using the magnetic gear.
Magnomatics have developed and patented a unique configuration of magnetically geared motor, the Pseudo Direct Drive (PDD) which is highly suitable for UAM applications.
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