Rim-Driven Electric Aircraft Propulsion

Today our aviation capability is built upon a carefully iterative evolution in technology over more than a century, in this time craft have become highly optimized machines with every incremental technological advance pushing the envelope of capability and economy. However it is widely accepted that our current progression towards electric aircraft requires significant innovation across most, if not all, aircraft subsystems. This is a gap that no single iterative evolution can bridge. A revolution is required.

Rim-driven fan (RDF) technology is not innately new as this technology has become successful in the marine industry in the last decade, however it has never been able to pass feasibility in aerospace applications. The approach did not merit serious investigation until aircraft electrification became a solid target for the industry, and with heritage architecture minimizing the certification risk for first movers it has been an under-developed area of research since.

A computer generated image of a future electric aircraft featuring RogersEV’s rim-driven propulsion system. (Image: RogersEV)

Our move towards electrification has meant the blueprints for this technology have been dusted off and recent studies from RogersEV — a U.K.-based startup developing the system — show the architecture has significant promise for aerospace application with large potential improvements in system efficiency, noise, complexity and safety.

Why do We Need a New Generation of Powertrain?

A graph comparing thrust vs efficiency (power loading) for different blade architectures. (Image: RogersEV)

Electric aviation is not only a requirement for the decarbonization of the sector, but it also makes sense from an economy of effort point of view. In classic combustion engine aircraft we typically can expect a powertrain efficiency of between 18 to 32 percent depending on the craft and stage of flight. The majority of this is thermal loss from combustion when converting the chemical energy of fuel into mechanical power. In comparison all-electric drivetrain efficiency is between 50 to 70 percent with electric motors resolving many of these existing losses. However, even given a threefold efficiency gain aviation gas still has 40 times the stored energy we can access.

Whilst the automotive industry is pouring immense capital into battery chemistry or hydrogen storage there is a significant gap before early successes are scaled and eventually made available to the demanding requirements of aerospace. Innovation in the powertrain is needed to bridge this shortfall and better leverage existing energy storage capability. To achieve this effectively we need to reach efficiencies of 70 to 90 percent by reimagining the classical electrical solution from the ground up.

A graph comparing estimated performance efficiency gains with HaloDrive compared to conventional approaches to electric aircraft propulsion system designs. (Image: RogersEV)

Benefits of RDF vs an EDF?

RDFs have several key differences to their cousins electric ducted fans (EDF), in the case of the HaloDrive system in development by Rogers these are the conjoining of the blades to the rim (forming a complete rotor) and the migration of the electro-mechanical motor elements to the rim. The benefits of these changes span multiple domains and offer interesting concinnity across sub-systems.

Aerodynamically we see approximately a 7 percent gain in thrust from the perfect ducting, where no leakage of air between blade and rim can occur, we would see this increase as we tolerate larger gaps needed to reduce structural mass and account for tip-erosion.

The motor, having been distributed around much larger radius, exploits the benefits of efficient operation and passive cooling. Being intrinsically a much larger diameter system it produces much more torque. Rim-distribution of the motor also dramatically increases surface area to volume ratio and in turn heat radiation.

This new architecture offers opportunity to explore ultra-efficient high solidity ratio blades, running at lower RPM with a significantly reduced acoustic signature and physical profile. Whilst heat and particle ingress tolerance will allow the system to operate in harsher environments the reduced complexity of having fewer moving parts will cut inspection, maintenance and certification requirements. These operational and performance benefits make the HaloDrive completely scalable and able to address a broad spectrum of aerospace market verticals.

Why Has No One Tried Rim-Driven Electric Aircraft Propulsion?

Well there are some pretty unique challenges when it comes to implementing this architecture of system. One key challenge is the bearing and air-gap challenge. For any motor, it’s air gap — the distance between its stator and rotor — typically kept between 0.5-1 mm, is performance critical. This fine tolerance does not allow a central bearing system at large diameter as such a design would leave too much rotor material exposed for deformation. However, due to the velocity at the rim and therefore relative “track speed” bearings at this diameter would also be unviable despite being able to prevent deformation.

The HaloDrive system overcomes this though the use of a core-less circumferential flux motor (CFM) architecture, technically a Lorenz machine the circular magnets sit around the periphery of the rim and run in a ring of toroidally wound coils. This arrangement captures undirected magnetic flux from almost the whole circumference of the magnet and so can tolerate much larger airgaps in the order of >2.5 mm.

This approach also solves another issue, frequency losses. At high frequencies conventional axial/radial motors suffer from hysteresis and eddy losses generated from manipulating the dipoles in the core laminations. When the motor is operating at the rim the switching speed and frequencies have to be exceptionally high, an effect the CFM is immune to in its core-less nature.

An overview of the RogersEV rim-driven propulsion system design. (Image: RogersEV)

When Will it Fly?

The HaloDrive project began life within RogersEV in 2019 where it was successfully developed against critical feasibility milestones. The technology was granted government funding in 2022 and is currently in development within a consortium of partners to rapidly explore the design space and prove efficacy. Aerodynamic development is supported by Sabe Technology Limited, a fluid dynamics consultancy with over two decades of Formula One experience.

Having just passed key technical milestones, RogersEV are looking to bring RDP technology to aerospace verticals ranging from drones through electric vertical takeoff and landing (eVTOL) aircraft and scaling up to narrow body commercial aviation. Prototypes of the system will be ready for testing by the first quarter of 2024 whilst early airworthy flight versions should follow some 12 to 18 months from then.

Why Now?

The answer, as with many of these things, is a combination of technology, certification and market forces. The high value aviation market has been notoriously difficult to enter with incumbents so large and integral to the existing market, however the call for revolution brings a somewhat more level playing field with startups racing to find innovation amongst the birth of a new market segment. With the development in adjacent drone markets growing at a frightening pace the tantalizing idea of compact, electric personal air vehicles was brought into focus as a natural progression between the drone and light aircraft markets.

A future electric aircraft prototype featuring the RogersEV rim-driven propulsion system. (Image: RogersEV)

The use case was simple, offer A to B air travel at a cost per passenger competitive with the likes of Uber and Lyft. However as we have begun to consolidate around the first movers in these spaces with some of the first reaching IPO in the last 18 months it is clear not everyone has been able to deliver upon the promise of electric aerial utopia. A large factor in this has been the red tape of certification, which has begun to descend as global air authorities struggle to regulate this emerging market.

This has led to an almost unanimous but silent pivot to medium occupancy larger aircraft targeted at high value intercity routes with investment risk in such entities being empowered though infrastructure projects such as vertiports as the world of UAM is divided amongst survivors geographically. As we move closer towards tomorrow’s next generation electric aircraft, we need to work to create innovative technological foundations today.

This article was written by Kynan Fletcher, Founder-Director, RogersEV Limited (London, UK). For more information, go here .