James Richmond explores the challenges in the development of electric propulsion.

Designing an electric motor for use in aviation may seem to be a simple task – after all, they’ve been in use in one form or another since the 19th century. But it has proven to be a complex engineering challenge, with a number of hurdles to overcome before the technology can be brought to market as a primary aviation power source. Innovators and regulatory bodies alike have a weighty responsibility resting on their shoulders, as how these products perform could shape public perception and dictate the initial rate of user adoption. So, what are the obstacles on the flightpath to certification, and how can we meet them head-on?

Innovation through regulation

Robust and accessible regulation is essential for the successful development of electric propulsion in aviation. As safety-critical components, there must be confidence in the performance and reliability of electric propulsion units (EPUs), both to keep passengers and pilots safe, and to inspire trust in shareholders and the general public. For conventional aircraft, development of these regulations has been achieved through decades of lessons learned. They have matured incrementally to keep pace with changes in technology. Electrification of aircraft propulsion, however, represents a step-change in aviation technology, with uniquely intricate interactions and complications that extend beyond the aircraft to include the infrastructure on which they will operate. While this might mean that the certification environment for EPUs is in its early stages, great progress has been made in paving the way, through the preparation of Special Conditions.

These Special Conditions, set out by the United States’ Federal Aviation Administration (FAA) and the European Union Aviation Safety Agency (EASA), have given original equipment manufacturers (OEMs) the guidance needed to begin real-world testing of their designs. The FAA currently issues these on a case-by-case basis for both EPUs and whole aircraft, while EASA’s SC-VTOL (VTOL aircraft) and SC E-19 (hybrid electric and electric propulsion systems) provide support for applicants striving for certification. However, the non-binding standards – or ‘Means of Compliance’ – which guide companies in the methods that can be used to comply with basic regulations, are likely to remain excluded from SC-E19 until a consistent means of testing and compliance is established.

Weight management

Weight represents a critical factor in all design decisions for electric aircraft, driven in part by the relatively low specific energy of batteries when compared to fossil fuels. This issue is particularly prevalent in thrust-borne aircraft like eVTOLs due to their sensitivity to changes in weight, driven by the absence of lift during take-off and landing that a wing would provide. In light of this, and with published specific power targets of up to 15kW/kg by 2030, innovators need to make trade-offs to minimise weight wherever possible.

One such trade-off might be to take advantage of the weight-savings afforded by air-cooled, rather than fluid-cooled, designs. This could cut up to 30% from the dry-weight of the motor, but – as with most weight-saving techniques – could come at the cost of creating challenges elsewhere. Increased innovation will be required to ensure that EPUs are adequately cooled during flight, particularly during hover where passive airflow is significantly reduced.

Ultimately, optimising the balance between EPU weight and power will be a challenge, but to achieve both sustainability and performance, gains in efficiency must be realised wherever they can be found.

Reliability and redundancy

The fault rates and failure modes of many vital electric propulsion components may be difficult to ‘pin down’ until sufficient historical data exists, from which designers can draw insights. Reduced complexity relative to their combustion-driven conventional counterparts may theoretically result in simpler maintenance requirements, but the rigours to which aviation might subject an EPU will require a greater degree of care and diligence, to provide assurance that their benefits are fully realised.

The EASA’s and FAA’s Special Requirements each stipulate that the likelihood of a catastrophic failure that would result in the loss of the aircraft must be kept below a given threshold. Often, these thresholds are difficult to guarantee on any single component. As a result, redundancies are introduced, to reduce the risk of failures by offering back-up mechanisms in the event of a single point failure. In EPUs, for example, a significant amount of heat can build up in power and control electronics during use, creating a potential point of failure. This can be mitigated through the addition of redundant systems, perhaps by splitting the electromagnets alternately across parallel sets of control electronics to ensure that, should any fail, the motor can still run with enough power to land safely.

Redundancy, as a critical contributor to safety and reliability, should be considered at both an engine and aircraft level, to enable continued safe flight and landing even if the EPU or other critical component fails.

Aircraft architectural diversity

Every company entering this space – from disruptive startups to aerospace incumbents – is starting from a similar point: a blank canvas, with more freedom for diverse designs enabled by smaller, powerful engines. This has resulted in the emergence of a vast array of system architectures, all seeking to maximise both their competitive advantage in economics, performance, and reliability and their share of the future market. From slow-rotor gyrodynes to distributed ducted fans; from liquid cooling for better thermal performance, to air cooling to minimise both weight and complexity; and even down to differing flux configurations. Each design presents its own positives and compromises, with customers left to decide which ones dominate.

Whatever the aircrafts’ configuration might be, the fact remains that each EPU design may differ considerably depending on its application. Each application brings distinct thrust, speed and, loading requirements to design for. These variations create a challenge for EPU developers looking to sell their products across the full spectrum of the market.

Modelling complexity

When entering the design and testing phase, it’s critical to gain a better understanding of where any weaknesses may lie. Doing so with physical prototypes is both expensive and time-consuming – a lack of understanding of ‘how’ the motor might fail will mean it’s unclear where efforts should be focussed during testing.

Finite Element Modelling (FEM) can provide a cost-effective way to analyse and identify these potential issues early in development, so the design can be quickly iterated upon, and tests can be tailored following the creation of a physical prototype.

A common approach is to model vibration, loads, and thermal cases individually, then to combine them into a cumulative ‘worst-case scenario’. While this approach may have its limitations, it provides an efficient means of indicating how the motor is likely to perform, and the potential location of any structural shortcomings that need to be addressed.

However, with a lack of empirical data to verify the results from FE models, any estimates must remain conservative to maintain safety standards. This is likely to become less of a challenge when testbench and flight data is collected and available for comparison but, in the meantime, it may result in prototypes that are – at least initially – overdesigned.

Overcoming challenges

While it may appear that Zero Emission aviation is in its early stages, the rate of development is moving at great pace: from the certifiably uncertifiable (but world record-breaking) Volocopter Exercise Ball in 2011, to the start of piloted test flights in the latter half of 2023. The rate of advancement seen in the technology has – out of necessity – been met by an equally impressive drive to evolve certification standards, to ensure that a robust framework of safety assurance and guidance is available. With the first certified electric engines set to start flying commercially as early as 2025, the engineering challenges faced in their development and certification will remain a significant factor to overcome in reaching the next watershed moment in aviation history.

As engineers designing and developing these systems, we recognise there is a need to balance the drive for technology innovation for competitive advantage, with the necessity of delivering safe, reliable aircraft. Publishing validated data, sharing lessons from safety incidents, and contributing to the development of unified standards across the market is key to building a trusted, safe technology that could become an integral part of how we travel in future. Trust in the technology, however, relies not just on the engine alone, rather it’s an aircraft level challenge, with complex interactions and cascading events that must be analysed during the design phase.

AtkinsRéalis is focusing on the future at this year’s Farnborough International Airshow and the industry collaboration that will help to shape it, including a jointly hosted event with the World Economic Forum (WEF) and industry partners on the Airports of Tomorrow initiative; realising the Future of Flight, with a showcase on the UKRI Future Flight Challenge; and talking about technology, systems and future skills in both civil and defence aviation. Please contact us if you would like to book an appointment at our chalet: [email protected]