Special issue on all-electric and hybrid-electric flight

Public and political pressures for environmental improvements are driving a wave of current interest in all-electric and hybrid-electric aircraft. These emerging new aircraft propulsion systems bring with them the promise of net-zero emissions and greatly reduced noise profiles. The main application focus is currently around smaller aircraft including general aviation, drones, urban air mobility, commuter, and electrical vertical take-off and landing (eVTOL) aircraft. In the slightly longer term, business and regional aircraft operators are also keen to adopt these novel propulsion systems and exploit their benefits.

The aim of this special issue of the IET Electrical Systems in Transportation journal is to showcase original research on enabling technologies for all-electric and hybrid-electric flight. For this publication, there was also a desire to feature case studies, which express the ambitions of the customer base and detail the key technical hurdles to be addressed by the supply chain. Many propulsion system demonstrations are in development, whether in a lab environment or progressing to flight trials, and submissions on original research addressing the key technical challenges were also encouraged. Technology improvements in electrical machines, power electronics, energy storage, thermal management systems, and electrical system integration (including advanced control and protection) are required to make the vision for a more electric aviation fleet a reality. Papers which detail specific relevant technical advancements or review a range of these technological contributions are also included in this special issue.

Despite the many aerospace industry projects underway with the intention of realising the potential of all-electric and hybrid-electric flight, there appears to be a shortfall in underpinning research to support these projects. It is hoped that this special issue will inspire researchers and encourage innovations that will help to meet the many challenges that lie ahead in developing a sustainable and environmentally friendly air travel ecosystem.

The papers selected for this special issue have been peer reviewed by the Guest Editors and specialist reviewers. They include case studies on industrial flight trials for all-electric and hybrid-electric propulsion systems, evaluations against other modes of transport, innovative propulsion system architectures, and novel power electronics converter solutions. The selected papers are summarised in the following sections.

This paper reviews the numerous engineering challenges faced by the ACCEL consortium comprising Rolls-Royce, Electroflight, and YASA in designing, developing, and flying an all-electric aircraft. Three world records were set by the project team for top speed over 3 and 15 km distances and for the fastest 3000 m climb as officially verified by Fédération Aéronautique Internationale. During its record-breaking runs, the aircraft also clocked up a maximum top speed of 623 km/h (387.4mph), making it the world's fastest all-electric vehicle. Power-to-mass ratio is one of the key characteristics of most high-performance vehicles and the ACCEL aircraft is no different. Project ACCEL pushed the limits of energy management, thermal management, and mass saving, while maintaining measured safety for the pilot and crew to ultimately achieve the world record-breaking flights. The blank sheet design of the Energy Storage System and Powertrain allowed for an appropriately optimised solution. The closing stages of the project included assembly, ground testing, and the exciting flight operation of the aircraft at MoD Boscombe Down.

This paper evaluates various modes of transport against the dual requirements of net zero carbon emissions and user convenience, in particular speed of travel, and cost of transportation. Results show that when operated across a whole country, battery-powered ePlanes have the lowest energy use, as measured by well-to-wing efficiency, of high-speed transport systems such as the UK's HS2 and conventional diesel rail systems, although this condition may not hold for extremely high passenger numbers per hour as seen in metropolitan areas with high-density populations. Various proposed disruptive technologies lower the cost of ownership when combined with changes in transport paradigm. With the advent of eVTOL aircraft, point-to-point journeys are achievable, which would further reduce journey times and alleviate the congestion from other modes of transport.

In this study a novel H-type hybrid-electric aircraft propulsion architecture is proposed to address the issues associated with the radial electric propulsion architecture. In a radial architecture, a fault on the main bus or failure of the generator leads to the shutting down of all the propulsion motors in that channel, thus increasing the burden on the remaining motors to maintain the power required by the aeroplane. As a result, it is necessary to oversize the motors for the purpose of compensating for the loss of other motors. To mitigate these issues, a new H-type architecture is proposed to isolate the faulted channel while still maintaining power flow to the remaining healthy motors. The proposed architecture minimises the oversizing of various components in the system as compared to a radial propulsion architecture. The sizing of components and the amount of overall mass reduction by using the proposed architecture is discussed in this study. A Hardware-in-the-Loop platform is used to validate the performance of both architectures for faults at different locations, and the results are presented.

This paper describes how, as a power electronic converter with bidirectional energy transmission, the Dual Active Bridge (DAB) has broad application prospects in electrified propulsion systems for hybrid-electric flight. High efficiency, high reliability, and long life expectancy are demanded for such applications. This paper proposes a new Pulse Width Modulation (PWM) optimisation control strategy which is based on the feedback temperature fluctuations of Gallium Nitride (GaN) power transistors to improve the life of the whole circuit. Detailed simulation results demonstrate the precise life of the GaN power transistors and verify the accuracy of the novel optimisation control strategy. Insulated Gate Bipolar Translators (IGBTs) are used to compare the life of conventional silicon-based power devices with the GaN power transistors. A Hardware-in-the-Loop platform test is used to further validate the proposed control method.

This paper describes the collaborative efforts between Tecnam, Rolls-Royce, and Rotax to equip a 4-seat Tecnam P2010 aircraft with a parallel hybrid-electric powertrain, the first of its kind ever developed for general aviation, to help reduce fuel consumption while maintaining, and even extending, the aircraft range. The High Power, High Scalability, Hybrid Powertrain (H3PS) project is set out to design, build, ground test, and demonstrate in a flight campaign such as a propulsion system. Creating a parallel hybrid-electric drive system, which brings together the electric and combustion engine worlds, creates a completely new set of challenges in the design, assembly, operation, and safety assurance of the aircraft which must be addressed for a successful business proposition. The paper reflects on some of the challenges faced, as this innovative and scalable powertrain was developed on the road to minimise emissions in the aviation industry.

Overall, we feel extremely privileged to have guest edited this special issue on All-electric and Hybrid-electric Flight for the IET Electrical Systems in Transportation journal. We are confident that our objective of including papers that will inspire interesting developments and new research in the areas of electrical aerospace propulsion systems has been met. While great strides have been taken to shape the future of all-electric and hybrid-electric aircraft, there are still many challenges to be addressed to ensure their widespread uptake with the consequential environmental benefits for all of us. It is imperative that industry and academia alike intensify efforts to close the technology gaps highlighted in these special issue papers.