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Design, build, test and flight of the world's fastest electric aircraft

2022; Institution of Engineering and Technology; Volume: 12; Issue: 4 Linguagem: Inglês

10.1049/els2.12059

2042-9746

Timothy Bingham, Matthew Moore, Taylor De Caux, Marco Pacino,

Reliability and Maintenance Optimization

IET Electrical Systems in TransportationVolume 12, Issue 4 p. 380-402 INDUSTRY ARTICLEOpen Access Design, build, test and flight of the world's fastest electric aircraft Timothy Bingham, Corresponding Author Timothy Bingham [email protected] orcid.org/0000-0003-1469-0199 Systems Engineering, Electroflight, Staverton, UK Correspondence Timothy Bingham, Systems Engineering, Electroflight, Staverton, UK. Email: [email protected]Search for more papers by this authorMathew Moore, Mathew Moore Systems Engineering, Electroflight, Staverton, UKSearch for more papers by this authorTaylor De Caux, Taylor De Caux Systems Engineering, Electroflight, Staverton, UKSearch for more papers by this authorMarco Pacino, Marco Pacino Systems Engineering, Electroflight, Staverton, UKSearch for more papers by this author Timothy Bingham, Corresponding Author Timothy Bingham [email protected] orcid.org/0000-0003-1469-0199 Systems Engineering, Electroflight, Staverton, UK Correspondence Timothy Bingham, Systems Engineering, Electroflight, Staverton, UK. Email: [email protected]Search for more papers by this authorMathew Moore, Mathew Moore Systems Engineering, Electroflight, Staverton, UKSearch for more papers by this authorTaylor De Caux, Taylor De Caux Systems Engineering, Electroflight, Staverton, UKSearch for more papers by this authorMarco Pacino, Marco Pacino Systems Engineering, Electroflight, Staverton, UKSearch for more papers by this author First published: 25 November 2022 https://doi.org/10.1049/els2.12059AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract Power to mass ratio is one of the key characteristics of most high-performance vehicles, a record-breaking ACCEL race 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 world record-breaking flights. The numerous engineering challenges involved in designing and developing the world's fastest electric aircraft have been solved by a relatively small team in a limited timeframe. The blank sheet design of the ESS and powertrain allowed for an appropriately optimised solution, swiftly moving into physical testing allowed for rapid development of the system and opportunities to iterate. In the closing stages of the project, building, assembling, and ground testing the aircraft under one roof allowed for an effective cohesive and exciting crescendo before flight operation of the aircraft at MoD Boscombe Down. 1 INTRODUCTION The aim of the ACCEL (ACCelerating the Electrification of Flight) project is to develop the technology and capability necessary to enable alternative energy storage and propulsion systems for the future advanced air mobility market rather than for the aircraft itself (Spirit of Innovation, Figure 1). [1] Previous electrification of aerospace projects include limited attempts at record breaking feats by early electric aircraft, however, the technology utilised is not transferable to what is expected of future market development. It was therefore decided that attempting to break multiple speed records over a variety of distances would best lead to the development of suitable technology for hybrid primary propulsion applications including sustained high-power demand across the entire Energy Storage System's (ESS's) capacity and fast charging for rapid energy recovery between flights. FIGURE 1Open in figure viewerPowerPoint Spirit of Innovation in flight Prior to ACCEL, adoption of electrical technology in aviation was limited to mostly low voltage non-propulsion systems. The project intended to exploit the established automotive and motorsport High Voltage (HV) electrical methodologies and components to expedite the development process. Subsequently, validating the automotive spec HV components feasibility for the primary or hybrid propulsion systems in the aerospace sector is an output of the project to influence and guide the design of next-generation electric powertrain components. Facilitating learning on the areas of technology improvement required to facilitate the adoption of electrical energy propulsion. Through doing so, this project will also provide the United Kingdom with an independent route to electric propulsion system and component understanding for aerospace. A key focus of the project is advancing high-power battery system technology and high-power density electric machines for aerospace use. An important theme of the project is to learn quickly, testing what technologies and practices can be pulled from other sectors such as electrified automotive and motorsport while gaining an understanding of aerospace specific solutions that will need to be developed to meet performance and certification requirements for future electric flight. ACCEL follows a long tradition of using competition as a catalyst for innovation through a multitude of methods. Use of performance to drive the development and implementation of technology, specifically power/energy density and aircraft system integration. Promotion of electric aviation through the excitement of speed while meeting aerospace levels of quality and safety. Setting the world speed and time-to-climb records for an electrically powered aircraft gaining publicity and industry exposure. Attracting electrical engineers, suppliers, and Small Medium Enterprises (SMEs) to aviation. The project set several 'markers' Highest power electrically powered aircraft. Simultaneously highest voltage, energy, and power density aircraft battery system. Fastest electrically powered aircraft. History shows that projects of this type stimulate global interest, with investment and innovation being powerful mechanisms in accelerating electric flight to be commercially viable and thus adopted worldwide. Keep It Simple 'KISS' philosophy was employed across the entire project. This approach paid dividends such as easing the learning curve of this new technology to established aerospace stakeholders in the project while reducing the demand for extensive testing of new safety-critical hardware and software and validation in alignment with the project's drive for fast-paced development. Pilot training was essential throughout the project as this technology application is so emergent, a new understanding of the HV electrical systems needed to be firmly founded to facilitate the evaluation of the course of action in the case of an emergency. The pilot was given complete control over the aircraft's thrust and Power Distribution Unit (PDU) control, ensuring that even in a critical failure case they had total authority over the aircraft systems as opposed to an automated shutdown potentially grounding the aircraft catastrophically. Despite the obvious possible danger presented by a HV insulation failure or a cell thermal runaway, the pilot was given the best chance of reaching safety in a scenario that would typically cause a ground-based electric vehicle to automatically shut down. The drive for sustainable aviation is unlikely to be a single solution but rather a combination of different energy storage and propulsion technologies, their allocation dependent on the use case. Given the goals set for the ACCEL project, several of these alternative technologies were considered before a lithium-based ESS and axial flux electric machine based powertrain were selected. Hydrogen-based ESS fundamentally lack the power density and output stability required for sustained flight, let alone the higher demand during a climb. Additionally, the failure case of hydrogen fuel cells can result in an extremely volatile event that at the time would not have been feasible to mitigate without compromising the aircraft's take-off mass, especially within the limited timescale of the ACCEL project. Lithium-based electrochemical cells however can meet the required high-power demand, provided sufficient thermal energy rejection is implemented for sustained flight. Energy density is considerably lower for a lithium-based ESS, however, the ability to better mitigate the cell impact during a failure event is crucial to meeting the safety requirement for flight. The relative feasibility to achieve this with lithium-ion cells is far more reasonable than for hydrogen thus far. Existing electrification of aerospace Pipistrel Velis Electro is currently the only Type certified electric powered aircraft fully approved for pilot training in day visual flight rules operations. [2]. Velis Electro is a full-electric derivative of the proven Virus SW 121, it is equipped with a Pipistrel type certified electric engine E-811-268MVLC, developed with partners EMRAX and EMSISO, and Pipistrel's three-bladed composite fixed pitch propeller P-812-164-F3A. The electric machine can produce 57.6 kW, is liquid cooled, and the cooling system consists of a radiator and two pumps. Two high-power axial fans are installed behind the radiator to cool the batteries during charging. The ESS, which is crashworthy, thermal runaway inhibiting and HIRF/EMI tolerant, is formed by two high-performance batteries (Pipistrel PB345V124E-L) connected in parallel, for a total nominal energy of 24.8 kWh and a nominal bus voltage of 345 Vdc. The two-seater aircraft, intended primarily for pilot training, has a maximum take-off weight of 600 kg with a cruise speed of 90 knots calibrated airspeed (KCAS) requiring 35 kW. [3]. Previous record attempts The Spirit of Innovation broke 4 world records for the Fédération Aéronautique Internationale (FAI) category Powered Aircraft, sub-class C-1c, group Electric [4]: Speed over a 3 km course Speed over a 15 km course Altitude Time to climb to 3000 m FAI is a non-governmental and non-profit international organisation with the basic aim of furthering aeronautical and astronautical activities worldwide, ratifying world and continental records, and coordinating the organisation of international competitions. It is the body recognised worldwide which is responsible for verifying the claims of world record-breaking attempts. For a record to be recognised by the FAI strict rules and detailed information and data need to be submitted to the organisation for review. Before the Spirit of Innovation, the only world record ratified by FAI for this category of aircraft, electric powered aircraft with take-off weight between 1000 and 1750 kg, was 'speed over a 3 km course'. The record was set in 2017 by Walter Kampsmann on the all-electric Extra 330LE with a speed of 342.86 km/h. [5] The Extra 330LE aircraft was powered by an electric machine developed by Siemens with support from Germany's Aeronautics Research Program, the SP260D, with a power output of 260 kW (348 hp). The propeller was driven directly by the electric machine without the use of transmission and could reach 2500 rpm. The aircraft featured two battery packs, each with 14 high-power Li-Ion battery modules with an energy of 18.6 kWh. [6]. With a speed of 555.9 km/h, the Spirit of Innovation broke the 3 km record by 213.04 km/h (132 mph). To put the sizable step in performance of the Spirit of Innovation into context, the same Extra 330 LE electric aircraft, in a slightly lighter configuration, also set the current FAI world record for time to climb to a height of 3000 m making it in 4 min and 22 s. The same record set by the Spirit of Innovation for the heavier category is 3 min and 22 s [7]. A further speed record was set over the FAI 15 km course, where the Spirit of Innovation achieved 532.1 km/h (330 mph) which exceeds the record held by the Extra 330 LE electric aircraft in the lower weight class by 292.8 km/h (182 mph). Additionally, an altitude record was set for the electric group sub-class C-1c of 3539 m. 2 UNDERSTANDING THE ENGINEERING PROBLEM To achieve the record flight whilst remaining within the predetermined project scope required the entire engineering problem to be comprehensively understood, captured, and then categorised in such a way that systems or components were creatively and efficiently utilised. Only by perceiving the entire project as a single engineering problem with multiple tiers of solutions could allow for such an amalgamation of systems to truly minimise the aircraft mass ensuring maximum power to weight ratio and maximum energy to weight ratio. All whilst maintaining appropriate levels of safety and redundancy within a limiting timeframe and comparatively small yet effective team. The energy density of lithium-based electrochemical cells is two orders of magnitude below that of hydrocarbon fuels. [8] An electric powertrains efficiency, whilst superior to that of a jet engine, [9] cannot rectify this significant deficiency of energy density and therefore an ESS will almost always be limited by its energy density. [10] Even considering next-generation cell technology that could deliver up to 500 Wh/kg at the cell level, there is insufficient energy for intercity flight without the aid of a fuel consuming hybrid ESS solution. Therefore, whilst the technology is maturing, projects implementing lithium-based electrochemical ESSs must focus on shorter, intracity high-power flights such as those achievable by Electric Vertical Take-off and Landing (eVTOL) as conceptualised by AIRBUS, Archer, Beta Technologies, Embraer EVE, Joby Aviation, Lilium, Supernal, Volocopter, Vertical Aerospace and WISK Aero. [11-21]. Whilst energy density is essential to the manufacturer's aircraft viability, the fundamental limitations of cells result in a power limited ESS during take-off and landing due to the required discharge rate at high and low State of Charge (SoC). A reasonable estimation suggests a 'lift and push' eVTOL ESS of 200 kWh energy will need to deliver up to 1.4 MW during a worst-case take-off scenario equating to 7 C nominal discharge rate or 8.5 C at low SoC. [21] Decoupled from the cell selection, this will generate a significant amount of thermal energy during operation that is required to be removed from the system to preserve cell life and reduce thermal runaway risks. A compounding factor is the viable business case desire to conduct back-to-back flights requiring rapid charging. The thermal limitation of the ESS becomes as restraining to the electrification of flight as much as the energy and power density issues from a technical perspective but even more so limits the commercial viability with accelerated cell ageing from exposure to high temperatures. In extreme cases, taking cell life span below 200 cycles or as low as 13 days of high utilisation when 80% capacity retention is considered cell End of Life (EoL). [21, 22]. The ACCEL project set out to develop and stimulate engineering capability to deliver aerospace certified systems that can meet the power and energy demands of the airframe manufacturers with a specific focus on ESS and powertrain enhancement over the readily available technology for automotive and motorsport applications. Aerospace technology differs from that required by automotive and motorsport industries for a multitude of reasons, as summarised in Table 1. Whilst these both provide a great foundation for the technology to be developed upon, in some respects an ESS to be used for flightworthy primary propulsion systems will have to both improve upon and roll back advancements made in these sectors. TABLE 1. Design criteria analysis for the three largest transport electrification sectors. Green, amber, and red markers signifying considered, dependant and critical design criteria respectively All sectors of electrification in industry are sensitive to ESS energy, or specifically gravimetric and volumetric energy density, with cost to the consumer being the driving factor behind enhancements for automotive and mass sensitivity driving development in motorsport. [23, 24] Aerospace ESS energy density enhancements however will be driven by both factors to ensure both technical and commercial aspects of feasibility are achieved. The maximum charge rate of commercially available cells is yet to meet the requirements of recharge times that allow for back-to-back flights during morning and evening peak travel periods. [25] Even more challenging is the accelerated cell ageing that is inherent with extreme fast charging and must be overcome to ensure a cell's useable cycle life spans thousands of cycles and not hundreds. Discharge rates however do currently meet eVTOL & Electric Conventional Take-off and Landing (eCTOL) requirements as proven by the ACCEL project, however, not at the high gravimetric energy densities required for commercially viable flight. [26] Unfortunately, the mutually exclusive design of 'power' and 'energy' cells ensures that the required high power capable cells will have lagging energy densities behind the market leading values. [27] Ensuring the aircraft can operate and generate a profit is likely to be highly limited by the rate at which the ESS cells are aged from such intense charge and discharge profiles of up to 15 times per day. [25] Whilst many other areas of the aircraft will incur significant capital expenditure, the ESS is likely to be considered as an operational expenditure, for example, a consumable and will regularly require refurbishment with new cells being installed as they reach EoL, typically deemed to be retaining 80% of their original energy. [28] A cell's lifespan is dictated by the stress applied to the internal chemistry under loads; however, certain conditions at which this occurs can expedite the ageing effects, particularly when this chemistry is heated above 50°C. [29] High ambient temperatures, high power loads at high SoC and high current loads at low SoC are cases where the limitations of charge transfer within the cell make it extremely susceptible to capacity degradation. [30] Hence why the typical flight profile utilisation aims to avoid these high degradation areas, Figure 2. Unfortunately, a typical flight profile requires the largest power draw for take-off and landing, during the two most vulnerable areas of operation within the SoC range. Cell thermal management is therefore one of the most critical design criteria for aerospace applications. Cell balancing, SoC calibration and assessing the impact of electrothermal performance degradation are all essential regular maintenance tasks on the ESS to aid a minimalist thermal management system in mitigating unnecessary cell ageing. FIGURE 2Open in figure viewerPowerPoint Cell State of Charge (SoC) utilisation for a typical flight profile Figure 2 highlights how the energy of the ACCEL ESS would have to increase to meet the requirements that are imposed on passenger focused eVTOL and eCTOL aircraft. For the same 55 kWh energy utilisation during flight, a commercially viable aircraft's ESS must be cable of delivering a minimum of 20 min emergency cruise. In addition to this, to ensure the product is financially viable, with regards to operational expenditure, sufficient degradation must be able to occur before the typical flight profile cannot be met anymore thus an additional 20% is built into the ESS to accommodate capacity loss through usage. The final 5% of unused SoC is usually determined by the inability to access specified capacity due to bus voltage drop under high load at low SoC (during landing) causing the cell to undervoltage subsequently introducing the risk of compromising the cell health as well as the powertrains' ability to perform consistently with such a rapid bus voltage drop, whilst power output is at peak request. Safety and redundancy requirements for an automotive, HV, lithium-based electrochemical ESS notably differ from that considered for use in aircraft. Primarily used in automotive products, but also motorsport, there is a focus on safety systems being automated and beyond the operator's control or ability to override. [31] Shutdown systems cutting the HV supply and thus isolating the rest of the vehicle in a non-destructive failure case, present the driver and any passengers the safest condition and offer the best chances of survival in nearly all scenarios for road-going vehicles. [32] Whilst this method is well suited to these vehicles, an aircraft does not possess this luxury. If this type of system was implemented and was triggered by a HV insulation resistance fault during flight, the PDU would remove the HV supply from the electrical machine by opening the contacting units, causing a total power loss to all propulsors, resulting in a potentially catastrophic scenario for both passengers and anything caught in the crash zone. One common school of thought implies that it is imperative for the pilot to have complete control over the ESS functionality and not rely on automated safety systems when an isolation fault or thermal runaway event occurs to ensure the best chance of continued safe operation until touchdown. There are also differences in thermal runaway requirements for aerospace, DO-311A specifies that an aircraft ESS must be able to contain a thermal runaway event for sufficient time such that the aircraft can safely land. [33] During this time the ESS must operate with the failed Energy Storage Unit shutdown, and therefore rely on a smaller total ESS to distribute the high-power load that is dictated by a landing manoeuvre. This level of ESS redundancy is not typically built into automotive or motorsport applications and therefore facilitates the need for a bespoke PDU that is controlled by novel power management techniques across all ESUs. An unprecedented HV utilisation in aerospace for primary propulsion systems is driven by the need for mass saving across the aircraft to reduce load current and therefore conductor mass for a given power demand. Carrying more than a megawatt from the ESS to a series of electric powertrains distributed across the entire airframe, unfortunately, incurs a significant mass penalty. However, copper and aluminium current carrying mediums are typically rated to a continuous value that is intended for upwards of thousands of hours of use for power transfer in automotive applications. For the applications in question, however, parts needed to meet a specification where load periods are defined in minutes and life span in hours requiring a different continuous rating to be supplied by the manufacturer and subsequently being able to downsize the HV current collector cross-sectional area and mass required for the specified current. Aerospace applications of HV battery electric systems and powertrains, therefore, require the evolution of current Commercial Off The Shelf (COTS) components to suit which are slowly being adopted by suppliers. This however is proving to be a slow uptake and projects such as this require innovative solutions to be implemented to ensure the proof-of-concept vehicles continue to accelerate the industry focus to components suitable for flight. 3 AIRCRAFT DESIGN Airframe selection It was apparent from the outset of the project that the scope of design should be constrained to the powertrain predominantly and not aircraft airframe design. Finding a suitable existing airframe was paramount, specifically, a proven high-performance aircraft was a priority. Air racing platforms were the obvious choice with their high aerodynamic efficiency at high speed, high strength, and provision for relatively oversized powerplants. The Nemesis NXT (Neoteric eXperimental Technology) met the unique set of requirements, being a proven Sport-class aircraft designed for air racing by John Sharp (USA) and holds many fuel combustion records achieving speeds of more than 650 km/h (400 mph). [34]. The Nemesis NXT is a two-piece fuselage, retractable gear, single engine, kit aeroplane of all moulded carbon fibre construction allowing a very sleek aerodynamic profile. The airframe is designed for the environment of high-speed low-level flying, and such has an inherent high strength. Only ten Nemesis NXT kits were ever made, with just four constructed to flying status. To permit timely successful completion of the ACCEL project it became paramount to secure an airworthy kit. An identical airframe in lesser condition provided the project with a ground test rig, known as the ION Bird, which was used for system integration and characterisation testing. Figure 3 shows the Spirit of Innovation being assembled at Electroflight's facility in Staverton. FIGURE 3Open in figure viewerPowerPoint Spirit of Innovation airframe partially assembled Powertrain architecture The ACCEL powertrain is a relatively conventional system that would be familiar in an automotive setting, comprising three galvanically isolated HV ESSs, each serving its own inverter and electric machine. The powertrain together is installed and mounted to the structural ESS case enclosed in the cowling, Figure 4, and could be operated detached from the airframe. The three separate channels are considered to be standalone powertrains, Figure 5, consisting of the following: Water glycol cooled channel 23.3 kWh, 756 Vdc, 250 A continuous Water glycol cooled Sevcon Gen 4 Size 10 inverter [35]. Dielectric oil cooled YASA 750R [36]. FIGURE 4Open in figure viewerPowerPoint ACCEL powertrain installed in aircraft FIGURE 5Open in figure viewerPowerPoint High-level powertrain overview YASA 750R is an axial-flux electric machine capable of 790 Nm and 200 kW peak power. The key benefit of using the 750R is its speed range of up to 3250 rpm, which facilitates direct drive to a typical General Aviation 2 m diameter 3 blade propeller [35]. The Sevcon Gen 4 Size 10 inverter has previously been coupled with a YASA 750R and complement each other with respect to their current capability and position sensor integration. [37] To reduce the complexity of the propulsion control system each inverter is individually served by its own independent potentiometer to provide a torque demand. The three separate potentiometers are connected to the same single torque demand lever operated by the pilots left hand. The propulsion system consists of a central propeller shaft driven by a stack of three YASA 750R electric machines & supported by grease lubricated bearings at each end. The bearings are mounted within aluminium bearing support housings to take the radial & thrust loads as required and 'shield' the electric machines from excessive external loads. The propulsion electric machine stack is attached to the structural ESS case via a tubular steel framework. A single MT-propeller electrically controlled three-blade constant speed propeller has been utilised with a maximum continuous power of 400 kW. Hung below the powertrain is the cooling system as a unit, which consists of nine coolant to air heat exchangers, inlet and outlet cooling air ducts and diagnostic instrumentation. The cooling system adopted dry break couples to facilitate simple assembly and bleeding of the cooling circuits. The PDUs, of which there are three, one for each galvanically isolated channel, are predominantly responsible for the control of the ESS. The PDU connects and switches the always active ESS to the rest of the HV system through a magnetically actuated fuse. Before the ESS can be directly connected to the system a pre-charge sequence is required. Pre-charge is the process of raising the system voltage through a limited current flow. A gradual voltage rise is required as capacitance is present in the system. Once the voltage is within a tolerable range of the ESS bus voltage the main HV positive contactor can safely close with a small current flow and reduced risk of contactor welding. Other functions/features are completed by the PDUs, such as monitoring the insulation resistance of the HV system to the chassis, measuring full HV bus voltage and current as well as housing a proportion of the Battery Management System (BMS), namely the supercell voltage balancing. A High Voltage Interlock Loop passes through the PDU and to all the HV connectors to enable the detection of a connector not properly mated. A decision was made to not include a separate charger control system on the bases of reducing mass, instead the main inverter supply connector is exchanged with the charger connector. The same control components that are used to connect the inverter are then used to pre-charge and connect the charger to the ESS channel. This methodology also provides an additional layer of operational safety in that the propeller cannot be unintentionally powered while the ESS is being charged and the aircraft is serviced, thus eliminating risk to personnel. HMI design An untrivial aspect of making a fully electric aircraft is to make it safe and easy to operate. This requires enough effort to be put into the Human Machine Interface (HMI) design (Figure 6). Not dissimilarly to the automotive case, some of the information relevant to the pilot is the same as per a conventional aircraft, but some critical information is new and unfamiliar to pilots that are used to traditional aircraft and that rely on instinctive actions trained and perfected in years of flight. The HMI design for ACCEL, therefore, needed to find the right balance between offering a familiar feeling to the pilot and at the same time includin