A new age in aviation will be ushered in later this year, when Rolls-Royce’s all-electric ‘Spirit of Innovation’ aircraft makes its attempt to attain a speed of 480 kph, topping the existing 336 kph record for an all-electric aircraft. The record-breaking attempt is part of Roll-Royce’s Accelerating the Electrification of Flight (ACCEL) initiative.
To develop a state-of-the-art battery and propulsion system for the ACCEL initiative, Rolls-Royce turned to Gloucestershire, UK based start-up Electroflight, which develops energy storage solutions for aerospace electrification applications and Oxford-based manufacturer of lightweight electric motors and controllers, YASA. Electroflight joined the ACCEL programme in February 2020 and announced its partnership with engineering simulation and 3D design software firm Ansys in October.
According to Rob Watson, Director, Rolls-Royce Electrical, “The characteristics that ‘air-taxis’ require from batteries are very similar to what is being developed for the ‘Spirit of Innovation’ so that it can reach record-breaking speeds.” Rolls-Royce completed the first taxiing of the Spirit of Innovation in March when the aircraft propelled itself forward using power from its advanced 500hp (400kw) battery and propulsion system.
Partnering for success
Electroflight used multiphysics simulations via Ansys to gain additional insights into its challenges during its battery design for the ACCEL programme. These simulations played a vital role in accelerating Electroflight’s battery development and verification processes, allowing the firm to meet tight deadlines set for the record-breaking project.
One of the primary challenges faced by Electroflight was optimising the 475 kg battery pack it developed for structural strength, thermal management and other critical performance criteria, said Douglas Campbell, Technical Director, Electroflight, in a recent article in Ansys Advantage Magazine.
Engineering simulation helped the Electroflight team address three critical issues: materials selection, structural integrity and balanced cooling. In addition, to cater to the battery pack’s size relative to the aircraft itself, Rolls-Royce and Electroflight are also designed to be integrated as a critical structural element for the one-seater aircraft.
While a lightweight engine is often considered the holy grail for high-performance IC engines, it is battery pack design that is most crucial for all-electric propulsion. This is because even the most power-dense lithium-ion batteries impose a significant weight penalty. Electroflight took a novel engineering approach to deliver the energy-storage performance required at this level, and its 475 kg battery pack features more than 6,000 battery cells delivering an energy efficiency of 90%.
Three separate battery assemblies are used in the lithium-ion battery pack, which also features an innovative cooling system that pumps a water and glycol liquid coolant mixture through plates between the batteries, offsetting the battery packs’ natural propensity to generate heat. The three separate battery assemblies provide electricity to three high-power electric motors developed by YASA. The electric motors use a proprietary axial-flux design to deliver a small and lightweight engine configuration producing more than 500 hp. With safety as the top priority, the Electroflight team engineered the aircraft to land safely even with just a single battery pack in operation.
Modelling a new reality
Electroflight relied on Ansys simulation solutions such as Mechanical, Discovery, Fluent and Granta Materials Selector to model and solve several challenges involved in the battery assembly’s design while dealing with significant time and cost pressures due to tight programme deadlines. It also utilised these solutions to understand the challenges related to sustainability and manufacturability, operating limitations and hence were able to optimise the battery design accordingly.
Another challenging situation related to the material used for the battery cells was solved using Ansys Mechanical and Granta. Electroflight had originally designed the individual battery cells to be mounted in a compact, back-to-back arrangement located on a polymer mounting plate. But it found that the 3D printed frame material originally selected for use displayed reduced stiffness properties as temperatures increased. Electroflight was now faced with a difficult challenge, where it needed to identify a material with a glass transition temperature greater than the battery operating temperature. A 30% glass fibre-filled polycarbonate was identified to be the optimal material using Ansys Mechanical and Granta.
The Electroflight team used Ansys Mechanical to obtain a complete structural and frequency analysis of the battery assembly, allowing it to adjust the assembly design. Amongst the notable changes made by Electroflight following the analysis was to design new clamps that avoided the resonant frequencies aligned with the propellor operating frequencies by adjusting the stiffness of the entire assembly.
Images courtesy: Rolls-Royce.