Mark Greenfield of Ultimate High talks to the team behind one of the most exciting electric aviation projects that should take to the air this year, with the ultimate aim to push the electric speed record beyond 300mph…
Words: Mark Greenfield Photography Rolls-Royce and Electroflight
16 March 2021
The Federation Aeronautique Internationale strictly defines how the 3km airspeed record is to be measured. Two specific points 3km apart, with a level 1km run on the way, which necessitates at least a 1km run out to prepare for the next leg back, and four runs in total, two in each direction to balance out the wind. Typically contenders will gently climb to height after each run, have a think and gain lots of potential energy to dive in for the next timed run. Most will take 19 or 20 minutes in total to make their record attempt. Electric aircraft were not a consideration when the rules were drawn up, so the challenge becomes even more interesting when your aircraft can run at max chat for around eight minutes. Possibly…
The aircraft here is the Spirit of Innovation, part of a Rolls-Royce-led project with Electroflight and YASA named ACCEL (Accelerating the Electrification of Flight), which hopes to not just beat the current 213.04mph electric record – set in 2017 by Walter Extra’s Siemens powered 330LE – but to smash it, aiming for a speed in excess of 300mph. Although the speed record might inevitably be attracting the headlines, there are far more significant issues here that are likely to have an impact on the future of aviation.
“it’s about enabling sustainable aviation”
CEO Warren East has committed Rolls-Royce to net zero emissions sustainable aviation by 2050 which means that to meet this ambitious goal the company needs to examine a range of options including improving the efficiency of existing engines, work on sustainable fuels, develop hybrid powertrains and focus on new disruptive power technologies like electrification and the use of hydrogen. And for the critics who suspect that electrification of aircraft is only a very distant goal, Rolls-Royce is working towards being the leading supplier of all-electric and hybrid-electric power systems across many markets. Just recently they announced a collaboration with Vertical Aerospace (based in Bristol, UK) for an Urban Air Mobility (UAM) aircraft able to carry four passengers up to 120 miles at 200mph and scheduled to enter service in 2024.
When that mission profile is considered, hovering from the top of a tall building, transitioning to level flight, decelerating into the hover at the destination 10 or 15 miles away and landing, it transpires that the energy and power requirements are very similar to those being developed in the ACCEL project. The ‘why’ is not primarily about going fast, it’s about enabling sustainable aviation; the use and industrialisation of high energy power batteries, control systems, motors and safety systems and all the associated challenges required to enable an air taxi to fly around a city, in an airworthy manner.
And the challenge is bigger than the propulsion system, it’s air traffic logistics, integration with other air traffic, cyber security, the whole charging infrastructure, how to pay for a ticket before getting on a taxi. Rolls-Royce typically works with an established supply chain of well-known aerospace companies but has recognised that the industry may be changing. Matheu Parr, Customer Director, Rolls-Royce Electrical, said, “We needed to recognise that as we move into more disruptive spaces there will be lots of new entrants, and Rolls-Royce need to learn how to work with them, fast, and ensure that it’s beneficial to both sides, creating value together.”
At the same time, Rolls-Royce is unapologetic about the interest that the programme is generating. Speed records inevitably attract attention, and Rolls-Royce is keen to showcase the project to a variety of audiences, hoping that it inspires the next generation of engineers with strong STEM outreach. The last year has obviously been very tough for aerospace. Rolls-Royce is hoping to demonstrate that there is still much opportunity in the industry.
The project started in 2018 with Rolls-Royce agreeing the ACCEL project with start-up Electroflight, brainchild of engineer Roger Targett, who among a variety of engineering achievements had already started down the electric speed record journey with his P1E, which was soon recognised as being underpowered with only 150kW. The basis for ACCEL’s aircraft came from the Nemesis NXT, an aircraft designed and raced by Jon Sharp for air racing at Reno, the holder of multiple speed and race records, and so successful that the prototype is now displayed at the US National Air and Space Museum. Already optimised for high-speed flight, the thin NASA Natural Laminar Flow wing contained no fuel, with the aircraft having a single 340 litre fuel tank in the fuselage right over the main spar (and C of G), so conceptually it seemed a sound starting point. Being a racing aircraft rather than a pure speed record machine, the aeroplane has a very efficient wing and is happy sitting at 3-4G in a turn without generating excessive amounts of drag.
The immediate problem was that only 10 kits had been constructed, and finding an unbuilt version proved problematic. The team were reluctantly considering the purchase of a pre-built aircraft, but were not keen to take on any third party build quality issues. Fortunately the very last unbuilt kit was tracked down to the back of a hangar near Paris, France, and was rapidly acquired, along with another NXT which had been damaged in a landing accident. This latter airframe was used for ground testing and, with a nod to such aircraft usually being referred to as ‘Iron Birds’ (as in not flying), was christened the ionBird.
“The total cost of the programme is just over £6 million”
Funding for the project is shared between the Aerospace Technology Institute, a Government agency that works with the Department for Business, Energy and Industrial Strategy to encourage UK innovation for the ACCEL project, and Rolls-Royce and YASA. The total cost of the programme is just over £6 million with Electroflight demonstrating the technology relevance by already having won additional business as a result of the project, and Rolls-Royce entering into additional UAM and commuter markets.
Once the kit (and damaged aircraft) had been taken to Electroflight’s hangars at Gloucester Airport, redesign and assembly of the aircraft started. The challenge for Electroflight was to develop a safe propulsion and battery system that developed the required power, while at the same time maintaining the highest possible safety standards. Most of the aircraft aft of the firewall is the original NXT kit, assembled to a very high standard by the technicians for the build at Electroflight. Replacing the original fuel tank (and part of the engine bay) is a strong battery case (with three internal subdivisions) that is a structural component of the airframe and transfers all of the engine loads from the three electric motors and propeller upfront through to the fuselage.
“The propulsion system display is required to monitor 6,000 battery temperature sensors”
The cockpit panel is new to reflect the different powerplant, and is a mix of steam gauges for height, speed and acceleration combined with a Garmin G3X Primary Flight Display and some bespoke state of the art technology. The propulsion system display is required to monitor 6,000 battery temperature sensors and several hundred other sensors, so a motorsport derived unit from Bosch has been used. Electroflight have their own ECU onboard control unit that processes the large number of complex inputs, summarising what is happening with the powertrain. The rest of the cockpit is standard switches, three small radios and circuit breakers for the more critical functions, all in a cohesive package.
Electroflight had already worked with YASA, an Oxford-based manufacturer of direct drive electric motors for the automotive sector, who were chosen to provide the powertrain for ACCEL and are partners in the project. The rotational speed of a wheel on a car is typically about 2,000 to 3,000rpm, which of course is very similar to an aircraft propeller speed. When the programme started there were no other companies that could make aviation direct drive electric motors available. Rolls-Royce subsequently purchased the Siemens eAircraft business in October 2019, but its motors were effectively a competitive product at the time that YASA was selected.
This YASA 750 R motor is a lower-speed, high-torque motor with leading torque and power densities, and has a ‘pancake’ design which allows the motors to be stacked one in front of the other, using three in this application for a maximum total output of 400kW or about 530hp. Each motor has a hollow female spline bearing, allowing a single multi-spline shaft to be slotted through all three motors, and the prop hub is directly attached to this shaft. Internal confidence among the project engineers on reliability is high, partially reflecting the intrinsic benefit that the simplicity of electric motors results in far fewer failure modes that can lead to locking or jamming, with only one moving part per motor, compared to the controlled explosions and multiple moving parts (pistons, conrods, camshafts etc) required by Internal Combustion Engines (ICE). The latter typically operate at an engine energy efficiency of about 30% compared to around 90% with electric motors. Both of these figures vary depending on a variety of factors like component temperature and ambient conditions.
The inverter in each system converts the DC power of the batteries to the AC required by the motor, and also controls the frequency of power from the batteries in order to manage the rotation speed of the motor. Without an inverter, the motor would operate at full speed as soon as the power supply was turned on.
While the motor is more efficient that an ICE, the electrical advantage is reversed when it comes to the power source. Chemical fuel – currently at least – is a lot more energy dense than battery cells. The automotive industry has been addressing this for some time now and electrical aircraft, while some way behind, are piggybacking off that progress.
The Electroflight battery system solution sits within one case that is split into three individually sealed units. Each battery unit is about 700 volts, completely isolated, and connected to its own inverter, which in turn is connected to an individual motor. There is no cross torquing between the three systems and the built-in redundancy and design philosophy means that a failure in any one system should not have an impact on the other two.
The individual cells are a little bit bigger than an AA battery – and are the same format that Tesla used to use in its cars – called an 18650.
Stjohn Youngman, MD of Electroflight (and building a Laser aerobatic aircraft in his spare time), explained.
“A key challenge is that the large number of batteries in the pack – more than 6,000 – need to be connected in an effective manner, and this is done by using very intricate Ultrasonic wire bonding that almost looks like jewellery on top of the cells.
“There are a huge number of connections as the maximum cross-sectional size is limited by efficiency issues and because there are so many discharging at very high power levels which means the amount of current generated by each cell is quite high. This also adds a redundancy element, as the more wire bonds there are then the more that can be lost before it becomes a critical problem.”
Another advantage of the electric motors are their thermal efficiency compared to ICEs, and this is easily demonstrated by looking at the apertures at the front of the aircraft for cooling air. As an example, the T67-M260 Firefly with 260hp has two very large air intakes directly to the cylinders and another chin air intake for cooling air. ACCEL, with just over double the power output, has just two tiny NACA intake cooling ducts.
Stjohn Youngman commented, “This is even more impressive when looking at ACCEL with the cowlings off, when it is immediately apparent that there is a substantial cooling system overhead – no fewer than nine individual cooling circuits – required to have full redundancy across the three powertrain systems. It was a big design challenge to figure out how to package the cooling system and highlights the benefit of having people on the team from the motorsport world (F1 and Formula E) to perfect the packaging.”
Nine circuits of cooling fluid moving around the propulsion system required a specialist radiator system, so motorsport cooling system specialist PWR helped develop the radiator system that can bring all nine of these circuits into a pair of radiator assemblies. The optimum operating temperature for the batteries is quite low and close to the ambient temperature. This small difference (delta T) makes it harder to get a heat exchange, so the engineers designed a lot of high-speed air flow through the radiators, with the system as a whole generating very low cooling drag. Cooling system optimisation will remain a key development area for aerospace electrification.
With Rolls-Royce involved in the project it’s easy to assume that the project is a massive business effort with the huge resources that such a large company can bring to bear. While Rolls-Royce has been intimately involved in the project with staff embedded inside Electroflight from the very start in 2018, the core team on the project has only been some 10-15 people strong, who have punched well above their weight in designing and building this aircraft in a couple of hangars at Staverton using tools and equipment that would be recognised by people who have built kit planes (alongside state of the art techniques from across aerospace and motorsport sectors). At the end of the day this is still a GA aircraft, albeit designed to be the fastest of its type in the world.
The programme has been delivered to date by this small and dedicated team of enthusiastic engineers, many of them aviation enthusiasts and many with PPLs and/or building their own aircraft, against the background and challenges of Covid-19 – making for a very efficient process – and one which Rolls-Royce say it has learned from. Families of the engineering team have been very supportive and the record attempt pilot, Rolls-Royce Director of Flight Operations and ACCEL Project Pilot Phill O’Dell, is very aware of the responsibility he carries on behalf of the people who have put so much of their lives into the project.
Perhaps this is a message for the future post-pandemic – teams need to be agile and more efficient. This programme is an excellent demonstration on what can be achieved by a small team of people. And certainly Electroflight has benefitted, with a series of partnerships signed since ACCEL started demonstrating its transition from start-up with three staff to a commercially viable, lightweight, bespoke battery systems company employing 25 people. Having Rolls-Royce as your first business partner obviously helps!
Stjohn Youngman is the first to highlight the benefits that Rolls-Royce have brought to the programme in terms of their enabling role, system safety lead, development of airworthiness, promotion of safety culture and now the flight test and operations schedule.
Rolls-Royce and Steve Jones have flown the ICE NXT to assess its handling qualities and expect ACCEL to operate from an aerodynamic perspective in a very similar manner. With the record attempts inevitably pushing the battery storage system to the limits, much practice has been carried out in similar aircraft gliding at the high speeds dictated by the racing wing should power be exhausted before getting back down on the ground.
As previously mentioned, the record attempt flight profile will be something of an optimisation nightmare. The original ICE NXT used pretty much all of the 20 minute time limit for the record run and spent a long time turning the aircraft around. ACCEL does not physically have the energy to do anything similar and it will look more like a Red Bull Air Race profile, having to turn the aircraft very efficiently at the end of each run, not pulling too much G so as to avoid induced drag but still minimising the time ‘wasted’ at each end of the record attempt runs. Former RBAR racer and Ops Director Steve Jones has worked with the Electroflight team for a number of years and will advise on the tactics required to help maximise the performance as well as flying the aeroplane himself.
ionBird was completed in March 2020 and final system integration is taking place now, with the Spirit of Innovation moving under her own power at Gloucester for the first time in March this year – always a big milestone for a new powerplant and aeroplane. There is still much to do before flight testing – and the record attempt – takes place at Boscombe Down, but the excitement is palpable, both for the record attempt and what the aeroplane represents. Phill said, “I’ve been in aviation for 30 years, but this is the most exciting time I’ve seen, and this aeroplane’s contribution towards carbon neutral and sustainability is remarkable.”
“…clearly delighted with what they have been able to achieve…”
Rolls-Royce, Electroflight, YASA and their suppliers are clearly delighted with what they have been able to achieve together around developing a credible electric aircraft along with the underlying technologies, and the new agile way of working for both lead companies should prove beneficial from a business perspective.
But underlying all that, there’s a strong team desire to absolutely smash the world electric speed record. FLYER will be following them on the next steps of that exciting journey.
