We are seeing rapid progress in the electrification of passenger transport, and there is much talk of the electrification of air transport in future (writes Wayne Ward). The latter of these took a major step forwards recently when the air speed record for electric aircraft was shattered.

The Rolls-Royce badged Spirit of Innovation aircraft raised the record from 213 mph (342 kph) set in 2017 over 3 km to 345 mph in 2021, significantly raising the bar for electric-powered flight, and in the process, becoming the fastest EV ever, eclipsing the record held by Venturi (Voxan’s owner) which previously held the record with a battery-electric streamliner car. The record over 15 km was also broken.

There is scope for the 3 km and 15 km records to be raised further; these records were set at 500 ft (152 m) whereas records are often broken at 5000-7000 ft, flown from higher altitude airfields (often Reno, the home of air racing). The Spirit of Innovation   team estimate that increasing altitude alone could add another 20-30 mph to the record.

Rolls-Royce is not looking to produce very fast single or twin-seat aircraft, but it is hoping to develop an electrically powered vertical take-off and landing capability, and this project provides useful data for that project. The application of new technology to light aircraft is a logical way for the company to develop new propulsion systems aimed eventually at commercial passenger travel. It also supplied the pilots for the development of the aircraft and the record attempts, all of which was done in 30 flights.

While the Voxan motorcycle project was an international effort, the development of the powertrain for the electric air speed record was a very British affair. Small innovative companies had backing from Rolls-Royce and the UK government. Breaking the speed record is part of a larger initiative called ACCEL (Accelerating the Electrification of Flight), which is well-funded by the government. The aircraft is based on the Sharp Nemesis NXT air racing aircraft, more usually powered by a six-cylinder Lycoming IC engine.

 

Propulsion motors

The drive for the propeller was provided by three YASA 750R three-phase AC motors designed and produced at its factory in the UK.

The motors are of a flat or pancake format, with the outside diameter of the machine being much larger than the length. At 368 mm outside diameter, they suit the ‘engine bay’ of the NXT very well.

The motors have a hollow output shaft, which allows them to be stacked to multiply the performance of a single machine. The large-diameter spline provided in the drive hub positively encourages this, with a large spline capable of handling much higher levels of torque than a single motor is capable of producing.

The 750R’s architecture is axial flux, and it provides high torque at relatively low speed. The combined three-motor drive provides an output of 400 kW (536 bhp), although on headline datasheet performance alone, a three-motor stack could potentially produce 60 0kW at peak performance.

However, when considering the application, cooling (or lack of it for aerodynamic reasons) is an important consideration, and it makes sense for various reasons to run the motors at reduced output. The combination of motor and controller (inverter) have a very broad ‘island’ of maximum efficiency below maximum torque, and operating in this high-efficiency area reduces the cooling requirement significantly.

The high efficiency area is at 93%, but at maximum torque it can fall to 86-88%. These are very impressive efficiency figures, but it is clear that the cooling requirement, which is proportional to [1 – efficiency], is doubled by running at the maximum possible torque.

It is usual to limit peak performance to only a very short period of time owing to the maximum operating temperature of the motor magnets. If this temperature is exceeded, the risk of demagnetisation is high. It is the main reason why such motors have both peak and continuous power and torque ratings. Peak power is generally only available for a few seconds before motor controllers start to limit performance in order to protect the motor from irreversible damage.

Here we can see the whole powertrain mounted in the aircraft. The battery mounts to the bulkhead, and the motors and power electronics mount to the structural battery case (Images courtesy of Electroflight)

Battery

Electroflight, based at and airport at Staverton in the UK, is nominally responsible for the battery in the powertrain, but the company actually managed the whole project and was actively involved in the manufacture, build and preparation of the aircraft and ground testing of the powertrain.

Doug Campbell, Electroflight’s technical director, was keen to emphasise the reliance of the project on the expertise of ex-motorsport engineers, especially those with experience of electrified powertrains.

The battery contains 6480 lithium-ion cells arranged in three channels in a bespoke outer casing. The cells are cylindrical and of the 18650 design (18 mm diameter and 65 mm long). The battery’s energy capacity is 72 kWh, and the whole assembly weighs 450 kg, giving an energy density of 0.16 kWh/kg.

That tells us that each cell has an energy capacity of about 11.1 Wh. The battery is capable of delivering 450 kW continuous and 750 kW instantaneous peak power.

 

Safety

The battery casing is structural, and is mounted to the aircraft bulkhead and serves as the mounting for the inverters and motors. It is a composite case with a patented cork-composite fire-protection lining.

The 6480 cells are built as three discrete batteries, each linked to its own inverter and motor, giving an important degree of redundancy. In the event of a problem, one-third of the powertrain can be shut down while two-thirds of it can continue to run. A stipulation by Rolls-Royce was that, in the event of a problem being detected, the plane should be able to continue to fly for at least 15 minutes.

The health of each individual cell was monitored by a temperature sensor, and the battery management system was modified from a McLaren product.

The battery has the same energy density as the one in a Tesla Model S, 100 kW, and weighs 625 kg. The Tesla Model S has a maximum power of 500 kW, so the power density is 0.8 kW/kg. At 0.888 kW/kg, the Electroflight battery comes very close to this.

However, the Electroflight battery has a far more arduous duty cycle than in a typical Tesla, with continuous high power required for the entire run. The Tesla’s performance makes sustained peak power driving a very unlikely scenario, while the air race records involve getting the plane up to full speed and maintaining that over varying distances, from 3 to 15 km.

Of course, at speeds in excess of 500 kph, these course distances take very little time. However, for a system with limited cooling, such sustained high-power operation represents a very challenging duty cycle. The permanent magnets in the motors can become demagnetised, which leads to an irreversible loss of performance, and the batteries will have a strict upper limit on operating temperature, beyond which there are very real safety concerns.

Perhaps surprisingly, the battery is cooled by a water-glycol mixture, which offers high specific heat capacity. A great deal of work went into proving that such a battery coolant was flight-safe.

Here are the internals of the battery, and the numbers are amazing – 6480 cells, joined by more than 100,000 wire-bonded connections and monitored by 6480 temperature sensors