Tom Shelley reports on the development that’s sparked an all-electric aeroplane
Electric motors and batteries have now reached the point where they can power a micro-light aircraft.
Such is the conclusion of a University of Cambridge lecturer and two UK aviation experts who make up a team developing the ‘Hummingbird’ machine, scheduled to take to the air before the end of the year.
While it is expected to provide some ‘serious fun’ for its developers and users, it is also part of a project to develop more fuel-efficient strategies for aircraft that may one day be hybrids, using similar strategies to those being increasingly used in cars.
The brains behind the project are Dr Paul Robertson, a university electrical engineering lecturer in the Department of Engineering; Paul Dewhurst, a director and chief flying instructor of Flylight Airsports, based at Sywell near Northampton; and Paul Welsh, an aviation certification engineer.
At a gathering of the Cambridge Energy Forum, Robertson explained that the project was only now feasible because of the development of rare earth magnets and lithium polymer batteries. Targeted at laptops and phones, these have capacities of up to 150 Whr/kg and power densities of 1,000 W/kg – figures that compare with approximately 30Whr/kg and 200 W/kg for conventional lead acid batteries for cars.
The other reason such a project can now be attempted, he says, is due to a change in the law last year, which excluded the smallest aircraft from airworthiness legislation.
“Almost all accidents in small GA [general aviation] aircraft are due to pilot error,” he explains. “Provided you have a pilot’s licence, you can build and fly what you like – to encourage innovation.”
To take advantage of this concession, aircraft must be single seat, weigh less than 115kg, have a wing loading of less than 10 kg/m2, a maximum take-off weight – including pilot and fuel – of 300 kg and a stall speed of less than 40mph.
Brushless electric motors are around 90% efficient and can deliver 6kW/kg, whereas a four-stroke petrol engine is about 30% efficient and delivers around 0.75kW/kg. The snag with electrics is that petrol possesses 12.5kWhr/kg. At 30% efficiency, that translates into 3.7kWhr/kg, whereas a lithium polymer battery has an energy storage density of only 0.15kWhr/kg. So while a 75kW motor saves 85kg of engine weight, this only corresponds to 13kWhr of battery capacity or 4kg of petrol.
The bottom line, then, is that, for an electrically powered aircraft that to be of any practical use, it needs to be aerodynamically very efficient and have relatively long, thin wings – as has been demonstrated by gliders and, in extreme cases, the Sunseeker and Nasa Helios aircraft, which are entirely solar cell powered.
The Hummingbird is going to have a single 10m wing and be 6m long. It will weigh 85kg empty and carry 25kg of Korean-made lithium polymer batteries. Lift-to drag-ratio will be 18:1 at 45 mph, which compares with conventional light aircraft at 10:1, airliners at 15:1 and gliders which are generally in the range 50 to 60:1.
Propulsion is by two new brushless electric motors developed by Plettenberg Elektromotoren in Baunatal, Germany. This company produces motors exclusively for serious model makers and also for small UAVs for silent aerial surveillance. The ‘Predator 30/8’ brushless motor can deliver up to 12kW, despite weighing only 1,55kg and being just 103mm in diameter and 74.5mm long. In level flight, the motors will only be required to each deliver 2.2kW of aerodynamic power, but have to be able to provide the full 12kW each for take-off. Each motor will drive a 31-inch three-bladed propeller through a direct drive. Robertson says that larger, slower rotating propellers would be quieter and more efficient, but would have to be driven through reduction gearboxes. These would involve increased cost, weight and complexity, and are planned for a later stage of the development.
The cell stack will consist of twelve 3.7V 40Ahr Kokam batteries. In order that these neither become damaged nor catch fire, an essential part of the aircraft is an electronic system that will monitor and manage each individual cell, ensuring voltage is in the range 3.2V to 4.2V, current does not exceed 200A and temperatures do not exceed 50oC.
The present state of development is that the motors, batteries, propellers and control systems are undergoing full-scale static tests. Run at 8kW maximum power, the arrangement produces 30kgf of thrust from each propeller. Based on the measured performance figures, the finished aircraft is expected to have a flight duration time of 40 minutes, range of 35 miles, maximum climb rate of 650 feet/minute and maximum speed of 80 mph.
Robertson suggests that the range could be increased by 8%, if the aircraft were able to carry a second set of batteries. But this would depend on whether the Civil Aviation Authority decides that batteries can be considered as fuel.
Since the Cambridge Energy Forum is focused on ways of reducing the need for fossil fuels and carbon footprints, he believes that the only truly eco-friendly way of charging the batteries would be to use wind turbines. While he is dubious about the worth of small wind turbines on town houses, he points out that airfields “are windy places” where an average 5.5m/s average wind speed is a realistic expectation. He then calculates that a 1.8m diameter turbine, which was 35% efficient, would produce 1500 kWhr per annum and, with a 67% charge efficiency, would provide 270 recharges and so 180 hours flying per year. That, he says, is a lot for a leisure aircraft. “An electric leisure aircraft is now just about feasible,” he concludes.
He sees the Hummingbird not so much as a prototype as an experimental aircraft to test electric and hybrid power sources. With it likely to take to the air in the not too distant future, what is likely to be the timing for the first hybrid aircraft? “In ten years’ time,” he predicts, hastening to add: “GA not airliners!”
A hybrid aircraft, as in a car, would only require the use of a very small internal combustion engine, running at maximum efficiency to keep the batteries topped up, with maximum motor power from batteries only being required for take-off and steep climbs.
Pointers
* Each ‘Predator 30/8’ brushless motor can deliver up to 12kW, despite weighing only 1,55kg and being just 103mm in diameter and 74.5mm long
* The cell stack will consist of twelve 3.7V 40Ahr Kokam batteries
* Based on measured performance figures, the finished aircraft is expected to have a flight duration time of 40 minutes, range of 35 miles, maximum climb rate of 650 feet/minute and maximum speed of 80 mph
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