Electric aircraft on the horizon

TRW Lucas Aerospace’s pioneering development of power-by-wire for the A3XX promises to produce big savings in weight and operating costs. John Dunn reports on how electrically-powered flight control systems are bringing the all-electric aircraft closer

Aside from the commercial gamble on the financial feasibility of the A3XX, Airbus is also banking on the abilities of its engineers and suppliers to produce lightweight component and system designs to meet its target of lowering the A3XX’s operating costs by 15-20% compared with the Boeing 747.

The gamble is already beginning to pay off. As a result of government-supported work at TRW Lucas Aerospace in Wolverhampton and Hemel Hempstead, the A3XX will become the first aircraft to specify an electrically-powered flight control system. It will use electrically driven hydraulic actuators, developed by Lucas for moving the rudder, flaps and other control surfaces, and which will act as a back-up for two conventional hydraulic systems, saving about 500 kg in weight compared with an additional hydraulic back-up system.

Airbus says the A3XX will also use variable frequency electricity generator technology developed by TRW. Compared with conventional, constant-frequency generators used to supply an aircraft’s electrical power needs, variable frequency generators use far fewer components and should cut flight operating costs by up to $16 hourly.

`It’s a hugely significant saving,’ says Peter Crouchley, chief engineer for electrical systems at TRW Lucas Aerospace. `Operators would bite your arm off for savings like that.’

Lucas introduced fly-by-wire on the Airbus A320 and under its present owner, TRW, is pioneering the concept of power-by-wire on the A3XX. The decision to use variable frequency generators and electro-hydraulic actuators is a further step along the road towards the so-called all-electric aircraft. The aim is that all mechanical and hydraulic engine and flight control systems will one day be replaced by digitally controlled electric motors and actuators. An on-board electric current will be delivered by high-power generators embedded in the engines, which will become integrated power units.

Out will go the heavy networks of pipes, valves and associated parts which are prone to wear and leaking.

They will be replaced by electro-mechanical actuators adjacent to each control surface, which will be linked by electrical cables rather than pipes. Engines will no longer need heavy, mechanical gearboxes to run ancillary equipment such as fuel pumps.

Active magnetic bearings will do away with the necessity for lubrication systems on generators. And hydro-mechanical engine control systems and bulky wiring harnesses will be replaced by embedded electronics communicating via a digital databus.

Massive savings

The potential savings in operating costs are huge. In a recent study for an all-electric military aircraft, TRW identified savings of up to 50% on maintenance and life-cycle costs simply by replacing unreliable mechanical and hydraulic systems with power-by-wire.

Work on the all-electric aircraft is focused on three areas: replacing flight control systems with electrical actuators; developing variable frequency power generators and power management systems; and introducing digital control of engines – replacing bulky wiring and hydraulic pipework and using electric pumps in fuel meters to remove hydro-mechanical equipment.

New electrical flight control systems are being developed, based on electro-mechanical and electro-hydraulic actuators. The Large Electro-Mechanical Actuation System, for example, is now operating as a proof-of-concept demonstrator. It uses a 270V DC electric motor driving a mechanical ball-screw to deliver 20 tonnes of thrust to a wing spoiler.

All moving parts in the ball-screw are coated with a layer of hard-wearing stellite alloy, diffusion bonded to their surfaces. This enables the actuator to run without oil in an emergency – it won’t seize up if the oil supply fails and will get the plane home safely.

The Electric Innovative Surface Actuation demonstrator has been developed to control the A3XX’s rudder and ailerons. This is an electro-hydraulic actuator incorporating a wet, brushless 270V DC electric motor which pumps 120 litres/min of hydraulic fluid and delivers 50kW of power. The A3XX will need three of these actuators to operate each half of its split rudder.

Electro-hydraulic flight control systems are favoured by Airbus, TRW says, because they mimic conventional hydraulic systems. But the more electrical power systems an aircraft uses, the more power it needs to generate. On the A320, for example, conventional constant frequency generators on each engine individually produce 90kW. But Airbus says the A3XX will require 150kW from each of its four engines.

A constant frequency generator uses a complex hydro-mechanical regulator to control the generator speed and produce a fixed frequency DC voltage, whatever the aircraft engine speed. The variable frequency technology developed by TRW Lucas uses electronics to modify the output of the generator. The result is a generator with just 120 components compared with the 400 or so in a constant frequency generator.

This makes it three times more reliable and the reduced size of the generator means more power can be produced for the same size and weight. TRW says operating costs for its new generator are just 50 cents per flight hour compared with $4.40 for the conventional generators on the Airbus 320. If fitted to all four A320 engines the new generators would save nearly $16 per flight hour.

TRW says these considerations will give aeroengine makers the confidence to integrate the generators into future engines instead of the generator being added as a separate bolt-on component.

First commercial application

The first commercial aircraft to use TRW variable frequency technology is Bombardier’s new Global Express business jet. It uses 40kW generators. TRW has now built a 120kW generator in a unit roughly 38cm long and 25cm in diameter which runs at 22,000 rpm. It was built to meet early estimates of the power needs of the A3XX, but TRW says it can easily be stretched to 150kW.

The company has also built an electric engine fuel pump. This reduces the amount of heat the pump transfers to the fuel and so cuts the load on an aircraft’s cooling system.

TRW is also developing a distributed digital engine control system in which high-temperature electronic systems are embedded in the various engine control mechanisms and communicate by a computer network-style data bus. On a big engine this could save more than 50kg by replacing the tangle of copper wires which surround a conventional engine with one cable.

But the all-electric aeroplane poses a number of risks and will require a fundamental change in aircraft design and back-up systems, says TRW. New electrical systems will have to be developed to separate the computer control systems from the flight control systems so that both cannot fail at once. And the engines will have to be able to generate their own stand-by power so they can keep running in an emergency.

Take-off for the world’s first all-electric aeroplane is decades away, but, says Mike Yates, head of systems integration at TRW Lucas Aerospace: `The A3XX represents a safe, yet adventurous step along the way.’