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With electric cars yet to breakthrough, are we ready for electric aircraft?

Aircraft are constantly evolving, but the fundamentals of propulsion have remained relatively unchanged for more than half a Century. For larger aircraft and military planes, the jet engine has reigned supreme, but could a 'systems approach' to aircraft design also include a radical change to engine design?

There is a huge amount of investment into aircraft design at the moment, particularly in the 'middle market', where aircraft such as the Boeing 737 and the Airbus A320 dominate. While individual aircraft may be upgraded and the overall design improved, Kevin Daffey, Global Head of Electrical Power and Control Systems for Rolls-Royce, believes a more fundamental change may be waiting in the wings. "Some of the key areas that I am involved in are electronics, electrical software and control systems," he said, "and what we are looking at is the 'more electric aircraft'."

The consequences of the 'more electric aircraft' – and eventually, maybe, an 'all electric aircraft' – could be changes in the way aircraft look, how they are powered and a much reduced fuel consumption, emissions and noise. Such an aircraft is still at the concept stage and reality may be decades away. However, such a goal requires a systems approach and that is already starting to filter through to new generations of aircraft.

Treating the aircraft as an integrated system was previously difficult as the airframe manufacturers, like everybody else, wanted competition in the supply chain. The heart of an aircraft is the engine and these are generally certified separately from the airframes. The engine manufacturers, primarily Rolls-Royce and GE, therefore had to supply their equipment with a simple interface in order to make certification easier and keep cost and complexity to a minimum in both airframe design and manufacture.

This design isolation between airframe and engine is something that Daffey believes is starting to change. "We have got to the point where trying to squeeze more efficiency out of the aerodynamics and other parts of the system will involve the airframe and propulsion provider working together to look at where we can get those efficiencies."

Whilst Rolls-Royce still only supplies engines, it has, for the last 10 years, been developing a capability around electrical engineering, electronics and controls to give it a greater ability to work with airframe manufacturers on optimising the engine with the other aircraft systems. "Like many other industries, you have to understand the system that you are putting your product into," said Daffey. "You have to operate at least one or two levels into the system – at least have a good knowledge of it."

One example is cabin air. This has traditionally been bled off the engine as a ready supply of cheap compressed air. It is technology with which aircraft designers are familiar but, because it takes energy to compress it in the first place, it may not be the most efficient use of the engine – particularly in some of the more demanding operating modes like take-off and climb. The Boeing 787 Dreamliner has broken with tradition and moved to an electric design for cabin air supply. Air is drawn in electrically, not from the engines, and then compressed with electrically driven compressors. Daffey commented: "There will be more draw of electrical energy from the engine, but overall, it is more efficient for the aircraft and actually saves fuel. Companies like Airbus and Boeing have been quite interested in this technology and we are expecting it is something that will become more normal for aircraft going forward."

Another example of the 'more electric aircraft' is changing the deicing method. Leading edges of the wings and the engine cells require deicing and this is generally done using compressed air from the engines, which has already been heated and can be ported about the aircraft using simple pneumatic valves. In the same way as using engine air for the cabin, this introduces inefficiencies in the engine and a new technology involving heated electric matting claims efficiency improvements over pneumatics. The new deicing technology is also being used in the Dreamliner and other airframe manufacturers like Bombardier are looking at it.

Other aspects at which Rolls-Royce is looking at include electrifying more of the engine systems. The oil system on an aircraft engine is driven by a mechanical pump via a gearbox attached to the engine's main shafts. "It is becoming very complex now," said Daffey, "particularly in very large engines, so we have been studying whether the oil system could be more effective and more efficient using an electrically driven system – similar to what is happening in cars."

Many accessories on a car – like oil or water pumps – are now driven electrically, rather than mechanically. Daffey added: "We have been looking to see if we can use that same architecture and philosophy. And we have found some benefits; for instance, we can keep the oil temperature more constant through the flight cycle by powering the pumps electrically and we can look at ways of scavenging oil from the various ports and bearings on the engine itself."

Electric fuel pumping is another area of investigation, as is the electrification of some of the linear actuators round the engine. Within the engine, things like stator vanes are currently moved hydraulically using fuel as the medium. Daffey said: "We have been thinking about how we can do that electrically – can we save weight and improve performance? This is where we need good power electronics and compact servo motors, as we do for the electrification of oil and fuel systems."

Putting power electronics around an aircraft engine obviously presents a challenge, with vibration and temperatures often going beyond the tolerances of normal components. "You need to keep the power electronics cool," commented Daffey. "You need to think how we are going to do that and what mediums are available – the air that is being drawn into the engine, the bypass air, the oil or maybe the fuel system." Fuel can be used as a coolant for parts of the engine, but if the fuel temperature is too high, performance can be affected. "There are lots of system trade offs that have to be done in introducing these new technologies and we have to really demonstrate there is a benefit," observed Daffey.

Demonstrating a benefit is not easy as aircraft operate in different phases – take off, climb, cruise, descend and land – and the system demands on the aircraft vary depending on the phase. The importance of each phase will also vary, depending on the length of the journey. It means most components and systems tend to be over specified to cater for 'worst case scenarios'. Daffey feels this is where the more electric aircraft comes into its own. "Moving things away from being mechanically operated, like fuel and oil pumps, to having them electrically driven means that can benefit from the fact that you are operating in different mission profiles. You can optimise the system for more of the operating profiles. We have found fuel savings of 1 to 2%, depending on the engine and the flight cycle."

Adoption of such technologies can be slow. Aerospace requires high integrity components and these are subjected to a vigorous certification regime. It is a regime aimed to ensure that systems work and meet the right levels of safety. But when it comes to introducing brand new technologies, Daffey said the certification process makes sense to everyone. "New technology must be able to be certified – so the certifying authorities must understand it and the airlines and the manufacturers of the aircraft must also understand the technology and accept it."

E-Thrust
It is, therefore, unsurprising that the move from the more electric aircraft to the all electric aircraft will take many years, but the concepts have been introduced.

There is a NASA version (to which Rolls-Royce is contributing) based on 'turbo electric distributed propulsion', while Rolls-Royce and Airbus EADS unveiled E-Thrust at the Paris Airshow. This is based on the concept of using a gas turbine to produce electricity, then distributing that electricity around the aircraft to a number of distributed fans that propel the aircraft. The advantage, according to Daffey, is that you can get additional operating efficiency by drawing some of the boundary air from around the aircraft into the fans and then ejecting it. "It is a more efficient way of propelling the aircraft and the fact that you are doing it with a number of fans means you are collecting more of this boundary layer air from around the aircraft. We are now looking at savings of tens of percents, compared to traditional aircraft."

While this has yet to be demonstrated on anything larger than model aircraft, Daffey believes the potential is enormous. "If you are generating electricity with one or two big power plants and then distributing it to a number of fans, it can improve the boundary layer and enable other technologies around deflected slipstreams – it means you could make the wings smaller, save weight and so on. Even though the propulsion system is heavier, the whole aircraft is lighter and therefore you save fuel."

Rolls-Royce has been working with the Universities of Manchester and Cambridge on developing very power dense electrical machines using high temperature superconductivity. This phenomenon has been around for some time but, with the exception of MRI scanners, has found limited application. But it could turn out to be an enabling technology for electric aircraft, as it allows very compact, very lightweight motors. Daffey said: "The E-Thrust concept has a high temperature superconducting motor. The challenge is that you have conductors operating at 30 or 40K. You have this wonderful juxtaposition of the world's coldest electrical motor and a gas turbine on the aircraft with some of the hottest temperatures that we produce in machines.

"We will have to look at the impact of thermal integration – we will have to integrate thermals, electrical and control systems and make the systems as effective as possible."

The technologies could combine to change the way that aircraft look; some of NASA's concepts include an aircraft that looks like a giant flying wing, a version with two fuselages 'bolted' together. The wing shapes on some aircraft are much more slender, with the fans around the outside. Daffey observed: "The whole idea is that, by liberating the aircraft designer from having two propulsion pods beneath the wings, they are now asking 'what if we could put propulsion around the fuselage?'. What if we could put it around all of the wings? What would that make the aircraft look like? Potentially, the future could look a lot different and it will be exciting – more electronics, more power electronics, more electrical machines, more system engineering."

Author
Tim Fryer

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