One of the most important and promising areas of technological research and development in designing modern commercial aircraft is directed at finding ways to make the architectures of aircraft power-generation and distribution systems more electric.
Traditionally, commercial aircraft have used pneumatic and hydraulic power to drive a variety of their flight-control and cabin environmental-control systems.
Compressed air for pneumatic can be drawn off easily as bleed air from aircraft engine high-pressure compressor modules, while hydraulic lines provide a very efficient, power-dense means of delivering power to actuators for vital aircraft systems such as landing gears and flight-control surfaces.
However, aircraft and systems manufacturers are devoting much effort to designing more-electric aircraft, whose flight-control and environmental-control power requirements are provided entirely by electric power-generation, distribution and actuation systems.
In the commercial-aviation industry, the first generation of the movement towards more-electric aircraft is led by the Boeing 787, the world’s first all-electric airliner in terms of system actuation and control. For military aviation, the Lockheed Martin F-35 Lightning II provides the same pioneering role.
UTC Aerospace Systems (UTAS) is a world leader in the aircraft power-generation systems business. Tim White, president of UTAS’ Electric Systems business, says the reason commercial-aircraft operators increasingly want more-electric aircraft is simple: they can provide greater fuel-efficiency.
White says that, counter-intuitively, replacing pneumatic and hydraulic systems on aircraft with smart (i.e., computer-controlled) electrical power generation and distribution substantially reduces the amount of wiring – particularly large wires – on a commercial aircraft.
For instance, says White, Boeing’s decision to make the 787 an all-electric aircraft allowed the company to remove some 20 miles of wiring the design would otherwise have required.
UTAS should know. It provides 26 systems on the 787, involving some 3,000 parts. According to White, about 600 more-electric aircraft (including F-35s) are now flying and together they have accumulated 3 million flight hours.
White estimates that, on every long-haul Boeing 787 mission – flights in which the aircraft spends a substantial amount of time in cruise – 3 per cent of the fuel savings the 787 offers over comparable, more-traditional aircraft derive from its more-electric systems and architecture.
In the past decade UTAS has invested $3 billion in developing more-electric aircraft systems, receiving more than 1,000 separate patents in the process. The company has also filed for several hundred more and is awaiting awards for them, according to White.
During that period, UTAS has built 15 research laboratories to support development of new aircraft electrical power systems, more than 12 of which the company has now developed fully. Each lab is devoted to development of a specific electric power system for a specific aircraft type.
Several labs have been devoted to Boeing 787 systems – a new lab, developed at UTAS’ R&D and manufacturing facility at Windsor Locks in Connecticut, has developed the air-conditioning and environmental control system for the 787-10. This compact unit delivers the equivalent of the central air-conditioning requirements for 15 large US homes.
As a result of this specificity, once UTAS has developed and thoroughly tested an electrical-power system in the lab, the system requires only a short period of certification testing in UTAS’ Aircraft Integrated Systems Laboratory for the aircraft type.
UTAS continues to invest, substantially, in at least 10 areas of research into more-electric aircraft systems. One area is to obtain an overall view of how to optimise more-electric aircraft architectures.
Another is looking at electrical power offtake from engine low-pressure spools (discussed in more detail in June 10’s Talking Point, ‘The world’s most innovative aerospace manufacturer?’)
A third is research into development of megawatt-class electrical power generators. The 787 requires about 1.5 megawatts of electrical power to drive its systems, whereas the older, more traditionally designed 767 requires only a few hundred thousand watts.
However, future aircraft – particularly next-generation fighters, but also large commercial aircraft – will require several megawatts of electrical power to drive all their systems.
From the research UTAS has performed to date, says White, “What we’ve seen is that you can get into the low-megawatt class, two to three megawatts, with conventional technologies,” such as synchronous generators and power offtake from the engines using accessory gearboxes.
However, higher electrical-power requirements will require using superconducting materials. This is because, at such power loads, “even a 1-2 per cent difference in [power transmission] efficiency” is a key consideration.
Another area UTAS is researching is increasing the power densities its electrical generation, conversion and distribution systems provide. Ultimately the goal is to reduce the weight of the components and system packaging required per kilowatt of power converted.
“If we can take 20, 30 40 per cent of the weight out [of a given electrical-power system], it’s significant at the aircraft level,” says White.
A fifth area of more-electric aircraft systems research for UTAS is to develop smart, solid-state power distribution systems. By eliminating electromechanical relays, such systems would reduce aircraft weight, deliver electrical power more efficiently, more responsively and more flexibly.
UTAS is also researching ways to replace (or potentially redesign, to improve efficiency) traditional ram air turbine (RAT) emergency-power systems.
Today’s RATs, which have saved thousands of lives, are designed as wind-driven turbines mounted within the underside of an aircraft’s wing or inside its fuselage, usually near the wing-body fairing.
When an aircraft is left without any electrical power, usually because it has run out of fuel and its engines stop operating, the RAT drops automatically into the airstream flowing past the aircraft. Its wind-driven turbine generates enough electrical power to provide emergency control of the aircraft’s flight controls and surfaces, hopefully enabling it to make an emergency landing.
Although airlines and manufacturers like RATs because they are fairly light and don’t require much maintenance (they are only overhauled after being deployed for any reason), White says UTAS’s research has identified two potentially promising ways of improving RAT efficiency or replacing it with a lighter, more efficient system.
UTAS has studied ducted-RAT configurations in which a wind-driven turbine would be embedded permanently within a duct – normally covered by a door, which would open in an emergency – located in the aircraft’s fuselage.
This arrangement would do away with the long arms on today’s RATs, allowing them to swing down into the airstream to position the wind-driven turbine below the aircraft. It would offer “an advantage, depending on the application”, says White.
Additionally, UTAS has conducted studies of how an emergency electrical-power system could extract power from the low-pressure spool of an engine which was closed down but whose fan was windmilling in response to the high-speed airstream flowing into its inlet.
The company is also researching designs for more-electric flight and environmental controls. By designing an aircraft to generate, directly, sufficient electrical power to drive its environmental control system, the aircraft’s electrical-generation system by default is powerful enough to perform other tasks now powered in other ways. “That architecture starts becoming mutually supportive,” says White.
One obvious follow-on idea would be to use the aircraft’s powerful electric-generation capability directly to drive its flight-control and landing-gear actuators.
“We’re continuing to invest in the enabling technology for that,” White says, adding that UTAS – like every other major aerospace OEM – is also researching how big-data analytics can help its customers.
Importantly, also, UTAS – in cooperation with sister United Technologies organisations Pratt & Whitney and United Technologies Research Center – is researching more-electric engines, its studies including exploration of hybrid-electric propulsion systems.
These propulsion-system concepts are looking primarily at taking gas-turbine engines scaled to provide optimal cruise performance (rather than full take-off power) and augmenting their thrust substantially for take-off and climb by providing large amounts of electrical power to the propulsors’ fans during those phases of flight.
Such concepts would require “very advanced electrical storage systems”, according to White. But should UTAS and its siblings succeed in their research efforts, their hybrid-electric propulsion concept would create “a more efficient engine”.