快猫短视频

Could a new generation of electric aircraft do for aviation what Toyota鈥檚 Prius has done for cars?

WHEN will the aviation industry clean up its act? Not any time soon, to judge by last October鈥檚 meeting of the International Civil Aviation Organization in Montreal, Canada. At the gathering, held to plan its response to climate change, the ICAO voted to continue making incremental improvements in fuel efficiency, a policy that has produced in over 20 years. The problem is that these small percentage point efficiency improvements haven鈥檛 matched the growth in air traffic, which averages over 5 per cent annually (快猫短视频, 24 February 2007, p 32). It looks like business as usual.

Yet elsewhere in the aviation world, there is a smell of revolution in the air. In 2009 NASA announced funding for the aviation equivalent of the automotive X Prize. Its will reward the light aircraft with the best fuel efficiency. It might be the $1.5 million prize money or a new spirit of innovation, but since then around a dozen innovative electric aircraft have been announced. This includes plans by US plane-maker Cessna to fly an all-electric version of its workhorse, the , early in 2011. 鈥淚 think people are waking up,鈥� says Randall Fishman, a pilot and engineer who has built his own 鈥渃lean plane鈥�, the . 鈥淎 lot of people are looking up and thinking, 鈥榃here is this going and how is it going to affect us?鈥� 鈥�

The recent history of the automobile industry may hold some clues. Since the late 1990s, a proliferation of agile start-up companies have turned out their own electric vehicles, set the pace for technological development and shown an enthusiasm unmatched by many established car makers. Although the limitations of battery technology mean the highway is not yet packed with all-electric vehicles, fuel-efficient hybrids have turned out to be a winning formula. Could it be a similar story in the air?

It鈥檚 starting to look that way. Certainly, the majority of new electric aircraft are the products of small teams of ambitious engineers rather than large manufacturers. But don鈥檛 expect to fly off to Mexico or Majorca on an all-electric airliner any time soon: batteries are too heavy. The prospects are far better for hybrids. Much like Toyota鈥檚 Prius, a hybrid plane could balance reasonable performance with green credentials.

鈥淢uch like Toyota鈥檚 Prius, a hybrid plane could balance reasonable performance with green credentials鈥�

Airbus has dabbled with the idea, sketching out a jet engine with a cunning clutch system that can run on kerosene, electricity or a bit of both when extra oomph is required. And in May Boeing revealed the Subsonic Ultra Green Aircraft Research (SUGAR) Volt, a hybrid airliner concept that would use both a kerosene-fuelled turbine and electric motors to turn its engines. This would help reduce fuel consumption to less than 30 per cent that of a conventional passenger jet. 鈥淚t wasn鈥檛 that long ago that people laughed at the idea of using electricity to power an airliner,鈥� says Boeing engineer Marty Bradley. 鈥淏ut I think we鈥檙e starting to show what you need to make that feasible.鈥�

Electric propulsion is not the only route to greener air travel. Running aircraft engines on biofuels or hydrogen could reduce their greenhouse gas emissions too. The main issue is supply: neither fuel is available in sufficient quantity to feed the world鈥檚 aircraft fleet and this problem isn鈥檛 likely to be solved in the near future (快猫短视频, 16 August 2008, p 34).

Electricity, however, is something most airports have on tap. Thanks to the 90 per cent-plus efficiency of an electric motor, the hourly running cost of an aircraft with an electric engine is expected to be less than half that of an equivalent biofuelled plane. Electric engines should be better for the environment too: although fossil fuels generate most of our electricity, the motors release no greenhouse gases at high altitudes where they do most damage, and are so quiet that even the crankiest neighbour wouldn鈥檛 object to a 3 am landing. So could electric aircraft ultimately be the shape of wings to come?

Not while batteries literally outweigh all these advantages. Pack a car with batteries and it will happily roll. Do the same with a plane and it, too, will happily roll. It just won鈥檛 take off. Consider a conventional 200-seat airliner that weighs about 115 tonnes at take-off. About a third of that weight is kerosene fuel. Providing the same amount of energy for an all-electric airliner would take almost 3000 tonnes of lithium-ion batteries. and others show that fuel cells are not much better (see diagram).

All of this means that right now the only practical electric aircraft are light sports planes with just one or two seats and a propeller. Yet with the cost of aviation fuel rising, pilots are waking up to the low running costs electric aircraft offer. Since 2008, when Fishman first demonstrated his ElectraFlyer at the AirVenture show in Oshkosh, Wisconsin, things have snowballed. In the last year or so, manufacturers in Germany, in France, in Italy, in China and US-based companies and , among others, have announced new electric engines or aircraft. Most of these companies are small, but to test an electric Skyhawk 172 is telling, given that over 43,000 conventional Skyhawks have already been sold, more than any other single aircraft type.

Anglo-Chinese joint venture Yuneec is building a 4-hectare factory in Shanghai to mass-produce four new electric aircraft. 鈥淭here must always be a beginning,鈥� says Brien Seeley, president of the (CAFE), based in Santa Rosa, California. 鈥淎nd what you might call the initial steps have come about in the last year or two.鈥�

Many of the key components in these aircraft were developed for electric cars. This crossover with the automotive world could be what finally makes electric planes take off, Seeley says. By developing and mass-producing new batteries, motors and control equipment for electric propulsion, car makers are laying the foundations for a new aviation industry.

Seeley expects other advances to come from NASA鈥檚 , a competition that is administered by CAFE and which aims to result in quieter, cleaner and cheaper aircraft through advances such as more efficient propulsion and new airframe designs. To win, competing aircraft must fly 200 miles (322 kilometres) in less than 2 hours using the energy equivalent of less than 1 US gallon (3.78 litres) of aviation fuel per occupant. The event will take place in July 2011.

Of the nine entrants registered so far, five have revealed they rely on electric propulsion and at least two of these are developing hybrid designs that combine a small piston engine with an electric motor and batteries. Competitor John McGinnis of Synergy Aircraft in Kalispell, Montana, says he is developing new technologies for the challenge that will help make it possible to one day fly non-stop from New York to California in a hybrid electric plane.

Improving motors and batteries through this kind of challenge could help smooth the path to commercial electric aircraft. 鈥淚 expect that experimenters and small companies will lead the way with electric propulsion,鈥� says engineer Ron Gremblan of the California Cars Initiative, a non-profit organisation that helped to develop plug-in hybrid vehicles. In particular, Gremblan thinks the project to develop electric engines for the Cessna 172 will provide major impetus 鈥渂y establishing electric airplane propulsion as mainstream: safe, certificated, and economically viable鈥�.

Meanwhile at Boeing, Bradley is hopeful that the competition could lead to breakthroughs for larger commercial aircraft too. He is especially keen to see battery improvements: for now, batteries simply can鈥檛 carry anything near the energy density of liquid fuels (see diagram).

Nonetheless, manufacturers including Airbus and Boeing are beginning to shift their attention towards electric propulsion. In 2009 showing how it might build a hybrid electric jet-turbine. And earlier this year, a group led by of a NASA-sponsored study to map out technology that could reduce airliner fuel consumption by 70 per cent by 2030.

The Boeing team began by considering a range of options, including changing air traffic control rules to allow more direct routes, improving gas turbine performance and almost doubling wingspans to add more lift. Combined, these factors allowed them to halve an airliner鈥檚 fuel consumption, but electricity seemed the most plausible option to get further, says Bradley.

Though electric motors are roughly twice as efficient as conventional jet engines, the researchers calculate that current battery performance means an all-electric airliner is far too heavy to fly 鈥� and they don鈥檛 expect this to change before 2030. Instead, the Boeing study concludes that the best way to fulfil NASA鈥檚 green design requirements is to combine conventional turbofan jet propulsion with electric power. This led to the design of the SUGAR Volt craft.

In this design, each of the aircraft鈥檚 two turbofan jets is supplemented by an electric motor, connected to each engine鈥檚 drive shaft through a clutch. During take-off and climb, the turbofans are driven by both kerosene and electricity, but once the SUGAR Volt reaches cruising altitude and less power is required, the aircraft can fly mostly on batteries. Shorter flights could rely almost exclusively on battery power, offering the greatest efficiency, while longer flights would require more kerosene. The study also suggests that a hybrid airliner could take off on shorter runways and be quieter too, thanks to the extra thrust available from the electric motor. As a next step, Boeing has applied for NASA funds to refine the concept of hybrid regional passenger planes. The design team expects to find out shortly if it has been successful.

If all goes according to plan, a seat on the SUGAR Volt could be yours in about 20 years鈥� time. Yet even if electric propulsion fails to transform long-distance air transport, Seeley believes it will still end up making its mark on the way we travel. In particular, he thinks electric planes have the potential to fundamentally alter the nature of transportation, by turning the personal flying vehicle (PFV) from a dream into reality.

PFVs are small planes with semi-automated controls that could replace cars for some journeys. Seeley has been working with NASA to develop this technology for several years. There are still plenty of challenges to overcome. For PFVs to be more convenient than cars, each neighbourhood would need its own small 鈥減ocket鈥� airport.

If flying cars were as loud and dirty as conventional aircraft, complaints from local residents would prevent such airports being built in the first place, and this is where electric propulsion could make all the difference. With unprecedented quietness and no emissions, Seeley foresees small electric vehicles flying from airports built close to the suburbs where commuters live and work, with no racket to disturb the locals. And with the fuel and environmental costs of traffic congestion rising rapidly, quiet, green aviation is the only solution to gridlock, he says. Seeley admits that controlling this traffic will be a headache 鈥� but one he is prepared to leave for another day.

鈥淲ith the costs of traffic congestion rising rapidly, quiet, green aviation is the only solution to gridlock鈥�

Electric plane makers might have cause to thank the auto industry for getting them off the ground, but if Seeley鈥檚 vision comes to pass, car drivers could be receiving a favour or two in return. So could the Prius evolve into a personal flying vehicle, or even into something like a hybrid airliner? We鈥檙e going to find out shortly, says Michael Dudley, chief of NASA鈥檚 Flight Vehicle Research and Technology Division at Ames Research Center in Moffett Field, California. 鈥淚f you look back at the history of aviation, there were all kinds of wild and crazy designs coming out. Over time they sort themselves out and you end up with optimum designs,鈥� he says. 鈥淓lectric propulsion is going through that same process. It鈥檚 a really exciting period.鈥�