Transport: As electric cars make steady progress on land, battery- powered aircraft of various kinds are quietly taking to the air

RANDALL FISHMAN, a retired jeweller from New Jersey, is a keen pilot who has spent a lot of time hang-gliding over the Hudson river when he should have been working. Despite some 30 years’ experience as an aviator, he yearned to fly in a new and purer way. So in 2006 he converted a powered hang-glider, removing its small combustion engine and replacing it with a battery and an electric motor. He was not trying to be green. “I just did not like all the vibration and noise from a combustion engine,” he says. “With electric power it was like that dream you had as a kid—of soaring and flying silently.”

Mr Fishman is among a growing group of aviators in America, Europe and Asia who are flying under electric power. What began with hang-gliders and microlights has moved on to full-sized gliders and now two-seater aircraft. A dozen or so electric aircraft are expected to turn up at the Green Aviation Show, which will be held at Le Bourget, France, this month. And they will also be out in force at the Experimental Aircraft Association’s annual gathering at Oshkosh, Wisconsin, in July.

Fly high, fly silent

The idea of flying electric has lots going for it. An electric motor can deliver a huge amount of torque, or turning force, which is what gives electric cars such rapid acceleration, and is good for turning propellers, too. Electric motors do not need a gearbox, which reduces weight and mechanical complexity. This means less maintenance is required and there are fewer things to break—which is comforting in an aircraft. And if things do go wrong, there is no tank of fuel to rupture and explode.

The problem is that batteries tend to be heavy. What has changed the game are lithium-ion batteries, which are a lot lighter than other rechargeable batteries. Paul Robertson of the University of Cambridge’s engineering department, who has also converted hang-gliders to fly under electric power, calculates that the energy density of a lithium-ion battery is 0.15 kilowatt-hours per kilogram (kWh/kg). The equivalent figure for petrol is 12.5kWh/kg, and although only 30% of this energy is captured, the energy density is still 3.7kWh/kg, or 25 times as much as a battery. Even so, Dr Robertson says it is feasible to build electric aircraft—provided they have efficient wings to provide lots of lift.

In aerodynamic terms, lift is a function of weight, thrust and drag (air resistance). The aerofoil of a wing creates more lift than drag, but exactly how much depends on its design and the speed of the air passing over it. A light aircraft typically has a lift-to-drag ratio of around 10:1; for a hang-glider, the ratio is about 15:1. (This ratio also means that an unpowered hang-glider flying at a constant speed moves 15 metres forward for every one metre it descends.) Dr Robertson is working on a twin-engined electric microlight with a wingspan of 10 metres (33 feet) and a lift-to-drag ratio of 18:1. This means it requires an unusually small amount of thrust in order to stay airborne. He expects to be able to fly it for around 40 minutes, and longer with additional battery packs. Flying electric, says Dr Robertson, is a pleasantly quiet and low-cost way of taking to the air.

A modern sports glider, with long, slender wings, is the most aerodynamically efficient aircraft, with a lift-to-drag ratio of around 50:1. Lange Aviation, a glider manufacturer based in Zweibrücken, Germany, started developing a self-launching electric glider in 1996. Some gliders can launch themselves with a retractable propeller turned by a small combustion engine. The propeller is usually mounted on a mast behind the pilot and raised for take-off (or to gain more height or distance once airborne) and then lowered back inside the fuselage. The trouble is, says Axel Lange, the company’s founder, combustion engines are noisy, vibrate a lot and require plenty of maintenance.

Lange’s Antares 20E is a single-seat glider with a retractable propeller powered by a 40kW electric motor and 72 lithium-ion cells mounted in the leading edges of its wings. On a single charge it is capable of taking off and soaring to 3,000 metres, though the same amount of energy could also be used for multiple launches to a lower altitude, or for range-extension during a mostly unpowered flight. The Antares 20E is thought to be the first fully certified and commercially produced electric plane. So far 50 have been sold. Mr Lange is also supplying the power system to Schempp-Hirth, another German glider-maker, which is developing a two-seater version called the Arcus E.

Making sparks fly: Solar Impulse (top); Yuneec E430 (middle); Antares 20E (bottom); Antares DLR-H2 (on next page)

Gliders are all very well, but the most exciting area of development is the construction of electric aircraft that are quite good at gliding but, like a small Cessna or Piper, can also be used for longer journeys. Mr Fishman, who went on to build a single-seat electric plane and then moved into the electric-aircraft business, is completing development of a two-seater called the ElectraFlyer-X. It will be able to fly for two hours and reach a top speed of 128kph (80mph). It can be recharged in around three hours from a built-in charger, has a 15-metre wingspan and a lift-to-drag ratio of around 30:1. The plan is to sell the ElectraFlyer-X in kit form for around $65,000; the batteries cost an extra $15,000. Regulations vary around the world, but in America and some other countries, home-built aircraft do not have to undergo the complex certification processes that commercial aircraft do, and can be flown as “experimental”.

Indeed, there are all kinds of rules in America and elsewhere for various forms of lightweight recreational aircraft. But very few were written with electric power in mind. That means it is slow and frustrating to obtain certification, says Tine Tomazic of Pipistral, a Slovenian maker of light aircraft, which has produced an electric version of its Taurus self-launching glider and has been granted permission to sell it in France. Mr Tomazic believes the lack of a universal certification process for electric aircraft is holding up a technology that is “mature and ready to go”.

Another company seeking certification is Yuneec. In January Tian Yu, the firm’s founder, became one of the first passengers to fly in his firm’s new two-seater E430 from the company’s airstrip, next to its factory and research centre near Shanghai. A sales office has been opened near London with plans to sell the aircraft for around $89,000, including batteries. Mr Yu was inspired by radio-controlled electric planes. He says the E430’s operating costs work out at just $5 an hour—about one tenth of those for a typical two-seater combustion-engine light aircraft. With a 13.8-metre wingspan and a 42kW electric motor, the E430 has a flying time of up to three hours.

Topping up the batteries

Some of the tricks being used by carmakers to make electric vehicles go farther on a single charge are being copied by aviators. Propellers can, for example, be used to “windmill” in a descending glide, thus working the motor as a generator and topping up the batteries. This is somewhat akin to capturing the kinetic energy of an electric car via regenerative braking—a feature found in the Toyota Prius. Similarly, hybrid aircraft could use a small combustion engine as a “range extender”, running at a constant speed and driving a generator to power the electric motor or top up the batteries. (The Chevy Volt, GM’s forthcoming electric car, uses this trick.) And thin-film solar panels can be applied to the wings to generate electricity—enough electricity, hopes Bertrand Piccard, a Swiss psychiatrist turned adventurer, to fly a solar-powered aircraft around the world.

Mr Piccard’s Solar Impulse has a wingspan of just over 63 metres, similar to that of a giant Airbus A340 airliner, but its carbon-fibre fuselage has only enough room for one pilot. The idea is that Solar Impulse will take off and fly under its own power using electricity from the 11,628 solar cells covering the upper surfaces of its wings and tailplane. With an average speed of 70kph, it will spend the day climbing as its lithium-polymer cells (which account for about a quarter of its 1,600kg weight) are recharged. It will then descend slowly under low power throughout the night to conserve energy. The idea is to circle the globe in five stages, each taking four or five days of continuous flight (which is about as much as a pilot could endure). A prototype flew in April and test flights will continue this year, culminating in a 36-hour flight. A second aircraft will then be built for the record attempt in 2012.

Another way to fly farther is to swap the batteries for hydrogen and use that in a fuel cell to produce electricity. Hydrogen is lighter than air, but compressing it, storing it in tanks and adding all the necessary equipment starts to tip the scales towards heavy again. Nevertheless, the German Aerospace Centre has converted an Antares 20E, the DLR-H2, to fly with a fuel cell. Boeing has also flown a Dimona motor glider with an electric motor powered by a fuel cell. And United Technologies, which makes Sikorsky helicopters, has flown a large model electric-helicopter powered by a hydrogen fuel cell.

The main reason for these projects is not to develop primary power sources for jet-turbine powered aircraft, but to build small, highly efficient fuel cells that could be used to make greener airliners. The fuel cells could be used as auxiliary power units to produce electricity when an aircraft is on the ground, for example, allowing its engines to be turned off. They could power electrically driven wheels, letting an aircraft taxi without using its main engines. Fuel cells are also being used to power unmanned surveillance drones. A small, fuel-cell powered electric helicopter might be possible, reckons David Parekh, director of United Technologies’ research centre. But for anything much bigger, the energy density of aviation fuel still looks to be unbeatable.

Perhaps that will always be the case. But it is worth remembering that just over a century ago, heavier-than-air flight seemed an impossible dream. The Wright brothers proved otherwise on December 17th 1903, making a series of short flights in a flimsy, single-propeller plane. It had a petrol engine producing 12 horsepower (8.9kW)—not much more than each of the four 6kW electric motors on Mr Piccard’s Solar Impulse. Wilbur Wright travelled just 260 metres on the longest flight that day; whereas Mr Piccard is off around the world, under electric power, using no fuel at all.