Solar Impulse, a 1-seater solar-powered plane,completed a 5695-kilometer bunny hop across the United States on 6 July. The feat deservedly got a lot of attention, but it’s just the tip of the iceberg: Electric flying has been a reality for quite some time, and it’s never been more practical.
Aviation is a slow-moving industry, but the daring designers of electric aircraft have made a lot of progress recently. During the past two years, as a number of key technologies—batteries, controllers, motors, and materials—have neared maturity and become easier to source at more affordable prices, there has been a minor boom of sorts in the offering of electric drives for small planes.
Because batteries still haven’t made the energy-density quantum leap that we all hoped for—gasoline’s energy density is still about 13 times that of the best lithium-ion batteries—most electric planes are self-launching gliders or motorized gliders. These have less stringent requirements in terms of range than standard aircraft, and their highly aerodynamically efficient airframes require less power to keep them airborne in all phases of flight. What follows is a sampling of the most innovative efforts.
The recent Solar Impulse flight has been both dismissed as a pointless exercise in renewable energy technology and praised as a daring Victorian-style adventure with exciting technology implications. Whichever side you take, the facts are that the people behind Solar Impulse pulled an impressive team together, managed to build a unique solar-powered airplane, and flew it first from Switzerland to Morocco and then across the United States.
Solar Impulse (tail number HB-SIA) is the first of two aircraft planned by the Switzerland-based group. As a test bed for numerous more or less innovative technologies and construction techniques, the HB-SIA is a collection of impressive numbers: It has the wingspan of a major airliner (about 63 meters), but its honeycomb-shaped, carbon-fiber-reinforced structure keeps the weight down to that of a small car (1600 kilograms); a fourth of that weight comes from the batteries. That low weight and its sleek design mean it can get by on four 7.35-kilowatt motors. It’s not built to be practical, and it isn’t: It flies at an average speed of 70 kilometers per hour, and it fears rain and winds that any student pilot in almost any plane could handle without too much worry.
Despite its pedigree, the fate of HB-SIA is sealed: The cross-U.S. trek will be its last. A new plane is being built in Switzerland—the HB-SIB. It will incorporate all the lessons learned over these past few years of intensive flight testing and publicity tours, in preparation for the real goal of Solar Impulse founders Bertrand Piccard and André Borschberg: a round-the-world flight in the spring of 2015. Godspeed, gentlemen.
With its 23-meter wing—designed by the Dutch aerodynamics guru Loek Boermans and built by Germany’s Lange Aviation—the Antares 23E is the most advanced electric airplane money can buy. The new wing profiles, along with the newly optimized wing/fuselage interface, give the Antares 23E a glide ratio of 60—meaning that from a height of 1000 meters the Antares 23E can glide for 60 km. The plane is an impressive machine, built with safety in mind: It’s one of the few production gliders equipped with a crash-proof cockpit, and the design and manufacturing process of each component has been thoroughly assessed by German aviation authorities. In fact, it’s the world first electric aircraft to receive certification by any aviation authority.
The aircraft’s motor and battery pack haven’t changed much from those of an earlier 18-meter-wingspan version. It has the same 42-kW brushless motor and the same SAFT VL41M lithium-ion batteries used on most European satellites and the F35 Joint Strike Fighter. However, by using a better battery-charging process, its engineers managed to squeeze 500 extra meters of climb out of it, bringing the maximum altitude to 3500 meters on a single charge. This albeit excellent piece of German engineering will set you back €205 000 (about US $260 000), onboard charger included.
Taurus Electro G2
Pipistrel is a smallish Slovenian airplane maker that has managed to climb to global relevance thanks to the relentless efforts of its founder, Ivo Boscarol. Its line of efficient, prizewinning light aircraft was completed in 2007 with the launch of the Taurus, a side-by-side-seat self-launching light glider. The Taurus G4 prototype, with its daring twin-fuselage design, won the prestigious 2011 Google Green Flight Challenge. Its commercial electric incarnation, the G2, launched the same year, features a 40-kW engine and a Li-ion battery pack that lets it climb 2000 meters on a single charge. What’s more, it comes with a solar-panel-covered trailer that can recharge the batteries in 5 hours.
Pipistrel should be credited for stating clearly that building an electric version of the Taurus was not an effort to go green but rather a technological improvement: The electric plane climbs faster and has a shorter takeoff run than does its petrol-powered version. As if this wasn’t enough, the enterprising Slovenian company has designed a highly streamlined new 4-seater called Panthera. The plane made its first flight this spring on a combustion engine, but Pipistrel plans to offer this sleek airplane in both hybrid and pure electric versions, with a 145-kW engine.
The Sunseeker family of solar gliders dates back almost 30 years. Developed and flown by the talented Californian engineer Eric Raymond, the Sunseeker I crossed the United States in the summer of 1990. Raymond, a protégé of solar flight legend Paul MacCready, took 21 flights and 121 hours in the air to make that historic crossing. After an upgrade of the Sunseeker’s electronics in 2009, Raymond decided to develop a whole new aircraft, this time with two seats instead of one. With solar panels carefully placed on the surface of the wings and on the tail, the Sunseeker Duo can cruise under the power generated by its own cells. It has some unique, elegant features, such as a sliding canopy that provides the best in-flight views in any class, swiveling tablet support for the next-generation avionics and maps running on the iPad, and—wait for it—seats that recline completely. So go ahead, stretch out and snooze if you like. Just make sure you have an alert copilot.
front electric sustainer propulsion
Another small but clever Slovenian company (we sense a trend), LZ Design had what now seems like a perfectly obvious idea. Rather than design complicated mechanisms for a retractable mast with an engine and a foldable propeller that sits in a bay behind the pilot, the company decided to put an electric engine in the nose of the glider and the batteries in the compartment that’s usually reserved for a small two-stroke engine, as is found in some of the world’s best self-launching gliders.
The brushless engine drives an ultralight carbon propeller (240 grams), whose blades fold elegantly along the curvature of the fuselage and fit snugly into thinly molded recesses when not in use. The drag of the hinge is as negligible as that of an insect wiper. (Yes, glider pilots are so obsessed with aerodynamic efficiency that they keep their wings clear of squashed bugs to reduce drag.)
A front electric sustainer propulsion system is installed on a few single-seaters, such as the light Italian glider Silent 2 Electro and the Lithuanian LAK 17. The company is searching for alternative energy storage options.
If you haven’t seen an Archaeopteryx up close, you could be excused for thinking that watchmaking is Switzerland’s finest example of workmanship. But the Archaeopteryx is a carbon-fiber jewel, so light it can actually be worn like a hang glider, and you can take off from the top of a hill simply by jogging a few steps.
With a ridiculously slow stall speed of 30 km/h, the Archaeopteryx can take advantage of minute amounts of rising air at low altitude, normally the domain of paragliders or hang gliders. But with the ability to glide 28 km after being dropped from an altitude of 1 km, it far outraces either of those flying machines.
Now the Archaeopteryx is about to get even better, with the addition of a removable electric propulsion kit. The drive can be retrofitted to existing gliders and adds only 16 kg for the battery pack and 7 kg for the engine, propeller, shaft, and electronics. The whole package, save for 200 grams of fixed slots and parts, can be removed in a few minutes without any tools.
But this isn’t a cheap machine, as it’s hand built to order; even though the final price for the electric version is not yet known, without the propulsion system it’ll set you back more than €70 000 (about US $93 000).
The funnily named Cri-Cri, a vintage 1970s design, is such a charming little airplane that it’s no surprise that there have been two different electric versions of it. In 2010, Airbus made one. The company added two engines for good measure, making it the smallest four-motor electric plane in the world by far. Shortly afterward, the Cri-Cri E-Cristaline—whose power train was developed by Electravia, a French company—became the world’s fastest electric aircraft, zinging about at 262 km/h, with Frenchman Hugues Duval at the controls. Duval went on to beat his own record the following year, bringing it to 283 km/h. That record was broken by Chip Yates in 2012, but it must be said that it was set at only 75 percent of the power available. The limiting factor, as it happens, is the airframe’s maximum design speed of 290 km/h. The Cri-Cri drapes over the pilot like a Savile Row suit: It has a wingspan of just 4.9 meters and a paltry empty weight of 78 kg. Despite coming in an extremely small package, it packs a surprising punch: It’s fully aerobatic, it’s a hoot and a half to fly, and it’s a guaranteed crowd pleaser whenever it lands. Needless to say, it doesn’t take up much space in the hangar.
After experimenting with the Cri-Cri, Airbus came up with a new all-electric design called the E-Fan. The product of Didier Esteyne, the same French engineer who electrified the Airbus Cri-Cri, the E-Fan hasn’t flown yet. It looks promising, though. Its main feature, noticeable even to casual observers, is a couple of electric ducted fans in lieu of normal propellers. Capable of aerobatics, the E-Fan is geared toward the training aircraft market, where the typical mission profile (lots of shorter flights from the same base rather than longer point-to-point cruises) suits the limitations of the batteries. While its final specs haven’t been fixed, it shouldn’t weigh more than 550 kg at takeoff (in line with the advanced light-aircraft regulations in the United States and some European countries). It will likely have two 30-kW engines and two lithium polymer battery packs made of 120 cells from the industry leader Kokam, for a total of 250 volts.
Yuneec E430 and ESpyder
Founded by Shanghai businessman and aviation enthusiast Tian Yu, China’s Yuneec International has a long and eventful history. Yu began designing and assembling a full-size light electric airplane, the V-tailed E430, in his own factory, which made remote-control toy airplanes. The first iteration of the E430 lacked the refinement needed to make it into more mature markets—there aren’t many light airplanes flying in China, due to airspace constraints—so Yuneec went on to acquire the production molds and designs of petrol-powered aircraft, mainly from Europe. Eventually, Germany’s Viva, the United States’ Spyder, and Slovenia’s Apis all found their way to China’s east coast and a new life, presumably at more affordable prices.
Yuneec cultivated its international ambitions when it planned a prototype to enter the CAFE Green Flight Challenge, but disaster struck when the German designer leading the project died on a test flight. After a period of introspection, the company spun off a U.S.-based commercial arm called GreenWing International to market its most advanced model, the ESpyder. A proven design, the single-seater ESpyder has finally matured into a production electric ultralight, with a declared endurance of one and a half hours. It’s one of the cheapest electric planes on the market, and its airframe is great for newly licensed pilots who want to go electric while flying a basic three-axis aircraft. The plane flies slowly—its maximum-level cruising speed is 110 km/h—but it’s fun, and its battery pack can be recharged in 2 to 3 hours. GreenWing has a promising pipeline, with a revamped E430 set to make its first flight in the next month or two.
Long-ESA/Flight of the Century
Chip Yates isn’t one for half measures. He holds an electric motorcycle speed record, and—although he had no pilot’s license—he decided he wanted to break the electric airplane speed record as well. He managed to get the license in two months. After completing the record flight, he made a dead-stick (engineless) landing when a battery developed a problem in midair. For this, he used a modified Long-EZ, a canard design by famed SpaceShipOne engineer Burt Rutan. (Yates will use the same plane for an upcoming record-breaking attempt next October.) Propelled by an enormous 192-kw-equivalent water-cooled DC brushless motor, it’s claimed to be the world’s most powerful electric aircraft flying.
Yates isn’t one for nuance either. By naming his venture the “Flight of the Century,” he doesn’t leave you guessing as to his goal. He wants to cross the Atlantic à la Lindbergh (literally so, with the same constraints) but in an electric aircraft. It would be with a new design, which is still on the drawing board. Since the energy densities of currently available batteries aren’t up for such a feat, Yates needed to get creative. His team came up with a solution dubbed Infinite Range. The technically ambitious plan is the electric equivalent of midair refueling, with a twist: the in-flight battery swap. Battery packs will be housed in autonomous unmanned aerial vehicles (UAVs) that, once depleted, can be jettisoned and glide down to recharge stations, either on the ocean or on the ground. An internal battery will sustain the aircraft, while a freshly charged battery pack will fly up to and connect with the mother ship.
A variation of this plan, for when the UAV-based approach proves impractical (and really, when doesn’t it?), specifies a bigger battery pack whose independent cells would be jettisoned midflight. Reducing the weight of the airframe by a fair chunk would increase the range by a significant percentage. Both solutions require impressive logistics to ensure the availability of battery packs on the flight path and the recovery of spent ones. (Yates operates under a strict “No Battery Left Behind” policy.) There are significant challenges to overcome, but Yates is a man with a plan and wants to get there in stages. He’s using the Long-ESA as a test bed for another in-flight recharging technique, a structure on the plane’s nose that would connect to another plane in flight for recharging, similar to midair military jet refueling.
EuroSport Aircraft’s Crossover
Presented this year at Aero Friedrichshafen, light aviation’s leading exhibition, EuroSport Aircraft’s Crossover is an innovative design from Portugal, a nation without much of an aviation legacy. Perhaps unburdened by tradition, its designers have been bold. The Crossover includes a number of clever features: Its two 3-phase 40-kW motors are mounted on retractable booms that can fold back into the fuselage. They do so, rather unusually, from the lower side of the fuselage rather than from the top. The engines are the same make and power found in the Pipistrel Taurus. Once this beautiful side-by-side twin-seater has reached a comfortable gliding altitude, both the landing gear and the slender engine booms can retract, leaving the Crossover with an extremely polished line, including a world-class wing-to-fuselage junction, which reduces drag.
The highlight of the plane’s design could have soaring pilots the world over salivating: EuroSport Aircraft says the Crossover can modify its wingspan in flight, from an agile, speed-friendly 9.6 meters to a more efficient, gliding-friendly 15 meters. Wingspan—wing surface, really—affects the wing load, and it’s one of the main factors that influence the handling of an airplane. (The wing load, which is the weight of the aircraft in flight divided by the surface area of its lifting parts, is usually lower in slower, easier-to-fly airplanes and higher in faster, more difficult aircraft.) It’s obviously difficult to change the wing surface in flight, so glider pilots have traditionally used a reverse approach to control it: They increase the weight of the plane by loading water—lots of water—into wing tanks. A racing glider today might load as much as 200 liters of water, bringing the wing load close to 60 kilograms per square meter. Because the increased weight reduces the ability to climb, on days with poor weather conditions the pilot can easily jettison the water. The Crossover’s ability to extend or retract the last section of each wing by about 2.5 meters in flight is really novel.
Because the Crossover is rated for a relatively high maximum takeoff weight of 600 kg, it’s able to carry a lot of battery weight: up to 180 kg, which gives it a very respectable 3-hour range. As the packs are modular, each weighing about 60 kg, the pilot can decide how many of those to load depending on the mission profile of the day.
A shorter version of this article originally appeared in print as "Five Electric Planes to Watch."
A correction to this article was made on 05 August 2013.
About the Author
Michele Travierso, a writer based in Shanghai, says he got the “airplane bug” when he was 4 years old. Now a licensed glider pilot, he has been following the electrification of such aircraft intently, so “10 Electric Planes to Watch” was a perfect fit as his first assignment for Spectrum. The designers, engineers, and enthusiasts involved in this effort are “visionaries and dreamers,” says Travierso. “Everybody wants [electrification] to work, because it could be so much better than the gasoline engine.”