Imagine sliding open the door to your hangar, unplugging the power cable snaking from your airplane to a quick-charging station on the wall and taking to the sky in a sleek four-seater powered only by the smooth, quiet, nonpolluting whir of an electric motor connected to rows of lithium battery packs nestled in the wing. It’s a techno-utopian fantasy right now, of course, but this is precisely the vision many in aviation believe needs to become a reality if we are to have any hope of breaking our dependency on increasingly scarce and expensive fossil fuels.
One encouraging sign that might help us edge closer toward this goal is what’s happening on the electric-car front. Long the domain of technophiles and eco-warriors, electric vehicles are coming on strong. As more car buyers show up at dealer lots kicking tires, automobile manufacturers are responding with a growing number of plug-in hybrid and pure-electric models. So far, the Tesla Motors Model S and Nissan Leaf lead electric-car sales — meanwhile, almost every carmaker wants in on the action, as the market evolves to include a wide variety of models, including the Chevy Volt and BMW i3, plus others from Ford, Volkswagen, Mitsubishi, Honda, Toyota, and even Bentley and Rolls-Royce.
The key to bridging the divide between airplanes powered by cylinders, pistons and injectors and those running on electricity depends largely on advances in battery technology — the all-important fuel source for future electric aircraft. Improvements in specific energy, power density and durability of tomorrow’s batteries must be realized before electric airplanes can even come close to matching the performance of their piston-engine counterparts. The good news is that growing demand for electric cars has put battery technology at the forefront of research initiatives in Silicon Valley and investment bets on Wall Street. Suddenly, building a better battery has become an obsession for some of the tech world’s brightest minds, as carmakers seek to make lithium-polymer batteries cheaper, lighter and longer-lasting.
Without the pioneering work underway in the auto and consumer electronics industries, the dream of a battery-powered electric airplane would probably be limited to ultralights and motor gliders capable of remaining aloft for brief periods before having to return to the ground to swap out battery packs or spend time plugged in to recharge. It’s doubtful we’ll see an electric-powered airliner anytime soon (check out p. 3 for Airbus’s contrarian view), but with improvements in battery technology it’s likely we’ll soon be flying four-seat, propeller-driven electric airplanes with the range to take us at least a few hundred miles from home base. If it happens, that will be a game changer.
Besides more powerful batteries, one of the big breakthroughs we’ll need to see before electric airplanes can deliver equivalent utility compared to today’s piston-powered aircraft is the ability to recharge batteries quickly or swap them out with fresh battery packs. After all, even if the battery technology existed to allow us to cruise for, say, four hours before landing to recharge, we’ll be in a bind if we then have to plug an extension cord from a wall outlet into our airplane and wait a day for it to charge up.
This will probably require new infrastructure including high-power, fast-charge stations at GA airports across the country. It’s an idea that could gain faster acceptance if the stations can be collocated with electric-car chargers similar to the Tesla Supercharger stations popping up in strategic locations along busy motorways. Presumably, the airplane chargers would need to be located on one side of the airport fence and car stations on the other — but that’s better than plugging into a regular wall outlet, which means waiting ages to replenish high-capacity lithium batteries.
But that’s getting well ahead of ourselves. Before we can even start the discussion about creating a nationwide fast-charge network for a fleet of mythical electric airplanes, manufacturers need to start building and selling the first electric airplanes.
Several companies are progressing toward this goal, though only one product — Pipistrel’s two-seat Taurus Electro G2 — has hit the market so far. Projects that appear to have the greatest chance of succeeding involve small one- and two-seat experimental and light-sport aircraft with estimated endurances of 30 minutes to perhaps two hours. That’s barely enough juice to justify venturing very far from the traffic pattern — more importantly, the cost savings of going all-electric versus sticking with gasoline will be hard to rationalize given the fuel economy of small LSAs. When you throw in the purchase price premium of going all electric plus the many thousands of dollars you’ll spend on eventual battery replacement, the calculus in support of electric airplanes becomes murkier still. At least you won’t have to pay to overhaul your engine.
Producing an electric airplane that can perform a similar mission to, say, a Cessna Skyhawk will be difficult without some key technological breakthroughs. The current lithium-ion and lithium-polymer batteries favored by carmakers for electric vehicles, for example, are heavy, lack the energy density of liquid fuel (although gasoline converts a lot of its energy to heat), and can’t match the range or payload capability of an internal combustion engine without designers turning to a hybrid approach or perhaps using solar cells atop the wing and fuselage. But there are drawbacks to both of these avenues. A hybrid drive including both an electric and internal combustion engine would be heavy. Photovoltaic solar cells — at least those that exist now — lack the recharging ability to keep our theoretical Skyhawk flying for very long as the batteries drain in flight.
China-based Yuneec International eventually hopes to bring the all-electric Greenwing GW430 to the United States as a kit.|
These technological hurdles are a part of the reason why we haven’t seen faster progress with the planned electric-powered Skyhawk on the drawing board at Bye Energy. It’s also why the solar-powered Solar Impulse has a wingspan of an incredible 208 feet. A big wing is important for two reasons: It provides more surface area for lots of solar panels, and the longer wingspan requires less power to lift a given weight. For the Bye Energy Skyhawk to succeed, more powerful and longer-lasting batteries will be needed. For solar-powered airplanes like the Solar Impulse, the breakthrough will come only when improved-efficiency solar cells allow for faster aerial recharging. Until we see power-to-weight ratios approaching the equivalent of a 200 hp recip, electric-airplane applications will likely be limited to small aircraft carrying only one or two people. Even Bye Energy’s proposed electric Skyhawk would carry just two people with an endurance of two hours.
Even if you’ve been keeping track of developments on the electric-airplane front, you might not have a good feel for where we are in terms of products in the pipeline. A surprising number of projects are underway, a few of them with backing from heavy hitters like Airbus and Boeing. Makers of unmanned aerial vehicles, such as the high-altitude solar QinetiQ Zephyr, are also exploring electric power, which could help spur development of civil aircraft down the road. (Although, UAVs don’t suffer from one of the unaddressed shortcomings of electric aircraft, namely how to supply cabin heat in flight, an especially important consideration in an airplane that can fly efficiently up high because its electric motor and batteries don’t need oxygen.)
LSA-maker Sonex has proposed an electric version of its Waiex (pronounced why-ex) that would carry two people using a brushless DC-Cobalt motor and lithium-ion and lithium-polymer batteries. The motor is said to weigh just 50 pounds, but the aircraft would be 75 pounds heavier than the gasoline-powered version because of the weight of the batteries. They would tip the scale at around 200 pounds total yet provide the equivalent operating time of only a couple of gallons of gasoline. Sonex, by the way, plans to house the batteries in “safety boxes” that would direct a fire or battery explosion away from occupants. As you probably know, fire is a major worry when dealing with lithium batteries in a crash — so too is the gasoline we carry in our wings.
Not surprisingly, much of the pioneering research into electric aircraft is underway in Europe, where gasoline prices are exorbitantly high and there is considerable pressure to reduce carbon emissions. Pipistrel in Slovenia has been the most proactive in the electric arena. The Taurus Electro G2 is claimed to be the world’s only electric airplane in serial production. Built on a glider airframe, the two-seater is said to climb faster and perform better at altitude than the version equipped with a Rotax engine. Power comes from a 40 kW (54 hp equivalent) permanent magnet AC motor that Pipistrel says can get the 675-pound Taurus Electro off the ground in 525 feet. At altitude, a push of a button retracts the propeller, transforming the Taurus into a pure glider. Its lithium-polymer battery packs can be recharged in about three and a half hours with the standard battery or five hours with larger packs. Pipistrel also sells an enclosed trailer with a solar-charging system for completely free flying.
The one-off Taurus G4 serves as a test bed for the under-development Pipistrel Panthera electric four-seater.|
Realizing that a motor glider, while fun, isn’t a practical everyday airplane for most pilots, Pipistrel has announced plans to offer versions of its slippery Panthera four-place single with hybrid and pure-electric power options — still, that won’t happen until improved battery technology is available. The goal, the company says, is to design the Panthera Electro with a 145 kW (around 200 hp equivalent) motor that will have the battery capacity to carry the airplane on trips of 215 nm before needing a recharge. The hybrid version, meanwhile, would include a “range-extending generator” — possibly consisting of a small gasoline engine used only for battery recharging — making longer trips possible.
Another European-borne electric airplane under development is the Elektra One from PC-Aero in Germany. The single-seater has the goal of providing enough battery/solar power for three hours of continuous flight. That would give it a similar range to the Panthera Electro but at a more leisurely 85-knot cruise speed. PC-Aero is aiming to certify the Elektra One as an ultralight in Germany, and hopes to tap into the U.S. market at some point. Electric-powered LSAs, as you might know, can’t fly in the United States yet, but the FAA is said to be working on rule changes that eventually will permit electric-powered production airplanes here as well.
A separate all-electric project out of Germany is the e-Genius, a two-seat motor glider powered by a 60 kW powertrain that has secured financial and technical backing from Airbus. Its designers say the e-Genius has a maximum takeoff weight of 1,874 pounds and is capable of flying 215 nm at a cruise speed as high as 126 knots before needing a charge up. Developed at the University of Stuttgart, the e-Genius claimed a distance record of 183 nm for electric-only aircraft as part of the NASA 2011 Green Flight Challenge. Future tests will seek to beat that distance.
Powered only by sunlight, the Swiss Solar Impulse can fly day or night by storing energy in its rechargeable lithium batteries.|
**Of course, that’s nothing compared with the vast stretches the Swiss-made Solar Impulse can cover. The Impulse’s designers are building toward a round-the-world flight next year that will take an estimated 20-25 days to complete with five stops along the way to swap pilots. The original version of the Solar Impulse, piloted by Swiss adventurer Bertrand Piccard, completed several long-distance flights, including a 26-hour journey in July 2010 that included nine hours of flying at night. With a wingspan similar to that of an Airbus A340, the wing structure is made of a lightweight carbon honeycomb with more than 11,000 photovoltaic cells on top. Under the wing are nacelles that house the Solar Impulse’s lithium-polymer batteries, 7.5 kW motor and twin propellers. The version of the craft that will attempt to circumnavigate the globe will be larger still, with a wingspan about the length of an Airbus A380’s.
While solar-powered giants like Solar Impulse might have practical uses as unmanned aerial platforms, they won’t make much of an impact for those of us who clamor for something that resembles the airplanes we now fly but with quiet electric motors instead of rumbling pistons under the cowl. The closet we’ve seen flying so far is the Yuneec Greenwing GW430 out of China. This two-seat LSA with a V tail and composite airframe is projected to have an endurance of two hours plus a 30-minute reserve and to cruise at 80 knots. The Yuneec PowerDrive 48 propulsion system includes a 48 kW motor (64 hp), controller and lithium battery packs that are designed for quick swap out. First flight of the airplane occurred in 2009, when it was called the Yuneec e430. That prototype was named the winner of the Lindbergh Prize for electric aircraft at EAA AirVenture in 2010. Work continues on the project, with the goal of eventually bringing the Greenwing GW430 to the United States in kit form. GreenWing International is the California company formed to handle U.S. sales of the GW430 and a smaller ultralight called the eSpyder.
Several other interesting electric-airplane projects are in the works, including an electric version of the tiny Cri-Cri single-seat twin that is able to stay aloft for 30 minutes. There was also the world-record speed dash set in 2012 by air racer Chip Yates in his Flight of the Century Long-EZ. The run lasted all of 16 minutes and reached a top speed of 175 knots before the batteries died and Yates had to make a dead-stick landing back at Inyokern Airport in the desert outside Los Angeles. Boeing, meanwhile, is supporting a project that added lithium batteries and a 200-pound hydrogen fuel cell to a Diamond HK36 motor glider as a replacement for its 80 hp Rotax. That airplane first flew successfully in 2008. Another interesting project involving an HK36 is the Diamond E-Star developed jointly by Siemens, EADS and Diamond Aircraft. The project uses a serial hybrid drive that turns the airplane’s prop with a Siemens 70 kW (94 hp) electric motor and generates power to recharge the batteries with a Wankel rotary engine.
Carmakers have mastered hybrids, and they are also investigating hydrogen fuel cells — here again is an area where advances in the automotive industry could serve as a vanguard for aviation technology. With the electric car continuing its long-awaited ascension, batteries will improve. It’s too early to tell whether electric-powered airplanes will be able to hold their own against gasoline and jet fuel-burning diesel models, but it might not be too far of a stretch to say that the era of the electric LSA is very nearly upon us. If battery technology advances far enough, maybe we’ll even get to take that sleek and nearly silent four-seater on cross-country trips in eco-friendly and hyperefficient style.
Dreaming of an Electric Airliner
Airbus parent company EADS unveiled the VoltAir concept in 2011 as a forward-looking view of airliner travel 25 years from today. As described, VoltAir’s two next-generation lithium-air battery banks would power two highly efficient super-conducting electric motors, which would drive two coaxial, counter-rotating shrouded propellers.
The batteries would be housed in the lower front section of the VoltAir, where they could be removed and replaced just like baggage at the terminal. Recharging would be accomplished when the batteries were out of the aircraft, so airplanes would simply land, swap out their depleted batteries for fresh ones and take off again. Not only would this arrangement make turnaround times similar to those of conventional refueling, but it would also reduce the weight and technical complexity of the airplane, EADS says. Not to mention the VoltAir would be exceptionally quiet.
Lithium-air batteries rely on oxidation of lithium to produce their current flow. First proposed in the 1970s, the technology is getting a second look because it holds the potential of providing five to 15 times the energy density of lithium-ion batteries.
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