SIEMENS

At first glance, this unique new aircraft makes an entirely unremarkable impression. From the outside, the DA36 E-Star motor glider is indistinguishable from its conventional sibling, which goes by the name HK36 Super Dimona. But a perceptive observer would have noticed a difference during the maiden flight of the two-seat motor glider, which took place on June 8, 2011 in eastern Austria, at the Wiener Neustadt air field: quiet operation and the absence of any aviation fuel odors.

That’s because this unique aircraft is the world’s first serial hybrid electric airplane, and it doesn’t use a combustion engine at all during takeoff and landing. A Siemens electric motor with an output of 70 kilowatts powers the propeller, drawing its energy from batteries mounted in the wings. Once the motor glider reaches its cruising altitude, the pilot switches on a small 30-kilowatt combustion engine. The sole purpose of this rotary engine is to provide energy for the electric motor through a generator, which simultaneously recharges the batteries.

That may sound like a complicated system, but it has very significant benefits. “Compared to the most efficient technologies currently in use, electrically powered airplanes reduce fuel consumption and emissions by 25 percent,” says Dr. Frank Anton, the initiator of electric aircraft development at Siemens Corporate Technology. “In addition, we prevent noise and emissions during takeoff and landing.”

How do electric drives save energy? “In conventional aircraft, the engines and turbines are designed for maximum output, but that’s needed only for the takeoff and the ascent,” says Anton. “As soon as cruising altitude is reached, about 60 percent of this output is sufficient.” Conventional engines are thus not only unnecessarily large and heavy, but they also usually run only at partial load, which is very inefficient — so they squander a large share of the energy contained in the aviation fuel.

Electrifying Takeoff. That’s nothing like what happens with the hybrid motor glider that Siemens developed with EADS and Austrian manufacturer Diamond Aircraft. The DA36 E-Star’s electric motor operates at an efficiency of close to 100 percent over a wide range of loads. After takeoff and ascent with the help of the battery, the rotary engine takes over the job of supplying power, and it can be run continuously at the most efficient operating point — if it sometimes produces too much power, the surplus can easily be temporarily stored in the batteries. The energy of the fuel is thus utilized in the most ideal manner.

“With the serial hybrid concept, we’ve separated the energy production and the drive, so that we can optimize both independently of each other,” says Anton. In the past, however, components for electric aircraft were too heavy to allow the technology to come into its own. But as a result of the increasing electrification of automobiles, manufacturers of motors, batteries, and power electronics have been making tremendous advances, which are now also benefiting the aviation industry. “The year 2011 is the year of electric flight,” says Anton.

In addition to the Diamond motor glider, the e-Genius also recently made its maiden flight, in May 2011. The e-Genius is an all-electric aircraft developed by the University of Stuttgart together with EADS and Airbus. The two-seater has a 60-kilowatt electric motor and batteries with a storage capacity of 56 kilowatt-hours. Thus equipped, the e-Genius completed a flight of 341 kilometers in June with an energy consumption equivalent to four liters of gasoline. And in addition to the DA36 E-Star motor glider, Le Bourget was also host to the “Cri Cri,” a miniature airplane powered by four electric motors that was developed by EADS, Aero Composites Saintonage, and the Green Cri-Cri Association. The Cri Cri can fly on electric power alone at a speed of 110 kilometers per hour for 30 minutes.

For aircraft manufacturers, electric airplanes are an interesting alternative because they help to reduce the adverse impact of air travel on climate — 2.2 percent of the CO₂ emissions caused by humans currently come from the jets and engines of aircraft. “With our prototypes, we’ve bought an admission ticket to electric flight,” says Peter Jänker, who leads an EADS team that is deeply involved in the field. “But the components need to become even lighter.”v

A good example of this are the batteries. Top-of-the-line lithium-ion batteries can now store about 200 watt-hours (Wh) of electrical energy per kilogram — aviation fuel holds 13,000 Wh, of which only about half can be used because of the poor efficiency of the turbines. New lithium-sulfur batteries could reach 2,600 Wh in a few years — today their storage capacity is 350 Wh/kg.

New Lightweight Design. Engines have to be made lighter too, since today’s electrical drives have an output of at most 1.5 kilowatts (kW) per kilogram of weight. “Our goal is ten kilowatts per kilogram,” says Anton, who has a pilot’s license himself and practices aerobatics. “And we’ve already come a big step closer to achieving that target since experts at Siemens Drive Technologies developed a motor with an output of 6.4 kW/kg — that’s four times today’s value.”

Lighter weight is achieved by means of a spectrum of measures. Mechanical components must be made not of metal but of lightweight carbon fiber, and because of the high number of poles in a motor, the magnetic field has to cover only short distances. This means that designers need less magnetic material, which is relatively heavy but doesn’t contribute anything to the drive. “With this patented design, Siemens is opening up a new chapter in the development of electric motors,” reports Swen Gediga of Siemens Drive Technologies.

Over the long term, these developments may mark the start of a new chapter in aviation, because the electrification of flying makes it possible to spatially separate energy production and drive. For instance, the combined weight of the combustion engine, the generator, and the battery add up to about 80 percent of the weight of this plane’s drive system, and they can be installed in the fuselage. The lightweight electric motors — which account for the remaining 20 percent of weight — would be mounted in the aircraft’s wings. “So there wouldn’t be any more heavy turbines hanging on the wings,” says Anton. “Instead, you could mount a number of swiveling electric motors with propellers at the wings, and some of them would only be used for takeoff. That would lead to a major reduction in energy consumption during flight.”