In the summer of 2016, an electric-powered aircraft - a two-seat Extra 330LE aerobatic plane - took off on its maiden flight, demonstrating Siemens' newly designed SP260D motor, which delivers an impressive 340 hp (260 kW) while weighing only 110 lbs (50 kg). (By comparison, a typical Cessna four-seater has an engine that produces less than 200 hp.) Unlike internal combustion engines, which only run efficiently within a narrow range of RPMs, an electric motor performs well at various speeds, allowing the SP260D to directly drive a propeller at 2500 RPM, delivering 1000 Nm of torque without the need for a gearbox.

Siemens tested the motor on an aerobatic plane in order to demonstrate its ability to perform under maximum acceleration conditions. The SP260D - a 95% efficient AC induction motor with a variable speed drive - is designed for use in small, fully electric aircraft and also in larger, hybrid airplanes.

Designing a High-Performance Motor

Achieving a power-to-weight ratio of five kW/kg in a large electric motor is no easy task, so Siemens approached it from every conceivable angle. They began with some of their existing motors, testing all of the components individually, and reducing materials whenever possible. Engineers found that the aluminum endshield, the part of the motor housing that supports the bearing and protects the motor's internals, was quite heavy.

To reduce its weight, Siemens developed a sophisticated computer model of the endshield. The software represents the endshield as 100,000 separate parts and then simulates each element's performance under various force conditions. At that point the algorithm conducts millions of trial-and-error simulations, eventually finding components that can be eliminated or reduced. The process helped engineers redesign the endshield, turning it into a filigree (lattice-like) structure with the same performance at less than half the weight. They're also working on a prototype made from a carbon-fiber composite, which will reduce the weight by another factor of two.

Inside the motor, the rotor's permanent magnets are configured into a Halbach array, which produces a stronger magnetic field with less material. The stator is made of an easy-to-magnetize cobalt-iron alloy. The motor's windings are surrounded by a special cooling liquid that conducts heat but not electricity. Lead engineer Dr. Frank Anton said, “We use direct-cooled conductors and directly discharge the loss of copper to an electrically non-conductive cooling liquid — which in this case can be, for example, silicone oil or Galden.”

Hybrid-Electric Airplanes

Siemens is working with Airbus to develop a hybrid-electric aircraft that uses a fuel-powered generator to provide energy to the electric motors, allowing the generator to run at its optimal RPM all the time, while the efficient electric motors vary their speed as needed. On-board batteries would deliver the extra power needed for take-off. Such a system could result in a 25% decrease in fuel consumption. In a couple of decades, we could be flying in an 80-passenger hybrid-electric aircraft.

Here's Dr. Anton describing Siemens' vision of electric flight:

Images and video courtesy of Siemens

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