Another step toward electric-powered air travel has been taken with the testing of a hybrid-electric powertrain for aviation. Slovenian light-aircraft manufacturer Pipistrel successfully fired up a 200 kW propulsor developed to deliver power equivalent to a typical general-aviation piston engine and which can run in three modalities: electric-only mode using batteries, generator-only mode or hybrid mode combining both power sources.

All of the powertrain components were developed by Siemens as part of the HYPSTAIR project to develop and validate hybrid propulsion system components to enable electric aircraft that can extend beyond the current range for such craft while reducing pollutant and noise emissions. Project HYPSTAIR partners are Pipistrel d.o.o. Ajdovščina, Siemens AG, MBVision, University of Pisa and University of Maribor.

Diagram of Siemens' hybrid serial propulsion system architecture. Image credit: Siemens.The drive motor, delivering 200 kW takeoff power and 150 kW continuous, and the generator, delivering 100 kW, feature a power density exceeding 5 kW/kg and dual windings with four power controllers for reliability. A further innovation is the human-machine interface, designed to simplify the operation of the powertrain: a single lever with haptic feedback is used to apply power and an integrated cockpit display to monitor the powertrain status and performance.

Following laboratory testing of components and the integration on a representative airframe at Pipistrel's aviation factory in Slovenia, the power-up tested all propulsion modes at low and high powers, driving a specially developed five-blade, low-rpm, low-noise propeller. Tests of takeoff power were performed using combined output of the generator driven by a turbonormalized engine and the high-performance battery custom developed to support high discharge rates.

“Siemens is developing electric drive systems with the highest power-to-weight ratio for aircraft propulsion," says Frank Anton, head of eAircraft and the initiator of electric aircraft development at Siemens AG. "Innovations developed for the HYPSTAIR hybrid-electric powertrain will be instrumental in making aviation more sustainable in the long run. As electric drives are scalable, we can expect that in the future larger aircraft will [also] use electric propulsion."

In the coming months, testing will continue to simulate typical mission profiles covered in the performance study and to validate the hybrid drive concept and performance.