Austria-headquartered Diamond Aircraft Industries, in collaboration with German automation conglomerate Siemens, has completed the first flight of a jointly developed multi-engine hybrid electric aircraft at Diamond Aircraft's headquarters in Wiener Neustadt, Austria.

Diamond Aircraft has developed a multi-engine hybrid electric aircraft that should address issues inherent if air traffic meets annual growth projections of up to five per cent. With today's technology, this growth would cause an increase in emissions and a depletion of fossil fuel reserves. So in order to keep future flying sustainable, the company is developing efficient drive systems that will contribute to the reduction in fuel consumption and aid in the protection of the environment.

The other objectives of the joint project are to develop an aircraft that is more efficient and that has a lower noise footprint. The hybrid powertrain will allow for full electric take-off capabilities without the added noise and pollution of a combustion engine.

To demonstrate this serial-hybrid technology, a DA40 aircraft was reconfigured to support a hybrid electric powertrain. In this system set-up, one combustion engine is powering two independent electric drive systems each consisting of one motor, battery and inverter. Two electric engines have been added on a forward canard, that combined can generate 150kW of take-off power. The diesel generator is located in the nose of the aircraft and can provide up to 110kW of power. Two batteries with 12kWh each are mounted in the rear passenger compartment and act as an energy storage buffer. With a dedicated power lever, the pilot can control the energy flow between the generator, batteries and motor. The pilot can select either pure electric mode (generator off), cruise mode (generator provides all power to the motor) or charge mode (generator charges the batteries). Under pure electric, the aircraft has an endurance of approximately 30 minutes. The hybrid system extends this to five hours.

This first flying testbed allows the development team to gain experience with a hybrid propulsion system built up of one genset and multiple electric motors, and a system architecture with distributed drives. The redundant architecture of the propulsion system, with two electric motors operating independently, increases aircraft safety. Moreover, energy production, energy storage and thrust generation are locally separated; this concept of distributed propulsion should enable aircraft designers to think in new directions at the very beginning of the process of design.

“This is the first serial-hybrid electric plane in the world with two electrically powered free-stream propellers and one combustion engine. A distributed propulsion architecture opens entirely new possibilities for the design of highly efficient aircraft; and we have now proved its technical feasibility,” says Siemens executive vice president eAircraft Dr Frank Anton.

The project started in 2013 under the supervision and finance of the German Federal Ministry for Economic Affairs and Energy (BMWi) and the Austrian Research Promotion Agency. It is split into two main work packages. Siemens has been responsible for the design and development of the electric powertrain. This was done within the framework of the German Aeronautical Research Programme LuFo. Diamond Aircraft was responsible for the reconfiguration of the aircraft and installation of the hybrid-electric powertrain. For this, it partnered with techno-economic and environmental risk assessment TERA Group and the Johannes Kepler Universität Linz (JKUL). The TERA group preformed the design and dynamic analysis of the generator drive unit. JKUL preformed the aerodynamic analysis and simulated the flow interaction between the propeller and airframe to achieve a high overall efficiency. The consortium partners in the hybrid electric multi engine plane (HEMEP) project play a pioneering role in the field of aviation in the field of electric drive systems, thereby enabling participating companies to become technology leaders for the future.

During the 20 minute first flight the aircraft demonstrated all modes of operation: pure hybrid, charging flight and full electric flight. In this last configuration the aircraft was able to fly pure electric and produced zero emissions. A full electric takeoff was demonstrated which lead to a significantly lower noise footprint.

Diamond Aircraft Industries head of flight test Ingmar Mayerbuch piloted the aircraft and says: “The first flight exceeded all our expectations. The combination of the hybrid powertrain and the configuration of the aircraft is just perfect. We reached 130 knots at medium power output and climbed to an altitude of 3,000 feet.”

The objective of future flight tests will be to determine the exact efficiency increase achieved in comparison to similar non-electric aircraft. Also the noise footprint will be determined. Future tests will also gather knowledge on the practical operation of hybrid aircraft. Different hybrid modes of operation will be tested to determine their effect on efficiency, range, and energy consumption.

“For us, serial-hybrid electric propulsion systems and distributed propulsion architectures are the key to a more sustainable flight future in higher power classes as well,” adds Siemens' Anton. “The Diamond flying testbed will help us to understand the requirements for these new propulsion technologies and to be prepared for the challenges of larger-scale applications.”

This is not the first hybrid electric aircraft developed through a collaboration between Siemens and Diamond. In 2009, the successful implementation of a hybrid powertrain has already been demonstrated in the DA36 E-Star. This initial project showed the opportunities this hybrid technology has in aviation. Lessons were applied over subsequent years to improve the propulsion systems, and this learning step lead to the success of this later project.

The first hybrid electric aircraft developed by Siemens and Diamond in 2009 was the DA36 E-Star. This initial hybrid powertrain project showed the opportunities this technology could bring to the aviation industry, and the subsequent improvements to propulsion systems lead to the success of this later project.