At the COP26 UN Climate Conference held recently in Glasgow, numerous countries reaffirmed the target of making air transport climate-neutral by the middle of this century. One important step in this direction will be the development of hybrid-electric propulsion systems for aircraft on feeder and regional routes. The DLR 'Do 228' research aircraft D-CEFD will be used to test electric propulsion technologies. It was handed over to the German Aerospace Center (Deutsches Zentrum fuer Luft- und Raumfahrt; DLR) by aircraft maintenance, repair and overhaul specialists General Atomics AeroTec Systems (GA-ATS) in mid-November 2021. Preparations are now under way at the DLR site in Oberpfaffenhofen to upgrade the aircraft into a flying technology platform. The first extensive project is being launched in conjunction with industry partner MTU Aero Engines. The research partners will replace one of the two conventional engines with a 600-kilowatt electric propulsion system powered by a fuel cell. This is to be developed by the middle of this decade and tested on the aircraft in flight.

"A fundamental examination of the suitability and efficiency of various propulsion options in combination with sustainable energy sources for all classes of aircraft is a prerequisite for achieving zero-emission air transport. All the technologies on which this objective is based need more research and development, and DLR is making its contribution," says Markus Fischer, DLR Divisional Board Member for Aeronautics. "The concept of combining electric propulsion units with fuel cells has a lot of potential for feeder and regional aircraft. We hope that the new 'Do 228' D-CEFD test platform will mark a significant advance for this technology concept." The use of electric propulsion systems and fuel cells calls for systems with a high level of performance, reliability and safety. The new research aircraft now makes it possible to test a wide variety of components and entire propulsion systems in detail under real operating conditions.

Speaking on behalf of MTU, Barnaby Law, Chief Engineer in the Flying Fuel Cell Department, said: "A liquid hydrogen and fuel cell system can power the aircraft of the future, all with no emissions apart from water. And that is precisely our goal – zero emissions." For an engine manufacturer like MTU, the development of an airworthy fuel cell represents a great opportunity: "The experience and data gained in this way, including in regulatory matters and airworthiness certification, are crucial for our further product development."

A highly reliable aircraft with striking livery

The 'Do 228' is renowned as a highly reliable aircraft within its category. Prior to being handed over to DLR, the aircraft was used for passenger flights for many years. It was completely overhauled for its new mission and given striking paintwork by General Atomics AeroTec Systems, in accordance with DLR's specifications. The aim was for it to be instantly recognisable. "Our 'Do 228' aircraft is already used worldwide to protect countries from coastal pollution using precise reconnaissance and surveillance technology," explains Harald Robl, Managing Director of General Atomics AeroTec Systems. He adds, "The DLR and MTU project represents an opportunity to make our own contribution to environmental protection in the future, using new propulsion technologies. In doing so, we can help to combat the climate crisis." General Atomics AeroTec Systems will continue to support the progress of the project as a technology partner.

Electrical powertrain with effective cooling

In the joint DLR-MTU technology project, the research aircraft will initially be used to test individual system components. Later, it will be extensively modified to test the entire hybrid-electric propulsion system developed by MTU. The aim is to develop an electric powertrain that is suitable for air transport, using liquid hydrogen as the energy source, and all the necessary additional components such as the cooling system. During the extensive reconstruction of the aircraft, the left-hand turboprop engine will be removed and replaced by an electric motor. The power required for the motor will be provided by fuel cells housed in the fuselage. This also contains a tank that provides the hydrogen required to operate the fuel cell. Fuel cells have no emissions, apart from water, and are highly efficient. However, they generate a lot of heat, which is why developing an efficient cooling system is also critical. The project partners are aiming for the 'Do 228' D-CEFD flying laboratory to have its first flight by the middle of the decade. Before that, extensive ground tests and preliminary trials will need to be carried out.

Collaboration for the first electric flight

The management of the planned research project with the 'Do 228' D-CEFD lies with DLR, which is providing the research aircraft and will conduct the flight experiments. DLR is also responsible for integrating the powertrain into the new research aircraft. DLR's expertise feeds into numerous areas of research, including aerodynamics and aeroelasticity. With this, DLR is contributing its overall systems expertise as an industry partner to the joint technology project. MTU has been tasked with developing the entire hydrogen-powered fuel cell powertrain, including the liquid hydrogen fuel system. All of the work and integration will be carried out jointly and in close coordination. Up to 80 experts will be involved.

The procurement of the 'Do-228' D-CEFD has been partly funded by the Bavarian State Ministry of Economic Affairs, Regional Development and Energy. The planned research work is currently financed by funding projects under the aviation research programme of the German Federal Ministry for Economic Affairs and Energy. Other funding applications have been made and are being processed.

DLR – research for climate-neutral air transport

The consequences of climate change demand action for climate-neutral air transport. This involves new technologies that will also ensure global mobility in the future. With its 25 institutes and facilities in the field of aeronautics research, DLR is driving this change forward with technologies for sustainable, environmentally compatible flight. Expertise from DLR's research programmes in space, energy and transport will also play an important role in this.

DLR has systems expertise in aeronautics research and sees itself in the role of an architect. DLR's goal is 'emission-free air transport', in order to achieve the climate targets that have been set. In doing so, the results of research must flow directly into the development of new products.

There is a considerable need for research and development on the path to climate-compatible air transport, which requires continuous funding and support. Much of this needs to be researched at a fundamental level, tested in practice and approved. DLR can do this with large-scale facilities such as its research aircraft, propulsion demonstrators and large-scale computers. In 2020, DLR published the white paper 'Zero Emission Aviation' together with the German Aerospace Industries Association (Bundesverband der Deutschen Luft- und Raumfahrtindustrie; BDLI). DLR is currently working on a Zero Emission strategy.