22. November 2021
SynergIE project

Hy­brid-elec­tric propul­sion sys­tems en­able more cli­mate-friend­ly air trans­port

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Aeronautics
Regional aircraft with distributed propulsion
Re­gion­al air­craft with dis­tribut­ed propul­sion
Image 1/3, Credit: DLR (CC BY-NC-ND 3.0)

Regional aircraft with distributed propulsion

High­ly ac­cu­rate flow sim­u­la­tions show that elec­tric propul­sion sys­tems al­low the thrust to be dis­tribut­ed over mul­ti­ple small­er pro­pellers. When the air­flow from these pass­es over the wing, it pro­vides in­creased lift and more ef­fi­cient aero­dy­nam­ics. This ef­fect en­abled the project part­ners to re­duce the wing area and the wing mass and to re­duce the drag caused by the in­ter­ac­tions be­tween the pro­peller air­flows and the wing’s wake tur­bu­lence.
DLR Air Vehicle Simulator (AVES)
DLR Air Ve­hi­cle Sim­u­la­tor (AVES)
Image 2/3, Credit: © DLR. All rights reserved

DLR Air Vehicle Simulator (AVES)

The flight char­ac­ter­is­tics of the hy­brid-elec­tric short-haul air­craft were eval­u­at­ed in the flight DLR Air Ve­hi­cle Sim­u­la­tor (AVES).
Finite-element model of the aircraft structure – dynamic analysis of the wing
Fi­nite-el­e­ment mod­el of the air­craft struc­ture – dy­nam­ic anal­y­sis of the wing
Image 3/3, Credit: DLR (CC BY-NC-ND 3.0)

Finite-element model of the aircraft structure – dynamic analysis of the wing

Among oth­er things, the struc­tural be­haviour of the air­craft was in­ves­ti­gat­ed dur­ing the project. This showed that the dy­nam­ic struc­tural be­haviour changes due to the propul­sion units be­ing dis­tribut­ed across the span of the wings. For ex­am­ple, the ini­tial flex­ing of the wing is re­duced quite con­sid­er­ably.
  • Together with Airbus, Rolls-Royce and Bauhaus Luftfahrt e.V., DLR has investigated the overall system of a hybrid-electric short-haul aircraft for up to 100 passengers.
  • Electric propulsion units distributed across the wings improve the aerodynamics and thus significantly increase efficiency.
  • The flight characteristics of the prototype have already been tested in the DLR AVES flight simulator.
  • Focus: Aeronautics, climate-friendly flight

The development of climate-friendly technologies suitable for everyday use in the air transport system of the future is at the top of the list of priorities for the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR). Among other things, research efforts are focused on new aircraft configurations that can be successfully operated commercially with significantly lower emissions and reduced noise pollution. Electric or hybrid-electric propulsion systems have potential for aircraft configurations that meet these requirements.

As part of the SynergIE joint project funded by the German Federal Ministry for Economic Affairs and Energy (Bundesministerium für Wirtschaft und Energie; BMWi), DLR researchers in Hamburg, Braunschweig and Göttingen, together with partners Airbus, Rolls-Royce and Bauhaus Luftfahrt e.V., have studied the entire system of a hybrid electric short-haul aircraft that can carry up to 100 passengers with distributed propulsion units on the wings. With this technology, the propulsion system is distributed across the entire span of the wings, which leads to more efficient airflow around the aircraft.

Aerodynamic advantages thanks to electric propulsion systems

"In conventional regional aircraft, the wings are often oversized in order to achieve good performance during take-off and landing," explains Martin Hepperle from the DLR Institute of Aerodynamics and Flow Technology. "These aircraft then have excessively high energy consumption during cruising flight." Highly accurate flow simulations show that electric propulsion systems allow the thrust to be distributed over multiple smaller propellers. When the airflow from these passes over the wing, it provides increased lift and more efficient aerodynamics. This effect enabled the project partners to reduce the wing area and the wing mass and to reduce the drag caused by the interactions between the propeller airflows and the wing tip vortices.

As a final design for an aircraft with distributed hybrid-electric propulsion, the researchers selected and evaluated a concept with turbogenerators in the fuselage and 10 electric motors along the leading edge of the wing as the best solution out of the various possible arrangements. The optimal design and installation of the propellers makes it possible to reduce the chord of the wing and size of the rudder, and thus reduce energy consumption by approximately 10 percent. "The special arrangement of the propellers allows us to compensate for the weight disadvantages of the hybrid-electric propulsion system," says Hepperle. "We were also able to design the vertical stabiliser to be smaller and thus lighter and with less drag in our multi-engine concept," he continues. "This concept can even compensate for the failure of two electric motors; it therefore offers greater operational reliability as well."

Flight characteristics investigated on the test stand

During the virtual first flight in the DLR Air Vehicle Simulator (AVES), DLR test pilots evaluated the flight characteristics of the hybrid electric short-haul aircraft. Particularly during the landing approach, it became apparent that the aerodynamic interaction between the propeller wakes and the wings strongly influences the flight characteristics of the aircraft. To compensate for the reduced effectiveness of the smaller rudder and vertical stabiliser, a research team from the Institute of Flight Systems developed a flight controller that enables yaw control – rotation about the vertical axis – using the rudder combined with differential thrust.

In the SynergIE project, the participants developed and established a universal 'software simulation toolchain' for future aircraft designs with distributed hybrid electric propulsion systems at DLR, in order to expand the overall evaluation capability of German research and industry. The interdisciplinary work includes the integrated aerodynamics of the wing and propellers in close interaction with flight mechanics issues for control as well as boundary conditions of the structure and aeroelasticity. In future, remaining questions on the aeroacoustics of the distributed propellers and on optimal flap systems for landing approaches are to be resolved.

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.

Contact
  • Falk Dambowsky
    Ed­i­tor
    Ger­man Aerospace Cen­ter (DLR)
    Me­dia Re­la­tions
    Com­mu­ni­ca­tions and Me­dia Re­la­tions
    Telephone: +49 2203 601-3959
    Fax: +49 2203 601-3249
    Linder Höhe
    51147 Cologne
    Contact
  • Vera Koopmann
    Com­mu­ni­ca­tions
    Ger­man Aerospace Cen­ter (DLR)
    In­sti­tute of Aero­dy­nam­ics and Flow Tech­nol­o­gy
    Telephone: +49 531 295-3405
    Lilienthalplatz 7
    38108 Braunschweig
    Contact
  • Martin Hepperle
    Trans­port Air­craft
    Ger­man Aerospace Cen­ter (DLR)
    In­sti­tute of Aero­dy­nam­ics and Flow Tech­nol­o­gy
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