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Free Radical: Innovative configurations and propulsive concepts for the 2030s

BLI aircraft configuration with wingtip fans (DLR)
BLI aircraft configuration with wingtip fans (DLR)

Targeting 20-30% fuel savings by 2035 for short-medium range aircraft application means employing radical new approaches. Clean Sky is addressing this ambitious challenge by means of a trio of interrelated initiatives in radical aircraft configuration, hybrid propulsion, and boundary layer ingestion (BLI).

Radical new approaches to aircraft propulsion, airliner configuration, and the way airflow around an aircraft is managed, are needed to ensure compliance with Europe's targeted energy efficiencies and emissions goals for the mid-2030s. Clean Sky's ongoing work on radical aircraft configurations, hybrid and distributed propulsion, and boundary layer ingestion envisages a minimum of 20% to 30% of fuel efficiency improvements by 2035 for short-medium range application.

‘The area of investigation is quite broad, with three categories of concepts being designed and validated by different research organisations,’ says Sébastien Dubois, Clean Sky Acting Head of Unit and project officer for Large Passenger Aircraft.

‘These three different concepts should eventually converge down, in 2020, to produce an ideal configuration that incorporates the best of each, to assess what the most relevant ingredients are, and what should be the ideal configuration. Further assessment will be carried out using a flying downscale model – a sort of drone – where we'll be assessing the key characteristics of such a configuration. The aim is, by the end of Clean Sky 2, to produce an aircraft concept that could contribute to the emergence of such a hybrid electrical aircraft for the next generation of European aviation,’ says Dubois.

Concept One: The DRAGON: Transonic distributed electric propulsion

The DRAGON (Distributed fans Research Aircraft with electric Generators by ONERA) concept is an A320-like electrically powered aircraft aiming at significant fuel efficiencies. The focus is on developing, de-risking and maturing transonic distributed electric propulsion.

‘There are benefits associated with distributing the electric propulsion and there are already lots of ongoing activities here in Europe and also in the US in this field,’ says Peter Schmollgruber, Programme Director for Civil Transport Aircraft at ONERA. ‘But what you don't see a lot is distributed electric propulsion for transonic speed. Our work within ONERA is to mature this technology. We believe the first aircraft to fly using hybrid-electric propulsion will have propellers, but we're preparing for the step after that – to have distributed electric propulsion for fast aircraft that will fly at the same transonic speed (around Mach 0.8) at which we're flying today.’

‘Right now we're finalising the assessments of benefits in cruise, and later in 2020 we plan to look at how this technology will behave at low speed for takeoff and landing conditions using numerical simulation,’ he adds, anticipating that distributed electrical propulsion could lead to about 19% fuel reduction with respect to a 150 passenger reference aircraft of 2014, flying at Mach 0.8, with a range of 2750 nautical miles. 

In 2020 ONERA will finalise its research activities with noise assessment and also specific mission points, such as takeoff and landing.

The DRAGON concept developed by ONERA
The DRAGON concept developed by ONERA

Concept Two: DLR: Coupling of the airframe and propulsion system

DLR is currently working on a combination of concepts including a boosted turbo fan, fuselage BLI, tip mounted propulsion and distributed propulsion, and is also developing methods and tools to better assess aircraft performances equipped with distributed propulsion. Conceptual studies in the pipeline focus around aircraft overall configuration, including BLI, and hybrid propulsion configurations with wing-mounted fans.

One possibility under investigation is the coupling of the airframe and propulsion system. By strategically placing the propulsors, the resulting interactions between the airframe and the propulsion system can be exploited positively, and DLR is looking at how such a configuration works in alliance with boundary layer ingestion. A linked area of research at DLR is examining various ways to distribute the propulsive units. However, several small turbofan engines would not be feasible, because they lose efficiency with their reduced size. 

‘One promising approach is the utilisation of electric engines that do not suffer from scaling effects,’ says Dr. Ulrich Herrmann of the Program Directorate Aeronautics and Coordinator of Clean Sky 2 LPA and ITD Airframe at DLR. ‘A hybrid electric power-train allows distribution of power – provided by efficient gas turbines – to remote electric machines. It also introduces more degrees of freedom related to the design of such a propulsion system. The research effort is dedicated to the investigation of the impact of individual technologies such as BLI. The impact assessment of techno-bricks is also covered at system level, where the mutual influence of all technologies – which are not always complementary – has to be considered.’

According to DLR, the potential environmental benefits of radical aircraft configurations are linked to reduced fuel burn. This is because the numerical simulation of a new aircraft concept at high accuracy level for all disciplines involved is extremely demanding. 

‘We think that a fuel burn reduction of about 3-5% and thus CO2 reductions seem feasible for radical aircraft configurations we currently analyse together with industry in Clean Sky 2,’ says Dr. Herrmann. 

Two of the three DLR aircraft configurations centre around the BLI concept. One, a 'canard' configuration, is based on the intention to exploit synergies between the unconventional airframe design and the hybrid-electric power-train. The tailplanes are repositioned in order to reduce the inflow distortion to the BLI fan – the vertical tailplane is split and positioned at the wingtips, and the horizontal tailplane moved to the front.

‘A second BLI aircraft configuration uses additional electrically driven wingtip fans,’ says Dr. Herrmann. ‘This allows for a reduction in the size of the vertical tailplane and hence reduction of mass and drag. The final evaluation of each of the team's concepts is expected to occur by the end of Q1 2020.’

‘Canard’ aircraft configuration with BLI (DLR)
‘Canard’ aircraft configuration with BLI (DLR)

Concept Three: NOVAIR: Optimising preliminary aircraft designs for HEP integration 

The NOVAIR project, a collaboration between Dutch entities NLR and TU Delft, is focused around preliminary designs for future radical aircraft configurations of large passenger aircraft that are optimised for the integration of Hybrid Electric Propulsion (HEP) systems. 
‘HEP, BLI and DEP have the potential to significantly reduce fuel consumption, and as a consequence greenhouse gasses, emissions, and noise levels. The possibility to electrically distribute power opens up new avenues in aircraft design,’ says Dr. Henk Jentink, Senior Scientist at NLR. ‘For example, by placing the engines over the wing, engine noise can be shielded more effectively. DEP enables a huge increase of effective bypass ratio and thus a reduction in fuel burn and emissions. Moreover, multiple small fans make less noise than single large turbofan engines. Integrating engines with the airframe will enable improved flow characteristics of the airflow at the inlet and the outlet of the propulsion system. NOVAIR will incorporate these benefits from the outset looking at aircraft configurations that make optimal use of HEP, BLI and DEP.’

Whereas NLR is mainly focusing on the scaled flight-testing part of the NOVAIR project, TU Delft is mainly focused on design studies and subsystem studies, such as distributed propulsion systems.

‘We're trying to combine what we've learned so far in wind-tunnel experiments regarding various setups of methods of distributing propulsion around an airframe, and then to apply that in conceptual aircraft design,’ says Dr. Maurice Hoogreef, Assistant Professor, Flight Performance and Propulsion at TU Delft's Faculty of Aerospace Engineering. 

‘We're looking at ways of distributing fans around the airframe. Some over the wing, to enhance the lifting capability of the wing itself. The nice part of having an electrified or hybrid electric power-train is that you can distribute that propulsive component around the airframe and position it where you hope to achieve benefit between the thrust produced and the aerodynamics – what we call aero-propulsive interaction. When you start coupling the propulsion system and the wing, then suddenly part of the lift becomes dependent on thrust, but also part of the drag starts to become dependent on thrust. That can be both beneficial but also penalising towards design – therefore this is what we're focusing on.’ 

One option TU Delft has studied is a configuration with a propulsive empennage system using a 'ring-wing' – a wing in the shape of a circle – a model of which was flight tested. If a propeller is added inside it sucks in air and enhances the lift generating capabilities of the wing, and small thrust vectoring vanes (like rudders) can be added behind the propeller to the aircraft to be steered. The ring itself provides a stabilising force in vertical and horizontal directions – the vanes allow control of the yaw and the pitching motion of the aircraft.

In 2020 TU Delft will continue with one of the most feasible technologies that has emerged from all of the various concepts and Dr. Hoogreef says it's likely that distributed propulsion will feature on the wing or in front of the wing.

Novel aircraft configuration integrating hybrid electric propulsion, investigated by the NOVAIR consortium
Novel aircraft configuration integrating hybrid electric propulsion, investigated by the NOVAIR consortium

Scaled Flight Demonstration

‘Scaled Flight Testing technology is a viable means to de-risk the introduction of disruptive aircraft technologies and aircraft configurations in the aerospace community, easing their Technology Readiness Level maturation process,’ says Dr Pierluigi Iannelli, Project Manager at the Fluid Mechanics Department at CIRA. 

‘A Scaled Flight Demonstrator (SFD) can be considered as a scaled version of a new aircraft configuration being developed, or a scaled version of an existing aircraft on which we aim to test some new technology, which is similar to real-size aircraft in one or more aspects, such as aerodynamics, flight dynamics, or aero-elasticity. The big advantage of using the SFD technology is that it could provide, at reduced efforts, a big amount of flight data in the early phases of the development process, useful either to assess the expected performance of the new configuration or to understand improvements needed in the ongoing development phase. According to these aspects it's expected that SFD technology could reduce overall development costs, saving time for the deployment of new aeronautical technologies and configurations in the aerospace market,’ says Dr. Iannelli.

Scaled Flight Testing is an integral asset to the demonstration processes of Clean Sky's Radical Aircraft Configurations/Hybrid and Distributed Propulsion/BLI concept project and is being developed by ONERA, NLR (in the frame of NOVAIR) and the Italian research entity CIRA. 

The hope is to integrate that on a four metre wingspan flying demo. The team will then integrate distributed propulsion to test in flight how such a system behaves, together with the other Clean Sky partners.

‘We can put radical configurations into a wind-tunnel, but that is a static condition so you don't really know how the aircraft responds if you start doing a manoeuvre,’ explains Dr. Hoogreef. ‘We're in this together. In a cooperative effort by TU Delft, NLR, ONERA, CIRA and Airbus we'll fly this demonstrator in about three years' time.’

Scaled Flight Demo – ground remote pilot station (CIRA)
Scaled Flight Demo – ground remote pilot station (CIRA)

The big picture

Dr. Ing. Lars Jørgensen, the leader of Clean Sky 2 Demonstration of Radical Aircraft in the Engineering/New Concepts & Capabilities/Future Project Office at Airbus in Hamburg, believes that this initiative could be a game-changer in clean aviation for Europe. 

‘As Europe is a world leader in the application of innovative energy solutions, these radical configurations, hybrid and distributed propulsion and BLI projects would provide the matching aviation element. The projects mainly address the Horizon 2020 ambition to reduce greenhouse gas emissions. The development of innovative products for this ambition is expected in turn to increase European competitiveness on the global market,’ he says.  

A clear advantage of the Clean Sky system is that aerospace innovators, large and small, public and private, can come together to develop technology in a new way, which would be extremely difficult without having the framework of Clean Sky, as Dr Hoogreef says: ‘It really enables us to get a direct link to industry partners to show our capabilities and also get input from them regarding what they're actually looking for, to steer the research a bit. It's also a good way for us to fund research. Clean Sky allows significant funding which allows us to pay for some PhD candidates for experimental campaigns, so that we can bring the research to the next step. What we give back is not only advancement in research, but also awareness amongst our colleagues and students who see what projects are going on under the Clean Sky umbrella. Some will want to be part of that, and it makes students aware of what companies and research institutes are out there in Europe.’ 

Peter Schmollgruber of ONERA agrees. ‘Clean Sky 2 is a tremendous opportunity to really perform research closely in association with industry,’ he says. ‘And it also provides the possibility to work on long term projects – these are extremely valuable opportunities for collaboration.’ 

For Airbus, the benefits are clear-cut. 

‘While Airbus's strength is to develop competitive products out of the research results, the research partners are exploring a wide variety of solutions through linking in results from fundamental research,’ explains Dr. Jørgensen. ‘Through Clean Sky 2 we're able to work on projects that go beyond national boundaries, and all the work by European research partners and Airbus is compared against each other, to identify innovative ideas as well as to align the different approaches. As Airbus itself is a European company this way of working is close to the Airbus DNA, resulting in very close collaboration.’