Military and Strategic Journal
Issued by the Directorate of Morale Guidance at the General Command of the Armed Forces
United Arab Emirates
Founded in August 1971


Getting Ready for Clean Skies: Aircraft Innovations for the 2030s

The year 2035 has been set for a programme of ambitious 20-30 per cent fuel savings for short-medium range aircraft. The Clean Sky initiative involves three interrelated initiatives: radical airliner configuration; hybrid aircraft propulsion; and boundary layer ingestion (BLI) to achieve this target.
“The area of investigation is quite broad, with three categories of concepts being designed and validated by different research organisations, which 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”, says Sébastien Dubois, Clean Sky Acting Head of Unit and project officer for Large Passenger Aircraft. “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. By the end of Clean Sky 2, the aim is to produce an aircraft concept that could contribute to the emergence of such a hybrid electrical aircraft for the next generation of European aviation.”
1. The DRAGON  
The DRAGON stands for Distributed fans Research Aircraft with electric Generators by ONERA. The concept is an A320-like electrically powered aircraft seeking significant fuel efficiencies, the focus is on developing, de-risking and maturing transonic distributed electric propulsion.
“There are already lots of ongoing activities here in Europe and in the U.S. 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.”
ONERA will finalise its research activities with noise assessment in 2020 via specific mission points, such as takeoff and landing. “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,” Schmollgruber adds, anticipating that distributed electrical propulsion could produce 19 per cent fuel reduction in an aircraft, flying at Mach 0.8 with a range of 2750 nautical miles.
2.  DLR: Coupling of the airframe and propulsion system 
DLR is currently looking at a variety of ways to couple the airframe and propulsion system, including a boosted turbo fan, fuselage BLI, tip-mounted propulsion and distributed propulsion, while developing better methods and tools to assess aircraft performances equipped with distributed propulsion. Future conceptual studies will focus on the aircraft’s overall configuration, including BLI and hybrid propulsion configurations with wing-mounted fans.
By placing the propulsors strategically, the resulting interactions between the airframe and propulsion system can be exploited positively, with the configuration working in alliance with boundary layer ingestion. Although use of 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 Programme Directorate Aeronautics and Coordinator of Clean Sky 2 LPA and ITD Airframe at DLR. 
A hybrid electric power train allows distribution of power to remote electric machines provided by efficient gas turbines, introducing more degrees of freedom from this propulsion system. While investigating the impact of individual technologies such as BLI, the impact assessment of techno-bricks is covered at system level because the mutual influence of these technologies is not always complementary.
As the numerical simulation of a new aircraft concept at high accuracy level for all disciplines involved is extremely demanding, the potential environmental benefits of radical aircraft configurations are linked to reduced fuel burn. As Dr. Herrmann of DLR notes, “We think that a fuel burn reduction of about 3-5 per cent and thus CO2 reductions seem feasible for radical aircraft configurations; we are currently analysing this together with industry in Clean Sky 2.”
Two of the three DLR aircraft configurations centre on the BLI concept where the first, 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 to reduce the inflow distortion to the BLI fan, while 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 to allow for a reduction in the size of the vertical tailplane and hence the reduction of mass and drag. The final evaluation of each of the team's concepts is expected by the end of Q1 2020.
3. NOVAIR: Optimising preliminary aircraft designs for HEP integration 
A collaboration between Dutch entities NLR and TU Delft, the NOVAIR project seeks to optimise preliminary aircraft designs for HEP integration. Focused on preliminary designs for future radical configurations of large passenger aircraft, it will be 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 part of the lift becomes dependent on thrust, but part of the drag starts to become dependent on thrust. That can be both beneficial but penalising towards design – therefore this is what we're focusing on.”
TU Delft has flight-tested a configuration modelled with a propulsive empennage system using a ‘ring-wing’ shaped as a circle. When a propeller is added inside, it sucks in air and enhances the lift generating capabilities of the wing, with small thrust vectoring vanes (like rudders) then added behind the propeller if desired for steering. 
The ring itself provides a stabilising force in vertical and horizontal directions, while the vanes enable control of the yaw and the pitching motion of the aircraft. For TU Delft, this is one of the most feasible technological concepts to have emerged, with Dr. Hoogreef confirming that distributed propulsion will likely feature on or in front of the wing.
Scaled Flight Demonstration
According to Dr Pierluigi Iannelli, Project Manager at the Fluid Mechanics Department at CIRA, “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.”
Dr. Iannelli emphasises that, “A Scaled Flight Demonstrator (SFD) can be considered 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 large 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.”

Developed by ONERA, NLR (in the frame of NOVAIR) and the Italian research entity CIRA, 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. Having integrated it on a 4-metre wingspan flying demo, in-flight lean Sky tests will examine how an integrated distributed propulsion system behaves.
“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.”
The Big Picture
Dr. Ing. Lars Jørgensen, the leader of the 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. 
He notes that, “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.”  
As Dr Hoogreef recognises, the Clean Sky framework can unite aerospace innovators to develop technology in new ways: “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. And it also provides the possibility to work on long term projects – these are extremely valuable opportunities for collaboration.”
Clear-cut Benefits for Airbus 
Dr. Jørgensen explains that 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.”
“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,” Dr. Jørgensen confirms. “As Airbus itself is a European company this way of working is close to the Airbus DNA, resulting in extremely close collaboration.”
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