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Clean Sky’s BALANCE: A smooth operator

A smoother engine nacelle means less drag, less fuel burn, and less emissions. Clean Sky’s BALANCE project is propelling the science of European nacelle design towards a seamless future.

From a distance it might look as though the exterior surface of a nacelle (the housing around the engine that provides cooling or heating, reduces engine noise, and protects the engine) is silky smooth. But closer inspection reveals that nacelles are actually a combination of different components with access doors and flaps, fastening devices, with small gaps and steps – all essential for routine access to the engine for inspections and maintenance purposes between flights. But even the slightest gaps or sur face roughness between these parts can disrupt the aerodynamic flow around the aircraft, creating drag and costing extra fuel burn.

”The surface of the nacelle structure has many places where it has joins, holes or bumps along its surface. The purpose of the BALANCE project was to research how the structure could be designed to make the forward portion even smoother to the airflow, whilst still providing the support to the engine” explains Iain Minton, UK Technical Director at the Technical Department of Safran Nacelles and the primary coordinator of the 31 month Clean Sky 2 project which was completed last September.

”The smoothness of the nacelle affects the drag of the aircraft” adds Minton. ”This has a large bearing on how much fuel is used by an aircraft, which in turn affects how much CO2 the aircraft produces. By researching this, the goal was to find solutions which enable the drag of the aircraft to be reduced by 1% at the aircraft level for a business jet, without increasing the weight or other negative features of the aircraft.”

To make the challenge even more arduous, all of this had to be achieved without increasing the cost of manufacture, and without generating extra workload for those maintenance technicians – a balance of operational accessibility and cost control, while seeking that holy grail of aerodynamics: laminarity.

”Laminarity is a powerful technology which provides aerodynamic benefit for the aircraft, but it’s very challenging to achieve because it means having very small stepsand gaps as well as very low surface roughness to the aircraft components. It’s very complex from a manufacturing point of view, because an engine nacelle is made from hundreds of components that interconnect. And we have to achieve all this without increasing the manufacturing cost” says Clean Sky’s Antonello Marino, Project Officer for BALANCE.

The project investigated options to reduce the number of aerodynamic disturbances present in the forward section of a nacelle, a process which included reviewing interface locations, methods of attaching parts, and impacts on engine integration to reduce drag without adversely affecting production or operating costs. Using knowledge of nacelle and engine integration, this project provided a full scale TRL5 demonstrator, modelled around the requirements for a business jet sized nacelle, supported by analysis and test results. The underlying purpose being to integrate the findings of the project into the future propulsion systems and aircraft applications for the 2020 to 2030 entry into service timeframe, and the project has developed nacelle architectural options, improved design approaches for reduced steps and gaps, and revised engine access points to enable means of developing better, competitive solutions for future business aircraft nacelles.

Of course, the raison d’être of this project – as with all Clean Sky initiatives – goes beyond solving a specific technical challenge, for in addressing the air flow around aircraft engines there’s a specific issue of European competitiveness at stake, says Safran’s Minton: ”Technologies of this type have been seen in the USA, such as on the Boeing 787. But to date no European manufacturer has been able to reduce the drag around the engine using these techniques to the extent requested by the Clean Sky 2 programme. Without developing these solutions, the risk is that Europe will not be able to remain competitive for the work in providing the nacelles for future aircraft. However, if solutions could be achieved, the European companies would be able to win more work in the marketplace”.

And beyond industrial leadership, this, in turn, could have societal implications for Europe too, points out Clean Sky’s Marino: ”Addressing the drag around engine nacelles is very important not just for the environment but there’s also a matter of social impact. You have to consider that more than 200,000 people work directly or indirectly in Europe’s aeronautical engines sector so this project contributes to keep this sector, at European level, very competitive against non-European players”.

In terms of measurable results from the BALANCE project, Safran’s Minton explains that ”several concepts were developed, and the one suitable for the aircraft being examined in Clean Sky 2 ITD-Airframe was chosen. This promised approach was developed with the airframer, though the solution chosen did not respond to all wishes, meaning more work is needed, but nevertheless predicted a reduction in drag of 0.3% without negative impact on the aircraft”.

”Results are promising taking into account that activities have been focused on an existing business jet engine which has raised quite demanding constrains for the project”– says Olivier Colin, Dassault Aviation.

Other positive outcomes Minton points to are that the project ”was successful in identifying several concepts which could work with different aircraft, and whilst the drag reduction was not at the level requested, the achievement made showed good results were achievable without need for complicated mechanical systems”. And in terms of deliverables, the project created a digital demonstrator which was shared with the relevant A/C manufacturer (Dassault Aviation). It also produced some manufactured articles to confirm that the ideas being reviewed were able to be industrially produced.

And in respect of the project’s social and environmental benefits, Minton emphasises that ”The project, in delivering almost 0.3% drag reduction, shows a route to reducing CO2 emissions on aircraft should this technology be selected for future aircraft. In addition, the development of this within Europe enables the market opportunities around business jets (which this project looked at as a baseline) to be captured. Over 10,000 new jets are required over the next ten years, thus enabling support to the European Gross Domestic Product should European nacelle manufacturers be able to win some of this market”.