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Clean Sky's Active Flow Control projects aid UHBR engine integration

Distortion prediction simulation of SBA actuator with optimised PJA actuator
Distortion prediction simulation of SBA actuator with optimised PJA actuator

Larger-fanned engines bring considerable energy efficiencies to aviation, but their installation poses challenges too. Clean Sky is advancing the science of active flow control in the context of engine/wing integration, to give European aeronautics some extra lift.

One of the key initiatives for addressing the environmental objectives of Clean Sky, and the broader Horizon 2020 ambitions, is the focus on aircraft engines, especially large fanned energy-efficient Ultra High Bypass Ratio (UHBR) engines. These hold the promise of achieving around 10% improvement in engine efficiency using the same amount of fuel, relative to today's generation of power plants. But to install UHBR engines without having to elongate the aircraft undercarriage to provide enough ground clearance (especially during take-off and landing) means mounting the engine closer to the wing. This in turn makes it necessary to have a cut-out in the slats (also known as high-lift devices) at the wing-nacelle junction to prevent interference between the deployed slat and the nacelle. However, the cut-out in the slat will cause local flow separations when lift is needed most, especially during take-off and landing, leading to the loss of 3-4% of the max. lift.

Clean Sky's Active Flow Control (AFC) initiatives aim to reinstate that lost lift by exploring, designing, prototyping and testing various active flow control devices. As well as helping with engine integration into the wing, an additional anticipated by-product of this could be an improved understanding of active flow control for more general application across other important European aeronautical projects.

Two different types of active flow control actuators (steady blowing and pulsed jet actuator) are being designed and investigated by Clean Sky. 

In a Clean Sky project called DECOROUS, which ran from 1 June 2016 to 31 May 2019, Berlin-based start-up Navasto developed a two-stage no-moving-parts fluidic actuator system, called the Pulsed Jet Actuator (PJA).

At Navasto, in the DECOROUS project, using a combination of numerical and experimental methods, flow control actuator systems at two different scales were developed, manufactured and qualified, including a 1:13.6 scale wind tunnel model. 

‘The actuator we developed is a candidate for integration because it does not have any moving parts nor any electrical components,’ says Dr.-Ing. Matthias Bauer, managing director at Berlin-based start-up Navasto GmbH. ‘The idea of this project is to develop an actuator that is extremely robust, does not require maintenance during its lifetime and is highly energy-efficient.

It's a pneumatic system; at the moment the plan is to use the bleed air from the aircraft — we just add a valve which allows us to take some of the bleed air and route it into our flow control system.’

Navasto put their actuator through two wind tunnel test campaigns in Kryo-Kanal Köln (the cryogenic test facility in Cologne, Germany). Subsequently, a full-scale actuator, provisionally designed for integration into an Airbus 320 flight-test aircraft, was developed, constructed and qualified in a laboratory. This demonstrator addressed a combination of aerodynamic requirements for overall aircraft infrastructure, noise emission and system health monitoring factors.

PJA actuator manufactured by SLM
PJA actuator manufactured by SLM

‘This Horizon 2020 project really gave us the chance to connect with other large players in the field, for example Airbus, DLR in Germany, NLR in the Netherlands, and several colleagues from Spain and France. The way Clean Sky 2 is organised gives us international access to research which could not be done on our own, and the whole Clean Sky framework is very helpful for us as a small start-up company to participate in serious development work where, usually, only very large companies could participate. Clean Sky was the only way we could do something like that,’ says Dr. Bauer.

The design of the full-scale PJA was then delivered to the CS2 FLOWCAASH (Flow Control Actuators at Aircraft scale manufacturing by Selective Laser Melting (SLM) with high aerodynamic performance for use in Harsh environments) project, which runs from April 2018 to 31 December 2020. As part of this initiative, a PJA and a Steady Blowing Actuator (SBA) were developed and manufactured by IK4-Lortek in Ordizia, Spain. 

In addition to the need to address issues of air flow separation, active flow control actuators also need to be compact, as light as possible, energy efficient, maintenance free and capable of being installed in confined spaces.

‘This leads to geometrically very complex components,’ says Dr. Ane Miren Mancisidor Telleria, researcher and specialist in metal additive manufacturing at IK4-Lortek. ‘Traditional manufacturing technologies like machining present limitations with regard to the shape that can be obtained. Thus, the need for using additive manufacturing arises.’

Consequently, IK4-Lortek used Selective Laser Melting (SLM), an additive manufacturing process which uses a high power density laser to melt and fuse metallic powders together, to manufacture its actuators.

‘In the FLOWCAASH project, a PJA and a SBA were made using a SLM process involving a titanium alloy (Ti6Al4V) without presenting any cracks. In the case of the SBA actuators, the design improvements performed from one iteration to the next has led to adequate aerodynamic behaviour and successful performance in harsh environment tests. For the PJA actuator, the FLOWCAASH consortium is still working on design improvements to achieve actuators with adequate aerodynamic performance,’ she says. 

There are a number of environmental reasons why SLM is an attractive manufacturing technique for AFC actuators: the technology allows complex and unique product geometries which minimise product cost and weight; in additive manufacturing, wastage is minimal and more than 95% of the remaining material (powder that has not melted) can be recycled; and toxic wastes such as lubricants, waste oil, polluted scrap, and absorbents, are negligible.

In addition, the SLM manufacturing method enables a PJA design consisting of only a few single parts, reducing the risk of leakage and thus increasing the reliability of the AFC system during flight operation.   

‘Optimising design and manufacturing of complex and unique product geometries is a key factor for EU industrial competitiveness, and for maintaining and extending the leadership that the EU has in the aircraft industry,’ adds Dr. Telleria. ‘Consequently, the FLOWCAASH project contributes to the Flightpath 2050 industrial competitiveness goals. Additionally, the project helps promote transnational cooperation between researchers from different organisations and facilitates access to, and transfer of, knowledge between partners and the scientific and technical communities by promoting open access.’

Wind tunnel test – active flow control – actuators supporting high lift of aircrafts equipped with larger engines
Wind tunnel test – active flow control – actuators supporting high lift of aircrafts equipped with larger engines

The two types of actuators developed through the FLOWCAASH and DECOROUS projects are advancing European aeronautical knowhow in active flow control actuation, yet there's still an important challenge to be resolved, from the Clean Sky perspective:

‘We have different technologies which are under investigation, which involved the manufacture of some prototypes which were tested in wind tunnel tests to show their benefits,’ explains Clean Sky project officer Sebastien Dubois. ‘We discovered quite late that implementing such actuators on an aircraft would require a high amount of energy in terms of mass flow from the compressor of the aircraft. That's why, despite the fact that the concepts and the benefits were validated, we now have to further mature the technologies and to investigate other routes to minimise the energy demand to actuate such actuators.’ 

‘The project will continue until around 2022/2023 complemented by a new activity that involves designing new actuators which will be more energy efficient, and which will be taken to TRL3/4, but with full understanding about the areas to be further investigated until fully proven, and the overall concept and the approach to be implemented, to mitigate the adverse effect of integration of a UHBR engine,’ says Dubois. 

The plan now is to focus more on the design and the development of an actuator that can be reliably deployed using a lower amount of energy, and to investigate and test it on an aircraft, because the less energy that is required, the greater likelihood there is that it will deploy without failure.

‘To do this we have a large ecosystem of universities, SMEs, industry members, and research organisations involved in different steps, either in the characterisation and design phase, or in the validation of principles, acquisition of data for simulations, or for defining and prototyping different actuator concepts for installation on to the aircraft,’ says Dubois. ‘This is a promising domain, which has been investigated for several years and which is now coming to the point of finding its route in terms of potential exploitation in association with clean technology investigations within the engine domain. If we are successful, active flow control would also open the way to additional gains in the field of aerodynamic effect and would also help reduce drag.’

The project contributes towards Demo 11 on Active Flow Control (AFC) led by Airbus part of Clean Sky 2’s Large Passenger Aircraft IADP. The two actuators are essential bricks to demonstrate the industrial maturity of the AFC technology.