For aerodynamicists it is quite clear that the flow path around an aircraft is decisive for its fuel consumption; more than half of the drag of an aircraft is caused by friction. It is therefore important to reduce friction by delaying the transition from laminar (low-friction) to turbulent flow downstream as much as possible. Researchers at the German Aerospace Center (DLR) have now taken another major step in this area, having recently tested a new system for hybrid laminar flow control (HLFC) for the first time.
A system gets even more “simplified”
Hybrid laminarisation is the key to reducing the energy consumption of aircraft, researchers at the DLR Institute of Aerodynamics and Flow Technology are confident. With this technology, part of the airflow surrounding the aircraft is actively absorbed through tiny holes in the surface, thus keeping it laminar and delaying the turbulent transition. The research and development of such an extraction system has a long history at DLR: The first extraction system was tested at the end of the 1990s in cooperation with Airbus. However, this was still very complex and not economical. A "simplified" system was subsequently developed and has been successfully tested in the wind tunnel (LuFo project VER²SUS) and even in flight environments (EU project AFLoNext) in 2018. The results were impressive: Applied to the fin of an A320, local laminar flow could be extended to half of the chord length, thus resulting in significant drag savings. This provided sufficient incentive for the researchers to continue on the way to the series production of such a HLFC system.
The following iterations were seen to further increase efficiency: "Previous approaches to a suction system invariably featured a complex chamber structure," explains Thomas Kilian from the Institute of Aerodynamics and Flow Technology, responsible for aerodynamic design in the VarPorHyL (Variable Porosity for Hybrid Laminar Flow Control) project. "These chambers were necessary in order to generate the desired suction distribution over the outer skin." Based on these results, and with the help of new design methods, researchers have now succeeded in substituting this "chambering" for a micro-perforated outer skin with variable porosity, where perforation is adapted to the local aerodynamic requirements. Micro-perforation was achieved using a laser beam drilling method developed especially for this application.
"This makes it possible to significantly simplify the production of the leading edge and at the same time better optimise them for mechanical loads," explains Matthias Horn from the DLR Institute of Structures and Design in Stuttgart.
A material mix enabling new opportunities for industrial implementation
A mix of materials is used in order to fully utilise the potential of this simplified design. The supporting structure is made of carbon fibre-reinforced plastics, while the outer skin is made of metal. This makes it possible to achieve a very light construction while at the same time ensuring the precise machinability of the outer skin. Both are key technologies that are of great importance for later industrial implementation.
As part of the VarPorHyL project, the DLR Institutes for Aerodynamics and Flow Technology and Structures and Design, in cooperation with their partner Airbus, have designed, optimised, and manufactured an original-size segment of a vertical tail leading edge.The wind tunnel test under representative flight conditions in the DNW-LLF low-speed wind tunnel, which is essential for the evaluation of the new design, proved that the new design performs just as efficiently as previously-known technologies. The new design also introduces additional advantages as, "the suction system without chambers can be built much more cost-efficiently," explains Thomas Kilian. At the end of the three-year joint project within the framework of the Aviation Research Programme (LuFo V) of the Federal Ministry of Economics and Energy (BMWi) in September, the researchers drew a positive conclusion.
Further projects at the EU level are already underway, in which the new chamberless design with variable porosity is being further developed and tested. "From a demonstrator part for the wind tunnel we are now making huge strides towards series production," concludes Kilian with satisfaction