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Adaptation of a generic wind tunnel model for attachment line transition measurements

Final Report Summary - CLEANSKY-LBD (Adaptation of a generic wind tunnel model for attachment line transition measurements)

Executive Summary:
The Hybrid Laminar Flow Technology (HLFC) for drag reduction is a key technology in aerospace industry to realize a reduced fuel consumption of civil jet transport aircrafts. For drag reduction caused by boundary layer suction, small holes at the leading edges of the aircraft wings and tail planes are needed. This project addresses the perforation and integration of suction inserts for flow tunnel tests. To realize such suction surfaces a laser beam drilling system was designed, constructed and build to drill fast and homogeneous the required holes at long distances using a state-of-the-art short-pulsed fibre laser. The laser system leads to drilling rates up to 400 holes per second. The target diameters in the range of 50 to 100 µm were achieved after drilling by chemical etching. As a result through-going and round holes free of burrs can be machined with tolerances of a few micrometres. Challenges were the drilling strategy and the clamping system to avoid thermal distortion and ensure a homogeneous drilling pattern along the whole distance. With this technology suction panels with a length of about 2 meters were manufactured and integrated into interchangeable inserts for the wind tunnel model.

Project Context and Objectives:
Hybrid laminar flow control (HLFC) is an active drag reduction technique. The transition from a laminar to turbulent boundary layer is marked by a sudden increase in the thickness of the boundary layer and a significant change in the local flow behaviour. The random variation of velocity and flow direction within the turbulent boundary layer results in an order of magnitude increase in the skin friction compared to that of laminar flow. By sucking a small amount of the air in the external flow through the skin surface, the transition of the boundary layer from laminar to turbulent flow mechanisms can be delayed. As skin friction drag accounts for nearly 50% of the total drag of a civil jet transport aircraft in cruise, technologies that enable laminar flow to be maintained offer the potential for enormous economic and environmental benefit [1]. For drag reduction caused by boundary layer suction, small holes at the leading edges of the aircraft wings and tail planes are needed. The economically drilling of small holes in the range of 50 to 100 µm in diameter on large areas is up to date crucial. Apart from processing time this is also due to the demand of close tolerances in case of suction holes. Additionally, minimization of the thermal distortion when drilling large areas gets more challenging. The main objective of the project is the transfer of the laser drilling process to an adequate system technology used for the fabrication of large suction inserts in the range of 2 meters made of titanium 3.7024 in 0.8 mm thickness showing three different bore diameters with close tolerances in the micrometre range.

Project Results:
The laser beam drilling system was designed, constructed and build to meet the requirements to drill fast and homogeneous the required holes at long distances. The solid state laser used in this set-up is a pulsed fibre laser with a maximum output power of 200 W. Triggering permits pulsing operation up to 1000 kHz. Since it delivers short pulses switchable between 30 to 240 ns high peak powers of up to 1 GW/cm² can be achieved. After the drilling system configuration laser parameters were identified to drill a large amount of holes in a short time with the following properties: through-going, round, free of burrs at laser exit. The actual drilling diameter should be smaller than the target diameter. The required target diameters were achieved afterwards by chemical etching. The investigation of drilled bore geometries and tolerances was performed by light microscopy and preparing cross-sections of the holes. This was the basis for the following steps to investigate and realize the production of wind tunnel inserts: manufacturing of flow test specimen and hole analysis, upscaling of the laser drilling procedure, manufacturing of suction inserts with different hole diameters, integration of the suction inserts into the model. The panels for the suction inserts were drilled under the same conditions as the flow samples. Challenges were the drilling strategy and the clamping system to avoid thermal distortion and ensure a homogeneous drilling pattern along the whole distance. In conclusion, three different suction panels with a length of 1997 mm and a width of 106 mm were manufactured and integrated into interchangeable inserts for the wind tunnel model. For the integration the panels were first formed to the required shape and then glued into the insert. The achieved bore diameters were 49 µm with a tolerance of 2 µm for the 0.5 mm spacing, 90 µm with a tolerance of 5 µm for the 0.9 mm spacing and 132 µm with a tolerance of 8 µm for the 1.3 mm spacing as specified. Drilling rates up to 400 holes per second and overall machining speeds up to 175 holes per second were achieved within the project.

Potential Impact:
Within this project the system technology and knowledge about the necessary drilling technology for inserts usable in flow tunnel tests was developed. Furthermore, the feasibility was proven in form of a full size model. The final results are a potential impact to the targets of the Clean Sky programme as the successful production of inserts for wind tunnel tests significantly contribute to the technological breakthrough and shorten the time to market of the suction hole technology, thus contributing significantly to reducing the environmental footprint of aviation. Additionally, the feasibility of an efficient large-scale drilling method for the series-production of perforated sheets especially in aircraft industry was shown. The steps necessary to bring about these impacts were a successful development and implementation of the system technology. The impacts were achieved as the tolerances can be controlled in a sufficient way according to the specifications. One aspect of the dissemination of projects results was the contribution of technical reports and specimen/inserts to the topic manager. Furthermore, the findings were published on conferences and magazines for a wider technical public than only the aerospace industry. Further publications, conferences and press releases are planned in 2015 for a wider dissemination of the foreground. This will be done in a way that not only inserts for flow tunnel tests will be addressed but also the possibilities for large-scale drilling with respect to other applications, like filters and nozzles in the automotive industry or rotor blades in the wind power industry, in the serial-production will be shown. An exploitable foreground is the knowledge and system technology for a very precise and homogeneous as well as fast drilling of perforated surfaces. Therefore, one plan for exploitation is to cooperate furthermore with the aerospace industry related to the technological development and delivering of suction surfaces with further increased dimensions of the panels. Additionally, the knowledge and system technology is helpful to find partners for the development of technology and products in other drilling applications.

List of Websites:
BIAS – Bremer Institut für angewandte Strahltechnik GmbH
Dr.-Ing. Andreas Stephen
Klagenfurter Straße 2
28359 Bremen
Germany

Email: stephen@bias.de
final1-cleansky-lbd-publishablesummary.pdf