Session: 03-12-01: Testing and Characterization

Paper Number: 121620

121620 - Structural and Aerodynamic Characteristics of Micro-Perforated Porous Sheets for Laminar Flow Control 

Compared to state-of-the-art transport aircraft, aircraft designed for laminar flow promise a significant performance increase by decreasing the friction drag of the wetted surface by up to 90%. To achieve laminar flow, current research focuses on natural laminar flow (NLF) and laminar flow control (LFC), where NLF achieves transition delay with an appropriate aerofoil design and high surface quality, whereas LFC achieves transition delay with active boundary layer suction. While NLF only results in a partial laminarisation of transport aircraft, LFC has the potential of a fully laminar aircraft but requires active systems for boundary layer suction.

The structural components enabling boundary layer suction are micro-perforated sheets. In recent developments, perforated sheets used as aircraft skins are thin, etched foils or thicker laser-drilled sheets. However, thin foils are susceptible to stability issues and metallic sheets add significant weight to the wing. Therefore, this study includes sheets with integrally manufactured porosities, such as carbon-fibre-reinforced plastics (CFRP), laminated on a needle bed tooling, and additively manufactured porous sheets. While the etched and laser-drilled foils and sheets investigated in this study are available on an industrial level, the porous CFRP sheets and the additively manufactured sheets are produced within the scope of this research.

The different porous sheet specimens are investigated regarding their porosity, hole geometry, and perforation quality. To determine the porosity and hole quality of the specimens, an algorithm is developed to detect porosities based on microscopic images and to return a statistical analysis of hole size, geometry, and their deviations. Micro-computed tomography (CT) analysis provides insight into the three-dimensional geometry of the holes. The combined data of perforation size, quality, and geometry allows a direct comparison of industrial available perforated skins to the additively manufactured porous sheets. The surface of the skins surrounding the holes is characterised by a study on the surface roughness, but limited to non-post-processed surfaces.

To connect the structural characteristics to the aerodynamic characteristics, the pressure drop depending on the suction rate is experimentally determined for all specimens. For this purpose, the specimens are mounted into a flow bench, where the pressure is measured in front of and behind the specimens. In combination with the measured volume flow, this results in a relationship between pressure drop and flow rate for each skin specimen.

The results of this study show that each micro-perforation manufacturing process has its unique perforation geometry and brings its own limitations. While etched foils show the highest hole quality in terms of geometric accuracy and repeatability, the skin type is limited to relatively thin stainless-steel foils. Their characteristic perforation geometry is an oblique cylinder. Laser-drilled perforations are not limited to thin foils or steel but show larger variations in diameter and less geometric accuracy in terms of deviation from cylindrical shape. Their characteristic hole geometry is a truncated cone with limited tapering.

While etched and laser-drilled skins achieve minimum equivalent diameters larger than 50um, perforated CFRP sheets were demonstrated in this study to achieve 180um. Additively manufactured porous sheets were limited to equivalent diameters larger than 250um. The characteristic perforation geometry of laminated CFRP sheets are perfect cylinders reflecting the shape of the needle. In contrast, the characteristic perforation geometry of additively manufactured porous sheets is a truncated pyramid with strong tapering.

This study shows that the pressure drop characteristics of each skin specimen strongly depend on the specimen's porosity. However, the characteristic perforation geometry and the absolute hole size also have an influence on the pressure drop. The results of this study allow a correlation of pressure drop, porosity, hole size and hole geometry. These relationships allow to identify hole types and porosities to obtain the required aerodynamic characteristics. Thereby, the laminated and additively manufactured porous sheets significantly widen the set of available porous sheets for extended hybrid laminar flow control.

Presenting Author: Hendrik Traub Technische Universität Braunschweig

Presenting Author Biography: Hendrik Traub studied mechanical engineering at Braunschweig University of Technology from 2010 to 2014. He wrote his Bachelor's thesis entitled "Studies concerning the automated design of a flexible wing" 2014 at Airbus Operations GmbH in the Aeroelastics department. From 2015 to 2019, Hendrik Traub studied aerospace engineering. He completed his studies with a master's thesis on the "Structural Optimisation of an Aircraft Wing using Rapid Analytical Methods" at the German Aerospace Center (DLR). Since then, Hendrik Traub has been a research assistant and doctoral student at the Institute of Mechanics and Adaptronics at Braunschweig University of Technology. Here he works in the Cluster of Excellence for Sustainable and Energy-Efficient Aviation on active laminarisation concepts and the mechanics of microstructures.