AN AUTOMATED ASSESSEMENT TOOL OF NLF CRITERIA FOR AIRFRAME JOINTS

Abstract

An ever increasing awareness of the ecological impact of air travel and associated regulatory measures demands for a cut in aircraft fuel consumption to reduce CO2 emissions and operating costs. Among improvements in engine technology or use of alternative fuels, the sustainment of laminar flow on surface areas of transport aircraft is seen as an important contribution to the solution of this challenge. The reduced friction drag of a natural laminar flow (NLF) wing can lead to a reduction in fuel consumption and thus reduction of CO2 emissions of up to 8 % on aircraft level [1]. Laminar flow's sensitivity to surface disturbances however requires specific shapes and high surface quality: Structural features like steps, gaps and surface waviness can cause early laminar/turbulent transition [2]. This calls for novel structural design concepts for laminar flow applications and tools to enable their practical implementation in aircraft operation. Through the course of several national and EU-funded projects, a multi-material leading edge concept using CFRP with an integrally bonded steel foil erosion shielding is being developed by DLR [3] with a distinct focus on operability. The leading edge and an associated interchange-enabling attachment concept are realized in a 2.3 m ground based demonstrator representing an outer wing section. A test stand is designed to recreate "wing on ground" and "cruise flight" surface deformations to enable interchange trials of the leading edge [4]. To enable an assessment against NLF criteria of the achieved step at the joint between leading edge and wing cover, an automated step measurement tool is developed and verified against manual assessments. Such a tool is a necessary step not only to validate suitability to support NLF on aircraft wings of the leading edge design and attachment concept. It also serves as a key contribution to a possible closed loop system supporting the assembly of airframe structures with intended laminar flow characteristics by providing direct feedback of the surface quality and informing on necessary adjustments to be made by the technicians. The paper will focus on the development of the assessment tool, framed by the results it delivered on the NLF leading edge installation trials.