CFRP Upper Wing Cover for Natural Laminar Flow

Abstract

Integral CFRP Wingcover for laminar Wing

Technologies for reducing the fuel consumption are in the focus of future transport aircraft development. A significant reduction of friction-drag can be achieved by extending laminar flow on the wing upper side. Therefore it is necessary to keep surface irregularities within close limits, as even small disturbances like waviness, steps and gaps or fastener heads can cause an early transition from laminar to turbulent flow. Aiming on those objectives, the DLR Institute of Composite Structures and Adaptive Systems worked on the design and manufacturing concept for an integral wing upper cover together with its industrial partners Airbus, Premium Aerotec and gom. Anisotropic properties of composites, different thermal expansion coefficients of fibers and matrices and chemical shrinkage of the matrix cause process-induced deformations (PID) in composite structures, which can disturb laminar flow. In addition, the structure is deformed by aerodynamic loads in flight (load-induced deformations LID). Therefore the compliance with aerodynamic requirements for the laminar wing needs to take into account both effects in a coupled design process. Numerical analyses confirm, that with the choosen design the superposed deformations are within the limits for natural laminar flow. The manufacturing concept is based on aluminium cores for the integral stiffeners, while the outer skin is produced on a precise female tool. Prepreg material is draped around single cores, which are aligned to stringer-rows. In the last step the skin is being co-cured with the stiffeners. The concept has been verified by the manufacturing of 1,0m x 0,6m test samples. The test samples have been analyzed, using an optical 3D measurement system, turning the attention to the surface waviness which is induced by the stiffeners. In the next step, the integral design is implemented in a 2,4 m x 1,5 m section of a full-scale wing upper cover. The structure is designed and sized for representative load cases and the cruise-flight pressure distribution is used for calculating the surface waviness. In close collaboration between sizing, design and manufacturing the aerodynamic requirements have been fulfilled without weight penalties. The tool consists of the female mould for the outer skin and 36 modular cores for the integral stiffeners. Hollow cores are flown through by hot air in the autoclave and allow faster heat-up rates and help to reduce process-time and energy consumption. A tolerance concept ensures the exact positioning of the aluminium cores and the specific use of their thermal elongation for the consolidation of the prepreg material during the curing-process. The high precision of the mating surfaces facilitates the assembly process of the wing box by reducing costly production steps like shimming. For the assessment of the surface waviness a tool chain has been developed to superpose process-induced deformations with load-induced deformations. As process-induced and load-induced deformations act in opposite directions, both effects compensate partly and result in a reduced waviness. The resulting waviness can be exported to a numerical transition analysis based on computational fluid dynamics.

Christian Ückert, Olaf Steffen, Dr.-Ing. Erik Kappel, Tobias Bach, Lars Heinrich, Prof. Dr.-Ing. Christian Hühne , Dr. Markus Kleineberg German Aerospace Centre DLR e.V., Institute of Composite Structures and Adaptive Systems, Composite Design Department Deutsches Zentrum fuer Luft- und Raumfahrt e.V., Ottenbecker Damm 12, 21684 Stade