HLFC leading edge in automated autoclave infusion

Kurzfassung

Maximising ECO-Efficiency of future aircrafts by minimizing friction drag is the main motivation of the “HLFC-HTP” project within Clean Sky 2. To achieve this, a HLFC (Hybrid Laminar Flow Control) approach at the HTP (Horizontal Tail Plane) was selected. This approach combines a suction system in the leading edge with a rather conventional, just surface optimized HTP box structure. Despite many other laminarity related projects in the past the major objective of the “HLFC-HTP” project is to provide a competitive suction system that is integrated in the leading edge structure and that is suited for series production without trading in aerodynamic drag reduction potential. The required suction pressure distribution at the leading edge is established by 22 chambers with adjusted vacuum pressure and distributed electrical compressors to provide the vacuum pressure. To minimize piping effort and piping losses the compressors are located directly in the leading edge with their respective outlets leading the exhaust through the spar into the HTP box. In order to address the space allocation aspect, the concept verification has been done on the basis of small-scale demonstrators with a representative cross section of the smaller, outer HTP leading edge. The major elements of the leading edge structure are a composite load bearing skin structure with a number of integrated spacers to build up the required chambers and a microperforated Titanium outer skin. The skin allows to remove the critical boundary layer and provides the required erosion resistance. In order to minimise tooling costs and lead time an open mould resin infusion process was selected to manufacture the rather complex composite prototype structures. Since the final series production was planned as an RTM (Resin Transfer Moulding) process it has been decided to use an autoclave and the option of a pressure assisted infusion. 5 different prototype structures with different preform configurations have been manufactured and assessed in order to identify the most mature and production friendly setup. Furthermore, a newly developed, fully automated “Digital Infusion Center” has been used for the final production sequence. The “Digital Infusion Center” accepts simple resin storage tin cans and uses microwaves to quickly adjust the target resin temperature. To record the infusion process the “Digital Infusion Center” precisely monitors the resin flow and other relevant process parameters. The Titanium skin is finally bonded to the inner composite structure using a sequential process with a specialized, pressure controlled bonding fixture. Currently the final prototype is prepared for the impact test that will show the structural performance of the novel design. Based on an estimated aircraft production rate of 15 A/C per month (long range, wide body) an assessment of the production sequence showed that series manufacturing cost could be reduced by more than 75% in comparison with the prototype production. The major cost reduction potential has been identified for the preforming sequence but also a lean and energy efficient, isothermal RTM infusion and curing approach would contribute to the cost reduction.