Natural Laminar Flow was one of the principal new wing technologies to be applied in the near future to reach the challenging fuel consumption and emission reduction objectives stipulated in ACARE’s Vision 2020. NLF air foils are shaped in such a way to avoid laminar to turbulent flow transitions in cruise which creates increased viscous drag, hence larger fuel consumptions and more emissions.
Krueger flaps are suitable as high lift devices for NLF, because they leave the upper wing surface clean when retracted. Furthermore, when fully deployed during take-off and landing, the main Krueger panels form a kind of deflector that protects the fixed leading edge from accumulation of insects or dirt and from surface erosion due to rain or hail during low-altitude flights. Such accumulations and erosions, resulting in wing surface discontinuities, could significantly increase friction drag by causing loss of laminar flow in the affected areas.
In the frame of DEAMAK, ASCO Industries N.V., a Belgian company involved in the development of the high-lift devices on all recent major aircraft programs (A380, A400M, Boeing 787, A350XWB, Bombardier C Series), designed and manufactured the Krueger flaps and the corresponding interface struts that were mounted on the new NLF outer wing sections of the A340 Flight Test Demonstrator. This aircraft was used within Clean Sky’s SFWA-ITD to among others assess the leading-edge insect-shielding efficiency of Krueger flaps during landing and take-off.
The proposed technical work was divided into four work packages dealing with the design, the manufacturing, the handling and transport as well as the validation and verification of the Krueger flap and wing interface struts. The project lasted 73 months in total. The budget was estimated to be €759,840.
Natural laminar flow (NLF) wings are among the key technologies being developed to reach ambitious reduction goals with respect to fuel consumption and emissions proposed in the Vision 2020 of the Advisory Council for Aviation Research and Innovation in Europe (ACARE). Krueger flaps are leading edge high-lift devices needed to increase the critical angle of attach at low speed flight phases and hence delay the onset of stall (sudden loss of lift force). They are particularly well suited for NLF wings since they deploy from the lower part of the wing, leaving the upper part smooth to facilitate NLF, as opposed to traditional slats used on most aircraft today. Furthermore, in extended setting, they protect the fixed leading edge of the wing against insect and debris impacts that could have a detrimental effect on the laminar wing characteristics in cruise phase afterwards, namely a preliminary transition of the boundary layer on the upper side of the wing from laminar to turbulent flow resulting in increased friction drag and hence jeopardizing the benefits of the NLF profile.
The Clean Sky Joint Undertaking-funded project 'DEsign And MAnufacture of Krueger flaps' (CfP DEAMAK) relied on the expertise of the project leader ASCO Industries - a family owned multinational headquartered in Zaventem, Belgium - in the development of high-lift device movables for all recent major commercial civil aircraft programs above 100 seats. Following the 6-year project DEAMAK within the Clean Sky Smart Fixed Wing Aircraft Integrated Technology Demonstrator (CS SFWA ITD) research program, ASCO engineers were be able to deliver the Krueger flaps mid-2017 for use on the Breakthrough Laminar Aircraft Demonstrator in Europe (BLADE). Practically, the outer wing sections of the Airbus A340-300 MSN1 were replaced by new NLF wing ones and several flight tests were organised within BLADE. At the end of the BLADE flight test campaign in 2018, 23 flight hours with Krueger flaps were foreseen to assess their so-called insect-shielding properties.
After a feasibility study including analysis of available space, stress, bird strike resistance, manufacturability and cost, ASCO researchers selected carbon fibre-reinforced plastic (CFRP) as material for the Krueger panels and developed a multi-cell concept. Following a detailed design phase during which ASCO conducted non-linear finite element bird strike analyses of the Krueger flap CFRP panel and metallic wing attachment structure, they have investigated manufacturing of the novel CFRP flaps via resin transfer moulding, including both experimental trials (coupon testing and production of sub-elements) and theoretical analyses enabling a design freeze of the structure.
In parallel a series of validation tests (from coupon level till reduced size risk reduction article production and testing) were carried out, enabling to give green light for the design and production of specific moulds for the full-scale parts.
During the last 2 years of the DEAMAK project, three Krueger Flaps prototypes were built at ASCO (mainly for RTM mould validation and US-NDT calibration purposes), all production process parameters (dry carbon fibre satin weave preforming parameters, RTM6 resin injection rates, curing times and pressures, etc.) were frozen and the flying shipset for BLADE was produced successfully. In parallel, all special processes like the preforming, resin transfer moulding, painting, drilling, blind bolt installation, visual and US-NDT inspections needed for the production, assembly and control of the DEAMAK structure on ASCO site were qualified by Airbus in the frame of BLADE, proving the ASCO team had invested sufficiently and had acquired the required knowledge to be able to produce airworthy composite parts.
Finally, the different Airbus design maturity gates were passed successfully and all required Permit to Fly inputs were prepared all along the DEAMAK project. The latter ranges from for instance stress notes and approved V&V matrices during the Engineering phase to several production related deliverables like FAI reports of all components shipped to the FAL and a FPQ QTR of the critical component (summarizing all successful assessments and testing done on the CFRP Krueger panel).