Keel beam design poses many engineering challenges for Airbus
Airbus Aerospatiale composite keel beam. |
The A340 fuselage extension required by the -500/-600 project has created a considerable engineering and manufacturing challenge for Airbus. The extension required engineers to develop a new large keel beam, measuring 16 m in length. The keel beam provides rigidity in the fuselage center area where the landing gear is located. Due to weight and rigidity requirements, a design consisting of composite panels fastened together to form a beam of trapezoidal shape was chosen. Two beams are used on each aircraft. The level of loads to be transferred in the keel beam required the selection of fasteners with diameters above 10 mm.
The keel beam is constructed by fastening composite panels and longitudinal beams with the thickness of components being between 8 mm and 20 mm. The panels are constituted by several layers of unidirectional carbon fibers cured in an autoclave specifically installed for this application.
Load repartition comparison between threaded fastener and Lockbolt.
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In addition to being a design challenge, the keel beam also posed many manufacturing issues. The thickness accuracy of keel beam components required the company to develop a controlled molding technology specifically defined to obtain constant panel thickness, but mainly to generate a constant thickness of radii on the longitudinal beams throughout the length of the beam. New rotating jigs, designed to take into consideration ergonomic criteria, allow the operators to face parts always in the best conditions to optimize the handling and the efficiency of drilling and assembly tools.
Titanium pins and lockbolts served as the preferred way of fastening the beam for mechanical performance, weight savings, and manufacturing reasons. In critical structural applications, the consistency and regularity of preload is key. The swaging of a collar, at the base of the lockbolt concept, eliminates the friction between parts that is the root cause of the preload scatter of threaded fasteners. The specific installation cycle of a lockbolt generates installation loads that flow the sealant and eliminate the requirement for retorquing after curing, which is often required of threaded fasteners.
Standard Huckcomp lockbolt. |
In addition to mechanical performance the ease of installation was another reason for the selection of lockbolts. The axial pull-push cycle required to install a lockbolt simplifies the installation process, which is accomplished with a hydraulic tool.
Another element improving installation parameters is the wider grip range of the lockbolt in comparison to threaded fasteners (2.4 mm vs. 1.5 mm). This extra range provides grip overlap, which reduces the possibility of incorrect installations, particularly in composite panels, in which thickness variations are more common than in metallic structures.
The A340-500/-600 keel beam development has resulted in the requirement at Airbus for lockbolt of diameters up to 15.9 mm for composite and metallic structures. For these larger diameters, the solution of a removable pin tail has been chosen to avoid the shock and noise that such diameters would generate with a standard type pin tail. In the removable pin tail lockbolt, the swage load is controlled by the tool pressure. The installation tools are consequently modified, adding electronic devices to monitor and record installation parameters.
Removable pintail lockbolt. |
The selection of an all-composite keel beam has permitted a saving of 400 kg on a total weight of 2000 kg for each of the two keel beams. The tendency for thicker panels and even larger diameters for the A380 are a reality that Airbus is facing. Therefore the company continues to develop technologies for very thick composite panels while its fastener suppliers consider expanding lockbolt diameters above the present limit of 15.9 mm.
Information was provided by Philippe Leseur of EADS Airbus France, and Massimo Cella and Arnaud Brunet of Huck Fasteners.
