Aircraft Structural Design and Build

There is no doubt that the cost of producing the “next generation” of transport aircraft has to be the lowest possible, so that the aircraft will sell at a price which enables profitable airline operation. Advanced design concepts need to be developed to achieve the goal of reduced manufacturing costs.

Current design practice for Aluminium structures is based on the use of semi-finished products and mainly four categories are purchased in the following product form:

– forgings 6%
– extrusions 13%
– thin sheets 17%
– thick plates 64%

Clearly the use of thick plates keeps a major share within the total amount of Aluminium products needed. However very large quantities of the raw material being machined away in most current applications, end up in an extremely high buy-to-fly ratio.

Cost effective design of Aluminium components would tend to reduce the utilization of thick Aluminium plates and to increase the use of extrusions and forgings or to enter into entirely new manufacturing technologies in order to keep the buy-to-fly ratio as low as possible. Extensive research and technology programmes were initiated some years ago, throughout the Airbus system, to acquire and validate the necessary advances in the most promising manufacturing techniques, such as

– aluminium castings
– integrally extruded wing and fuselage panels
– welded structures.

Just to mention some predominant examples:
Casting is the most consistent “near-net-shape” process. The cost saving potential of investment castings is considerable for Aluminium alloys. Due to the good surface quality, practically no finish processing is required.

The successful example is an A320 bulk cargo door, which has now resulted in an industrial application study for the A340-600 Airbus passenger door.

Extruded panels for fuselage structures with integral stringers or alternatively, welded-on stringers compare very positively with today’s riveting process in terms of weight and cost reduction. On riveted structures there is a substantial additional volume of stringer material and additional skin thickness to compensate for the rivet holes.

Currently, the Airbus partners traditionally producing fuselage structures have made investments in large C02 laser beam welding machines capable of producing integral panels, about 4m wide and about 10m long. Initial test panels are being manufactured to optimize the process and to collect statistical data. High welding speeds up to 15m/min and a high degree of automatization allow a reduction in manufacturing costs of 20% compared to the automatic riveting process.

Aluminium extruded panels.

For wing structures the technology for heavy and very long extruded panels is under investigation. This principle has been used for many years by the Russian aviation industry, mainly on large transport airplanes designed by Antonov.

The large An 124 wing structure, for example, is built up from 44 extruded panels, up to 28m long. Indeed, machining is still necessary to a certain extent, but first investigations show a double profit which could be obtained from reduced buy-to-fly values as well as increased structural efficiency over conventional riveted structures.

Airbus wing design provides outstanding aerodynamic performance. However achieving this requires the use of complex double curvature wing panels, particularly near the root end where thickness is greatest. This presents an exacting manufacturing challenge, whether considering our current skins with fastened stiffeners or integrally stiffened panels. A process called Adaptive Creep Forming (ACF) is being developed by Airbus Industrie partners in consortium with various research organisations, which provides for rapid panel forming to a precise contour with software control of adaptive tooling. The ACF process also supports our automated wing box assembly project. This system, currently at the demonstrator stage, is to provide a jig-less, tool-less high throughput assembly, using machine visual feature recognition and laser position scanning. This manufacturing process promises maximised use of factory facilities. It offers increased production rates and flexibility to introduce any developed or new products that are within the machine geometrical capacity, in principle by software changes alone.

Welding of thin fuselage structures, using laser beam technology, for welding of thick wing panels, the friction stir welding technology is currently being investigated with very promising results.

Friction stir welding is a non-fusion solid phase welding process based on research in a consortium with the TWI. It is a continuous hot shear process. A non-consumable rotating tool made from a material harder than the work piece is passed along a joint between two closely butted sheets.

The friction heat creates a plasticized region around the immersed welding pin which consolidates behind forming a solid phase bond.

This process causes a more limited heat affected zone than conventional welding. This allows retention of a high proportion of parent metal properties for high strength heat-treated aluminium alloys.

It is also suitable for long joints and high tracking speeds, using machines similar to those for milling. It offers the possibility of low cost, high integrity assembly of integral wing structures.

These few examples of innovation in airframe design and manufacturing give an indication how Airbus can improve overall aircraft efficiency.

Advanced metal and composite materials combined with new manufacturing technologies are the possible means to face the economic challenge for meeting the expectations of the airlines for a 15% to 20% improved operating costs over current large transport aircraft.

One should not deny, however, that significant efforts will be required in innovative technical and industrial approaches, in a world in which progress has become an increasingly self generating process.

With limited opportunities in the current climate, UK companies like Marshall Aerospace are sustained by some long term contracts. MA has begun the structural design and build of the aero engine nacelles for the new advanced light jet, HondaJet.

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