Aerospace components have contributed to the evolution in the efficiency of airframe and aircraft structures over the last 15 years. There are very few truly revolutionary technologies, like the jet engine, and continuous improvement is the sum of many small steps. On today’s generation of commercial aircraft, aerospace composites provide a weight saving of the order of 1% of the Operating Weight Empty. The competitive environment in commercial aviation drives us to seek further ways of reducing Direct Operating Costs (DOC). The largest contribution is the first cost followed by fuel.
It would be convenient if we could concentrate our research and technology in the largest sector of the ‘pie’. It is unlikely that this approach would result in a large enough improvement in DOC so we have to try to improve in all sectors. The implication for structures is that we have to reduce weight while reducing the overall manufacturing cost of the aircraft components.
One option is to increase the use of aerospace composites. The current generation (A320 and A340) typically have around 15% by weight of composite in the structure. If the main wing box was to be composite, this proportion would increase to 40%. However, to realize the potential weight saving presents several formidable technical and economic challenges. The solutions to these challenges are beyond the current state of the art.
At BAe Airbus, the Composite Wing Programme has researched the most effective way of making a wingbox from carbon fiber composite. The importance of reliability and maintainability fleet engineering of the structure in service is recognized.
As the development costs of large technically advanced aircraft continue to escalate, the need to have a product which meets the requirements of the customer and hence will sell successfully in the market becomes ever more critical. The substantial investment which is required to introduce a composite wing box onto the next generation of large civil airliner demands that the product must achieve customer approval and if possible exceed expectations as soon as it enters into service.
Thin composite laminates, typically on removable structures, is where the majority of the non-metallic parts exist at present. The industry introduced advanced composites cautiously to ensure the capabilities of these new materials hence the majority of carbon components used on current civil aircraft is of removable aircraft structures. Moving aerodynamic surfaces, radomes, fairings, and engine cowl doors are all typical examples of composite components and in-service experience is showing that there are advantages and disadvantages for both materials. For example, composites eliminate the corrosion problems which metallics suffer (good design and manufacturing practices must be exercised at the carbon to aluminum interfaces), but are more prone to erosion on exposed edges while honeycomb structures, of either material, is generally more complex to repair than monolithic.
For structure specific to wings, trailing and leading edges contain many lightweight composite parts which, due to their location, are vulnerable to impact from service vehicles and other hazards while on the ground. (This is equally applicable to metallics but less so for a wingbox which is protected by these ‘bumpers’). If damaged, it is generally accepted that such components are more costly to repair than their equivalent metallic parts. Damage assessment, preparation, cleaning and curing of bonded patches are time-consuming operations and it is of paramount importance to an operator that this type of work can be completed within the scheduled maintenance downtime. Also, availability and interchangeability of repair materials needs to be improved and this again risks leading to longer downtimes; storage of the materials if required is expensive and often have to be purchased in large quantities.