Words on the wing

Wrapping up a successful week during a hot and humid Farnborough Airshow by announcing a total of 74 orders and commitments valued at more than $4.25 billion, Bombardier celebrated a milestone in surpassing 500 firm orders and commitments for its CSeries aircraft. Four new CSeries customers were announced during the airshow, which sees the customer list grow to 20.

With over 40 years' experience in the design, manufacture, certification and support of advanced composite components, Bombardier Aerospace, Belfast is a centre of excellence for composites. Today, the Belfast operation has a composite manufacturing portfolio of some 30 components, including the design and manufacture of the advanced composite wings for the CSeries aircraft – representing a £520 million investment in the programme – and the composite wing skins and spar components for the Learjet 85 business jet.

Colin Elliott, Bombardier Aerospace, Belfast's vice-president, engineering and business development, begins by providing an update on the manufacture and testing of the CSeries advanced composite wings.

“The progress made on the manufacture of the integrally stiffened skin panels and spars has exceeded our expectations. Right at the start of the programme, recognising that the resin transfer infusion (RTI) process was new and therefore carried a degree of risk, we set about documenting any and all potential hazards and creating detailed risk mitigation plans. Over the next couple of years, we did a lot of research and development work, aimed at optimising the process and understanding the limits of the process capability. This work has really paid off and we have been producing really high quality parts from the start of production with little or zero non-conformances, such as porosity, reported.

“In terms of the geometric form of the panels, it has long been recognised that designers have to make allowance for variation in the final thickness of composite components. This is typically done by designing in a positive gap (so that you can be sure that mating parts will not clash or foul against one another), and then filling that gap with shim material during final assembly. During the R&D phase I just mentioned, we found that the RTI process created much more consistent parts and therefore we are able to significantly reduce the amount of shim material that we might need to use. With the introduction of local ‘intensifiers' during cure, we were actually able to totally eliminate shim in a number of areas, giving us a good pick-up in final assembly time and in the final weight of the assembled wing.

“The wing assembly has gone well too, thanks to the reduction in shimming, but also to the R&D we did on drilling holes and installing fasteners. In many areas of the wing, particularly in and around the main landing gear, we are drilling close tolerance holes through a stack up of different materials like titanium, then carbon, then more titanium and then aluminium alloy, so it is critical not just to control the diameter of the holes, but also to make sure that the internal bore of the hole in the softer materials does not get damaged by the swarf from the harder materials.”

Technology transfer

The Belfast facility has refined the resin transfer moulding (RTM) process, an advanced composites procedure which allows for the design and manufacture of components in a more integrated way than the traditional composites process, and has developed its own patented method it calls resin transfer infusion. RTI is a hybrid of RTM and autoclave processing involving dry fabrics to create the structure, into which the resin is injected after it is placed in the autoclave. Belfast has developed the RTI technology to manufacture large one-piece wing skins and structural spars for the CSeries composite wing.

“For the RTI process, we can use individual plies which are 2.5 to 3 times thicker than those used in traditional prepreg materials because the thickness of a pre-impregnated ply is limited in order to ensure the right resin saturation level. So we need many less individual plies of dry fabric to achieve the right strength and stiffness, thereby significantly reducing the labour content and cycle time.

“Regarding the materials testing element, we have to design for certain levels of assumed damage so our test specimens undergo high energy impact damage to simulate worst case damage that might happen in service. We then scan the damaged areas to assess exactly the size and depth of delamination, i.e. how far through the thickness and how many layers of the composite have become delaminated. We take all the scans beforehand and perform static and fatigue or endurance testing and then re-scan it to understand whether there has been any growth in the size or distribution of the delamination through the thickness. This is built into the basic design so that we can be confident there are no structural integrity concerns throughout the entire service lifetime of the aircraft.”

A material whirl

Along with other next generation passenger airliner offerings from Airbus and Boeing, nearly half of the CSeries airframe comprises composite materials, whilst aluminium-lithium makes up about a quarter. With rumours in the aerospace manufacturing industry suggesting that this ratio could change in favour of Al-Li, I ask Elliott whether Bombardier expects the balance to tip in favour of more advanced metallics over the coming the years?

“Of course the metal manufacturers are fighting back and going to great lengths to develop new alloys with better properties at lower cost and we fully expect that this will continue. However, it all comes back to horses for courses. There are limits as to just how thin you can make the structure in metal or composite because, for example, you still have to cater for accidental damage in-service, so improvement in mechanical properties does not always translate into a directionally proportional reduction in weight. Also, you have to consider the operational environment of the aircraft: a business jet with relatively low production rates has to be thought of differently to a commercial airliner with high production rates and very long production runs, so it's never as cut and dried to say metal is better than composite, or vice-versa.

“The choice of metallic versus composite materials includes so many factors, such as the experience and capability of the manufacturer, the type of aircraft and whether it be a business, regional or large commercial jet aircraft. The size, total sales, of the aircraft programme will have an impact on this decision too. If you're only planning to sell 300 aircraft then maybe the investment you'll need to set up a composite wing manufacturing plant won't give you a good financial return. But if you're planning to sell thousands of aircraft over 20 or 30 years, then it absolutely could be the right solution. There is no right or wrong answer; it just depends on the market, the application, the history, the experience and the culture of the individual company.

“I think the aerospace industry as a whole has demonstrated that composites can and do work, and can provide a weight advantage and a robust solution which is repairable and meets all the airline's requirements. We know we can do it and the airlines and general public are comfortable that we've proved it works and is safe. Does this mean that every new plane from now on is going to have composite wings? No, I don't think this is the case – it is horses for courses.”

I ask Elliott if any ongoing R&D activities are taking place at Bombardier, and whether the partnership with the Northern Ireland Advanced Composites & Engineering Centre (NIACE) and the two local universities has proved to be a fruitful one so far.

“Yes, for example, we're working with Queen's University on building a computer simulation of our wing factory so that we can mathematically model the flow from the raw material coming in through the fabrications area, through all the sub-assembly and final assembly lines. We have a very sophisticated model of the entire facility which will help us manage potential bottlenecks when production rates ramp up.

“We continue to explore new materials and new manufacturing processes, and much of the Universities' work is focused on this. We're also doing a lot of working looking automation for composites manufacturing.”

Onwards and upwards

Elliott is satisfied with the way the Belfast wing facility has been set up and is now fully operational, but I ask whether in retrospect, he wished that Belfast had done anything differently.

“No, I don't think so. So far, it has worked well for us. We started the production with the minimum level of equipment required, and now we are at the point of building up to the complete production rate capability, so we have a lot more equipment being installed. But in terms of the functionality and performance of all the equipment that was designed and made initially, I think this has all gone really well.”

In conclusion, I finish by asking Elliott to sum up any lessons learnt for Bombardier Belfast and its journey in optimising the production and manufacture of the CSeries advanced wing?

“If we were to do things differently, we would probably go to a larger number of suppliers to help spread the load. To be honest, parts supply was really difficult for the initial sets, so spreading the load at start-up and then later consolidating into a smaller number of suppliers might be a better approach.”

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