In a large building in Belfast very near where thousands of laborers hammered thick steel plates to massive ribs and fittings using thumb-size rivets to build the ill-fated Titanic ocean liner, Bombardier Aerospace is carving out its own advanced technology niche, building wings for new aircraft models almost entirely from composite materials. In an odd coincidence, the famed Harland and Wolff shipyard that built the Titanic now focuses most of its efforts in the composites business, manufacturing enormous power-generating wind turbines.
For Bombardier, the Belfast facility is more than just a manufacturing center. The capital of Northern Ireland is a relatively high-cost area, and to keep the 1,100 employees there productive, they have to add value to the products that they make. “We’re not going to be sustainable if we just manufacture things,” said Michael Ryan, vice president and general manager of the Belfast facility. “We manufacture, integrate and support our products, with progressively more value added.”
This means that for the CSeries airliner and the Learjet 85 business jet, Belfast is responsible for designing, manufacturing and integrating the jets’ composite wings, including systems, flight controls and high-lift systems. Belfast engineers also developed the patented resin-transfer injection (RTI) technique used to manufacture the CSeries and Learjet 85 wings.
The composite wings are made in four main parts, with a front and rear spar and integrally stiffened top and bottom skins, all using composite materials. The wing ribs are aluminum (some titanium parts are used near the wing root and for the landing gear mount). The reason for aluminum ribs is that composite ribs would have to be much thicker and heavier to handle out-of-plane bending or shearing, according to Ryan. “Composites are good when you apply the load along the plane of the composites,” he said. “We can make lighter ribs in aluminum than carbon.”
The RTI process has significant advantages over more traditional composites manufacturing techniques such as resin-transfer molding (RTM) and composite layup. With layup, carbon fiber prepreg (pre-impregnated with resin) is laid into a mold, then the material is held tightly to the mold using vacuum bagging (applying suction to a layer of rubberized material laid on top of the carbon fiber), then the whole piece is baked in an oven or autoclave. An autoclave uses temperature as well as pressure to finish the piece.
RTM is simpler, involving placement of dry fiber into a mold of the final product. For example, North Coast Composites, Cleveland, Ohio, uses RTM to make rudders for the Gulfstream G250 business jet. Once the carbon fiber is placed in the mold, the mold’s two halves are bolted together, then resin is injected into the mold. The quality of the final product is dependent on the quality of the mold tooling, the goal being avoiding the need for a lot of final machining after the part is removed. Just for the G250 rudder, the mold tooling weighs 38,000 pounds.
While it is possible to make tooling large enough to build a wing using RTM, the tooling would be massive; Bombardier would have to borrow the gantry cranes–named Samson and Goliath–from Harland and Wolff to lift those, and maneuvering such heavy and huge bits into the autoclave would be impossibly tricky.
RTI uses one side of the tooling (the outer mold-line tool) as the hard surface to form the finished outer side of the part, like the upper or lower wing skin. Like RTM, dry carbon fiber is laid onto precision jigs, but the inside of the part is not formed by the tooling but by vacuum bagging material. Each ply of the carbon fiber–cut into the desired shape by an ultrasonic cutter–used for RTI is three to four times thicker than the prepreg material used in layup construction, which simplifies the construction process and lowers the chances of making mistakes. And prepreg has a limited shelf life, usually 30 days.
“We don’t have that issue,” said Colin Elliott, vice president of engineering, business and product development. “The outer mold-line tool is conventional,” he explained. “The clever stuff is how you vacuum bag it and keep the pressure on. We call it a flexible mold-line tool.” Bombardier Belfast is experienced with prepreg manufacturing, as it makes the Global Express horizontal stabilizer using prepreg materials and has been making composites parts for more than 40 years.
Once the part is laid up in precision production assembly jigs then vacuum bagged, it is placed into the autoclave, a 70-foot-long, 18.5-foot-diameter monster. Bulbous vats outside the autoclave squirt resin through pipes and tubes into the part, in the proper proportion needed to strengthen the carbon fiber. After a cure cycle involving precise temperatures (up to 370 degrees C) and pressures, the part is removed and sent to a five-axis machine tool for final trimming.
The machining is done with a high level of precision, not just the trimming of the composite material using waterjets (holes are drilled with mechanical cutting heads), but the way the part is held in place. Vertical holders called pogo sticks apply suction to the part in exactly the aerodynamic profile of the wing while the finishing is done in a tightly choreographed numerically controlled process. After finishing, each part undergoes nondestructive testing using ultrasonic scanning techniques, then they are sent to the paint booth.
While some might assume that composite parts are lighter than the same part made of metal, that isn’t necessarily the case, according to general manager Michael Ryan. “The overwhelming advantage is fatigue,” he said. And that advantage grows as the part ages because it doesn’t corrode. “Even though there’s no weight advantage, it still pushes the choice [toward composites].”
To prove the ability of composite structure, the Belfast facility has been running tests on a CSeries wing, which is the first three quarters of the important wing structure minus the last 12 feet to the tip. The wing root is mounted to a dummy fuselage structure that replicates the actual mounting scheme, and the wingbox structure includes the titanium landing gear mount. During testing, 12 hefty hydraulic actuators pushed on the test wing, forcing it to deflect 24 inches at the outboard end and endure 150 percent of the ultimate load, which it did without breaking. The wing structure is equipped with 2,100 strain gauges.
“That [hydraulic test rig] will break anything we need to break,” said Neil Campbell, head of the experimental and ground test facility. And on composite parts tested to destruction, he added, “It’s actually very quiet. When it breaks, you have a bit of a bang. But that’s not what we want on this job. Once it’s broken, you can’t test another failure case.”
Of course, there is much more to Bombardier Belfast than the new 600,000-sq-ft composites manufacturing and assembly facility. As quiet as the activities in the new composites buildings are, next door there is a bit more noise as assembly technicians rivet together traditional ribs, stringers, bulkheads and sheets of metal to build Challenger 300, Global and CRJ fuselages.
Later this year, the first production CSeries composite wings will begin flowing out of the Belfast facility in preparation for the new jet’s entry into service in 2013.