Bombardier’s Belfast facility preparing for composite wing manufacturing
In a large building in Belfast near where thousands of hard-working laborers hammered thick steel plates to massive ribs and fittings using thumb-size rivets to build the Titanic, Bombardier Aerospace is carving 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. Belfast is a relatively high-cost locale, 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, v-p and general manager of the Belfast facility. “We manufacture, integrate and support our products, with progressively more value added.”
This means that for the C Series airliner and the Learjet 85, 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 C Series and Learjet 85 wings.
For the C Series, Bombardier has released design data for the composite tooling, modeled parts in a digital mockup, defined design and routing of system components and begun releasing detail parts drawings. Bombardier has also developed the strategy for structural test and certification and for HIRF and lightning-strike evaluation and agreed on those plans with certification authorities. Later this year, the first production C Series composite wings will begin flowing out of the Belfast facility in preparation for the new jet’s entry into service in 2013.
The Learjet 85 wing, while mostly composite, has aluminum ribs and root structure as well as other parts made with aluminum. Delivery of Learjet 85 spars “is in line with our Mexico facility schedule,” a Bombardier spokeswoman told AIN. The Mexico facility is building Learjet 85 subassemblies, which will be shipped to Wichita for final assembly. The Learjet 85, too, is scheduled to enter service in 2013.
Streamlined Composites Process
The composite wings are made in four main parts, with a front and rear spar and integrally stiffened top and bottom skins, all made of composite materials. The wing ribs are aluminum (some titanium parts are used near the wing root and for the landing-gear mount). Bombardier uses aluminum ribs because 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 ribs lighter 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 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 of Cleveland uses RTM to make Gulfstream G250 rudders. 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 hinges on the quality of the mold tooling. The goal is to avoid 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 the tooling, and maneuvering such heavy and huge assemblies 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, such as the upper or lower wing skin, according to Colin Elliott, vice president of engineering, business and product development. As it is in the RTM process, dry carbon fiber is laid onto the tooling. However, with RTI 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 used in layup construction, which simplifies the construction process and reduces the chances of making mistakes. And prepreg has a limited shelf life, usually 30 days. “We don’t have that issue,” said Elliott. (Bombardier Belfast is experienced with prepreg manufacturing, as it makes the Global Express horizontal stabilizer using prepreg materials, and has been making composite parts for more than 40 years.) “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.”
Once laid up in precision production assembly jigs and vacuum bagged, the part 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 non-destructive testing using ultrasonic scanning techniques before entering the paint booth.
Ultimate Load Testing
While some might assume that composite parts are lighter than the same part made of metal, that isn’t necessarily the case, according to 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 three-quarters of a C Series wing–the important structure minus the last 12 feet to the tip. The wing root mounts to a dummy fuselage structure that replicates the actual attachment arrangement, and the wingbox structure includes the titanium landing-gear mount. During testing, 12 hefty hydraulic actuators push on the test wing, forcing it to deflect 24 inches at the end of the wing and endure 150 percent of the ultimate load, which it tolerated 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 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.