composite
CONSTRUCTION Building airplanes like full-scale flying models
eople have been building flying machines with composite materials since they first looked skyward. For some reason, this material seems to attract brothers. The first were named Wright, and Wilbur and Orville’s composite of choice was wood. At the turn of the last century, it’s all they had. Welding was in its infancy, riveting was how one turned iron into bridges, cloth was woven from natural fibers, and resins existed in nature, not a chemist’s lab. A mere 16 years after the Wrights’ first powered flight in 1903, Allan and Malcolm Loughead made the next step in composite construction. In a 21-foot-long concrete mold they laid three thicknesses of spruce plywood strips, well saturated in casein glue, in alternating directions. Before they bolted the cover on the mold they put a rubber bag over the spruce lay-up and then filled it with air. The next day they had a clean, smooth fuselage half for their S-1. In 1926, the brothers changed the name of their company to the phonetic pronunciation of their name—Lockheed—and the following year they introduced another composite speedster, the sleek Vega, made famous by aviators such as Wiley Post and Amelia Earhart. The technology they developed led to other composite wonders, like the de Havilland Mosquito and the Hughes HK-1 Hercules, better known as the Spruce Goose. Before the next set of brothers, Alan and Dale Klapmeier, added their innovations to the history of composite aircraft construction, scientists and chemists had woven glass, carbon, and graphite into
STORY AND PHOTOS BY SCOTT M. SPANGLER
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Mark Heimer, a technician trainer, teaches “blue coat” Aleasha Omlad how to lay up wing skin.
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The left fuselage half awaits its turn in the clamshell fixture, which mates it with its right side, and everything that goes inside.
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cloth and brewed a whole range of epoxy resins. When the Klapmeiers decided to design and build airplanes, starting with the VK30, they chose composite construction because it offered the greatest opportunity. “We had our fingers in each of the construction methods when we chose composite,” said Dale Klapmeier, co-founder and vice-chairman of Cirrus Design. The brothers learned tube and wood construction when they rebuilt an Aeronca Champ. “Before that we had a Cessna 140. It needed some rebuilding after I flipped it over, so we’ve hammered lots of rivets. And then we bought a Glasair kit.” There is no right or wrong construction method, said Alan Klapmeier, Cirrus’s chairman and CEO. Each of them has advantages and disadvantages for an aircraft’s particular mission. “To build a composite Super Cub would be a little pointless,” he said, “but it gives us the aerodynamics, crashworthiness, and esthetics [we want], and it’s what we know how to build.” The cost of getting started was another factor in their choice. As homebuilders they knew it was possible to build the necessary molds and fixtures at an affordable price, because they’d done it in their fam-
There is no right or wrong construction method, said Alan Klapmeier, Cirrus’s chairman and CEO. Each of them has advantages and disadvantages for an aircraft’s particular mission. ily’s Baraboo, Wisconsin, barn. Add the potential for innovation, and the decision was clear. “Building airplanes out of aluminum is a very EAA Sport Aviation
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mature process,” Dale said. “It’s optimized, and we’re never going to build an aluminum airplane better than Cessna can.” For inspiration they looked at the LearFan, among others, and they had a big poster of it on the wall while they were building their first airplane. An important lesson they learned from these inspirations was that replicating a metal aircraft in composites negates many of the material’s advantages, Alan said. Chief among them is the ability to use great big parts. There’s almost an inverse relationship between size and amount of work: an aluminum fuselage is composed of many pieces that must be joined by rivets; a composite fuselage has two halves that must be glued together. The opposite is true with smaller components, and this is “part of the reason we don’t do composite ailerons.”
Two-Part Production No matter what construction method a builder uses, airplanes are created by hand. All told, Cirrus has a bit more than 600 individuals dedicated to all facets of building airplanes. They are roughly divided equally between two shops, the assembly center in Duluth, Minnesota, and in Grand Forks, North Dakota, where they make 180 different parts. “The SR20 and SR22 each have about 130 [composite] parts,” said Director of Manufacturing Eric Hartwig. “Many of them are common to both airplanes, but the wings are different. The biggest part is the fuselage half, the longest is the [one-piece] main wing spar, and the smallest is the leading edge rib for the horizontal stabilizer.” One part in his office seems out of place. On Eric’s wall is a wooden wing rib. Noting that he grew up around sport aviation and attended EAA’s annual convention religiously, “I built that rib at one of the Oshkosh workshops; I’m not a pilot, but I’ve always been a builder… that’s what appeals to me,” he said. In an average month Grand Forks builds 7,500 composite parts. Each week Grand Forks sends 15 ship-sets to Duluth, and each week Duluth turns them into 15 airplanes. Production flows without interruption because “we are about 12 ship-sets ahead of them,” Eric said. To maintain this lead, Cirrus just added another 90,000 square feet to the 67,500-square-foot facility it opened in 1998. And no matter where you go, except for the curing ovens or storage rooms, it’s always 68°F and the relative humidity is 30 percent—or less. For all the creative shapes it makes, composite construction is a rigid process that doesn’t tolerate deviations. Working in the proper environment, builders must perform the proper steps in the prescribed time. Any amount of moisture can cause quality problems, Eric said, such as “porosity, small trapped bubbles in the laminate.” 46
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Two Cirrus technicians prepare a horizontal stabilizer for a “squish test,” which ensures the proper bond thickness between the ribs, spars, and skin. The computer-controlled fabric cutter marks and cuts each piece with minimum waste. With the parts held fimly by a jig, a Cirrus technician drills their necessary holes.
The “clamshell” mates the fuselage halves around the interior bulkheads, holds everything in precise alignment, and heats the joints to cure the glue.
“People putting together glass airplanes really have to take incredible care,” said Dale. “If they have a part ready to bond on Sunday night, but they don’t get to it until next Saturday, they have to treat it as though it’s not been prepped; you have to start over to make sure it’s right.” The worst-case scenario, said Alan, is a builder who prepares parts for bonding but decides to clean and lubricate something with WD-40. “Oh, and then we’ll bond these two parts together. That’s not going to be a happy ending. You don’t spray (any type of lubricant) in the building …until you’re done with all the composite parts. You will find no silicone spray in this whole building.” Cirrus creates most of its parts out
For all the creative shapes it makes, composite construction is a rigid process that doesn’t tolerate deviations. of E-glass (the E stands for electrical, because it’s used to make circuit boards) with a 7781 weave style, said Eric. “It’s an eight-harness satin, which means a bundle of glass fibers under seven other bundles and over one…or is it under and over one? (Either way, it’s a tight weave) almost like cloth for a shirt. It has very few open spaces in it; that affects the coarseness of the finish and how well it drapes or conforms to complex shapes without wrinkles.” In a few parts, such as the fuselage roll cage and all spars, Cirrus uses unidirectional S-glass, which provides additional strength in these load-bearing components. Both types 48
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With every piece held in alignment with pins, a multi-armed tool combines the parts that compose the cockpit floor and bulkheads.
of glass are pre-impregnated on one side with a thermostatic resin, and in the composite vernacular it’s known as OST (one side tacky) pre-preg. Its biggest advantages is that it limits the builder’s exposure to the resin, to which some are allergic, Eric said, and it allows close control of resin content. Because the resin cures with heat, it resides in a drive-in freezer kept at 0°F. “The material has a shelf life of 30 days at room temperature,” Eric said, and the cold stops the curing process. Each roll is 50 inches wide and 225 yards long, and each day the computer numeric controlled (CNC) cutters turn 27,000 square feet of material into lay-up plies one layer of cloth at a time. The cutters also label each ply, and “roughly 70 to 75 percent of the material that comes in goes out as airplane parts.” Each part is laid up by hand, and safety is a part of every step. New employees, called “blue coats” for the shop garments they wear until they get uniforms, get safety training related to their job, Eric said. Safety and proper procedures are reinforced on the job as experienced technicians work with their blue coat. Laying up a wing skin, Mark Heimer is teaching Aleasha Omlad how to apply the plies of OST, which is like working with contact paper. With a smooth plastic blade or a piece of cloth that slides over the pre-preg’s dry side better than their latex gloves, they smooth the cloth. They measure overlaps the proper distance, cutting off anything extra to control weight. To position foam core (which provides additional rigidity and impact protection) and locate island plies that reinforce something like a fuel tank filler, they place a detail-locating template in the fixture and poke a fine tip Sharpie into the holes. When they’re done, another crew EAA Sport Aviation
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The “glue” that holds the components together is a twopart epoxy precisely mixed by a computer.
will vacuum bag the part and prepare it for curing. A barrier film separates the part from the breather layer, which ensures equal vacuum force over the part. The edge of the mold is lined with tacky tape, which holds the bag layer in place. After installing the vacuum ports, the crew does a leak test that measures the bag’s integrity. Then it’s off to one of three 12-by-15-foot natural gas curing ovens. Curing is a five-hour process that tops out at 260°F for 100 minutes. After the parts are connected to a vacuum source and dotted with thermocouples, a computer ramps up the heat
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2 or 3 degrees a minute. Because there’s such a huge variety of sizes, shapes, and laminate thicknesses, the heat is controlled by the part last to warm up. “Everything has to be at the same temperature before it’ll take the next step up,” Eric said. After curing, and before the part is released from the mold, a drill guide is attached to precisely locate the part on its assembly tooling. Once released, the flashing is removed and the remaining holes are drilled. Before the translucent parts head to Duluth they pass over the inspection department’s light tables, where technicians look for problems such as ply separations or inclusions. “The blue plastic backing off the pre-preg is our most common inclusion,” Eric said. In the factory’s lowhumidity environment, the pre-preg builds a static charge that tends to pull everything toward it. You can’t see through the ply during lay-up, but the parts are easy to inspect once
Out of the clamshell, this fuselage will soon lose its flashing, which helps hold the part in position during assembly.
they’re cured, so attention to detail is crucial. So is keeping on schedule, for curing composites wait for no one, which is one reason the Grand Forks facility runs 24 hours a day with three shifts.
Model Building Assembling composite airplanes has come a long way in the last decade, and “we’re gluing them together now like the plastic models (we built) when we were young,” Dale said. Except the airplanes are full-scale and come in semi-trailers instead of a box. Liz Anderson and Tammy Armstrong build the cockpit. Working with a many-armed fixture, using plastic pastry bags, they apply the computer-mixed two-part epoxy to bulkheads and other parts that con-
nect to the cockpit deck and swing the arms into their closed positions. When they’re done, the cockpit fixture mounts a pedestal in the “clamshell,” which holds the fuselage halves in place around it. These fixtures not only hold the pieces in place, but also heat the joints to cure the glue. After curing, the clamshell releases
the fuselage, which moves about 10 feet to the trimming booth. Because of particulates in the air, such as the composite dust created by trimming the flashing off the fuselage, the booth changes the air in the building every 5 minutes. “If you threw a handful of talcum power in the air, it would be gone in five minutes,” said Mike Coon, director of product
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Courtesy of Cirrus Design
Cirrus pioneered the use of airframe parachute technology and has been a leader in installing safety features in its aircraft, such as this SR20/22-G2 model.
assurance (PA), which is what Cirrus calls its quality assurance program. The wings and horizontal stabilizer follow a similar production path. On the long list of things the PA inspectors check, bond thickness—the depth of the adhesive between the two parts being glued together—is right at the top. For certain parts the
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bond thickness is 80-thousandths of an inch, he said. Depending on the parts, there are several ways to verify the depth. “You have dry pin checks and wet pin checks,” he said, explaining that the pins are akin to feeler gauges used before (dry) and after (wet) the glue has been applied. In some cases
the inspectors use a backlight inspection, which reveals air bubbles in the bond. To verify the bond thickness of joints inspectors can’t get to, which is the case with skins on the wings and horizontal stabilizer, inspectors do a “squish” test, Mike said. Before they lay down the glue on the joints, they cover the ribs and underside of the wing skin with thin plastic sheets and then close everything up. After the glue cures they lift off the skin and inspect the bond thickness that’s trapped between the two sheets of plastic. If it’s good to go, they repeat the process without the barrier sheets. Monitoring the working environment is also on the PA to-do list. In each of the Duluth assembly center’s bonding shops are temperature and
humidity monitors. These shops are far removed from the finish area, where platoons armed with pneumatic orbital sanders massage large components—fuselages, wings, and horizontal stabilizers—before they are mated. As with every other aspect of construction, safety is key. Everyone wears respirators, goggles, gloves, and earplugs. At periodic intervals, Cirrus technicians stop working to run through a stretching routine designed to offset the strain of the repetitive tasks they perform. And they work four 10-hour days, which gives them three-day weekends. No matter what the future of composite construction holds, Alan Klapmeier said, getting hand labor out of the equation is the key to success. One possible future might be thermoplastic. “You heat up your sheet, form it, and it cools instantly, and there it is.” With the future development of this technology, composites would approach the mass production
One possible future might be thermoplastic. “You heat up your sheet, form it, and it cools instantly, and there it is.” With the future development of this technology, composites would approach the mass production capabilities of metal. capabilities of metal. Another possibility is RTM—resin transfer molding, Alan said. “You load up a mold with a preformed glass carbon or some other kind of fiber, close the lid, hit the button, resin is injected, and the air is sucked out. It’s all a matter of reducing the man-hours and cycle times, increasing strength. But the key will be cutting hand labor.” It’s possible, however, that somewhere in the 21st century another Louis Béchereau is at work. The original was the engineer for Deperdussin, a French aircraft manufacturer.
His contribution was the composite monocoque fuselage, three plies of tulip wood strips formed around a mold. Powered by a 140-hp Gnome rotary engine, the resulting singleseat monoplane won the 1912 Gordon Bennett Race and set a new world speed record of 108 mph. Because Béchereau was a decade ahead of his time, aviation turned its back on his innovation in favor of biplanes. Given the growing number of composite aircraft in today’s fleet, only time will reveal future revolutions in aircraft construction methods.
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