“My first exposure to the concept which later became C-PLY was as a student in Dr. Stephen Tsai’s ‘Composite Design Workshop’ offered by Stanford [University],” says Skillen. “The approach made a lot of sense,” he recalls. “Rather than try to make laminates that approximate isotropic materials, C-PLY is designed around the fiber properties and ply orientations (angles) that enhance laminate performance.” (See “Learn More,” p. 64.)
C-PLY combines several positive traits: It is a noncrimp fabric (NCF), it features anisotropic biaxial construction, and its fibers are oriented at a low angle â€” it is sometimes described as “bi-angle” due to its 0°/Î¸° construction, where a low value of Î¸, e.g., 20° to 30°, reduces interlaminar stress, enhancing load transfer between plies without matrix cracking. Further, its spread-tow construction makes it remarkably thin.
Although quasi-isotropic symmetric lay-ups (traditional “black aluminum”) are commonly used in composites because they mimic the properties of the metals they replace, computer-based analytical tools enable detailed ply-by-ply analysis, allowing designers to exploit the benefits of low-angle anisotropic design. The end result is better properties for the same weight or as much as 40 percent less weight than an aluminum structure for similar performance.
The use of a larger number of very thin plies reduces interlaminar stress and enhances toughness. “Like in a beetle shell, an optimized composite theoretically would have layers just a few fibers thick,” explains Skillen. C-PLY uses 12K, 24K, 48K or 50K tow, spread very thin but without gaps. The 12K spread tows VX Aerospace has used so far (0.003-inch/0.076- mm ply thickness) are much thinner than most unidirectional (UD) prepregs and weigh in at only 75 g/m2 (2.2 oz/yd2).
VX Aero expects to have a steady source for the new material. Chomarat is installing a production line (scheduled to be operational by mid-year 2014) capable of producing any stitched multiaxial configuration, but optimized to produce 100-inch/2.5m wide NCF with angles from 30° to 90°. Multiaxials with angles of less than 30° are available in a reduced width. Skillen emphasizes that the stitched two-ply fabric VX Aerospace has been using â€” totaling 0.006-inch (0.152-mm) thickness and 150-g/m2 (4.4- oz/yd2) weight per layer â€” is tailored for structural loads but is far easier to place and much less equipment-intensive than automated tape placement.
Currently, two composite aircraft projects occupy the company’s 17,000-ft2 (1,579m2) production space. Both will make extensive use of C-PLY. The first is the Falcon, a sleek, low-wing, high-performance sport aircraft esteemed by some as “the Ferrari of light aircraft.” Originally built in Hungary, a handful of the planes made it to the U.S. before the manufacturer, Corvus, went bankrupt. A groundswell of demand spurred an effort to put the plane into production in the U.S. Via a memorandum of understanding with Renegade Light Sport Aircraft (Deland, Fla.), VX has assumed responsibility for design, engineering, tooling and manufacture of the Falcon airframe, while Renegade controls sales, marketing and FAA certification.
The other aircraft is a 1:4 scale version of the VX Aerospace designed VX-1 KittyHawk. Its unique blended wing/fuselage is designed to exploit aerodynamics and potential alternative fuels to cut flight cost per mile by a factor of three while offering more internal volume and payload. Like its Wright Brothers namesake, the KittyHawk is poised to score a first of its own: It will be the first aircraft to fly with C-PLY laminates.
Skillen believes that the new material combined with OOA processing can enable small composite airframers the opportunity to produce structures with the same precision and performance as those fabricated by big aerospace OEMs and primes, but at a fraction of the latter’s delivery time and cost.