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FOCUS ON DESIGN 5 6 | H I G H - P E R F O R M A N C E C O M P O S I T E S L E A R N M O R E @ w w w. c o m p o s i t e s w o r l d . c o m Read this article online at short. compositesworld.com/ROTORwing. and pressures as high as 275 psi/1.90 MPa); a 4-ft by 9-ft (1.22m x 2.7m) auto- clave (650°F/343°C, at 250 psi/1.72 MPa); and a walk-in oven, 9 ft by 30 ft by 8 ft (2.7m by 9m by 2.4m), able to achieve cure temperatures of up to 650°F/343°C. Symmetry déjà vu For economy, the wing was conceived as a constant section — that is, one long rectangle — rather than a tapered con- fguration, and its pieces are symmetrical in shape. "Not only are the wings sym- metrical in their upper and lower skins," says AC&A;'s manager Steve Smith, "they are symmetrical left and right. A single tool makes four separate halves: upper and lower/left and right wing. They are unique in that regard." If a wing is dam- aged on the aircraft, the user does not have to keep right and left wings on hand as spare parts; one wing section will re- place either the right or left wing. Similar design simplicity made it pos- sible for AC&A; to make a single tool for each major assembly — not only for the wing sections, but also for the fuselage and the four tail fns. Aluminum tools were used "because they are about half the cost of a composite mold and ade- quate for the size," says Smith. For mold release, "we are life-long us- ers of Frekote 700," from the Loctite line of mold release agents from Henkel (Bay Point, Calif.), Smith says. For all the skin sets, AC&A; hand-layed carbon fber prepreg from Axiom Materi- als (Santa Ana, Calif.), made from bidi- rectional 2x2 twill T300 (Toray Carbon Fibers America, Flower Mound, Texas) and Axiom AX 5201 toughened epoxy in a balanced laminate and ply schedule that Smith describes as "only a couple of plies." The wings have a ±45° orientation on the outer skin for torsional stiffness, Godlasky says. Each wing has four actuators, or ac- tuator-controlled ailerons (ailerons and actuators are visible in main drawing, p. 54), made from the same materials and process. "In VTOL mode, the ailerons are used for collective control (more or less lift) and cyclic control (tilting the ro- tor) of the VTOL function," Godlasky ex- plains. "As the rotor rotates around the body, the ailerons change pitch in every quadrant in order to make the helicopter pitch forward or roll right or left. For ev- ery revolution, these servo-controlled ac- tuators must go to full up/down position per second for directional control." Additional composite structures in- clude a nose cone that is bonded to the fuselage and contains fight electron- ics and surveillance sensors, as well as the cowlings for the engines. These ele- ments are each made in upper and lower sections, from the same materials and process. All parts were vacuum-bagged and cured in AC&A;'s autoclave to the indus- try standard of 250°F/121°C at 90 psi/0.62 MPa for two hours. Autoclave cure was indicated, Godlasky says, because of "some tight corners in the part that I think would be diffcult to mold without pressure." Rivetless bonding The skin sets and other parts — about 40 in all — are bonded together with Henkel's Hysol 9330.3, a Loctite EA (ep- oxy adhesive). The thixotropic formula is said to have high peel strength and ex- cellent environmental durability. Bond- ing of the parts is, of course, critically important because of the centrifugal and gyroscopic forces applied during lift-off and landing. "We didn't use rivets, but we designed the geometry of the mating pieces — the joints — to minimize peel," Godlasky says. Bonded parts include the cowlings; two wing spars that are off-the-shelf unidirectional carbon fber tubes over- wrapped with ±45° fabric and machined to size; and bulkheads inside the fuse- lage made from machined aluminum billets. Notably, six wing ribs (three per wing) were 3-D printed by AC&A;, using Fortus 900mc fused deposition model- ing equipment (supplied by Stratasys, Eden Prairie, Minn.), from acrylonitrile butadiene styrene (ABS). The ribs were bonded across the 0.75-inch/19-mm chord of the wingskins. The 3-D printing technology enabled dimensional fdelity within 0.007-inch/0.178-mm per layer, di- rectly from CAD 3-D models, Smith says. AC&A; assembled the frst article, a static model, but Dzyne assembled the fight-test article. Scaling up for success Dzyne expects a production contract from AFRL, pending a successful fight test. At that time, the carbon fber com- posite airframe design will be scaled up for ROTORwing production models. Page says the full-size UAV will have a 25-ft/7.6m wingspan, weigh only 250 lb/113 kg and carry 35 lb/15.8 kg of pay- load for 20 hours or 70 lb/31.8 kg for six hours. Normal flight orientation This graphic indicates the normal flight position for wings and tail. Note particularly the position of the tail fins, which have rotated forward and are locked in horizontal position. Source: Dzyne Technologies Take-off orientation This graphic illustrates ROTORwing's VTOL lift-off configuration. The position of the ailerons during rotation tilts the attitude of the inscribed rotational circle, which changes the craft's flight direction. Source: Dzyne Technologies 0714hpc FOD-OK.indd 56 6/17/2014 11:13:29 AM