Design Results

• Production of single-use, consumable wings for weapons or UAVs is automated, using the pultrusion process.
• Wing design meets flight performance requirements despite sacrificing some aerodynamic details for more efficient, lower-cost manufacture.
• Pultrusion permits integral stiffening ribs to be incorporated into the wing in a single-step process.

 

The unmanned aerial vehicle (UAV) has become a potent weapon in the U.S. Armed Forces arsenal. These remotely operated “drones” have made headlines by providing military commanders a means of gathering intelligence and delivering ordnance with precision. A growing trend in the UAV market involves relatively small, single-use UAVs that, equipped with cameras and wireless communication gear and spared the concern of pilot fatigue, can loiter for days on surveillance missions and might be deployed in “swarms” to guide and protect soldiers and convoys or act as defensive decoys when launched from a manned aircraft (see “Learn More,” at right).

Designing an expendable UAV from composites presents an obvious challenge: How to produce a viable aircraft capable of carrying mission-specific communicators, sensors or other equipment at a cost low enough to justify a single flight?

One option is to use low-cost, automated methods for key parts. KaZaK Composites Inc. (Woburn, Mass.) has demonstrated the feasibility of pultrusion with innovative designs and manufacturing strategies for UAV wings, as well as other high-value parts for space missions and military applications.

To make expendable aircraft feasible, production and material costs have to be kept to a minimum, says KaZaK’s president Jerry Fanucci. Although pultrusion often fits the bill, “we don’t think of ourselves as a pultrusion company,” he maintains, pointing out, “we design composite parts, and if pultrusion is the right solution, then we use it.” According to Fanucci, KaZaK is a full-service design and production company, capable of developing not only UAV wings but also entire unmanned aircraft, including selection of the airfoil shape and definition of vehicle aerodynamics. “It ultimately depends on the performance requirements, but if pultrusion is the answer, then it works for aerospace as well as for industrial applications.”

Designing for low-cost manufacture

As most composites engineers know, pultrusion is a continuous molding process. A creel system pays out reinforcing materials, in the form of unidirectional roving, woven cloth, multiaxials, braided preforms and/or nonwoven mats that are continuously drawn through a moving production line by a system of alternating pullers. Reinforcements, in some cases, pass through a preheating furnace, which heats and dries them to improve resin wetout. Materials then are positioned or shaped as required. For example, woven cloth could be folded over or placed off-axis as it passes through one or more forming devices. The materials then are passed through an open resin bath with a doctoring system that afterward removes excess resin. Alternatively, it enters an enclosed injection die, where resin is forced under pressure directly onto the reinforcements. The wet-out materials then immediately pass into a heated steel pultrusion die, where they are compressed to the final part shape and cured. When the cured part exits the die, it can be cut to the desired length as it moves, with a traveling cut-off saw.

Pultrusion produces parts with constant cross-sections, most commonly simple tubes, rods, beams or flat plates — but any complexly-shaped two-dimensional profile is possible, and even tapering profiles can be considered, notes Fanucci: “A common misconception about pultrusion is that it can only produce parts with unidirectional reinforcement but, in fact, almost any multi-angle laminate can be achieved by using pre-plied broadgoods or inline filament winding.” As a continuous automated process, pultrusion can produce very high-quality parts with tight tolerances of virtually unlimited length, he adds.

“Because the reinforcements are always under tension, you can achieve very high fiber alignment and straightness,” he says, contending that such precision is often unattainable “with a hand layed part cured in the autoclave, where the autoclave pressure can cause some distortion.”

Costs are significantly lower for pultrusion than for any other composite production methods due to several factors. Dry reinforcements and liquid resin together cost less than prepregs, and labor costs are very low because the process is automated. Other cost savings can be gained in the tooling. Pultrusion dies are short, normally only 2 to 4 ft (0.6 to 1.2m) in length, so they can be produced fairly economically. “No matter how long your part or the number of copies,” says Fanucci, “the tool cost is the same, and relatively low compared to a layup tool.”

Evolving concepts for manufacturable wings

With an eye to these advantages, the company pursued and received two Small Business Innovation Research (SBIR) grants to design wings for weapons. The grant funded programs that evolved into a design for a telescoping wing for a swarm-type, long-loiter UAV, as well as the design for the company’s own disposable gliding UAV, developed for the U.S. Navy as an expendable, GPS-controlled sonobuoy delivery system.
To keep part costs as low as possible, some design compromises had to be made, notes Fanucci. One of the wing projects, for example, involved an SBIR customer that provided the external aerodynamic wing shape, with an approximate 2-inch/50-mm chord. Previously developed as part of the client’s overall vehicle design, and necessary to achieve the UAV system’s performance, the wing shape was non-negotiable. But the means by which KaZaK might provide the stiffness required to combat air loads and produce sufficient lift in the wing was not prescribed and included a range of solid, hollow and internally ribbed options.

Based on the given wing shape, KaZaK employed a finite element model based on air loads and stiffness requirements. The goal was to determine the composite laminate structure that could best handle the loads by manipulating the fiber architecture while staying inside the required outer aerodynamic shape. The company primarily used ANSYS software from ANSYS Inc. (Canonsburg, Pa.) and SolidWorks from Dassault Systèmes Solidworks Corp. (Woodland Hills, Calif.) to create the multi-ply wing laminate. But Fanucci says the company also has access to other FEA and CAD programs, when required by an application. The FEA analyses prompted selection of a hollow wing design with lengthwise interior stiffening ribs and a wall thickness of approximately 0.065 inch/1.65 mm.

During the selection process, an important consideration was “design for manufacturability,” where the company looked at available reinforcement material forms and considered how difficult it might be to pultrude prospective designs: “Anyone looking at pultrusion is interested in cost minimization,” says Fanucci. “So we often iterate several designs with the customer to arrive at a compromise that gives the needed performance, provides the best cost/weight value and is also easiest to manufacture.” The lengthwise stiffening ribs, for example, could be pultruded integrally with the wing, eliminating extra manufacturing steps. However, one client design feature, a several-degree root-to-tip twist to improve stall performance, was eliminated. Although KaZaK previously had produced twisted, pultruded wings, Fanucci points out that the cost is higher and, in this case, the marginal improvement in flight did not justify the additional expense: “For a consumable UAV, cost drives the design more than aerodynamic performance.”

When the wing shape and laminate design model were complete, the next step was to design the root end termination, where the pultruded wing attaches to the vehicle body. Depending on loads and other details, this feature can be a pivot or other deployment mechanism, in metal or composite, and is usually bonded into the hollow wing as an insert at the root end. Fanucci explains that for some contracts, internal telescoping mechanisms are needed inside the wing. These enable the wing to deploy, spanwise, as the root section pivots. With all of the various design elements in place, KaZaK turned to designing the pultrusion process.

Pultruding skins and ribs in one step

To make the hollow wing with interior stiffening ribs, the selected reinforcements included a stitched unidirectional carbon fabric manufactured by Fabric Development Inc. (Quakertown, Pa.), quadraxial carbon fabric supplied by Vectorply Corp. (Phenix City, Ala.) and continuous carbon tows from Grafil Inc. (Sacramento, Calif). The selected resin was aerospace-grade Epon 862 epoxy, supplied by Hexion Specialty Chemicals (Houston, Texas). The epoxy formulation is a proprietary “tweaked” version, modified with accelerators to ensure rapid cure at the pultrusion line speed (~12 inches/300 mm per minute).

The pultrusion line had to be tweaked as well. Multiple unidirectional tows were fed in conventional manner from the creel and through the first of two resin-injection dies. To ensure that the resin’s viscosity would be low enough to thoroughly wet out the fiber, it was preheated prior to injection. As the impregnated reinforcements exited the die, they were pulled through a progression of “cards,” or machined plastic blocks (see center photo, below), that bundled the tows together and gradually manipulated and positioned the bundles around a stationary airfoil-shaped mandrel, which was machined with grooves at the rib positions. Simultaneously, layers of wet out quadraxial fabric were pulled through formers that folded the materials around the outside of the mandrel, enclosing and securing the tow bundles in place just before they entered the second injection die, which is required to ensure complete wetout of the laminate stack. Immediately afterward, the layup was pulled through the heated die and cured. Parts were cut to length, and the metallic root end terminations were bonded.

The resulting wing has an intricate interior cross-section and a fiber/volume fraction in excess of 60 percent — approximately the same as the typical autoclave-cured prepreg part. Subsequent mechanical testing, including cantilever and four-point bending tests, showed the wing withstands loads predicted by the model, consistent with its laminate design, says KaZaK.

Providing low cost and high quality

Often overlooked by manufacturers of aerospace parts, pultrusion — as demonstrated here — is clearly a viable candidate for certain applications, where the very low cost and automated process can deliver high-performance parts. “The best-performing composite structures are not very practical if they can’t be economically manufactured,” observes Fanucci, noting that KaZaK has performed numerous trade studies that show how pultrusion can reduce the cost of carbon fiber structures in a variety of markets by 50 percent or more, compared to other fabrication techniques, yet deliver comparable fiber volume and performance.

One promising application is in the marine market, for production of very wide, cored panels with integral joints, intended for fast construction of naval deckhouses or slab-sided ships. “The panels represent a whole new rapid assembly paradigm for naval vessels,” he contends. “They easily save more than 50 percent labor over resin infusion methods, and it makes composite ship structure approach the cost of steel fabrication, with better performance.”