NOV 2018


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NEWS 21 Composites Gliding into Competition then more detailed concepts, studies and design calculations. And at the end, we asked the most important question: How is this supposed to be manufactured?" In 2012, the group started again with a clean sheet, using design tools that included a lot of paper and pencil, in addition to ViaCAD software by punch!CAD (Cedar Rapids, IA, US) for 2D and 3D modeling and drafting. Aerodynamic analysis and design of airfoils and wings operating at low Reynolds number was carried out using open-source web software XFLR5. Also important were spreadsheets used for calculation of structures, loads, composite characteristics, weight and balance, bill of materials, etc. Clearly, composites were the only material option for the small craft. ey would deliver the lightest possible weight, which would enable foot launching while delivering tailorable flight charac- teristics and very high glide (lift:drag) ratio. e xxtherm2's glide ratio is greater than 35, thanks to wing refinements and the use of a laminar airfoil, compared to the original xxtherm1 iteration. e design also had to balance the demands of aerodynamics with portability, given that one must be able to disassemble and pack a hang glider in a container for car transport to competitions. e group wanted the aircraft parts to fit within a 4.8m/16-ft-long container, which could be placed on a small trailer and weigh, together with the container, less than 75 kg. e wings, each 4.8m long, are built in three parts, to enable breakdown and transport, yet deliver performance while facili- tating foot-launch. Wehren points out that an aircraft's planform should be designed with the center of gravity and the aerody- namic center of the aircraft in the same location: "In a light, foot- launch aircraft like the xxtherm2, the main wing spar would cross the fuselage at the place where the pilot sits. So, I moved the wing forward in front of the pilot. And to compensate, the outer wings are swept back to move the aerodynamic center back to the pilot. is way, the aircraft is balanced when the pilot is lifting it and also balanced in flight, with no aerodynamic trim control needed during foot launch." To determine where the wing segments detach/assemble, Wehren explains that it depended on the bending moment in the spar: "e greatest bending moment in a spar is in the center of the wingspan. By designing the outer wings' detach point more than 2m away from the fuselage, the bending moment is reduced to roughly 40% of the maximum, making the coupling of the wings much easier and lighter in weight." He adds that the spar of the inner wing has a pocket at the end, into which the stub spar of the outer wing is inserted. A spring-loaded bolt secures the outer wing, while the inner main wing is fixed on the fuselage with bolts in bushings. e xxtherm2 craft's wings, tail and flaps are made with carbon fiber, a 160-g/m 2 (4.7-oz/yd 2 ) spread carbon twill (Style 67442), over Nomex honeycomb core, Type CS 3.2-29, both from the supplier for all the project's materials: distributor Swiss-Composite (Fraub- runnen, Switzerland). Wehren notes that the honeycomb sandwich had to be overdesigned, beyond the stiffness needed for air loads: "ere exist some challenges for the aircraft panels caused by spectators and other curious people, who look with the fingers Breaking down for easy transport The glider is broken down for transport in a relatively small trailer, shown here hitched to a tow vehicle. Source | Wehren Emcom xxtherm2 Using bifunctional flexible mold technology The prototype xxtherm2's fuselage tube being infused on a flat table, without a typical mold, using the process technology called bifunctional flexible molds (BFM). Once cured, the panel will become the mold supporting additional composite plies needed to complete the laminate. Source | Wehren Emcom xxtherm2 instead of the eyes. is leads to some higher strength demand for sandwich panels, in the realm of 5 kg per thumb," he quips. e craft's cockpit was designed as a hybrid shell of carbon fiber and ultra high molecular weight polyethylene (UHMWPE) (using Dyneema UHMWPE fiber from DSM Dyneema, Geleen, e Neth- erlands), with a stiff carbon frame and thermoformed side skins, cored with a polyvinyl chloride (PVC) foam (Airex C 70.75, from Airex AG, Sins, Switzerland). e tail is a conical monolithic carbon fiber tube, constructed using the same materials used for the wings and flaps, and with internal composite stiffeners and frames and protective outer ply of 105 g/m 2 glass fiber twill in a resin infusion process. A focus on manufacturability To fabricate the xxtherm2's parts, Wehren, his son Hannes and the rest of the group developed a low-cost, efficient manufacturing process technology they call BFM, for bifunctional flexible molds. All parts of the airframe are produced with this technology, via resin infusion, using Hexion (Columbus, OH, US) L-285 epoxy resin. Instead of typical solid female molds or male mandrels, the process uses thin composite laminate sheets that function in a dual capacity: Flexible molds that become part of the component. Says Wehren, "It

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