Unmanned Systems Technology 001 | UAV Factory Penguin C | Real-time operating systems | Hirth S1218 two-stroke twin | Base stations | ASV C-Enduro | Composites | Datacomms

20 composite production is such that the inner and each outer wing section abut in a perfect join. The sections are joined by a square-section aluminium spar plus an alignment dowel. The spar slots into both mating wing sections, from each one of which a bolt threading into a nut located within secures the joint. The beam is designed to bend in the event of a crash, thereby hopefully protecting one or both wing sections with which it connects. Each outer wing section has a (longitudinal) rib only at its inner edge, where it abuts the inner wing section. That rib is a carbon fibre over plywood production. Internally it has three (transverse) spars, one of which mounts the aileron. Each aileron is literally cut from the wing then bonded back with silicone to form a hinge on the lower side. On the upper side is a plastic film attached to the aileron that slides into the wing so as to maintain a continuous aerodynamic surface as the aileron articulates. The tail booms are hollow carbon fibre tubes using solid skins (no core) with a short aluminium tube bonded on at each end; the tubes are designed as clamps to secure the boom to mating inner wing and tail tube connections. The mating tube slips inside, and the respective boom clamp is tightened to secure it. The advanced composite components in the Penguin C airframe and power module are formed around moulds made from aluminium or, in the case of larger items, glass fibre. The latter is easier to handle but will in turn have been formed using aluminium masters. The skins are not made from pre-preg material. The logic of that, explains technical director Konstantins Popiks, is that pre-preg adds expense and calls for the use of an autoclave, making it even more costly. UAV Factory’s approach is to use a combination of carbon fibre and glass fibre weaves in up to eight layers, together with room-cured epoxy and various consolidation techniques. This, says Popiks, gives comparable results at a lower cost. Sometimes the multi-layered skins are a combination of carbon fibre and glass fibre; in less critical areas they are glass fibre only, to minimise cost. Core options include aramid honeycomb (a non-metallic honeycomb made from aramid fibre paper coated with phenolic resin) for flat areas such as the interface between the wing and the fuselage, and foam for the wing itself. The wing is room-cured and is made in two halves around a male mould, using a conventional vacuum bag technique to give integrity to the composite moulding. Spars in the wing are made from carbon fibre-skinned, aircraft-grade plywood. Originally the Penguin fuselage was made in upper and lower halves that were bonded together. That was in recognition of the fact that it is fiendishly difficult to make a cigar-shaped monocoque structure in one piece, even when using an advanced composite material rather than metal. The drawback though is that the joint represents another time-consuming process, adds weight and is at the cost of structural integrity and precision. Those considerations led UAV Factory to develop its own process for manufacturing the Penguin fuselage in one piece. In essence, the fuselage is formed in a female mould that splits into two halves, and an inflatable pressure bag is used to form the internal shape. The pressure bag consolidates the composite moulding, and the resin is cured at room temperature. The mould and pressure bag are reusable, and with skilled hand work in UAV Factory’s composite shop a perfect one-piece fuselage is produced each time. The fuel tank also has to be made as a one-piece item since any joint could allow leakage, but it is manufactured using a different technique. It is laid up inside a two-piece solid female mould in two halves using different overlaps between each ply layer and a high-temperature fuel-resistant epoxy resin. The two halves are then closed around a flexible bladder, which is pressurised up to 10 bar to form the internal shape of the tank, and it is then cured in an oven. The bladder pressurises the moulding, which has the effect of pushing the overlaps together to ensure the structural integrity of the finished item. As with the fuselage process, the bag and mould are reusable. A similar oven-cured technique is used to produce the duct supplying cooling air to the engine. In this case, instead of a bladder a silicone plug is used and the lay-up is created around this. The plug expands when heated, pressing the moulding against the external mould to consolidate it. The firewall, which is part of the power module, needs extremely precise internal and external forms, so match-die moulding is used, involving the use of precision-machined metal internal and external moulds. The part is laid up around the inner mould, then the outer mould is put around it and the entire assembly is room-cured inside a hydraulic press. Popiks notes that this is a relatively quick and consistent method of producing parts that have a consistent thickness and good internal and external finish; its drawback though is the need for two (albeit reusable) moulds. Advanced composite production at UAV Factory November 2014 | Unmanned Systems Technology