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86 Focus | Composites Applications UD composites find most use in applications that do not demand stiffness or strength in a 90 º direction and which require extremely lightweight structures. High-altitude long-endurance (HALE) type UAVs in particular can make use of materials tailored with UD material and a highly accurate resin content in order to achieve the composite’s target aerial weight while giving just enough strength for the atmospheric conditions they will face. The points in an airframe that need the greatest attention include the primary load-bearing structures, such as the front and rear wing spars, where tailoring the length of such parts from fuselage to wing tip can be critical to avoid carrying any more weight than is absolutely necessary. These are followed by secondary load-bearing points such as the ribs inside the wing. On the wing surface, a thin, lightweight ‘film’ can be enough to provide a covering. Composites are also increasingly being used in non-structural components in unmanned vehicles. For example, specialist materials such as E-glass, and occasionally aramids and quartz fibres, have been used as reinforcers for radomes housing UAV comms systems. The high RF transparency and low dielectric constants of these materials minimise the loss of telemetry through them, and are paired with low-loss epoxies to bind them without affecting electromagnetic signals. Even more niche applications may follow, with composites for unmanned spacecraft components replacing previous materials. The development of cyanate ester resin systems capable of withstanding 300 C, for example, may come to replace the titanium in heatshields, and metal matrix composites may be used in heatshields, exhausts and engines. Outside of heat-bearing points on spacecraft, general structural areas such as parts of solar arrays – and anywhere else where metals are used – may be able to save costs and weight by using composites. As HALE UAVs and unmanned spacecraft systems develop and mature, a pattern of each sector informing the other has emerged over the past few years. This will continue and grow, as high-altitude UAVs are on the cusp of encountering phenomena that space vehicles routinely experience. For example, outgassing – where moisture and volatile compounds evaporate out of composites when exposed to a vacuum – poses a major risk to electronics and solar arrays if the compounds condense onto them. Extremes of temperature can also trigger expansion and contraction of resin systems, so it may be vital to select carbon-epoxied materials with low coefficients of thermal expansion for high-altitude applications. Manufacturing The application of resin can be conducted in different ways. Hand lay- ups – in which resin is impregnated into dry fabric material manually with a brush or roller – continues to be widespread as a simple, low-cost means of preparing continuous fibres for curing (which is the next stage of manufacture). However, as unmanned vehicles are designed for increasingly niche applications and February/March 2018 | Unmanned Systems Technology With unmanned systems operating increasingly in space or at very high altitudes, it has become critical to test composite material components to prevent outgassing (Courtesy of CRP USA) The points in an airframe that need most attention include the front and rear wing spars, followed by the ribs in the wing