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92 roll of tape, ready to be laid down in a composite preform in the angles, positions, thicknesses and lengths defined for the component. Following this, only minor differences in curing are required, such as greater care around the expansion or contraction of the tooling, and during removal of the part after the cure. Possibly the most prominent advancements in thin-ply composites over the past few years have been in tubular parts. Tubes are efficient shapes for carrying loads, and are often used to form the spars of wings or the structural spar of a tailplane. For such parts, it is of course critical that composite manufacturers can optimise the thickness and strength of the tubes precisely, otherwise aircraft can suffer torsional problems along the wing. Minor degrees of flexing can be withstood, but the incident angle of the airflow should not change substantially during deformation of the wing. This and other issues can be particularly pronounced given conventional manufacturing methods. The most common approach to producing tubes of composite material – roll wrapping – involves winding lengths of prepreg around a mandrel until a target thickness is achieved. However, the beginning and end of each diagonally wrapped layer can experience inconsistencies or defects as they are manually and sometimes haphazardly cut to fit the required shape of the ends of the tube. The manual nature of roll wrapping can also mean inconsistencies in the thickness running along or around the tube, meaning uneven or unpredictable distributions of loads across a tubular part. The difficulty in consolidating thick roll wrapped tubes can also produce a higher rate of voids in the material. Other approaches have therefore been developed. One system lays down composite tape preform by winding it unidirectionally in a helical pattern around a mandrel or, for bidirectional tubes, winding two tapes simultaneously around a mandrel so that their relative orientation fits what has been specified for the user’s vehicle application. After that, the tube is cured using pressure and heat. Such a method is intended to provide greater homogeneity and accuracy of thickness along the tube, to help discourage the aforementioned structural weaknesses that could deform wings or tailplanes beyond safe parameters. Metal matrix composites Ongoing advancements in metal matrix composites (MMCs) enable them to provide greater stiffness and strength, and resistance to fire, radiation and fatigue than traditional metals and alloys. They can be used in any area of a vehicle where aluminium or other metals would be used, maintaining structural integrity while reducing weight. They are produced using various methods. One is die-casting, which involves incorporating reinforcement from materials such as carbon fibres, ceramic fibres or silicon carbides, and infiltrating the chosen material with a matrix, which can be aluminium, titanium or some other lightweight metal. A key difference though between these and polymer matrix composites is that MMCs have far better transverse properties. Conventional polymer matrix composites tend to have low stiffness and strength in a transverse direction to the fibres, and can break if excessive loads are applied at such angles. An aluminium-ceramic MMC on the other hand can achieve up to 180 kPa transverse to the fibre direction, at a density of 3.4 g/cm 3 – more than twice the stiffness of aluminium when normalised by density – and it can also have four times the stiffness of aluminium in the same direction as the fibres. In addition to potentially providing highly durable hull materials for large vehicles, and around exhaust vents and other heat-prone areas, MMCs are also increasingly being used in the manufacture of rotary engines and other powertrain systems, replacing steel components. Rotating parts will therefore have lower weight and inertia, improving overall engine efficiency. February/March 2018 | Unmanned Systems Technology Focus | Composites Advances in tubular composite parts is especially important for unmanned systems owing to their application as spars in wings or tailplanes (Courtesy of Lentus Composites)