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

61 often an epoxy resin. Pre-impregnated woven fabrics or unidirectional tapes (termed pre-preg) already contain the matrix resin before lay-up into a component, and the resins contain latent hardeners that are activated by elevated temperatures to fully cure the material. The curing process cross-links molecules, and this can be an exothermic reaction, so care must be taken in controlling the process temperatures, particularly in thick components. To stop the resin curing at room temperature, refrigerated storage is required, and pre-preg composites have a finite shelf life. An alternative to epoxy resins is to use cyanate ester-based resins, which provide excellent high-frequency radio wave transparency as well as very low outgassing for space applications. Outgassing is the release of trapped molecules or gases from a material placed in a vacuum – the gas can then condense onto a satellite’s instruments or optics for example. These resins can also have a very high glass-transition temperature (above 350 C) as well as high toughness, making them ideal for long-term service in extreme environments. PEEK (polyether ether ketone) is a particularly popular matrix choice for hard-wearing functional parts exposed to high temperatures, although it is comparatively expensive. Manufacture Reinforcement fibres are handled as a bundle, called a ‘tow’, of unidirectional pre-aligned fibres, typically counted in the tens of thousands (12K being 12,000 fibres per tow, for example). A filament winding process forms nominally cylindrical components by winding a continuous tow of reinforcement around a pre-form. The tows used range from 1K to 50K fibres and can be wound onto a mandrel in a pattern to provide the desired properties, typically useful for producing shafts or pressure vessels. Usually tows are woven to give a fabric for lay-up in a mould. Spread-tow cloths take each tow strip and weave them in alternate directions to provide a cloth with strength in more than one direction. The particular benefit of spread-tow fabrics is that the intersection between each weave is very flat, so the fibres remain almost straight, whereas more conventional woven fabrics ‘crimp’ the fibres to a greater degree where each weave crosses over another. Despite this crimping, the most common form for composites is as a woven fabric, with unidirectional tows of fibres interlaced at 90 o to each other, giving bi-directional properties. Here, each element of the weave is typically much smaller than in spread- influence the strength and stiffness of the material as required. Aramid fibres are based on an aromatic polyamide, with a wide range of materials more often known by trade names such as Nomex (a meta-aramid) or Kevlar (a para-aramid). Meta-aramid fibres typically have high temperature resistance, while para-aramid fibres have excellent mechanical properties for a given weight. Glass fibres may not provide the same strength-to-weight performance as carbon fibres, but they are comparatively ductile and cheaper. Quartz fibres are most often used for housings such as radomes, given their high-frequency radio wave transparency. The reinforcement fibres are then combined with a polymer matrix, most Composites | Focus Unmanned Systems Technology | November 2014 Spread-tow carbon fibre (Courtesy of TeXtreme) UAV using thin-ply composites for airframe components (Courtesy of NTPT)