Uncrewed Systems Technology 047 l Aergility ATLIS l AI focus l Clevon 1 UGV l Geospatial insight l Intergeo 2022 report l AUSA 2022 report I Infinity fuel cell l BeeX A.IKANBILIS l Propellers focus I Phoenix Wings Orca

99 Materials and manufacturing Propellers these days are made from many different types of material, including various kinds of wood, metals and polymers as well as composites of them. Each has its place in the airborne and maritime worlds – for instance, wooden propellers are popular in missions where there is a risk of explosions, such as gas facility inspections or gas leak detection, where metal or carbon props might produce a spark and hence ignite any gas present. However, carbon composite is the most prolific material by far, with carbon fibre props being practically ubiquitous in uncrewed systems around the world. Certified carbon fabrics are now especially important for self-driving transport vehicles such as eVTOLs and autonomous water taxis, as well as uncrewed aerial freight vehicles and medical logistics UAVs that might have similar or even higher safety requirements. And with uncrewed systems now being held to the same safety standards as commercial and general aviation aircraft, propeller manufacturers are expected to keep close track of parameters such as the shelf lives of prepregs and resins to ensure their materials are not just safe when purchased, but safe through to the point of use. There are many different techniques for making carbon propellers, starting with the cutting of carbon fabric. This can be performed manually, automatically or semi-automatically, and with different tools from scissors to ultrasonic blades, lasers and waterjets. The subsequent installation, impregnation and curing of layers into the mould can then be similarly carried out in a range of permutations of manual and automated methods, with some approaches such as compression moulding and bladder moulding having been applied in UAV propellers that need to work at particularly extreme altitudes and in harsh environments. In general, however, carbon propellers are labour-intensive to make, so mass production has been difficult to achieve. Some prop suppliers however are taking alternative routes towards achieving the bulk outputs that the uncrewed systems market is requesting. One approach, inspired by the automotive and crewed aerospace industries for instance, forgoes the conventional wet layup in favour of a hot-press system. Here, prepreg carbon fibre sandwiches a foam core inside a metal mould, with high compression and heat then being applied. Going with this process stands to take a supplier’s typical 7-hour cycle time for the wet layup and autoclave sequence down to 30 minutes, as well as reducing the overall production costs by 50%. The use of a metal mould and the hot-press technique is critical to these improvements. Whereas fibreglass moulds had previously been used, which allowed only one propeller to be made per mould and per day, now multiple props per mould per day can be produced, and with greater precision and repeatability. Also, the hot-press machine’s degree of compression can be regulated to within 0.05 mm, which can be pre- programmed and automated over a timed period. That is key, as different epoxy resins will have differing curing cycles over time, compression and temperature. That means the hot press can be closed to the exact and correct degree needed to squeeze the epoxy optimally and thereby distribute it evenly across the breadth of the fibre. It can also be done as the resin is being heated to the temperature at which it achieves the necessary liquefaction for being manipulated in this way. The hot-press process consists of around 40% manual work – cutting the fibre, layering it with resin and preparing the layers in the mould – and 60% automated or semi-automated pressing and curing. The repeatability of the process and consistency of propellers made using it also saves time in the design and validation processes, as key tasks such as balancing and validation are inherently accelerated. The precise control of the hot press is such that, after curing, propellers can be trimmed using a CNC machine to any tolerances required, something that conventionally would be done by hand. As might be guessed though, the hot- press approach requires much more expensive tooling than conventional methods, so it might be more sensible to use it only for high-volume production such as 200-300 props at a time, and opt for wet layups with glass fibre moulds or less expensive methods for smaller batches. Propellers | Focus Uncrewed Systems Technology | December/January 2023 With propellers for uncrewed vehicles starting to exceed diameters of 2 m, larger moulds must be made that last longer or can be produced more easily and flexibly (Courtesy of MagCAD)