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

100 Focus | Propellers It should not be assumed however that just because this method and others are taken from elsewhere in the composites world that any composites manufacturer can be contracted to design and produce batches of an ideal propeller. Most general carbon fibre parts manufacturers do not have the necessary understanding to optimise propeller fluid dynamics, mechanical properties or testing techniques, so it falls to specialist suppliers to feel their way towards taking the best from both worlds. Propeller protection Given the need for propeller safety among regulators, and longevity among buyers and users, considerable r&d in recent years has gone into protection systems capable of ensuring that UAVs and other uncrewed systems can keep flying amid impacts on or exposure to erosion of their prop blades. The impacts and erosion can come from more than bird strikes during flight or gravel flying about during take-off. Even rain and particles of dirt in the air are enough to cause hazardous erosion to the leading-edge profiles of propellers, threatening their structural integrity, aerodynamics and practical lifespan. This erosion is so problematic because the leading edge of carbon fibre props typically represents a point of structural weakness, where a process of delamination and eventual rupturing of the material’s outermost layers will occur. That means a near-total loss of lift by the blade, probably leading to a propulsion failure and ultimately a crash. The optimal protective material for propeller blades depends highly on an uncrewed platform’s working environment. In a desert for instance, rubber would probably be the longest- lasting and most comprehensive protection against sand and gravel. Alternatively, in a wet, dust-free, freshwater environment such as a reservoir, titanium would be best. So far, the optimal solution being adopted for UASs is an electroformed nickel-cobalt alloy that is custom- designed to suit both the outer aerodynamic profile and inner mounting profile for each propeller. This is a solution taken from crewed aircraft in defence and general aviation, particularly among some successful high-end military UAVs, and its mechanical and chemical properties are such that it is the second-best solution available for both desert and wet environments – it has about 98% of rubber’s effectiveness and 97% of titanium’s qualities. That therefore makes it ideal for the many real-world operating areas where uncrewed systems have to withstand a combination of moisture and dirt. It also combines the flexibility of nickel with the hardness of cobalt to result in something that can be mounted over propellers to provide sufficient physical safeguarding without being too brittle to function as part of an aerofoil. These parts are produced in galvanic tanks, with the nickel and cobalt distributed through an electrodeposition process onto stainless steel mandrels shaped like the propeller blade surfaces they are intended to mount on. The mandrel is highly polished to ensure that the nickel-cobalt layer growing around it does not adhere to it too strongly to be lifted off. This solution wins out over alternatives, owing to qualities such as high geometric customisability thanks to the nature of the electroforming process. For instance, bending or forming a metal sheet to fit precise aerodynamic tolerances over a blade tip is extremely difficult by comparison, and even if aerodynamically optimal geometries can be achieved this way, doing so would weaken the metal sheet and hence the degree of protection gained. The customisability of electroformed blade protection also extends to the material’s thickness. It can be varied to have, say, a thicker layer of material extending forwards from the blade tip, and a thinner layer running back along the blade chord, where material is needed primarily to ensure a broad contact area between the blade and its protection system for adhesive to be applied. As an example, an autonomous helicopter might have up to 1.5 mm of nickel-cobalt electrodeposited on its propellers’ blade tips, and 0.25- 0.3 mm elsewhere. For vehicles working in extreme environments and with critical safety requirements, the tip protection might be 5 mm thick. For small UAVs, less protection thickness is needed of course, with variations stemming from the use December/January 2023 | Uncrewed Systems Technology Using a hot-press approach with metal moulds enables faster manufacturing of propellers, with lower unit costs and greater repeatability (Courtesy of Mejzlik)