Unmanned Systems Technology 012 | AutoNaut USV | Connectors | Unmanned Ground Vehicles | Cobra Aero A33i | Intel Falcon 8+ UAV | Propellers | CES Show report

Use of a strong foam core to consolidate pre-preg material during compression moulding allows a thinner outer layer for composite material propellers (Courtesy of Mejzlik Modellbau) 73 Propellers | Focus that injection-moulded plastic tends to suffice. For runway-launched MALE-class UAVs, however, aluminium continues to offer the rugged durability and ease of production that stem from its traditional use in general aviation. For USVs and UUVs there is a wider range of metals for their props than UAVs, and in general they derive greater benefit from them. Aluminium and stainless steel offer longevity in a harsh subsea environment, along with the stiffness and strength to allow more freedom to achieve an optimal design. Although heavy, stainless steel is five times stronger than aluminium, and can therefore be cast much thinner, reducing drag while also producing thrust more efficiently thanks to its rigidity – although in the event that damage should occur, aluminium is far easier to repair and cheaper to replace than steel. On top of this, the fact that aluminium breaks more easily than steel reduces the probability of damage to a USV’s drivetrain if it hits rocks or other debris, as USVs are inherently less capable of spotting and avoiding such debris, and the prop can absorb more of the shock of impact when it breaks. In the pursuit of lighter and more geometrically complex propellers, composite materials – particularly carbon fibre composites and those pre- impregnated with their matrix material – have soared in popularity for UAV use (and also to a lesser extent in USVs and UUVs, where weight constraints are relatively less critical). A number of forming methods and techniques have emerged to ensure that working with such complex materials results in the desired propeller. Cost can be reduced by using low-temperature materials and moulds, and by glueing two separately cured propeller halves with an adhesive in a secondary process, but co- curing halves together in a single effort can yield far greater durability and integrity than curing separately and bonding afterwards. Moulding methods should also be considered by UAV designers looking to use composite propellers. Bladder moulding is a comparatively inexpensive means of producing a strong propeller with fine edges, if relatively tricky to scale down for smaller props. For UAVs demanding greater strength-to-weight ratios though, compression moulding (in which formation and curing relies on the use of high heat and pressure inside the cavity) can yield propellers that are thinner and more geometrically complex. For such ‘upmarket’ propellers, the use of high-density foam cores such as polyurethane to fill and consolidate a thinner composite outer layer allows props to be made that are 30-50% lighter than equally strong alternatives. Composite propellers can also vary widely in their binding matrix, with materials ranging from thermosets such as epoxy resins to thermoplastics such as nylon. Although the former is stronger, and the latter more flexible, both offer strength, durability and resistance to damage from water and heat, making them suitable across the range of unmanned vehicle types. Even hubs for ground-adjustable propellers can be made from composites, opening a new avenue for future weight reduction. And while the durability of composites relative to aluminium is low, it is not uncommon now to see guards or skins of aluminium or stronger materials such as nickel to be mounted on the leading edge, which can easily be filed off and replaced if damage should occur. A woven stainless steel skin can also be wrapped around composite propellers in situations where weight constraints come second to damage concerns. Wood still remains viable for UAS propellers. Aside from being much lighter than metals, wooden props are easy to produce owing to minimal tooling costs, so new design iterations can be quickly prototyped, and replacements easily generated for UAVs in damage-prone applications. The relative fragility of a wooden prop also means a lower risk of engine damage in the event of an accident. Prototyping can also be quickly accomplished using plastic propellers, either through injection moulding or additive manufacturing (AM). While notably less strong than metals or composites, plastics scale down far better than carbon composites, and are far lighter than metals, making them a useful option for micro-UASs. AM also offers a useful option for future propeller development. While many prop manufacturers regard the surface finish and overall strength of laser-sintered propellers to be inferior to injection- moulded or CNC-machined parts, rapid advances in this field – from refinements in simulation and modelling techniques, to new production methods, and innovations allowing composites to be printed – may lead to the rapid spread of new propeller materials and designs. Extremely quick prototyping is already possible, allowing for timely and cost- effective experimentation with new blade shapes and propulsion systems. Unmanned Systems Technology | February/March 2017