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

70 Focus | Propellers should closely consider is fitting a duct around their propellers. Aside from the increased protection against debris and collisions for the blades and improvement in course stability, using a ducted propeller significantly boosts efficiency at speeds below 10 knots, making them highly applicable to AUVs. That may not apply though to USVs with more critical missions, as beyond 10 knots drag becomes an issue. Ducting also increases the risk of cavitation by the formation of low- pressure areas as blade tips accelerate. However, new electric rim-driven thruster technology, in which the motor is built into the duct rather than the propeller hub, provides enough torque to operate efficiently with lower tip speeds while eliminating the gap between the blade tips and the rim, countering this risk. The scalability and flexibility of the technology means a cavitation-free design can be scaled down or retrofitted to a variety of USVs and UUVs. Ducting UAV propellers boosts efficiency as well as safety by shrouding the blade edges, but above diameters of 8-12 in the ducts will lose efficiency owing to excessive weight. In addition to ducting propellers, foldable blade designs are expected to become more popular for safety and transport of small UASs, as they would reduce the aircraft’s footprint when not in use, and eliminate the need to remove propellers and re-attach them later. They should also find favour with designers of wave- or wind-powered/sailing USVs for their ability to greatly reduce the underwater drag of a fixed propeller. Variable pitch A fixed-pitch propeller cannot be perfectly oriented for multiple flight stages, as the optimal pitch changes for taxiing, take-off and cruising, nor is feathering possible; their use is analogous to a car with only one gear. By using a variable-pitch propeller (VPP) however, the need for complicated CFD analyses is reduced because minute improvements to blade shape and profile are no longer needed to compensate for imperfect pitch. By altering the pitch of a propeller’s blades, both generic and customised blades can be adapted to different conditions. In particular, fixed-wing aircraft, including VTOL-capable UASs, can benefit from an optimised lift-position propeller that changes into a cruise position as it reaches its operating height. In addition to the basic principle of adjusting towards an ideal pitch during different phases of flight, VPPs often offer a ‘constant speed’ capability through the use of a governor that monitors engine rpm and changes the blade angle within a preset range to counter the forces acting on the blades, maintaining a target rpm. This enables the operation of the propeller and the engine/motor to sustain the most efficient revs and power level over a range of airspeeds. And although full feathering is ordinarily used only on twin-engined aircraft, UAS developers with an eye on safety may take interest in the ability of a VPP to turn its blades to a maximum pitch, nearly parallel with the airflow, to prevent ‘windmilling’ in the event of an engine failure. This is where the flat face of the propeller blades February/March 2017 | Unmanned Systems Technology Sweeping back the tips of propeller blades can yield increases in efficiency and reductions in material erosion and noise (Courtesy of Sensenich) Folding propeller blades decrease the need to remove them during transport, and gives reduced drag when the engine/motor is not required (Courtesy of Aerovate)