Unmanned Systems Technology 026 I Tecdron TC800-FF I Propellers I USVs I AUVSI 2019 part 1 I Robby Moto UAVE I Singular Aircraft FlyOx I Teledyne SeaRaptor I Simulation & Testing I Ocean Business 2019 report

40 Focus | Propellers serrated trailing edges, for multi-copter and fixed-wing flight. Results from modelling and testing propellers with sawtooth serrations indicate that they produce no particular aerodynamic advantages over conventional trailing edge designs. However, the findings show that they produce less noise, with greater reductions coming from larger, sharper ‘teeth’. Further testing is needed though, as some findings suggest that at very high revs the serrations might actually generate more noise than smooth trailing edges, and might be best suited to low- to-medium revs. The serrated trailing-edge approach is a biomimetic design, inspired by the serrations on bird feathers. Biomimicry is also spurring explorations into other blade shapes, including some designs that imitate cicada wings or maple seeds for example. Speed tests and anechoic chamber tests have been run on propellers with geometries and volumes matching these shapes. Results so far indicate that they can generate equivalent thrust to conventional propellers with the same power input, but with reduced wake regions, tip vortices and therefore less noise. For all the technological advances made in their designs, however, fixed-pitch propellers will often be a compromise between two optimal geometries. A fixed-wing aircraft’s propulsive efficiency relies on a steeper pitch during take-off and ascent than in cruising flight, owing to the higher power output needed at those times. Variable pitch Variable-pitch propellers (VPPs) provide for this by hydraulically, mechanically or electrically rotating the blades until the optimal pitch is reached. Multi-copters consume more power in hover than in motion, and might similarly benefit from integrating VPPs, as some new designs have done. A key drawback with these propeller types though is their complexity. Using a VPP introduces several potential points of failure, and fitting six or eight of them on a heavy-lift multi-copter could pose too much of a public safety risk in national airspace. For electric or hybrid-powered fixed- wing UAVs, however, electric VPPs have continued to develop. Belt-driven electric VPPs are now available for users seeking to increase the energy efficiency of their systems, by having a motor running at lower revs than its propeller. For HALE systems, which need to maximise endurance and consume as little energy as possible, a belt- driven VPP could save input power and alter pitch to suit the thin operating atmosphere. Such systems are not ideal for maximising flight speed though, so defence and special forces UAVs for example might be better off using more conventional direct-drive electric VPPs, to ensure surveillance payloads can be launched at short notice for instance. Passive VPPs have grown in popularity over the past few years. They are mechanically simpler than most other VPP configurations, and operate by having the hub system embedded with a specific take-off pitch angle and cruising pitch angle. Actuation between the two is triggered by changes in air resistance on the blades as the craft changes airspeed and altitude, transitioning between ascent and cruise. The hub system is tailored to match June/July 2019 | Unmanned Systems Technology Belt-driven variable-pitch propellers can provide highly efficient propulsion, but for some UAVs a direct-drive VPP would be preferable (Courtesy of Hawk Aviation Technology) Passive VPPs have specific pitch angles embedded into the hub to optimise propeller efficiency without needing complex hydraulic or mechanical arrangements (Courtesy of Aerovate)