Unmanned Systems Technology 007 | UMEX 2016 report | Navya ARMA | Launch & recovery systems | AIE 225CS | AUVs | Electric motors | Lethal autonomous weapons

73 Electric motors | Focus brushless motors have the windings on the case and the magnets on the rotating shaft. The AXI motor, however, combines a little of both, but with a twist. This brushless motor has the magnets on the case and the windings around the shaft, but rather than rotating with the shaft, the windings are fixed. The magnets actually rotate around the windings (along with the housing), then transmit that force to the shaft (which runs through the windings) by means of the back plate which is pinned to the rear of the shaft. This allows a relatively small motor to generate a lot of torque for its size, the outcome being the ability to spin large propellers with a light motor and no gearbox. The motor generally shows an efficiency range of around 70- 75% when used within recommended propeller ranges, making it comparable to a good cobalt brushed motor but less efficient relative to comparable brushless motors, although the low weight and the ability to spin larger, more efficient propellers without a gearbox – and the lower associated losses – may more than make up for the lower efficiency numbers. Its design yields a novel wiring arrangement – the wires exit the side of the motor, near the front. The motor is generally recommended as a seven- to ten-cell motor, with optimum efficiency between 25 and 30 A, but it is capable of being operated at up to 40 A for periods of up to 60 s. Hybrid option Given some of the limitations with electric motors, one area of particular interest is the development of hybrid electric- ICE propulsion systems for improved operational UAV capability, such as longer range, along with quieter and more efficient operation. Current UAV ICEs are typically sized to provide enough power and speed for take-off, which can vary according to the UAV, leading to a propulsion system that may operate below peak efficiency during other operational conditions; combustion engines are also noisy because of their exhaust emissions. On the other hand, electric motor- based systems are quiet, but have that limited range (unless they can be charged in flight by another energy source) so a hybrid system using an ICE and an electric motor would broaden a UAV’s operational capability by combining the advantages of quiet and efficient operation of electric-based propulsion and those of energy-dense liquid fuel-based ICEs. It should be noted though that the added weight of two energy systems may reduce some operational capabilities. To that end the US Air Force Research Laboratory has awarded contracts to several UAV manufacturers to develop just such a hybrid system, the objective being to anticipate and meet the current and future needs of small to medium- sized UAVs. Future prospects Most unmanned system manufacturers are saying it is not the technology itself in electric motors that is the stumbling block to their greater adoption but the energy density of the power source. One says that if methanol or hydrogen fuel cells became practical in terms of a steady supply of power, electric motors would be more attractive, and although fuel cells are making steady progress they are not there yet. The other alternative would be improved battery packs that could yield more energy per kilogramme, lithium- polymer batteries being the likely choice. Acknowledgements The author would like to thank Jeff Ratcliffe at NWUAV, Jan Vaclavik at Model Motors, Prof Barrie Mecrow at the School of Electrical and Electronic Engineering, Newcastle University, Dr Joseph Barnard at Barnard Microsystems and Prof Peter Malkin at the Propulsion Engineering Centre, Cranfield University, for their help with researching this article. Unmanned Systems Technology | April/May 2016 The AXI 2826 brushless outrunner electric motor (Courtesy of Model Motors)