Unmanned Systems Technology 027 l Hummingbird XRP l Gimbals l UAVs insight l AUVSI report part 2 l O’Neill Power Systems NorEaster l Kratos Defense ATMA l Performance Monitoring l Kongsberg Maritime Sounder

Quest for power Finding a suitable engine proved a fraught process. Bishop had wanted to use a rotary originally because of its mechanical simplicity and high power-to- weight ratio. However, the UK company he chose to supply the engine filed for bankruptcy two weeks after accepting Reference Technologies’ deposit. That frustration motivated him to seek an alternative, a search that led to 3W International/Sky Power and its air-cooled two-stroke twins, first the 17 hp SP- 170, which provided enough power for prototype flight testing and validating all the aerodynamic calculations. However, it was soon clear that better fuel efficiency and more power would be needed for the XRP version, and Sky Power’s new SP-180 SRE Hybrid seemed a natural fit. Bishop stresses that its power figures have been validated on a dynamometer, giving him confidence that it will perform as advertised. Combined with an article this magazine ran on the engine ( UST 25, April/May 2019), this influenced his decision to invest in the development and integration of the SP-180 SRE Hybrid. Mounted on CFRP tubes that attach to the duct walls, the engine draws in air through an upward-pointing intake and sends spent gases through the exhaust pipe to a silencer in the form of a perforated annular aluminium tube attached to the bottom of the duct. “The engine is very reliable, and achieves significant time between overhauls,” Bishop says. “And it has logged hundreds of hours of test time in all environments.” The SP-180 SRE Hybrid’s integral generator was also an important factor in choosing it, he says. In a 48 V electrical system, the generator can produce about 13 kW to power the propulsion motors, utilities and payloads, although Bishop emphasises that 15 kW will be available with a new, higher voltage inverter that will reduce the current. As it is, he notes, 13 kW at 48 V works out at 270 A, which is a large current for the main power bus of a relatively modestly sized vehicle.