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70 permanent magnet AC machine outputting 2-3 kW, although Hirth is evaluating more powerful systems in case of market demand, and its prototype is now being tested. The plan is to mount the production version on the flywheel side of the crankshaft within the engine’s current outer dimensions for ease of integration. “We have seen widespread demand for ways to make the whole UAV lighter, particularly from UMS Skeldar,” Bitter says. “A starter system by itself is effectively dead weight during flight – once it’s cranked the engine, you’re carrying around 1 or 2 kg that isn’t contributing anything to the ongoing operation of the engine, so it made sense to replace it with something more useful.” With this installation, Hirth estimates that the overall weight of this starter/ generator will be half the combined weight of the starter and most generators its customers use. For the design and supply of this custom starter/generator, Bitter says Hirth is talking to Acutronic Power Systems (formerly Sullivan) and ePropelled to determine which of the two can provide the optimal solution (including the power management unit) to the quality and reliability required. Oil injection The 3507 is designed with an oil injection lubrication system for supplying two-stroke oil to the engine and compressor, as well as using scavenged oil from the compressor for added engine lubricant. “We have two separate oiling maps in our ECU for the compressor and the engine,” says Bitter. “Four pumps go to the engine to lubricate the crankcase, the crankshaft, the bearings, the pistons, piston rings and so on, while one oil pump goes to the compressor, for its crankshaft and piston.” Of the four engine pumps, two feed into the side of the crankcase, near the pistons’ big ends, and the other two feed into the bases of the cylinders near the pistons’ TDCs. As some oil going to the compressor will not fully burn, a drain pipe runs February/March 2021 | Unmanned Systems Technology Hirth’s 42 Series engines are based on the same design principles and philosophy as the S1218 (as investigated in UST 1, November 2014), and incorporate improvements derived from new studies in CFD, manufacturing and other competencies. The 4201 is the first of this series. Like its predecessor, it is a two-stroke, flat-twin boxer displacing 183 cc, made mostly from aluminium (apart from its crankshaft and con rods, which are made from steel). It weighs 5.7 kg and outputs a top continuous 10.67 kW (14.3 hp). Compared with the 7.6 kg S1218’s output of 11.18 kW, the 4201 has a 27.2% higher power-to-weight ratio. The first 4201 prototype was built in 2016, and there followed two years of r&d to improve its balance of materials and documentation, along with its manufacturability for high-rate serial production, while still being a precisely From the S1218 to the 42 Series The 4201 engine is based on the design of the S1218, albeit with a 27% improved power-to-weight ratio The 4202 engine will feature rounded cooling fins on its cylinders, along with other modifications for improved combustion of heavy fuels