90 Insight | SkyPower’s SP engines the head and body separately, but the result is a lower overall weight than if they were made separately and then attached with screws, as well as then needing gaskets between body and head. In addition, a heat transfer from head to body is faster and more complete, owing to the absence of any physical separation between top and bottom. “It’s impossible to get the transfer ports through CNC machining for now. It would lead to really sharp edges instead of smooth channels, and airflow from the crankcase for efficient scavenging depends on that, while additive manufacturing comes with too many cost and optimisation problems for the time being,” Schudt notes. The crankcases are CNC-machined from EN AW 5083 aluminium, and the reconfigurable nature of CNC machinery (unlike casting moulds) means end-users can have different customisations added, such as mounting points for gear or belt wheels to drive ancillaries. Each crankshaft is forged from 1.5918 (15CrNi6) case-hardened 62 HRC steel as seven separate pieces (including the con rods) before being CNC-machined to their exact tolerances. After that, they are heattreated, then pressed together into the complete shaft, with the single-piece con rods mounted on their crank pins. The crank sits in four ball bearings (the only exception being the SP-275 TS CR, which has three ball bearings holding its crankshaft). The bearings are sealed, deep-groove designs to prevent fuel leaks or incomplete compression between sections of the crankcase. The con rods are forged from the same case hardened steel as the crank, and run using needle bearings. Meanwhile, the pistons are cast from aluminium alloy JIS H5202, grade AC9A, with T6 tempering. Hybridisation Electrifying an engine line can be difficult to offer a commonality of parts for, as different UAVs and missions could theoretically need widely differing levels of electric power and sizes of motor/generator. Sky Power’s approach however has been to develop what it sees as a onesize-fits-all (albeit customisable) set of hybrid components. It reasons that most UAV manufacturers want a growing amount of electric power from their powertrains, and those who don’t can scale what is effectively the regenerative braking of the starter/generator via its controller, the SGC-352. The SGC-352 weighs 500 g, and can be set to operate in various modes depending on a UAV’s operational requirements. These include three modes for generating electric power, a battery charging mode for replenishing energy as quickly as possible, a battery current control mode, a phase current control mode, an rpm control mode, and naturally an engine starting mode. In principle, the system operates as an ESC for the starter/generator (the SG151) via trapezoidal 120o block commutation, with Hall effect sensors for close position tracking and speed control. Its enclosure is IP64-rated, for sealing against dust ingress, and protection (if not fully sealing) from water spray in any direction. It enables the SGC to function in temperatures from -40 to +85 oC. It produces power at a standard voltage of 48 V, although that can vary up to 75 V or down to 20 V. 100 A of phase current is typically produced, although peaks of up to 350 A are possible. Upon receiving a specific starting trigger signal from the autopilot over the CAN bus, the ECU executes a start function that commands the SGC 352 to trigger a load in the starter/generator for drawing a control current (the exact values for that current being mapped in the SGC 352). Notably, the starting function can only be executed if the system is stopped – that is, if the rpm sensor reads the engine speed as zero. October/November 2023 | Uncrewed Systems Technology All the cylinders for the SP-110, 170, 210 and 275 are cast from aluminium to account for the number and geometry of transfer ports
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