92 Insight | SkyPower’s SP engines “Starting electrically this way means there’s no accidental engine starting caused by, say, a maintenance technician accidentally grabbing or flipping a propeller blade,” Schudt adds. Battery recharging is managed using a charge current map that induces current in the windings proportional to the crank speed and shaft power. The output current therefore matches whatever engine the SGC and SG151 are installed on without reconfiguring. For example, when the SP-210 is running 3000 rpm and outputting 4 bhp of shaft power, the SG151 produces 1 kW of electric power and 20 A of current. At 4500 rpm and 7 bhp, 2.4 kW and 50 A are generated, these being the maximum continuous electricity outputs for safe recharging. The mapping can be changed for different requirements, although that might come with a risk of overcharging and damage to the batteries. Also, a few thermal limiter functions are written into the controller to prevent damage to the engine or starter/ generator from overheating. For instance, if the CHT sensor reads 180 oC or higher, the maximum allowed load in the battery will be reduced as needed, to scale back the burden on the engine from the generator and hence allow the cylinder head to cool. Integration Despite all these commonalities, some limitations exist as to how far identical components can be designed or supplied for different UAVs. Sky Power says exhaust pipes and mounting brackets most often need to be tailored for the end-user’s airframe. The exhaust system will be routed differently depending on the integrator’s preference for the routing and discharge of exhaust gases, and noise suppression around the hull, and depends particularly on the location of the engine. Design of the mounting plate is dictated by a combination of CAD files of the UAV showing its engine bay and the downstream propeller selection, the latter having a strong impact on the momentum and vibration of the engine. “Producing the mounting systems is a trade-off between strength, weight, cost and lead times. The brackets and dampers are always CNC-machined aluminium, which allows flexible design and adjustment to each engine and UAV,” Schudt says. “And the g-forces of a catapult-launched aircraft will typically imply the strongest, heaviest kind of bracket we make, with the highest number of connection points. “Before that, we’ll additively print the design and perform initial integration and practical checks. We’ve found plenty of times that a bracket or plate that works great on paper is impossible to mount in practice, because of tiny details like a lack of space for inserting screws into the firewall.” Summary Limitations aside, Sky Power says parts commonality in engines brings benefits to UAV manufacturers and operators including fuel efficiency, inventory management, and maintenance. That said, the question arises as to whether an engine company could exploit this sort of approach to components with engines more complex than two-strokes. Makers of four-strokes for instance, with their camshafts and valvetrains, or Wankels with their unique sealing and cooling considerations, might struggle with reducing the variations in these sorts of parts between engine sizes. But with Sky Power also developing its SP-360 DRE (dual rotor engine) and SP540 TRE (triple rotor engine) as modular expansions of the SP-180 SRE (featured in issue 25, April/May 2019), a precedent is being set for multiple ‘different’ Wankels using common rotors, apex seals and so on. Naturally, Sky Power would also benefit from not complicating its manufacturing lines and inventories with new parts for new engines. And given the UAV industry’s tendency to order small batches of high-end subsystems, other engine manufacturers – and indeed of other components – can be expected to find ways of adopting this approach to its products. October/November 2023 | Uncrewed Systems Technology The SG151 starter/generator is controlled by its SGC 352 controller to optimise for different power and energy modes on different engines
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