Issue 45 | Uncrewed Systems Technology Aug/Sept 2022 Tidewie USV Tupan | Performance monitoring | Bayonet 350 | UAVs insight | Xponential 2022 | ULPower UL350i and UL350iHPS | Elroy Air Chaparral | Gimbals | Clogworks Dark Matter

86 Dossier | ULPower UL350i and UL350iHPS bushings holding the shaft come with an automotive-type coating, to avoid the need to apply it to the shaft or its seats. “Like the crankshaft, the con rods are somewhat overdesigned,” Denorme says. “The 4140 steel alloy gives great toughness and tensile strength, and it can be hardened very easily. “The con rods mount on the crank pins using split bushings, and each one attaches onto the piston pin using a brass bushing. The piston pin is made from an undisclosed hard material, so we go with the brass to have the pin move against something soft. “That, and the nature of brass, make for very good lubrication without needing special coatings. It’s a well-proven approach in automotive engines, and the piston pin is similarly a well-proven type used in automotive engines, which we get delivered with the con rods. “After taking delivery of the con rods we also check their weight to ensure they’re no more than 5 g apart. If any of them are, we’ll set them either side of the crankshaft before performing a final full weight test so they balance out.” Combustion The cylinders fire in the order 1-3-2-4, a sequence used in all four-cylinder ULPower engines to minimise vibration. Ignition is performed using automotive-type coils for reliability, with the FADEC wired directly to the spark plugs for supplying power and control signals, in addition to calculating the variable spark timing that is key to the engine’s effective ignition advances. Two spark plugs sit in each cylinder for redundancy, and fire at the same time. Once the fuel-air mixture passes through the cylinder head, via a system of overhead poppet valves, they enter a ‘squish’ zone designed into the combustion chamber that narrows to a gap of less than 1 mm between the piston and the cylinder head at the former’s TDC. The turbulence created by this, and the shape of the piston crown and cylinder head, is aimed at causing August/September 2022 | Uncrewed Systems Technology The UL350i’s crankcase is 6082 aluminium, cut from billet as two halves that split vertically down the middle, lengthwise with the crankshaft, and held together by 10 studs. The cylinders are 42CrMo 4 V steel, and notably (for UAV spark-ignited engines) they are linerless and use no coating, as the engine’s relatively slow speeds make it unnecessary to do so. Like the crankcase, the cylinder heads are cut from 6082 aluminium, as it is ideal not only for machining but also providing the necessary strength for the combustion chambers. The cylinders join to the crankcase using Viton gaskets, while a proprietary material gasket sits between each cylinder and head. Each head bolts to each cylinder using six M8x1x35 screws of 12.9 steel. The company has also sought to maximise the size of the cooling fins cut around each head, and has found that, as far as power-to-weight efficiency is concerned, “the larger the better”. The crankshaft, camshaft and con rods are made from high-strength 4140 steel alloy; these parts are also nitrided for added hardness. They are each milled from a single billet, with forging as a potential option for the crankshaft if production quantities justify investing in forging tools. The crank is located within five plain bearings and a single ball bearing for thrust. As the crankshaft is monolithic, each con rod splits at its big end, with the two pieces held together again by two M8x1x35 bolts made from 12.9 steel. The pistons are forged from aluminium and stamped into shape for minimised porosity and exact combustion chamber geometry. Three piston rings are mounted, with the bottom one acting as an oil scraper and support rings above and below it. Anatomy The UL350’s crankcase is milled from 6082 aluminium in two parts, with the split running vertically down the length of the case