69 are a bit low, systems integrators can compensate by reducing weight at the vehicle level,” Bogdanov says. “They can’t compensate for an engine that fails in the field, and the loss of a high-end payload can be tens of times that of the engine. Consider the loss of the mission, especially one in urgent medical deliveries, search-and-rescue or defence operations, and then an engine failure can cause a loss that is priceless.” Hence, UAVHE has recently unveiled two supercharged Wankels with forced induction to prevent performance drops at high altitude: the RW1-79, an aircooled, 22 hp hybrid range extender, weighing 5.2 kg; and the RW1-300, a liquid-cooled, 62 hp engine, weighing 17.7 kg and featuring a gearbox for driving a propeller. In addition to throttle-less air intake, both engines are single-rotor designs and incorporate UAVHE’s proprietary, two-stage fuel-injection technology, as well as an 11 kW starter-generator and a plethora of judicious metallurgical choices made for optimising reliability. From two-strokes to rotaries Metallurgy has been the critical factor in UAVHE’s decision to make the jump from two-stroke engines to Wankel-type rotaries, being core to how it has chosen to address what it viewed as the key problems of traditional Wankel designs. Since its first engine, the company has used atypical metal components; for example, the PT1-124’s singlecylinder liner is cast iron, rather than the conventional nickel silicon carbide. “Coating the interior of an aluminium cylinder with nickel silicon carbide is a great route to cost-effective engine manufacturing, but it means the cylinder and liner have different coefficients of thermal expansion,” Bogdanov says. “After around 10 hours of operation with repeated expansion and contraction cycles, microscopic cracks could form at critical points in the liner, and tiny shavings of very hard nickel silicon carbide could then scatter throughout the engine, risking damage.” UAVHE has opted for cast iron sleeves in its two-stroke cylinders, along with forged aluminium cylinders, heads and crankcases, rather than casting them. “The cast iron sleeves aren’t cheap and they’re difficult to CNC-machine well, but they can even outlast the forged UAVHE RW1 rotary | Dossier Consider the loss of the mission, especially urgent medical deliveries or defence operations, and then an engine failure can cause a loss that is priceless Uncrewed Systems Technology | April/May 2024 The engines’ cast iron sleeves are expensive and difficult to machine well, but they can last thousands of hours and make for an effective lubrication surface The side walls are also cast iron to ensure similar mechanical and thermal properties of all combustion-exposed faces
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