68 The era of UAVs has ushered in a variety of new approaches to power and propulsion as uncrewed aircraft manufacturers race to offer improved flight endurance, lower maintenance overheads, better fuel efficiencies, less vibration and other advantages over their competitors. Some powertrain types, previously consigned to minimal, niche use cases, have experienced commercial uptake and success among UAVs, such as Wankel engines and proton exchange membrane fuel cells. Some readers may eagerly rush to point out the perennial issues of these solutions, but the fact remains that they are today widely flown on both military and civilian platforms alike – for the various advantages they provide and for modern innovations that overcome key drawbacks or barriers to their use. While technologies such as sleeve valves and rotating cylinder valves have also contributed to highly successful and mass-produced aircraft engines, their use faded out after World War II, partially due to being outpaced by advancements in turbine engines. But with the tighter engineering tolerances now possible through modern design, material and manufacturing techniques, engines using rotary valves are making a comeback, as we have seen with RCV Engines (Issue 5) in Dorset, Britain and Strange Development (Issue 33; now Alpha-Otto Technologies) in Michigan, USA. Further west in Washington state, Avadi Engines is a company that has gained attention for its lightweight, compact, low-vibration engine designs, particularly its MA-250 prototype, which used a sleeve valve for the highly reduced part count (and hence simplicity, weight and maintenance advantages) over conventional, poppet-valve four-strokes. More recently, the company, led by CEO Landon Wilkinson, has thoroughly and definitively reworked the design of its flagship product, integrating the rotating Rory Jackson investigates the benefits of a modern take on sleeve-valve engines, featuring a single rotating cylinder and a piston with two connecting rods Spin for the win August/September 2024 | Uncrewed Systems Technology The XMD-250 is a 13 kW, single-cylinder four-stroke, configured with a rotating liner valve system for simplicity and efficiency gains (Images courtesy of Avadi Engines)
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