Unmanned Systems Technology 007 | UMEX 2016 report | Navya ARMA | Launch & recovery systems | AIE 225CS | AUVs | Electric motors | Lethal autonomous weapons

48 W ankel-type four- stroke rotary engines using an air-cooled rotor have made a comeback as power plants for UAVs. Such engines are inherently more compact than a comparable reciprocating engine, and lighter too, in effect providing the power density of a two-stroke rather than a four-stroke. Flight endurance and engine dependability are of paramount importance to UAV platform developers, and contemporary rotary engines are now well proven in both respects in this arena, where weight and packaging considerations are the other dominant factors. At low rpm the brake mean effective pressure (bmep) and specific fuel consumption (sfc) of the Wankel-type engine is inherently inferior to that of a conventional four-stroke owing to its more complex sealing arrangement compared with the latter’s piston rings. However, higher rpm, as applicable to UAV usage, reduces this gas leakage disadvantage and it is offset by higher mechanical efficiency, especially with the air-cooled rotor type. The sfc can then match the best four- strokes. Having an induction system without valve gear results in volumetric efficiency of 110% or so, providing high bmep. Advanced Innovative Engineering (AIE) is one of a number of companies currently developing four-stroke rotary engines for the UAV market, others include Austro Engines (profiled in UST issue 3, Summer 2015) and Rotron (profiled in UST issue 4, Autumn 2015). Based in the Midlands region of England, AIE is exploiting a number of new patents filed by David Garside, a pioneer of the small-displacement Wankel-type engine (see sidebar: Background). The key patent for AIE’s first UAV engine, the 225CS profiled here, is a new cooling concept called the Self-Pressurising Air Rotor Cooling System (SPARCS). Garside notes that SPARCS addresses two deficiencies of air-cooled rotary engines. The first is cooling of the rotor being only marginally adequate, particularly in high ambient temperatures; the second is the drawback of lubrication of the bearings and all sliding surfaces being achieved by a total-loss system. Ian Bamsey investigates a rotary engine for UAV use that exploits an ingenious system to reduce internal temperature, thereby increasing efficiency and reducing wear Cool running AIE’s 225CS brings innovative technology to the UAV power unit market April/May 2016 | Unmanned Systems Technology