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

56 the pressure and hence cooling capability of the system falls to match. This is advantageous in avoiding over- cooling, as can occur with conventional systems during long, low-power gliding of a UAV in very low ambients.” Bailey says the only disadvantage with Compact SPARCS is the somewhat limited heat exchanger capacity, since that item is cast into the engine structure. “So if you wanted to do something different with the engine, you might not have that option without redesigning the engine. By contrast, using an external heat exchanger [as in regular SPARCS] allows you to vary its size according to application.” He adds, “The operation of the SPARCS system doesn’t rob the engine in respect of power density: the power to drive the internal fan is never more than 1.5% of engine output. On the contrary, by providing a cooler rotor, it allows it to operate at higher power levels, as well as increasing the time between overhauls of the engine.” 225CS engine core A steel eccentric shaft runs in a pair of needle roller bearings, one housed in each endplate, and twin counterweights compensate for the eccentricity of the shaft with the rotor mass. One counterweight is integrated with the propeller drive hub, hence it is external, while the other is integrated with the SPARCS fan and the timing wheel in the non-drive endplate/cover. In all air-cooled rotor-type rotaries, and also in the SPARCS system, five parallel passageways allow the flow of cooling gas through the shaft and rotor assembly. Two are through the eccentric of the shaft and one through each corner of the triangular rotor. The connecting openings in the two endplates are bounded in their size by having to be located within the locus of the inner edge of the rotor side seals, as those seals rotate and orbit. Maximising the heat exchanging internal surface area of the rotor in conjunction with minimising the pressure drop of the airstream is a key design area for this type of engine. The steel rotor is formed as inner and outer parts. The original Fichtel & Sachs engines used a one-piece assembly, but this resulted in cracking problems when the power was increased during the development of the Norton motorcycle engine.  The inner part includes the rotor ring gear and the rotor bearing outer race, this being induction-hardened in the bore. The size and shape, including the depth of the combustion chamber pockets formed by the rotor, controls compression ratio. “Our pocket design is a simple one, a single rather than dual pocket that is easy to machine – hence April/May 2016 | Unmanned Systems Technology Illustration of the operation of the SPARCS cooling system The 225CS rotor is produced by Tata Steel SPARCS doesn’t rob the engine of power density: the power to drive the internal fan is never more than 1.5% of engine output