Unmanned Systems Technology 005 | Selex ES Falco UAV | Sense and avoid systems | RCV Engines DF70 | DSEI show report | Fuel cells | CUAV Expo, InterDrone and CUAV Show reports | SLAM

46 Dossier | RCV Engines DF70 UAV boxer twin a belt or gear-plus-shaft drive from the crankshaft to the VRV atop each cylinder. A belt drive does not tolerate the dust or sand associated with desert operation, leading RCV to concentrate on the purely mechanical gear drive system for its UAV engines. DF70 design work embraced advanced finite element analysis, which was sub-contracted to specialist engineering consultancy AAL. This embraced the likes of crankshaft behaviour and the optimisation of the VRV and its bore under cold conditions and at running temperature. Ricardo Wave engine simulation software was used to help optimise engine performance, including consideration of noise. Mason notes that the software had to be ‘tricked’ into assuming that the engine under evaluation used poppet valves. The DF70 first ran on the dyno in 2008. Development work since then has focused on durability and optimising fuel consumption. When it comes to noise, a more effective silencer has been developed that costs what Mason terms “a small performance loss” for those customers that want to prioritise stealth. In addition to the DF70, RCV currently offers a 35 cc single and a (downsized) 50 cc twin for the UAV market. The first application of the DF70 came in 2010, in a fixed-wing UAV called the Spear from Brock Technologies, which had originally been electrically powered. The manufacturer reported that the switch to a heavy fuel-drinking DF70 “significantly increased” payload capacity, top speed and endurance. Brock was supplied by Ultra Electronics, which had been licensed the right to market the DF70 from RCV. That same year, Ultra Electronics also supplied the engine for a small unmanned helicopter developed by Flint Hill Solutions. Since 2013, RCV has provided DF70 and DF35 engines for a further nine applications, mostly undisclosed. The DF70 The DF70 is almost square, with a 36 mm bore and 35 mm stroke for 71.25 cc. It is a pure boxer, with each con rod having its own crankpin, arranged so that in effect the two pistons ‘box’ towards and away from each other. The offset of the two crankpins is kept to a minimum to minimise the couple associated with it – it is just 18 mm. None of the webs are counterbalanced. “Since this is a small-displacement boxer, we don’t need counterbalancing,” notes Mason, “and we don’t require any vibration dampening either.” The engine has a built-up steel crankshaft that allows for the use of ball bearings as the main bearings, and needle rollers for the big ends. The small end also uses a needle roller bearing, which is prudent given that the engine’s lubricant is mixed with its fuel, in two- stroke fashion. It is also in the interest of low friction. Steel I-section rods are driven by light alloy slipper-type single-ring pistons. The DF70 has an aluminium structure, with the cylinders formed separately from the crankcase. The piston bore and the VRV bore are both formed linerless within the respective aluminium alloy cylinder, and both bores are coated with nickel silicon carbide. The VRV is shaft-driven from the crankshaft, and the use of two bearings at the bottom end of the shaft has been found to enhance the life of the timing drive. The hardened steel VRV is DLC coated inside and out so that its portion of the combustion chamber is thus treated. Atop the cylinder is a timing drive gearbox sandwiching the gears that drive the VRV and having upper and lower elements, the former simply sealing the assembly. Dec 2015/Jan 2016 | Unmanned Systems Technology The DF70 is almost square and is a pure boxer: in effect the two pistons ‘box’ towards and away from each other DF70 crankshaft