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45 sleeve had its top closed and it had an opening in the bore wall between the top dead centre position of the piston and the roof. This liner/sleeve was arranged to rotate about the cylinder’s main axis at half crankshaft speed. As it rotated, its aperture uncovered an intake port, then a spark plug, then an exhaust port. Thus was charge admitted to the combustion chamber, ignited and exhaust gas expelled, with the cylinder acting as its own valving through the rotation of its liner/sleeve; that rotation was obtained by gearing from the crankshaft. To keep things simple, the rotating cylinder of this 10 cc engine directly drove the propeller via a shaft extending from its roof, with the unit then horizontally mounted. Being primarily cylindrical in external form, this compact engine neatly fitted the cowling of a model aircraft using a puller propeller. It was very effective for its size, and model aircraft enthusiasts liked the noise it made. Inventor Keith Lawes called it the Rotating Cylinder Valve, giving rise to the name of the company created to commercialise the concept, RCV Engines. RCV quickly gained a firm foothold in the market, and there was soon an alternative engine that had a drive for the propeller directly from the crankshaft to suit certain applications. The cylinder drive variant was offered in 10, 15 and 20 cc versions, and the crankshaft drive version in 10, 15 and 22 cc versions. About 15,000 examples of these model aircraft engines have been sold over the past ten years. Six years ago the licence to produce them was sold to another company, Weston UK, and since then RCV has concentrated on developing the concept for other applications that need small-displacement four-strokes: it has explored markets for the likes of handheld power tools, scooters and UAVs. Model aircraft run on methanol, which is easier to burn than gasoline or heavy fuel. Nevertheless, RCV’s rotary valve concept is inherently free from detonation, and the company has successfully developed power units for gasoline and heavy fuel-burning UAVs. This work came about through a contract awarded in 2000 by DARPA, the US Department of Defense agency responsible for developing emerging technologies. DARPA engaged RCV to develop a heavy fuel-burning 44 cc boxer twin using its rotary valve concept. In view of the horizontally opposed rotating cylinders, the crankshaft was replaced by a ‘cam track’. “That cam track system wasn’t particularly reliable but the combustion system worked well,” says RCV’s commercial director Brian Mason. In 2004, a continuation of the DARPA commission saw RCV develop a 35 cc heavy-fuel version of its single-cylinder suitable for the Honeywell RQ-16A T-Hawk, a ducted-fan VTOL micro UAV suitable for backpack deployment. Given the absence of the cooling benefit of methanol fuel and the larger displacement, that version had the cylinder oil-cooled. The downside of that though was the added weight and complexity of the cooling system. To optimise power to weight, RCV developed a 60 cc boxer twin using the concept of a belt drive to the top of the cylinder rather than a bevel gear drive to the base of the cylinder. A customer requirement for more power led to a 70 cc rotary cylinder engine being built, but this proved to be problematic as the rotating cylinder concept had reached its thermal limits. RCV switched to an arrangement whereby the cylinder bore remains stationary throughout the length of the stroke, and just a top combustion chamber ‘hat’ rotates – the vertical rotary valve (VRV). This new development called for RCV Engines DF70 UAV boxer twin | Dossier Unmanned Systems Technology | Dec 2015/Jan 2016 The origin of the DF70 was a tiny internal combustion engine devised for model aircraft whose cylinder liner/sleeve had its top closed The DF70’s cylinder and vertical rotary valve