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56 Dossier | Cobra Aero A33i we have only two, which halves the seal friction,” Hilbert says. The crankcase integrates the engine mounting flanges and the generator mounting flanges. The airframe pick-ups and generator placement are compatible with those of the most popular UAV single-cylinder two-stroke in this power range, easing transition of existing airframes to this alternative. The cylinder The seal between barrel and crankcase is a paper gasket, and the barrel is attached to the crankcase by four 5 mm studs at its base flange. In turn, the head is attached to the barrel by six 3 mm studs using an O-ring seal. Hilbert says a separate head has certain advantages. “When the head and barrel are one piece, the bore can’t be honed straight through the top of the cylinder, and the application of the Nikasil coating is similarly compromised. From the maintenance standpoint, the ease of inspecting the piston crown for damage is another advantage of a removable head.” The barrel and the head are 356-T6 aluminium alloy castings; the barrel is made in two pieces. The inner piece is an insert akin to a liner (sleeve), but more complex and less extensive. It is designed to form the inner walls of the transfer ports, thus (before it is fitted) providing uninterrupted access to the outer walls of those ports. There is also uninterrupted access to the inner walls through machining its exterior. Thus can the transfer ports be fully CNC machined, which isn’t possible using a conventional one-piece casting. After machining, the insert is an interference fit into the outer section of the barrel and the two parts are not intended ever to be disassembled. The cylinder is bored after the fit, then sent out for nickel silicon carbide coating, then back in house it is finish- honed. Once the barrel, which we should consider linerless, has been finished there is no visible transition between the main cylinder and its insert. However, the lower transition line is located such that the piston ring remains above it. Hilbert notes, “Since we fully CNC each port, the timings are accurate side to side to within less than a thousandth of an inch. You avoid the inconsistency you get when ports are cast. It also allows you to do things that aren’t cast-able and to attain surface finishes and features on the inside of the port that nobody else has ever been able to do. “This approach is patent pending, but it is an expensive process. We will have a high-end version offering this cylinder and a more affordable version using a conventional one-piece cast cylinder.” The cylinder cooling arrangement includes an additional ‘tongue’ fin that extends over the exhaust port flange, contacting this notorious hot-spot to assist heat dissipation. There is also a fin at the base of the cylinder where likewise none would normally be seen. That base fin provides a more robust connection to the cooling shroud and minimises leakage. February/March 2017 | Unmanned Systems Technology The A33’s cylinder is made in two parts to allow full CNC machining of the transfer ports Since we fully CNC each port, it allows us to attain surface finishes and features that nobody else has been able to do