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83 ancillary components discussed above. Gamberini estimates that the standard- issue GAS418S crankshaft has up to 19 gears mounted on it, compared with the seven or eight at most that might be found on other UAV piston engines. “What we can say is that the main groups of gears can be seen as starter gears, supercharger gears, propeller reduction gears and camshaft gears, and we can also disclose that there’s a gear for the mechanical fuel pump,” he says. “Getting this crankshaft right was maybe the most difficult project of the overall engine development, and it took years to find the perfect configuration to run our systems. Our success in making gear- driven aviation superchargers comes as much from our crankshaft r&d as it does from the superchargers themselves.” Zucchini adds that the balancing of the crankshaft, in terms of getting the counterweights right, was outsourced to an unnamed Italian company with 60 years of experience in crank design. Oiling The oil’s pressure is usually between 3 and 6 bar, and the team suggests that the oil temperature should stay between 35 C and 112 C for safety during flight. There are typically between 4 and 5 litres of oil in the GAS418S at any given time, with a carefully engineered and customised system for storage and distribution of oil throughout the engine. Given the relatively high number of gears and con rods in this engine versus most others in the UAV space, Flygas has designed a comprehensive forced lubrication system, with a plethora of oil channels drilled throughout the crankcase and crankshaft to ensure the oil is distributed to the various bearings and gear teeth. Zucchini explains that the oil for the crankshaft lubrication is introduced radially from a lubrication hole located on each of the three main journals. The oil then travels along the crankshaft’s length, and exits from the lubrication holes on the crank journals. “Oil feeding is ensured by the high pressure provided by the pump,” he adds. “It is still possible to deliver oil radially from the main journals, because the centrifugal force across the whole envelope of crank speeds is not strong enough to prevent correct oil adduction.” Gamberini says, “This is obviously a more expensive approach than just pumping oil onto the crankshaft and letting centrifugal force do the work, but it’s far more reliable. Given how critical the mechanical drives are to the functioning and lifespan of our engine, we can’t leave any aspects of the lubrication to chance.” While some channels will deliver oil, others (those generally pointing downwards) will serve to drain it from the crankcase to the sump, to avoid any losses during operation. The GAS418S can be supplied with a dry sump (separate from the engine block) if installed vertically, or a wet sump if sitting horizontally. In either case the sump measures about 230 x 220 x 150 mm. Future plans Flygas is continuing its r&d into ways of improving the GAS418S engine and its ancillary components. For example, the team is developing a two-stage supercharger that will enable performance to be sustained at even higher altitudes than the GAS418S. This will probably be unveiled with an upcoming ‘high altitude’ version of the engine called the GAS418 HA, which is expected to weigh 3 kg more than the GAS418S in exchange for the increased operating ceiling (exact figures for which are due to be available after sufficient flight testing). The company is also working on an electronically managed turbo system, to enable greater boost power when needed while retaining the ability to switch off the system (both manually and autonomously) when appropriate to prevent back-pressure, knocking and overspeed-induced compressor damage. “That isn’t necessarily for increasing forced induction: UAVs will need more and more electrical power as we go into the future,” Gamberini says. “So that project is aimed at making a turbo- hybrid system for our engine, with the exhaust driving a turbine linked to an electric generator. That would make our engines optimal for replenishing batteries and powering payload systems, without inducing the kinds of losses associated with standard generators and turbos. “Turbo-hybrids are well-known in motorsport of course, and combinations of turbochargers and superchargers were used in World War II. At no point since our founding have we needed to discover new technologies to make our engines work: history has given us everything we need.” Unmanned Systems Technology | February/March 2022 GAS418S Four-stroke Four-cylinder 1800 cc boxer Gasoline (95 octane or higher) Mechanical centrifugal supercharger Liquid-cooled EFI (added carburettor optional) Dimensions: 664 x 664 x 449 mm Weight: 78 kg Maximum power output: 132 kW (180 bhp) at 5800 rpm Operating power output: 97 kW at 5000 rpm Maximum torque: 224 Nm at 4200 rpm Operating torque: 185 Nm at 5000 rpm Maximum speed: 6200 rpm Operating speed: 5000 rpm Maximum fuel consumption: 61 litres/hour Operating fuel consumption: 37 litres/hour Compression ratio: 9.1:1 TBO: 1200 hours Specifications