Issue 45 | Uncrewed Systems Technology Aug/Sept 2022 Tidewie USV Tupan | Performance monitoring | Bayonet 350 | UAVs insight | Xponential 2022 | ULPower UL350i and UL350iHPS | Elroy Air Chaparral | Gimbals | Clogworks Dark Matter

89 ULPower UL350i and UL350iHPS | Dossier bottom, to help with the consistency of valve timing and force. “Every part engaged or actuated in the valvetrain is made from 4140 chrome molybdenum steel alloy – in fact, so are the crankshaft and con rods, for the strength that alloy gives,” Defoer says. “The tips of the valves are given a hardening treatment to last longer against the friction of the rocker arms, and for similar anti-wear reasons the cam followers have a special proprietary coating that we adopted after taking inspiration from some Formula One engines.” ULPower also includes a proprietary design feature in the cams themselves. Although the exact nature of the feature cannot be disclosed, it reduces the wear experienced by each cam follower over time, again helping them and their coatings last longer. “Also, the cams all have 114 º of lobe separation, except in the UL350iHPS, which has 110 º ,” Defoer says. Forced cooling The cooling system is contained in a box-like shroud that sits around much of the engine. It weighs 12 kg on ULPower’s four-cylinder engines and 15 kg on its six-cylinders. The system consists of an aluminium impeller fan that is directly driven by the crankshaft. The fan sucks 1 m 3 of air per second into an airbox for distribution to the cylinder heads at 3000 rpm. Defoer adds that the fan takes only about 1 hp to move. The eight-blade rotor-impeller is milled in-house before being mounted onto the crankshaft by way of a fixed coupling sub-assembly. Two ram airboxes extend backwards from the rotor housing surrounding the fan – one sits over the left cylinder heads and the other over the right ones. The internal airflow dynamics have been designed to ensure an equal distribution of air to every cylinder and head, and hence an equal performance boost from the extra cooling. “The shroud was originally made from composite, but for reasons of easy mounting as well as maintenance of the engine, we switched to making it all- metal,” Denorme says. “The size of the cooling system is such that a manufacturer wishing to take advantage of the UL350iHPS would be better off designing and building an all- new aircraft around the engine, rather than trying to use the UL350iHPS as a drop-in replacement for the UL350i.” It is important to note that the cooling system does not rely solely on air for heat exchange. It also contains an integrated oil cooler that sits beneath the fan, acting such that the lubrication also provides cooling throughout the inner engine. While the oil is usually cooled by the air rush during flight, this approach guarantees that the oil temperature never rises above 120 C, to ensure effective cooling. Oil system Oil is stored in a cast aluminium sump, although larger sumps welded from aluminium can also be supplied. These were standard issue in previous iterations of the engines, and are still used by some aerobatics customers. “We bolt a wet sump at the bottom of the engine, so oil drain or return happens naturally through gravity,” Defoer says. “Oil drawn from the sump first passes through a coarse filter before passing through our pump – and we install quite a big pump to generate enough oil pressure and flow rate to extract heat from the engine. It’s a 25 mm-thick rotor inside the pump on the UL350, while on our six-cylinder engines the oil pumps contain two 20 mm rotors. Uncrewed Systems Technology | August/September 2022 The UL350iHPS is built with a forced-air cooling system for rotary aircraft; its aluminium fan draws in a cubic metre of air every second to enable a top power output of 150 bhp Our partners in Ostend have a lot of experience in rally car engines – it’s their expertise that informs key design targets for the cylinder interior