Unmanned Systems Technology 019 | Navya Autonom Cab | Batteries | UGVs Insight | UAV Factory UAV28-EFI | Swiss Aerobotics Hummel | UMEX 2018 report | Antennas | Oceanology International 2018 report

65 UAV Factory UAV28-EFI | Dossier Again belt-driven from the rear of the crankshaft (the pulley located between the crankcase and the propeller drive) this brushless item, rated at 200 W, runs at a different gear ratio and has active rectifier electronics to increase efficiency. Popiks notes that it generates less heat and is easier to cool. It works with an integrated electronics module for the entire aircraft. “That has a lot of functionality,” he adds. “It comprises a stack of PCBs, one of which carries out the GPU function, another the autopilot and so on.” Performance Some customers put a premium on stealth. Although machined from a block of aluminium, the UAV Factory silencer has walls as thin as those of a typical drinks can, and inside are various chambers with many internal perforated walls; glass fibre matting further aids sound absorption. The carefully packaged unit contains a lot of resonators, which absorb different acoustic channels, so it is a hugely complex production. Thanks to it though, the engine has a low acoustic signature: measured from 30 m it registers only 59 dB, while at 5000 rpm it is reportedly undetectable beyond 700 m. The UAV28-EFI runs on RON 98 gasoline as a default (RON 95 is also a suitable option) and 2% of oil is mixed in to provide lubrication. The engine runs from 1500 to 8500 rpm and produces maximum power of 3.3 bhp at 8500 rpm with maximum torque of 2.5 Nm at 8150 rpm. Normal cruising engine speed is in the region of 4500 rpm, while the normal maximum engine speed for take-off is around 6500 rpm. “We de-rate the engine by running to a maximum of 6500 rpm to squeeze out extra reliability,” explains Popiks. In cruise mode, the quoted fuel consumption is 400 g/kW/h. “Our aircraft needs a certain amount of thrust to fly in cruise mode,” Popiks says. “Let’s say we need 300 W to fly in cruise. What is very interesting is that as we install more powerful payloads and data links, the amount of power we consume on board the aircraft gets close to the amount we consume for level flight. A 200 W generator takes about 230 W from the engine, so that has a major effect on fuel consumption in cruise mode.” With the ongoing work since UAV Factory decided to bring power unit development in-house, trials have shown that 1000 hours can comfortably be exceeded with only standard maintenance procedures. Those procedures amount to external visual checks, a fresh spark plug at 50 hours and fresh silencer packing at 100 hours. The air and fuel filters will also be cleaned or possibly replaced at 100 hours, and the cylinder will be removed for inspection. Nevertheless, currently the TBO is set at 300 hours. “That is a very safe limit, to ensure maximum durability,” explains Popiks. “One problem is that customers don’t always do the standard maintenance, even after training. During the 300 hours there is even the possibility of running the engine outside operational parameters for some time, and the engine will continue to work fine. “Don’t forget that the engine doesn’t have redundancy – the [single] ignition, the [single] injection and so on. That means every component is flight-critical, and the only way we can maintain a super-high reliability of the system is not to push components even close to their potential lifetime. We proactively replace a lot of components even though they appear to be fine.” Popiks adds that the use of a powerful external starter means “we don’t need a compression relief valve. Such a valve adversely affects the shape of the combustion chamber and is prone to carbon build-up, so it’s a threat to MTBF.” Since everything revolves around MTBF, also avoided are two-stroke innovations such as compressed air- assisted fuel injection, exhaust heating of the fuel and crankcase, and active exhaust valving. “We have various ideas coming from our engineering department, but since our main concern is MTBF, anything that increases complexity is unwelcome,” Popiks says. “Any complication like that will fail at some time.” Thus it is that the UAV28-EFI is a straightforward but very dependable UAV power plant. Unmanned Systems Technology | April/May 2018 The very effective silencer developed by UAV Factory