Unmanned Systems Technology 008 | Alti Transition UAS | Ground control systems | Xponential 2016 report | Insitu Orbital N20 | UAVs | Solar power | Oceanology International 2016 report

59 were already a leader in reliability through development of the existing engine, but we wanted to push the bar with the N20. The goal is zero engine failures, especially as any failure can cause the loss of an expensive payload. “We have more than 20 years of learning from the development and operation of our existing engines. The N20 is a combination of our experience and that of Orbital.” Overseen by Insitu, Orbital drew the N20 from the aforementioned clean sheet of paper. Since Insitu funded the project it owns the IP, aside from the proprietary incorporated Orbital DI technology. The 50 cc N20 produces 2.4 kW. It runs on Jet fuel (A, A1, JP5 and JP8) while an N21 version runs on certain types of gasoline or Avgas, although the only difference between the two is in the engine calibration firmware. The engine is air-cooled, has an upright cylinder and is used as a pusher in the ScanEagle (in other words, the propeller is at the rear). The engine core has been designed to be adaptable to a variety of airframes, of which the ScanEagle is the first. The engine core is augmented by a fuel supply system, an oil supply system, a muffler, a generator, a propeller and an engine management system. Orbital supplies the fuel pump, the fuel injector and the ECU while Insitu has collaborated with a propeller manufacturer to develop an application- specific item. “The N20 has already met all its targets in respect of power, fuel consumption and reliability,” Beloy reported as this article was being written, and preparations were in hand for the first production examples to begin operation. Engine design considerations We asked Beloy, given the clean-sheet approach, why not opt for a change of engine type, perhaps to something more radical? “Reliability is the chief requirement,” he explains. “There was the temptation to use something different but it was considered more important to use well- proven technology.” Supercharging and turbo-supercharging were ruled out on grounds of cost and reliability, as well as added weight, size and complexity. “We make sufficient power without the additional complexity,” he says. A compression ignition engine would require a higher compression ratio, and with that a stronger, heavier engine architecture, and it would create more vibration, hence the preference for continuing with spark ignition. “However, it must in all circumstances be possible to ignite jet fuel in a controlled manner, and Orbital’s technology does this,” Beloy emphasises. Spark ignition of jet fuel normally implies running hot, which in turn causes the build-up of deposits in the combustion chamber. That not only reduces the time between overhauls but also brings a danger of pre-ignition. Orbital’s technology makes the engine run cooler, overcoming this drawback. It also helps meet the key challenge of the vast range of operational conditions to which the ScanEagle is subject, from the cold of the Arctic to the heat of the desert. The ScanEagle has always been powered by an air-cooled two-stroke single. It was originally powered by a modified model aircraft engine, although that 28 cc unit has been much modified over time. Insitu wanted to stay with an air-cooled two- stroke in consideration of power density. Minimising weight was a key target, hence the continuing use of just one cylinder. Compared with a twin, a single is lighter and more fuel-efficient. It also implies a craft with less aerodynamic drag and helps make the engine more affordable. However, compared with the Insitu N20 UAV 50 cc two-stroke single | Dossier Unmanned Systems Technology | June/July 2016 We wanted to push the bar with the N20. The goal is zero engine failures, especially as any failure can cause the loss of a costly payload The powertrain package for Insitu’s ScanEagle UAV incorporates the N20 into a module that can be removed from the craft without disconnecting any fluid lines