Unmanned Systems Technology 026 I Tecdron TC800-FF I Propellers I USVs I AUVSI 2019 part 1 I Robby Moto UAVE I Singular Aircraft FlyOx I Teledyne SeaRaptor I Simulation & Testing I Ocean Business 2019 report

65 The engine was conceived by the company’s founder Roberto Papetti, with engineer Filippo Marino as lead designer and fellow engineers Marco Costa, Mattia Durantini and Luca Narcisi providing the project’s CFD, ECU and prototype production work respectively. Papetti’s key principles for the development of the RES centred on making the system small and light enough to be swapped into a battery- electric 4 x 4 road vehicle, in place of where a spare tyre might sit beneath the back seat. With this emphasis on convenience, Papetti and his team wanted to ensure a low cost for each unit. As a result, heavy emphasis was placed at each design stage on minimising the cost and weight of each part, making the engine as quiet as possible, and keeping the form factor within the rough dimensions of a vehicle tyre. At the same time, Papetti’s investigations into unmanned aircraft propulsion led him to notice that even the largest multi-rotor electric UAVs were significantly constrained in their payload capacities, the main cause being the low power densities and large physical footprints of present battery technology. “We then suspected that our RES could be applied to electric multi-rotor UAVs to extend their endurance, because the power density of the system was far higher than what could be achieved with battery packs of equivalent size and weight,” Papetti says. “These suspicions were confirmed by running simulations and comparing the two in terms of their power-to-weight ratios.” The company also determined that the RES could alternatively be used for a non-electric UAV’s propulsion. This alternative configuration, now called the UAVE, was created by modifying the RES’ crankshaft, replacing the RES’ generator with a starter/ generator, designing new cam profiles and programming the ECU to operate the engine at 5000 rpm (instead of the RES’ 4000 rpm operating speed). The starter generator is a 300 W, 12 V system that provides start-up power for the engine and is also designed to supply the onboard battery for powering the UAV’s electronics – other than these changes, the RES and UAVE are mechanically and operationally identical. The company now promotes the former for electric UAVs, the latter for fuel-powered UAVs. As Papetti notes, “Our company’s experience lies largely with four-stroke engines and components from the race motorcycle field, and two-stroke UAV engines suffer from a few key problems in our view.” The first, he says, is excessive fuel consumption, which adversely affects the ability of UAVs to run missions of more than four or five hours. Increasingly it is the primary factor that UAV designers rail against in their discussions with engine companies. “Two-stroke engines also experience increasing difficulties at higher altitudes, as their efficiency is significantly affected by air density, air temperature and humidity, unless their spark timing and compression ratio are strongly mechanically regulated,” Papetti adds. “That limits their applicability for major UAV uses such as defence and police surveillance.” Marino notes, “Also, the volumetric efficiency is affected by fluid density and temperature. Two-strokes need to fill two chambers with air – above and below the piston – meaning two separate volumetric efficiency levels, and these both worsen with altitude.” “Four-strokes have only one chamber to fill with air though, so the volumetric efficiency changes less with altitude. Also, the exhaust gas clearance in two- stroke engines suffers problems owing to the external air density causing waves of pressure in the exhaust tubes; four- strokes experience this problem far less because of their different architecture.” Engine components As well as being designed for small size and low weight, the preliminary phase of development was also driven by the recyclability of component materials, in order to improve the suitability of the RES for ‘green’ automotive purposes. The need to abide by environmental quality was balanced against the need to keep production costs down, and materials were selected to optimise that balance, along with the Robby Moto UAVE | Dossier Unmanned Systems Technology | June/July 2019 To adapt the original range extender design for fuel-powered UAVs, the company swapped out the electric generator for a 300 W starter/generator (Author’s image)