Issue 39 Unmanned Systems Technology August/September 2021 Maritime Robotics Mariner l Simulation tools focus l MRS MR-10 and MR-20 l UAVs insight l HFE International GenPod l Exotec Skypod l Autopilots focus l Aquaai Mazu

48 weights that would result from the motor- prop permutations we were looking at.” Experiments were then run across combinations of motors and propellers (on both the upper and lower motor- propeller assemblies for the MR-10), with assistance from nearby experts at the University of North Dakota (UND), to identify the optimal configuration. The lower propeller in a coaxial pairing is often expected to suffer from an ‘interrupted’ airflow caused by its partner above it. MRS addresses that by using a larger pitch on the lower propeller to increase its efficiency. “It was some of the professors at UND who determined that we’d get more efficiency if we used higher-pitch propellers on the bottom,” Hodny says. “It just so happens that with T-Motor, if you want more pitch you need to order bigger- diameter blades. Once we’d incorporated the new prop arrangement, we were hitting our thrust targets while producing way less heat and noise from the motors and ESCs.” The change has led to a layout of four 71.12 cm upper props and 76.2 cm lower props for the MR-10, while the MR-20’s props measure 73.66 cm on top and 78.74 cm below. Currently all the company’s propellers are fixed-pitch types, but MRS plans to incorporate hinged propellers in the future for easier transportability. Further ahead, the company intends to fit variable-pitch propellers (VPPs) on its aircraft, to maximise the aerodynamic efficiency of the two UAVs across all their flight envelopes. “We’ve received the first engineering proposals for VPPs, and we’ll be looking to have production of those – along with our other powertrain components – moved either in-house, to somewhere local, or at the very least within the US, such is the importance of resilience in supply chains in this age,” Nickell observes. Hodny adds, “We’ve also sponsored a research project at UND’s mechanical engineering department, which provided us with exact designs for new carbon fibre fixed-pitch props that we plan to manufacture in the US.” Energy is supplied by lithium-polymer battery packs. On the MR-10, this is a 6S 22,000 mAh pack, with an optional second battery of the same size, while the MR-20 can integrate either two or four packs, each of which is also 6S, with 30,000 mAh. MRS reports that flight tests with the full 30 lb, four-pack, 120 Ah complement are continuing, but results so far indicate that 5 lb can be carried for up to 50 minutes under this configuration. “What we want by the end of development is a fully integrated powertrain,” Nickell says. “We want smart batteries for efficiently powering the ESCs, ESCs that are perfectly compatible with the e-motors, and propellers optimised for maximum lift – all the while with a minimum of EMI from the various electrical components. “But if we can’t get that whole package from a vertically integrated UAV powertrain developer then we’ll analyse and integrate the ideal components from different suppliers ourselves.” August/September 2021 | Unmanned Systems Technology Minimising (and eventually eliminating) solder joints was a prime motivator behind the in-house development of the power distribution board (pictured) Two Here3 antennas are installed for GNSS and heading information