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

46 Digest | MRS KittyHawk MR-10 and RavenHawk MR-20 Simulation and testing Optimising the UAVs’ structural parts and materials took several steps, the first of which were in-house simulations using AutoDesk and similar programs. The accuracy of these was enhanced with additional external data and finite element method logics, which were sourced in order to account correctly for changing thrust inputs and power outputs, so that mechanical stress levels (and the flow thereof) could be precisely mapped out across the different transients and flight modes the UAVs might undergo. Much of that testing made use of in- house bench tests, initially performed solely on subsystems, before they were assembled into the prototype KittyHawks and RavenHawks. “For example, we have a dynamometer on which we can install any combination of battery, e-motor, ESC and propeller, up to and including a 72 in prop,” says Nicole Hodny, r&d and production engineer at MRS. “We can then run a fully autonomous testing cycle on whatever powertrain combo we’ve mounted; it’s written in C++, stepping up or down various throttle settings over time and how quickly the dynamometer jumps between those. The system will also output any data we need, including current, voltage, rpm, thrust, torque and propeller efficiency.” A larger, rather unique testing rig, constructed specifically for controlled indoor flight trials of VTOL UAVs, came into play afterwards. This system features a tether extending from above the multi- rotor craft. That allows for direct observations and measurements of basic flight worthiness, avionics and fatigue testing, including how the hull materials, batteries and drives handle different aspects of the flight envelope. The tether allows for safe ‘catching’ of the aircraft if power is lost unexpectedly or needs to be shut off for any reason. “We have a kill switch wired into the GCS so that we can remotely cut off the battery if needed,” Hodny adds. “While the test rig and ceiling are only 32 ft high, the pulley and tether ensure we can catch it and lower it safely, so we save on r&d costs by breaking far fewer subsystems during testing.” Once the team was satisfied with controlled, tethered indoor flight tests, the prototypes were taken outdoors for real- world trials, to be assessed and refined amid the gusts, weather and other qualities of North Dakota’s segregated test airspaces. Propulsion As mentioned, the main architectural difference between the two platforms comes down to their propulsion arrangements. Both have eight electric motors integrated as four coaxial, counter-rotating pairs mounted at the ends of the carbon fibre arms using attachment decks made from additively printed polycarbonate. At the time of writing, the motors used were from the T-Motor U8 Lite series, although MRS plans to move its supply chains to within the US in the near future. The KittyHawk uses eight 24 V motors, while the RavenHawk is driven on larger (but similarly constructed) 48 V units. However, MRS is investigating motors of even higher voltage to reduce the current, given the undesirable impacts that high currents can have on safety, wear and power efficiency. The Pixhawk autopilot outputs commands in PWM to the ESCs, which are currently 70 A T-Motor Flame units, although MRS plans to switch to ESCs from a US source in the months ahead. “Early on, we explored the options of a quad- or octocopter that seemed most viable for ensuring a stable, distributed lift platform,” Nickell explains. “We’re looking into hexacopter designs too, to evaluate their efficiency and see what trade-offs might need to be made between motors, props, cost and manufacturability.” In pursuit of greater aerodynamic efficiency, MRS has drawn on a few sources of r&d to optimise the diameter, pitch, materials and overall shape of its props. “Initially we used props from T-Motor, who have done a surprising amount of work in terms of laying out their motor-propeller combinations and analyses,” Nickell says. “With that as a basis, we did a lot of further analysis internally into what kinds of thrust outputs we needed across our airframes and components, and how they related to the total and unit-level August/September 2021 | Unmanned Systems Technology The company’s coaxial motor configuration uses larger and higher-pitch propellers on its lower rotors for increased efficiency