Unmanned Systems Technology 042 | Mayflower Autonomous Ship | Embedded Computing | ElevonX Sierra VTOL | UUVs insight | Flygas Engineering GAS418S | Ocean Business 2021 report | Electric motors | Priva Kompano

60 Digest | ElevonX Sierra VTOL reaches a preset altitude limit (the user can set this to be, for example, 2 m). When that limit is reached or passed, the algorithm allows the main propulsion motor to start assisting with station-keeping against strong winds by compensating for any undue motion in GNSS position or inertia not requested by the autopilot or GCS. “The Sierra VTOL continues to climb as a quadcopter to final take-off height, which is typically 25 m. After that, the main propulsion motor kicks in and starts accumulating forward velocity,” Langus says. “Once the airspeed reading is higher than the minimum 58 kph it gradually decreases the power of the VTOL motors until they are off and it’s flying as a normal aircraft. “We found this whole procedure to be very stable over a long testing programme. We started trials using the algorithm’s default values, and then after many small modifications we tailored it to the optimal transitioning procedure.” As noted by other manufacturers with alternative UAV configurations, having four VTOL propellers on a twin- boom VTOL aircraft can disrupt its aerodynamics and therefore efficiency, which is why UAV Factory for example has developed prop-retraction mechanisms for its Penguin C VTOL. ElevonX has not skirted around this issue: it has installed what it feels is a mechanically much simpler and more cost-effective passive propeller alignment system. This has permanent magnets in stationary sections of the VTOL motors (one in each motor) that are too small to have a major effect on the rotors’ existing permanent magnets or their efficiency during take-off or landing, but just powerful enough to attract the motors and propellers to align with the booms once the ESCs stop receiving power. “For this size and weight of aircraft, we think that is the optimal solution,” Langus comments. “I know that in larger VTOLs, electrical control systems for forcibly aligning or even retracting their very large propellers can be an absolute necessity, but for the Sierra VTOL, just a magnet is sufficient.” Propulsion The electric motors and ESCs used for both VTOL and forward propulsion are from T-Motor. As with Hitec, its large product portfolio enabled Langus and his team to find products that closely matched the specs ElevonX needed. “We knew what the MTOW of the Sierra VTOL was going to be, and that made the selection of motors, ESCs and batteries quite simple,” Langus says. “From there we selected the proposed set-up found through T-Motor’s datasheets, and we’ve found them to be quite reliable.” The VTOL motors are U7s, which typically weigh 255 g each and produce up to 4.6 kg of thrust (with up to 842 W input power). The forward propulsion motor is an AT4130, which weighs about 400 g and produces up to 3.2 kW and 7.7 kg of thrust. Langus adds, “When the Sierra VTOL is accelerating from hover to flight, the AT4130’s output peaks at around 1.5 kW before scaling back to around 320 W for cruise. All the while, having electric propulsion means incredibly low vibration, which is great for all onboard electronics, so the payload needs almost no vibration damping and produces very high quality imagery and video.” The ESCs are oversized for the motors, in order to have enough capacitors for electrical ‘headroom’ to store the power spikes these safety-critical components must endure (and thus prevent them from exploding). They also provide a larger cooling surface to prevent overheating, which in ElevonX’s experience is an even more frequent cause of ESC failure. Both the motors and ESCs are exposed to and thus thermally managed by propeller wash, and although the propulsion motor receives less airflow than the VTOL motors, this lower degree of cooling matches the generally lower power it draws for cruise compared with hover. The VTOL booms contain 6S lithium- polymer batteries designed to handle the high currents necessary for vertical flight and hovering, with C rates of 90-100. They are dimensioned for 2000 mAh, which is enough to enable two take-offs and landings between swaps. The propulsion motor uses lithium- ion batteries for their higher energy density. They are swappable based on the payload, to up to 45,000 mAh if a small payload is used, or less if a larger payload is used (given the finite amount of space in the fuselage). All the battery compartments are located beneath February/March 2022 | Unmanned Systems Technology The Sierra’s booms contain U7 motors from T-Motor and 6S lithium-polymer battery packs