Issue 41 Unmanned Systems Technology December/January 2022 PteroDynamics X-P4 l Sense & avoid l 4Front Robotics Cricket l Autonomous transport l NWFC-1500 fuel cell l DroneX report l OceanScout I Composites I DSEI 2021 report

28 Dossier | PteroDynamics X-P4 UAVs. Similar repair logistics missions could be provided for armies, air forces, oil & gas platforms and remote industrial facilities, as well as delivering emergency medical products or aid packets. “Mapping or intelligence missions might seem straightforward and less urgent than all these delivery and relief operations, but we can certainly do them,” Corgnati adds “We can adjust the wings and tailplane to trade off between speed, endurance, payload stability and so on.” To carry all the different items across these missions, a payload bay is installed in the front as standard. A single hinge under the nose allows this section to swing open, to reveal a square- shaped 1 cu ft volume for installing packages. If needed, this section can be turned into a mounting point for a retractable camera gimbal. “We want even the most inexperienced personnel to be able to use the aircraft,” says Corgnati. “The payload is housed internally to maximise aerodynamic efficiency. Access requires no tools, and all airframe components are retained without having to be removed.” Flight control and avionics The VTOL-transition systems we have previously featured in this magazine rely largely on complex algorithms for transitioning that took enormous development time and rigour. PteroDynamics has a strikingly different story. As Dr Petrov discovered in the maiden test flight of his balsa prototype in 2016, the Transwing transition approach requires very little of the UAV’s autopilot in terms of sensory inputs, processing load and control outputs. “I made that first flight and transition with a $10 KK2.1 flight controller,” he recounts. “I programmed it with a really simple gain scheduling algorithm, to handle the gradual change of controller laws from a quadcopter configuration to a comparatively simple fixed-wing configuration. It’s a linear change from one set of rules to the other as the wings unfolded, and that was all – and it worked.” Specifically, the strength of the autopilot’s PID control feedback loop (much like those typically used in distributed-lift UAVs) is linearly reduced by the position of the worm drive, rather than a hard, binary switch, as the wings unfold into their fixed-wing angle. “All the flight controller needs to know is when it’s supposed to transition,” Dr Petrov adds. “It doesn’t physically need to look at the altitude or airspeed, and we know from hundreds of test flights that the transition will be very benign and controllable, without any significant loss or gain of altitude. The most common effect is some loss of speed as the aircraft decelerates during inbound transition, or speed gain during outbound transition.” Therefore, the main purpose of the flight controller is not to handle any complex transition algorithm, but to ensure an appropriate robustness of flight autonomy and hence consistent endurance for the end-user. “We have developed a Matlab model of our aircraft to better understand the interactions between the aerodynamic and propulsion forces, hinge flexing and feedback control actions during the transition regime,” Dr Petrov comments. “We have analysed multiple implementations of this model spanning a range of sizes and weights up to 5000 kg MTOW. We concluded that the transition flight envelope is robust and independent of the size and weight of the aircraft, even with the simple linear controller gain scheduling. We have also developed RealFlight models of the X-P2 and X-P4 to test the flight regimes using the Ardupilot SITL environment and the native RealFlight stabilisation systems.” To date, PteroDynamics has flown Transwing aircraft using three different flight controllers: KK2.1, Ardupilot and Applied Navigation’s Quattro FMS. “While the simple KK2.1 controller was sufficient to stabilise our aircraft, we have progressed to other, more mature flight control systems that enable December/January 2022 | Unmanned Systems Technology PteroDynamics has tested (and keeps in store) a wide range of BLDC motor and propeller pairings, to mature and optimise the full range of its UAVs’ flight envelopes