USE Network launch I UAV Works VALAQ l Cable harnesses l USVs insight l Xponential 2020 update l MARIN AUV l Suter Industries TOA 288 l Vitirover l AI systems l Vtrus ABI

26 Dossier | UAV Works VALAQ family problem of landing in gusty winds. “Another key might be the inclusion of 4G comms as standard to serve as a second comms link and extend operational range,” he says. Deep understanding The main challenges in the VALAQ’s development lay in achieving a deep understanding of its aerodynamics – the polar plot that shows the relationship between the coefficients of lift and drag in particular. In addition, the multi-copter propulsion systems, controls, aircraft physics and performance and their mutual interactions had to be understood and embodied in the autopilot software. In refining the airframe’s structure, its strength and weight had to be optimised. Along with the stiffness needed to eliminate a vibration problem, that led to a switch from a solid foam-cored structure to an all-carbon fibre airframe. Another necessary trade-off was the time spent in vertical flight and overall range and endurance. That meant the autopilot’s programming had to ensure that the transition to forward flight would be as fast as possible while remaining stable and safe. Developing the software for the autopilot was the principal task facing Ortiz as, during 2014, he thought through the idea for a novel UAV that would be the subject of his final project for his masters in aerospace vehicles course. He knew he could use his aeromodelling skills to build a proof- of-concept vehicle, but he also knew he needed a partner to help with the software, so he teamed up with fellow student Salva Puig to pursue the project. Puig was working towards his masters in propulsion systems. The two began to study programming in the C and C++ languages, and within three months they had gained enough knowledge of them to understand how to program the autopilot. At the time, Multiwii was the most accessible software, so they adapted it to create the code they needed to control the transition process. Building and flying this proof-of- concept vehicle successfully earned Ortiz and Puig a good grade in their final thesis, bagged them the best project in the helicopters section of an Airbus- sponsored engineering award and sowed the seeds of further development. They built a second prototype in 2015, logging a lot of flight data that they used to improve the autopilot code. During flight tests with this second vehicle, which had a slightly smaller wingspan than the first, they encountered vibrations that caused major control problems, prompting the switch to a stressed skin monocoque made from carbon fibre reinforced plastic (CFRP) for the vehicle’s next iteration. Flight control development While early autopilot development work was centred on the Multiwii system, the current autopilot is built on the Pixhawk Cube architecture by Hex Technology, and the software is a PX4 flight stack customised by UAV Works. It commands the motor controllers and the elevons, the latter through open-loop digital metal-gear slim servos to which they are directly attached, and which serve as the hinge at one end of each control surface. During the development process, the way in which the transition between vertical and horizontal flight is managed has changed. On the first prototype, the method was to accelerate vertically until the airspeed exceeded the wing’s stall speed, at which point the multi-copter control system was switched off and, using the elevons alone, the aircraft was pitched forward into horizontal flight. “This was very inefficient, but because the whole flight envelope was not yet understood it was the safest approach,” Ortiz explains. Subsequently, he says, the company worked to improve the efficiency of the transition progressively using the second and third prototypes before arriving at what they regard as the best method. These days the aircraft is capable of stable flight at all speeds up to 120 kph, so the control strategy is to make the transition by increasing forward speed while holding a constant altitude, using a combination of propeller thrust and elevons, then using the elevons when above the (theoretical) stall speed of the wing to pitch the vehicle over into horizontal flight. June/July 2020 | Unmanned Systems Technology Rendering of the VALAQ showing a propeller above and below each wing to enable fine control as a quadcopter in VTOL mode as well as forward thrust in horizontal flight without a tilting mechanism