Unmanned Systems Technology 011 | C-Astral Bramor ppX | IMUs | Autonomous farming | UAV Turbines UTP50R | London Show report | Advanced materials | Las Vegas Expo report

24 Working first in a simulated environment and then carrying out a great deal of real-world testing, the company compared many different configurations. “Very soon it was clear that we could achieve really extraordinary efficiency. Before that there was not much relevant data or research,” Trost says. “At that time – 2005-6 – there were a few studies of NASA’s BWBs for commercial aircraft, but they were based on much higher Reynolds numbers for higher speeds at airliner scale. At low Reynolds numbers it is a different kind of aerodynamics.” The Reynolds number of an object moving through a fluid, such as a wing moving through air, multiplies the fluid’s density by the velocity and a characteristic dimension such as length, and divides their product by the fluid’s dynamic viscosity (a measure of its resistance to shearing). Viscous forces are more important than inertial forces at low Reynolds numbers, and they produce laminar rather than turbulent flows and are characteristic of small UAVs operating at low speeds. The design goal of a 25% increase in endurance pointed C-Astral towards the potential efficiency of a BWB configuration, while the need for high levels of stability for survey accuracy meant it had to solve the stability problem. “There is more lift generated across the entire body with less drag on a large surface area, which results in a higher lift-to-drag ratio,” Trost says. “Of course, the pitch stability is the main trick here – how to design a stable aircraft and still keep the performance high.” C-Astral’s solution involves a mix of natural stability achieved with some twist of the wing and a degree of active stability control through the autopilot. Although it operates autonomously, Trost emphasises that the ppX is stable enough to be flown easily by remote control. Rather than using standard NACA aerofoil sections, C-Astral developed and tested its own proprietary designs over several years, with high stability and a high lift-to-drag ratio being the main priorities. Trost says the ppX’s glide ratio is close to the range normally associated with high-performance gliders. To maintain the correct height above ground when surveying in the presence of thermal updraughts, which the autopilot naturally detects from altitude sensors and accelerometers, the system reduces the ‘throttle’ (to save additional power) by keeping the same altitude. The motor is also switched off automatically for descents, allowing the propeller to fold back and the UAV to glide. The blended winglets are a distinctive feature of the delta-wing airframe. They serve two main purposes: to minimise induced drag, and increase directional stability. Drag reduction is achieved by preventing ‘leakage’ of air from the high- pressure area under the wing around the tips and into the low-pressure area above, which normally results in the formation of drag-inducing tip vortices. Their effect on stability is very much like that of conventional vertical stabilisers. Aerodynamic development depended on manual calculations as well as software, with the final configuration emerging from the combination of these calculations and the results of prototype flight tests. Tools used included a commercial computational fluid dynamics package and some undisclosed open source programs. C-Astral did not use a wind tunnel but instead relied on rapid prototyping and flight testing with a battery of sensors to measure the angle of attack, power consumption and current, stability, vibration, speed, pressure and more. “We can get very precise measurements of flight characteristics and compare different sets of wings and aerofoil sections in real environments,” Trost says. Most of the aerodynamic development, Trost reveals, was solved by using extensive flight testing and tuning the December/January 2017 | Unmanned Systems Technology The curved transparent tubes near the wing root reveal any dirt in the pitot-static air data system and provide access for the operator or maintainer to blow it through with compressed air