50 Digest | Dufour Aerospace Aero2 the hinge and how you throw the wing, and on the relative placement of the tail. “There’s also a lot of data and feedback on the CL-84 and XC-142, which were highly rated at the time – they saw use by over 100 pilots in more than 1000 missions, and their aerodynamic performance, handling and control were all superb, despite their mechanical and workload drawbacks. So we’ve been able to use that to start with a basic draft for the Aero3, which was then scaled down to become the Aero2.” On top of these, another key advantage of the tilt-wing configuration is that a wide range of airspeeds can be achieved at any given wing-tilt angle. The transition corridor is therefore not restricted to a narrow set of airspeeds and angular rates, as it is in some other VTOL-transition techniques. As Kent notes however, “Those crewed tilt-wing aircraft were expensive, and the technology of the time meant their mechanical efficiency was poor. They needed multiple large turbine engines to provide the power needed for vertical flight, and for failure conditions each engine had to be capable of carrying the full 5 or 15 t of aircraft weight in VTOL. “All this power was transmitted through driveshafts throughout the aircraft. And the control system was also mechanical, so they had rods and cables throughout. They also had a mechanical controls mixing computer, which was an incredible machine but very expensive to develop, build and maintain.” As well as using more efficient electric and electromechanical systems along with precise digital controls, much of the Aero 2’s anatomy has been shaped around balancing the craft sufficiently amid the slight movements to the CoM imparted by the 90o pitching of the wing during VTOL. During lift and hover, the CoM moves backwards, placing extra load on the tail rotor. Some past tilt-wing models have been prone to cross-winds during take-off, hover and landing, owing to the wing’s large surface area. Dufour has therefore designed the Aero2 so that in windy conditions, when the propellers are inactive, the wing will be horizontal, so wind exposure on the ground is the same as a conventional fixed-wing aircraft. As soon as the propellers start working, ambient wind about the wings is absorbed by the much higher rush of air coming from the props. “When we transition into cruise, the wing pitches downwards into its horizontal position, and that throws the weight forward, giving greater stability for cruise by moving the CoM ahead of the centre of lift [CoL],” Kent says. Bendrey adds, “The CoL also moves backwards during transition, giving us a good static margin of stability and hence good balance for moving in and out of fixed-wing flight.” A linear actuator from TiMotion is integrated for controlling wing tilt, to cover the speeds needed and dynamic loads imparted during transition. It also holds the dynamic loading in a stable manner during cruise, as higher airspeeds – a key requirement for many UAVs working in industrial and emergency medical deliveries – will generate higher g-loadings. “We can pull up to 4.5 g during fixedwing flight, so the actuator needs to hold that much,” Bendrey comments. “There aren’t many parts available that have been optimised for a 200 kg UAV application, but the TiMotion actuator has an ideal degree of safety for our initial aircraft, which will be flying over sparsely populated areas. “We’re also working with Maxon to find a redundant actuator, which we’ll need when we start flying over populated areas.” The company adds that, while the aircraft would not survive a full prop failure in hover, it is extremely unlikely to happen – it has found the probability of that to be less than 0.0001%. In any case, hovering takes place only in closed or controlled ground spaces, so in the event of such a failure, no individuals would be harmed. Propulsion and power To provide the power and torque needed at the speeds the company wanted, a customised motor derived from a general aviation design was initially adopted. Since then, that design has been further modified to enable power and control redundancy. “We’ve had bespoke motors developed by Geiger Engineering in Germany, and Plettenberg provides the tail motor,” Bendrey comments. “Based on EASA’s Special Condition for Light UAS guidelines, and how rules and requirements have evolved since then, we’ve taken steps like developing and certifying our own flight computer to meet DO-178C,” he adds. “We also ensure at least 30 minutes of flight from onboard reserve energy, through October/November 2023 | Uncrewed Systems Technology The tilt-wing configuration allows a wide range of airspeeds at any tilt angle, so transitioning is not restricted to a narrow set of airspeeds and angles
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