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67 50 km range for commuting, and this is where the scale model is positioned,” he says. “With a nominal 3 kg you can go 25 km out and 25 km back – it’s a plane so you get efficiency from the fixed wing and the variable-pitch prop for vertical and horizontal operation. The bigger you go, the longer the range gets as the speed scales up and you can carry more batteries.” The move to a full-scale aircraft proved challenging, but the scale model was suitable for other applications. One area that proved interesting was for delivery and parcel applications, as the platform was designed to carry more weight than other UAVs. It could also be used for monitoring and surveillance applications. Weiss’ design reduces drag by replacing stabilisers with a deployable landing ‘stand’, with no mechanical components. As a result he had to develop a simple and light mechanism to replace the traditional V-tail. The Hummel’s current design uses a rear (main) wing with winglets to provide an elliptical lift distribution in cruise conditions. The design was validated and improved using computational fluid dynamics. The approach taken by Swiss Aerobotics was different from the traditional hybrid VTOL platform. Instead of moving wings or motors to transition from vertical or horizontal flight, Weiss developed variable- pitch, cyclic-pitch rotors. The 60 cm rotors vary the lift to direct airflow over different parts of the airframe. This allows the UAV to move from horizontal to vertical operation by raising its nose. “The transition from vertical to horizontal is easy – it’s just thrust, and is fully automatic,” says Weiss. “Then, for the reverse, you increase the angle of attack without climbing so that the props create an airstream over the wings, then you pull up into the vertical and have the rotors bring it down vertically. “You do that by increasing the yaw angle to allow you to create more lift, and at a certain point you rotate the UAV back so that you can transition from horizontal to vertical operation without increasing altitude.” Swiss Aerobotics Hummel | Digest With a nominal 3 kg you can go 25 km out and back – it’s a plane so you get efficiency from the wing and the variable prop Unmanned Systems Technology | April/May 2018 NASA’s Puffin manned aircraft was the inspiration for the unmanned Hummel (Courtesy of NASA)