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71 Swiss Aerobotics Hummel | Digest links would then be encrypted, and the data would be sent via a serial link to the 4G network. However, the link is likely to be customised for customers with different radio technologies at different frequencies, says Weiss. The range requirements for the comms system depends on the application, he says. “In a populated area the range is not an issue. For example, a security company tasked with protecting a sports arena with a large quadcopter would have good 4G cellular coverage, so the comms range is not a problem.” Safety Safe operation is a key concern for the design of the Hummel platform. “One thing that must not be underestimated is safety,” says Weiss. “If there are two engines with 60 cm diameter rotors and 2 kW of power it’s like two open lawn mower motors in front of you, so you need professional operators. That can be a limitation for disaster relief delivery applications, for example, where you would need skilled staff on the ground to prevent further injuries.” That is part of the challenge of building larger versions. “If the Hummel hits the ground then essentially you’ll have daggers flying through the air – that is one of the challenges that comes with scale,” Weiss says. “Smaller quadcopters can use soft plastic propellers.” The blades weigh 40-50 g and run at 3000 rpm, so there is a considerable load on them, with a radial force on each blade of up to 100 kg. Weiss had aluminium moulds manufactured, and laminated the blades in the moulds to produce the specific design, rather than 3D or additive printing. “If you go to 3D printing of propellers you would need to go to titanium, which is costly,” he explains. Capture system The main focus is now on the system for catching a rogue UAV, which tends to be based around a net that is fired to capture the UAV in flight. “If you look at how net systems work, there are four barrels with four weights, and in between them is a container that holds the net,” says Weiss. “A pressure release drives the weights out that drag the net to catch the target.” The challenge is that the barrels have to be at an angle to open the net, but there is a trade-off between the angle of barrels, the size of the net and the distance it travels to the target. Ideally a counter-UAV needs to carry several of these to ensure it captures the target if the first attempt fails. This drives a demand for a larger platform such as the Hummel. “The four barrels present a relatively large cross-section, and it isn’t easy to mount many of them to a single platform,” says Weiss. “I had a system like that but started to look at a very lightweight, straight barrel version that allows a more compact system with four times less cross-section that will allow several systems on the UAV.” The design of the net system and its range then influences the accuracy of the computer vision targeting system. “I found that with a simple pyrotechnic charge I can get a range of up to 40 m for the net, and that is being increased. The targeting system will be less Unmanned Systems Technology | April/May 2018 The structure of the Hummel UAV The rotors are built from carbon fibre in aluminium moulds