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8 Platform one The demand for higher performance powertrains that combine propellers with high-end electric motors has led to a new design from MagCad and Plettenberg Elektromotoren (writes Nick Flaherty). The blade geometry of the latest MagCad propeller has been completely recalculated and implemented in CAD. The design was developed with Plettenberg to optimise the motor and propeller for a fixed-wing UAV. The propeller is hand-built from a single sheet of carbon composite, and provides a maximum standing thrust at 5000 rpm of 50 kg, at a propeller output of about 11 kW. This provides a flight speed of 34 m/s with a propeller efficiency of 74%. At 4000 rpm the thrust is 30 kg (about 5.5 kW propeller output), providing a flight speed of 27.5 m/s with the same efficiency. The design of the propeller is optimised for this power range, said Manfred Greve at MagCad, which meant working with motor and controller developer Plettenberg. For example, the shape of the propeller’s tip can make a major difference to the induced resistance, which depends on the speed of the motor. “We are the only manufacturer with wing tips [on the propeller],” said Greve. “The controller gives you the rpm you need with electric motors, while the shape of the wing tip depends on the operating point of the propeller. “At low rpm you get up to 7% efficiency out of the wing tips, but if the propeller is used at a higher rpm and lower pressure then the wing tip sees more resistance.” MagCad measures the performance of the propeller design in simulation and then on a test stand. Greve said, “Propellers are like tyres in Formula One – if the improvement is 2 or 3% in the motor but the propeller is standard, you will not feel the difference. Airborne vehicles April/May 2020 | Unmanned Systems Technology Prop optimised for UAV motors “For example, 70% of the thrust comes from the outer surface, 20% from the middle and just 10% from the inner surface. So what we did was build propellers that look like a spoon with a thin arm on the inner side, while the real blade is on the outer edge.” The aim is to provide the lowest resistance flow over the blade. “Each time the propeller crosses the fuselage there is a pressure difference, and that creates noise which reduces the efficiency,” said Greve. “If there’s noise, it comes from turbulence, which in turn significantly reduces efficiency.” The torque from the motor is also a key consideration. “Sometimes you have fast- changing rpm for multi-rotor UAVs for manoeuvring, and most of the time we build hollow propellers for less weight so the torque can increase or decrease the rpm faster,” he said. The propeller design is then tweaked with the powertrain to get the optimum efficiency. “Customers send us an existing powertrain, and we put it on the test stand and use a different motor, controller or propeller. That gives us a 7-30% improvement in flight time or thrust. “We have a good relationship with motor companies such as Plettenberg, and the motors we’ve worked on have had efficiencies of 92 or 93%, which is higher than usual in this business. That means we are always at the highest efficiency to give more flight time or a higher climb rate,” he added. Previously MagCad has developed propellers up to 3.9 m in diameter for a UAV that can carry up to 1000 kg for 30 minutes. The design of a propeller can be optimised with the motor and controller to deliver the highest possible efficiency for a UAV