Unmanned Systems Technology 012 | AutoNaut USV | Connectors | Unmanned Ground Vehicles | Cobra Aero A33i | Intel Falcon 8+ UAV | Propellers | CES Show report

6 Mission-critical info for UST professionals Platform one With an increased focus on autonomous vertical take-off craft such as the Airbus Vahana (opposite page), a pan-European consortium has developed two electric turbofans (ETFs) for designers of static thrust and VTOL-capable fixed-wing UAVs (writes Rory Jackson). Neva Aerospace has developed two types of ETF specifically with UAV developers in mind. The Athena 1ET is a 600 g single-propeller system which, in normal operation, consumes 0.26 kW and produces 1.3 kg of force (kgf), with a maximum thrust of 2 kgf using 0.47 kW. The Athena 2ET weighs 700 g and uses two contra-rotating props to generate a maximum thrust of 2.3 kgf for 0.65 kW, or 1.5 kgf for 0.3 kW in normal mode. Both ETFs use a carbon composite turbine shroud that is 250 mm in diameter and 200 mm long. The four- bladed propellers, 195 mm in diameter, are currently made from polylactic acid (PLA) but a carbon composite will be available in the near future. “Carbon will be lighter and less subject to deformation, leading to improved performance,” said Prof David Brotherton-Ratcliffe, director at Neva Aerospace. “Like the propellers, the propeller hubs are made from PLA, which allows both PLA and carbon blades to be changed with ease, while the weight penalty is small. “Neva’s turbines are relatively simple mechanically. The inner rotors on our two-stage model are not directly connected; there are two separate electric brushless motors and electronic speed controllers [ESCs],” he said. The secondary motor, ESC and propeller also provide redundancy in the event of a system failure in the 2ET. Instead of being installed in the hubs, the motors are located in the inner support arms, which are anchored to the turbine shroud (the hubs are then directly connected to the motors). The propellers themselves have been developed by Neva member Schuebeler Technologies using SimScale, a cloud computing-based CAE program. “The aerodynamics are optimised in an iterative CFD simulation so that many parameters like aerofoil shape, blade depth, maximum camber, rpm, number of blades, sweep and angle of attack can be investigated,” said Sandro Pinent, managing partner of Schuebeler. “With the CFD analysis, 3D effects that appear in the fluid can be visualised, and measures can be taken to optimise the geometry.” Coaxial rotor configurations have not always delivered a clear performance advantage; however, for VTOL applications, a major advantage of a twin counter- rotating system is intrinsic control over the turbine torque. For a VTOL turbine this is useful in that developers are not obliged to use both left- and right-handed pairs of turbines to control a UAV’s yaw. “The increased efficiency available by using twin counter-rotating rotors, which are able to effectively eliminate global vorticity, is fairly well balanced by the motor power/weight scaling, which slightly favours a single large motor over two smaller motors, and the greater simplicity of a rotor-stator system,” said Prof Brotherton-Ratcliffe. That means the two variants enjoy similar capabilities in terms of effective and gross power loading (weight-to- power ratio) figures. For example, the 1ET’s normal operating thrust of 1.3 kgf is congruent with 5 kg/kW, while for the 2ET it is 5.3 kg/kW when producing the same thrust. The key difference between the two is the 2ET’s improved torque control. Propellers focus, page 68 Airborne vehicles Electric VTOL turbofans… The Neva Aerospace turbofans come in two versions – one using a single prop, the other a contra-rotating model February/March 2017 | Unmanned Systems Technology