Unmanned Systems Technology 026 I Tecdron TC800-FF I Propellers I USVs I AUVSI 2019 part 1 I Robby Moto UAVE I Singular Aircraft FlyOx I Teledyne SeaRaptor I Simulation & Testing I Ocean Business 2019 report

38 still far from ideal for bulk production, it is now widely used by propeller manufacturers for rapid prototyping of new designs, in thermoplastic or printed composites with chopped fibres. Acoustic optimisation Real-world testing is vital for ascertaining qualities that computer simulations can occasionally fail to predict. For example, testing propellers for balanced weight across their shape is common for preventing weight-related oscillations of the prop during flight. Greater attention than ever is being paid to the noise generated by propellers. This is as much due to the correlation between noise and inefficiency (and vibration) as it is a function of the expansion of commercial UASs into populated airspace and the need to reduce noise pollution. By using an anechoic chamber and one or more sound sensors, a propeller manufacturer can measure the variations in noise level at different revs, from either a single propeller-motor pairing or several of them. This latter set-up could more accurately capture the overall noise from a multi-rotor system. Many rounds of testing might follow, depending on the UAV’s flight envelope and configuration. If the acoustics from a multi-rotor delivery UAV for example are being tested, sound sensors might be placed downstream of and perpendicular to the propeller’s airflow. That would test the noise heard by civilians as the UAV flies over them or lands next to their offices or dwellings. Carrying out tests of just the engine or motor is also vital for determining how much noise is being contributed by the propeller alone. Again therefore, it is important that UAV manufacturers know which other propulsion components they need before seeking a propeller optimised for their application. Testing and reducing propeller noise will be even more important for autonomous urban air taxis, as the passengers may be very close to the props for hours at a time. Modelling and testing propeller vorticity and noise is vital for determining how the wake of the blades might beat against the cabin windows and windshields, as any growth in the urban air taxi market will depend on passenger comfort as well as safety. A comprehensive testing regimen for urban air taxi propellers could therefore mean having multiple rounds of tests with several different prototypes, with different blade counts (although the optimal blade geometry might vary, as the solidity ratio increases with blade count). While a two-bladed propeller emits only two acoustic pulses per rotation (compared to three and four for a three- bladed and four-bladed prop respectively), collectively they are louder than the noise per rotation of propellers with more blades, assuming the same overall lift is being generated from each prop. While more blades might theoretically mean more drag and manufacturing expense for the propeller, if the same thrust can be maintained while making the ride less stressful for air taxi passengers, it will become increasingly important for propeller and aircraft manufacturers to develop (or partner with) teams with large-scale acoustic testing facilities. In addition to acoustic testing, air taxi propellers might benefit from some form of real-time integrity testing to ensure they are working well enough to assure passenger safety. For example, some helicopter blades have multiple fine-wire filaments woven into their layers, with an electrical circuit continuously passing through it to confirm integrity. Should the smallest crack occur, the circuit is broken and the monitoring system can then activate an alert. Such tests are rare for unmanned aircraft propellers that are already in use. June/July 2019 | Unmanned Systems Technology Propeller hubs must provide a strong core for the blades while also being carefully analysed to ensure they do not create unnecessary losses (Courtesy of KDE Direct) Many UAV propeller manufacturers now have their own test stands, facilities and other equipment (Courtesy of Xoar International)