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

35 optimal pitch varies depending on the power requirement. Among such manufacturers, carbon composite is emerging as the industry- standard material, thanks to its high strength-to-weight ratio and the wide range of carbon prepreg suppliers around the world. Fibre glass and wood continue to be used intermittently – wooden props can be particularly useful for oil and gas inspections, where ATEX certification mandates non-explosive, low-friction materials. However, carbon fibre tends to be preferred among UAV designers seeking to meet a weight target. Some of the changes in propeller manufacturing being spurred by the new UAV markets might be obvious. As ever-larger autonomous (and typically electric or hybrid) aircraft are developed for heavy-lift duties and transporting passengers, propeller blades must be made far bigger than those the UAV manufacturers typically work with, to provide the necessary lift for ascent, hover and descent in these urban applications. For a typical two-bladed UAV propeller to provide such lift, the speed of the motor would need to be quite high, but as the rpm and diameter of the propeller increases, the speed of its tips rises to close to the speed of sound. That exponentially increases drag (without a corresponding increase in thrust) and creates noise and structural problems, so the larger propellers must come with more blades to generate sufficient lift without relying on high motor speeds. That could mean further increases in their material costs, however, and adding more blades generally means extra weight and surface area for the propeller, creating more drag that the UAV and its motors have to contend with. Propeller design and analysis There are various blade parameters to be taken into account when analysing a propeller for a particular UAV and its end- use. These include the blade diameter, airfoil, pitch, chord and blade count. There is a growing understanding of just how many factors influence the effectiveness of a propeller in a given application. Approaches to propeller analysis and design, such as using modelling software for blade elements, momentum and fluid dynamics can show all the ways a propeller can be optimised. The parameters listed above can be explored in further detail, for example by examining the way pitch and chord are distributed along the length (or ‘twist’) of the blades. Hub diameter should also be considered, as the hub should be as small as possible while still providing a strong core for the propeller. And once the vehicle and mission parameters have been detailed by the customer, the designer can input factors such as engine power, rpm, air density and the forward speeds the UAV will be moving at (including if the craft will spend more time hovering than moving). Alternatively, the customer and designer might choose to omit such parameters in order to achieve a simpler and quicker simulation run time, if having a shorter overall lead time takes precedence over a thoroughly optimised propeller design. Propeller designers can then calculate a range of useful qualities to define the eventual shape the bespoke prop should take. These can be found by using established (and increasingly popular) CFD software such as SolidWorks or STAR-CCM+, or by way of in-house simulation capabilities and amassed data on UAV propeller characteristics, or a combination of both. For example, a propeller’s optimal helix angle – the angle between its plane of rotation and its effective pitch (the actual path the blade ‘cuts’ through the air) – can be determined, thus enabling significant optimisation for the prop’s intended use. In another case, a propeller with a low pitch and low helix angle will suffer minimal drag but might provide low thrust. Alternately, one with a deep pitch and large helix angle would provide powerful thrust but induce potentially significant drag. The solidity ratio – the ratio of chord length to pitch – can also be analysed. This gives a measure of how much of a propeller’s disc area is taken up by solid material versus air. Increasing it, by increasing the blade chord or the number of blades for example, can increase the power imparted by the prop to the air. Propellers | Focus Unmanned Systems Technology | June/July 2019 Larger bespoke propellers are being developed for applications such as autonomous package delivery and HALE solar-powered UAVs (Courtesy of Rockwood Composites)