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

69 designed and operated correctly (compared with the 70% efficiency that fully submerged propellers for UUVs and USVs should achieve), but studies indicate that in many cases that figure is less than 40% when using off-the-shelf propellers without a thorough analysis of what is best for a UAV’s unique specifications and requirements. Fortunately then, there are various options for the design, simulation and production of propellers. For example, software tools such as Nastran provide finite element analysis to investigate the strength, stiffness and fatigue across the different blade elements, and improvements in computational fluid dynamics (CFD) and computer processing power allow faster and easier aerodynamic analysis of new designs. These and other advances have generated a range of propellers with new designs and materials for unmanned systems engineers to choose from. Blade shape Diameter and pitch are the first parameters to consider. A greater diameter implies greater blade area, higher thrust, cruise efficiency and climb performance. For marine props, heavy loads merit larger diameters, but for a small, fast USV a smaller diameter ought to be more suitable. The pitch meanwhile can vary from relatively coarse (greater horizontal angle), as on a fixed-wing UAV for cruising, or relatively fine (more vertical angle), for a multicopter UAV designed for extended loiter times. There are limits to what diameter and pitch can achieve, however, and the power plant must also be accounted for. A fixed-wing aircraft powered by gas engines will typically use propellers with greater diameter and pitch than a multicopter of similar weight. But too large a diameter and the blade tip speed approaches Mach 1 (a Mach number of less than 0.72 is preferred for limiting noise, and drag and torque resistance). Application-based design must also take other aspects of the blade shape into account and closely observe operating lift coefficients according to aerodynamics, mechanical dynamics and acoustics. For modern UAVs, two blade characteristics in particular demand attention. The first, high Reynolds number effects, are found when examining UAV props that are simply scaled-down versions of other types, calling for major redesigns, most prominently in the length and distribution of the chord stretching from each propeller’s leading and trailing edges, and from motor shaft to blade tip respectively. This can potentially result in a relatively fan-shaped blade with a low aspect ratio in order to maintain high efficiency. The other key characteristic is blade tip thinness. General aviation-size tips are rarely thinner than 0.125 in, partly owing to durability concerns, although minimum inertia limits are also a key design criterion for internal combustion engines (ICEs), and thinner tips risk reducing inertia when compared with propeller mass. But with UAVs relying less on launch and recovery using runways (where thin propeller tips are discouraged owing to the risk of damage from small rocks and debris) and more often being designed to use electric propulsion instead of ICEs, advances in design and materials will greatly reduce acoustic signature and improve efficiency through thinner tips. This may also lower gyrostatic moment, enabling multicopters to accelerate and brake faster, meaning smoother flight and reduced vibration. Sweeping the blade back along its leading edge also improves acoustics and efficiency, and reduces material erosion, while keeping within diameter and weight limits, but can still incur disadvantages. Centrifugal force induces loads on a ‘scimitar’ blade as if to straighten it, while straight blades also minimise design effort and tooling requirements. Modern CFD techniques have also allowed experimentation with other forms of sweep, including forward leading edge sweep, different degrees of tapering along the blade and even winglet tips, with various results hinting at improved efficiency and reduced noise, albeit given a dearth of specialised tooling or techniques for mass- producing such designs. For marine propellers, increasing the ‘rake’ – the angle at which the blade tilts perpendicular to the hub – might increase top speed while restricting acceleration, and causing instability for fast USVs through bow lift. However, for a particular pitch, diameter and thrust output, the power and torque requirements are largely unaffected by parameters such as blade thickness, hub size or rake for well-designed propellers. Investigating these can yield counter- intuitive findings, including cases where increasing blade thickness can improve hydrodynamic efficiency. What USV and UUV designers Propellers | Focus Unmanned Systems Technology | February/March 2017 Finite element analysis simulating stress on a propeller due to aerodynamic loads in flight. Blue represents the lowest magnitude of stress while red is the highest (Courtesy of DARcorporation)