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

72 induce high drag and could cause severe engine damage should the propellers start generating negative torque owing to being turned by the airflow and driving the engine, instead of the other way around. While multicopters and unmanned marine vehicles may benefit less from VPP technology, because of their smaller operating envelope and the general absence of a mechanism to change the pitch, there is no reason why it cannot be scaled down from the types used on fixed-wing aircraft and large container ships, and a few prototypes are currently being developed and tested. VPPs can be actuated in one of three ways: hydraulically, electrically or autonomously. The most tried and tested of these is hydraulic actuation. To adjust pitch, high-pressure oil is directed to the propeller, usually through a hollow crankshaft, although set-ups for hydraulic actuation have been designed for UAVs smaller than general aviation aircraft that lack such a component. This flow moves the piston back, and this backward motion is transmitted to the blades through a series of actuating pins and links. Once the target rpm is reached, oil flow is reduced and the piston stops moving, locking the new pitch in place until the governor establishes oil flow once more. As electric actuation becomes a viable option for UASs that cannot use a pressurised oil system, so too do self- actuating propellers that adjust themselves in flight, and these are available for small- to-medium UASs. Also called passive variable-pitch propellers, they forgo hydraulics and electronics, working instead in a similar way to the constant throttle in an all-terrain vehicle. By transferring the load from the propellers to the hub, the hub can react by setting the pitch of the blades to the most efficient position for the type of flight at the time, having been pre-calibrated for the UAVs specifications and flight envelope. The key constraint of such systems is the size of the hub, given the scale of the components to be installed inside it. However, propellers with diameters ranging from 14 to 37 in could use the system, in vehicles using heavy fuel motors or electric motor systems. Blade count The use of two-bladed propellers in the water is rare, given the need for very wide diameters for enough blade area to generate sufficient thrust. Waterborne unmanned vehicles therefore tend to use three or more blades, with USVs in particular benefiting from choosing four or five blades to maximise the number in the water at any time. But for USVs, UUVs and UAVs alike, there is a trade- off between efficiency and smoothness when selecting the number of blades. Casual observers might assume that two-bladed propellers are a universal standard for UAS prop designs, as three-bladed props would be for marine craft. Their low weight, design simplicity and low material cost compared with propellers with more blades make them the most common for many UAV classes up to 200 hp. And as already mentioned, too large or heavy a propeller leads to excessive consumption of energy when generating propulsive force, so minimising the number of blades mitigates that and optimises propeller efficiency. That said however, there is still widespread use of propellers with three or more blades. The most obvious reason is that for UAV engines exceeding 200 hp, or for small heavy-fuel engines that produce higher torque than similarly sized electric motors, additional blades are needed to exploit the increased power and generate more thrust, particularly as it becomes unfeasible to extend the existing blade diameter any further. Increasing the number of blades for a given pitch can also increase load- carrying capacity, as well as acceleration, which translates to shorter take-offs for UAVs launched from runways. The most efficient blade count for a UAV is a function of the application and factors including engine power, operating rpm, diameter constraints, performance and acoustic requirements. It is also worth noting that adding more blades does not have a huge impact on efficiency – estimates tend to put the expected efficiency loss of adding a blade, without any change to mission or operational factors, at only 4%. Although it is unwise to generalise without a specific mission and set of parameters to hand, a three-bladed prop can have the same or slightly better efficiency than one with only two blades. Acoustic benefits, however, are unarguable. A two-blade propeller produces two pressure pulses per revolution, whereas a three-bladed propeller will produce three smaller pulses per revolution for the same amount of total thrust, making the latter inherently smoother and therefore quieter. The three-blade may also have a smaller diameter than the two-blade it replaces, which also reduces tip speed and noise for UAVs. For underwater propellers, the risk of cavitation also increases with tip speed. Selecting a prop with more blades can mean reducing rpm to mitigate that risk, adding further incentive to break with the three-blade standard. Materials Metal propellers are seldom used on small or tactical UASs, as their weight is prohibitive. Also, machining metal for very small propellers tends to be expensive, and for small or micro-UASs in particular the loads are low enough February/March 2017 | Unmanned Systems Technology Electric rim-driven thrusters combine the protection and efficiency of shrouding underwater props with improved torque and reduced risk of cavitation (Courtesy of TSL Technology)