Unmanned Systems Technology 014 | Quantum Tron | Radio links and telemetry | Unmanned Aerial Vehicles | Protonex fuel cell | Ancillary systems | AUVSI 2017 Show report

59 Protonex fuel cell for ScanEagle | Dossier assuming no payload on board. Also, with the fuel cell there is less loss of fuel weight during a mission so less of a concern over managing the centre of gravity (although that is not a particular concern with the ScanEagle provided you get it right when you launch it).” The operational cost of the ScanEagle FC is anticipated to be one advantage over the ICE. The base price of the complete FC propulsion system is currently “a little higher” than that of the ICE system, according to Osenar, although that differential would fall as FC system production volumes grow. At the same time, over the life of the product there is expected to be little or no change of the components of the fuel cell system, hence an overall saving on operational cost. “The mean time between overhauls should also become non-existent with the fuel cell system,” remarks Knapp. “With fewer moving parts, reliability is inherently better. When you carry out the failure modes and effects analysis, it is now really down to the very few mechanical parts, some filters and the mean time between failures of solid-state electronics.” Osenar adds, “Also, the key is that none of those things operate at [elevated] temperature, unlike an ICE. The temperatures are all very manageable.” Knapp says, “The electric motor we are using for propulsion is primarily used in kidney dialysis machines. The manufacturer says their life is tens of thousands of hours in that application.” A major advantage of the ScanEagle FC is that vibration is far lower than with an ICE, which helps the user to obtain superior images. Noise is also lower. “The fuel cell system runs at less than propeller noise – in fact, it is really only propeller noise that is left,” notes Knapp. “The electric motor obtains the required level of torque at lower rpm, allowing the use of a slower propeller for a given level of thrust. Reduced tip speed means less propeller noise and higher efficiency. We have specified the propeller to take advantage of that. “Customers want the lower level of vibration, the reduction of noise and the increased reliability offered by the fuel cell. Plus, moving drums of ICE fuel around can be problematic – the FC system requires water, electricity and a small box to generate fuel.” Unless it is supercharged, an ICE suffers from severe derating with altitude owing to the fall in air density. On a ‘standard’ day, between sea level and 10,000 ft, the ICE loses about 50% of its power at any given engine speed. The fuel cell system is not as badly affected. “There is a derating, but only in the region of 5% from sea level to 10,000 ft,” reports Osenar. “You do have to ramp up your air blower to push more volume through it as the oxygen content is diminishing, but you can compensate for most of it.” Knapp says, “Most of the ScanEagle’s life is spent at around 3000 ft above ground. But ground level can change, as can atmospheric conditions. For example, you can have hot days when you are only 1000 ft above sea level but the density altitude is that of 8000 ft. So you can get into that 10,000 ft density altitude realm quite easily, especially if you are flying in the mountains.” Another consideration is that it is difficult to turn an ICE on and off when in flight; not so with an FC though. The ability to switch off the electric motor in flight and glide can be advantageous. Clearly, the ScanEagle FC offers some enticing advantages over its ICE counterpart, even if it lacks the mission range. In theory, that gap could be much reduced by using liquid hydrogen and redesigning the vehicle specifically for FC operation. That may well be the next step in this intriguing cooperation between Protonex and Insitu. Unmanned Systems Technology | June/July 2017 The ScanEagle’s fuel cell module sees the two stacks mounted longitudinally side by side behind the balance of plant section and ahead of the control electronics. The balance of plant is surrounded by the four circular heat exchangers