Unmanned Systems Technology 027 l Hummingbird XRP l Gimbals l UAVs insight l AUVSI report part 2 l O’Neill Power Systems NorEaster l Kratos Defense ATMA l Performance Monitoring l Kongsberg Maritime Sounder

32 Power from the alternator can be supplemented by a Grepow/Tattu or Panasonic lithium-polymer battery pack. In response to a large power demand from the control system, the battery pack powers the propulsion motors in parallel with the engine-driven generator. As the vehicle approaches the commanded speed, and power demand falls, the generator can handle the propulsion requirement and recharge the batteries. “We have designed the system so that the batteries provide starting power and then, once the engine is running, the direction of power flow reverses,” Bishop says. With lithium-polymer chemistry, it is important to pay particular attention to battery management, he says. Before every long flight, which could be up to eight hours, the checklist recommends that the batteries are replaced with a fully charged set. “We can run a number of missions and let the system recharge the batteries, but at some point you remove them and do a balance charge to make sure you always have balanced batteries going into a long flight,” he adds. “We are looking at [lithium] iron chemistry for a more consistent power output, but the downside there is that when you’re out of power, you’re out. “The power graph for lithium-polymer will decline in a nice, predictable curve, but the iron chemistry delivers high power until nearly exhausted and then in a couple of milliseconds you’re empty. That’s not good for an emergency descent because it’s very hard to predict, but we are looking at different battery chemistries as they become available.” As well as starting and hybrid boost power, the batteries also provide a measure of redundancy. “If the engine were to die, we have enough stored energy to take us from about 3000 ft above ground level [AGL] to a safe landing,” he says. Redundancy and parachute One benefit of using multiple electric motors and a large reserve of thrust over weight is the opportunity to build in some extra redundancy, Bishop notes. “We can tolerate the loss of up to three motors,” he says. “There is a caveat though, in that you can’t lose all three on one side. You could have two together and then skip and lose another one on the other side. If you run everything at maximum you can get it on the ground safely. If you lose only two, you can continue the mission, but you are going to really tax the remaining six outboard motors and duct motors.” A parachute can be fitted as an insurance policy for the aircraft and whatever expensive sensors it is carrying. On the Hummingbird XRP, it is housed in a cylindrical container mounted on one of the hard points. The lines supporting the UAV under the parachute are attached steel ‘leader’ cables that in turn attach to fittings on each landing gear leg, close to where they mount to the structural girdle. The cables are attached using Velcro along the upper duct covers and come across the top of the lip, staying out of the airflow, and attach to the parachute harness. As the parachute deploys, the cables and the parachute itself are forced upwards to clear the rotors. Flight testing of the parachute installation is planned for later this summer at a site in southern Colorado, in which the company is cleared to operate at up to 15,000 ft above mean sea level, and where the Hummingbird will be accompanied by a manned spotter aircraft. Comms suite The basic comms system for command & control and telemetry includes radios operating in the 900 MHz, 1.2 GHz and 2.4 GHz bands, with 2.4 GHz suitable for still images and lower resolution video, and 5.8 GHz preferred for transmitting live video. In terms of output power, they normally operate at 1 W depending on August/September 2019 | Unmanned Systems Technology The Sky Power SP-180 SRE Hybrid is packaged with an integral alternator and produces all its power in electrical form (Courtesy of Reference Technologies)