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54 “That was a major challenge. It ended up requiring a very holistic-type approach to it – it wasn’t one ‘magic’ thing that solved it.” Warshaw adds, “You solve one problem and find another, and again and again. It was lots of little things – ten years of trial and error, with some of the best engineering minds in the world applied to it.” “Yes,” agrees Frigerio, “it was a very difficult nut to crack, but we can now predict what we are going to see from a rotor dynamics point of view, so we have really come to understand that in the context of these small engines. We now have a very reliable bearing system, and have engines running for 1500-plus hours. “But operating at 100,000-plus rpm, we also had to develop our own gearbox. And normally small UAVs have fixed- pitch propellers so we had to develop our own variable-pitch propellers as well – a variable-pitch propeller being critical in a single-spool, constant-speed design where the propeller has to operate within a small range of speeds.” The recuperator We asked: in the case of a micro-turbine is the efficiency inherently less than that of a larger version because the clearances and consequent blade tip leakages are proportionally larger? “That is true,” Frigerio says. “As you get smaller, the clearances don’t scale down accordingly. The implication of that is in terms of fuel efficiency, and that is where the recuperator comes in.” UAV Turbines’ recuperator is an air-to- air heat exchanger designed to exploit the waste heat of the exhaust to raise the temperature of the charge air exiting the compressor, ahead of combustion. The two flows don’t mingle; the exhaust runs over passages carrying the charge air from the compressor to the combuster, thus heating the air. “In the recuperator you are transferring heat from something that is hot to something that is cool, and the bigger that temperature difference is, the easier it is to move that heat in the direction you want,” Frigerio notes. The exhaust from the turbine is split into two passages, each travelling in the opposite direction at 90 º from the turbine axis, and there is a heat exchanger each side. Frigerio says, “The way you get more efficiency from your simple turbine cycle is by raising your pressure ratio. The higher the pressure ratio, the higher the efficiency – up to a point. But you can’t do that with a small engine because of the clearances and difficulty in cooling the hot parts. “We can run an efficient compressor at a low pressure ratio, but the disadvantage is that the air comes out of it relatively cool – in the region of 400 F (200 C). If you could run a higher pressure ratio, the air would instead be coming out at about 600 F (315 C). Then in burning fuel with it, you would get its temperature to 1800 F (980 C). When you heat air it wants to expand, and that expansion happens through the turbine, to drive it, the compressor and the propeller through the gearbox. “However, with the recuperator you can recover some of the waste heat from the exhaust gases back into the air coming out of the compressor. And since you kept the temperature of the air leaving the compressor low, and the exhaust temperature is around 1400 F (760 C), you have a very big delta temperature, and it is easier to transfer energy back into the cycle. “So the charge air, is taken from about 400 F (200 C) as it exits the compressor up to about 1100 F (600 C) entering the combuster. You still have to get it to about 1800 F (980 C), and you can do that with fuel. But instead of having to burn enough fuel to heat the air up from 400 F (200 C), you are only doing it from 1100 F (600 C). That means you burn roughly 40% less fuel. December/January 2017 | Unmanned Systems Technology The UTP 50 hp recuperated gas turbine seen from below