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51 The challenge UAV Turbines was conceived as a means of applying fundamentally simple, hence inherently dependable, light and cost- effective gas turbine engine technology to applications at the other end of the power scale to normal aviation. Indeed, it was the use of a micro-turbine in a model aircraft that sparked the idea: why not apply state-of-the art aerospace engineering to make such a power plant relevant to small vehicles outside the hobby sphere, including UAVs? UAV Turbines’ engineering director Fred Frigerio says, “The turbojet is well- suited to high-speed model aircraft; the problem comes when you want to make it more fuel-efficient and increase its life. If you are going to be flying slow, you need to make it into a much more efficient turboprop engine, and that is where it starts getting difficult. “Gas turbines work on the principle that the speed at the tips of the turbine blades is extremely high. In the case of a small turbine, that implies a shaft speed of 100,000-plus rpm, and how do you reduce that to a 6000 rpm propeller speed? You need a gearbox of some kind, and that takes you out of the model aircraft arena. That gearbox is itself a significant challenge. “The other problem we had was that existing model aircraft gas turbines were using compressors from automotive turbochargers, and the efficiency of those is not exactly what you need. What happens if you start trying to make your own compressor? Now you are moving into an area in which you need some serious knowhow, and it starts getting expensive.” This project began in 2000, and for the first ten years concentrated on developments funded by US government contracts. Only since 2011 has UAV Turbines concentrated on bringing its technology to the general UAV market. Warshaw notes, “One thing about the initial development phase was that there was a feeling that this is established technology; we are just making it smaller – how hard can that be?” As our research for this article revealed, the short answer is: far harder than anyone in the company suspected. “At the start we thought combustion was going to be the biggest problem,” Frigerio recalls. “The conventional fuel delivery method was not going to work. Normally spray nozzles atomise the fuel into the combustion chamber, but that wasn’t going to work because our fuel flow is so low relative to regular turbines, and you end up with orifices in the nozzle that are so tiny they would clog up too easily. “We did some work with different nozzles but in the end we solved it by going to a vaporiser tube; that is to say, you have a metal tube that you heat up. Instead of trying to make a spray of the fuel, you pass the fuel through that tube with the heat of the combuster itself warming it, and it passes out of the end of the tube perfectly vaporised.” Warshaw adds, “Of course, there are all sorts of forms the tube can take. We didn’t get it right first time; it took a lot of effort, a lot of trial and error by very gifted and highly experienced engineers to make it work properly.” The fuel supply pump is electrical and, operated by the engine control unit, is responsible for the fuel metering. It sends the fuel to a stainless steel manifold around the combuster, which directly supplies the vaporiser tubes set in a ring around the combuster to avoid the danger of hotspots in the turbine. UAV Turbines UTP50R 50 hp recuperated gas turbine | Dossier Unmanned Systems Technology | December/January 2017 The UTP 50 hp recuperated gas turbine engine core instrumented in the UAV Turbines test cell