Unmanned Systems Technology 011 | C-Astral Bramor ppX | IMUs | Autonomous farming | UAV Turbines UTP50R | London Show report | Advanced materials | Las Vegas Expo report

55 UAV Turbines UTP50R 50 hp recuperated gas turbine | Dossier “Of course, you will lose a small amount of pressure going through the recuperator, so that is a loss. Also, you want the recuperator to be as efficient as possible, but that implies heavy weight, so the challenge is to make it as light as possible while still withstanding the operating pressures and temperatures. “Our recuperator is essentially a radiator composed of many very thin plates of high-temperature alloy; it is a micro-channel recuperator design. On a regular radiator the passages will have turbulent flow, but when you go to a very tight micro-channel the flow is laminar, which is better for heat transfer, as is the additional surface area for a given volume. That minimises the size and weight of the recuperator by achieving the highest possible value of heat transfer per unit of volume. “But if you think about the passageway for the air from the compressor, through the recuperator to the combuster, that is quite convoluted. It was quite a design challenge to get the air to go through the recuperator as efficiently as possible. You can’t just do that with computational fluid dynamics, you have to make physical parts to rig test, redesign and re-test.” Another gain of the recuperator, according to Frigerio, is that “it works like a muffler, taking some of the exhaust noise out. The exhaust has no silencing, it simply comes out at the sides. Our turbine makes a high-pitched sound, which attenuates really fast in the atmosphere, especially at high altitude, and is quieter than a piston engine, but the loudest thing is always the noise of the propeller.” Materials The engine uses no advanced coatings but a variety of materials, although not for the rotating assembly. Frigerio reports, “We ended up with a single spool with the compressor, shaft and turbine as one integrated casting, thus using the same material throughout. “Normally you wouldn’t do that, as the weight of the compressor is a significant element of the overall weight of the engine. But once you get down to a micro-turbine it is less crucial, and the trade-off versus making the various elements as different castings wasn’t worthwhile. We eliminated a lot of cost and complexity by having just one casting. “The material we use is a high- temperature nickel-based superalloy. Although it’s an established superalloy, we don’t want to be precise as to its specification. It is difficult to machine, and our part is very complex and is cast rather than machined from solid, so there is very little machining required of the casting.” But isn’t the cost of a high- temperature material to cope with the demands of the turbine adding expense? “Yes, but overall it is less costly than making the compressor as a separate element,” Frigerio says. “Plus that makes the compressor more durable too – if a little heavier!” The gearbox is at the front of the engine, thus in a cold location, so its case is cast aluminium for minimum cost. The engine bearing housing immediately behind it is likewise cast aluminium, and this second section transitions into the engine intake. Behind that is the compressor housing, which is titanium, machined from solid. At the rear is the combuster housing and the turbine-recuperator-exhaust section, both of which are made from a nickel- based superalloy, cast and machined. The various sections are attached using flanges with bolts or clamps, according to requirement. Careful design overcame the problem of the differential in expansion of the various materials, Frigerio reports. Drivetrain The engine is connected to the front of the rotating assembly shaft via a gearbox. The gearbox’s first-stage drive arrangement is a derivation of a planetary geartrain. In this case there is no ring gear. Frigerio says, “The high-speed input pinion drives planet gears in opposite directions; these gears are straight cut. As in a planetary gearbox, the radial component from the gears meshing is offset by the component on the other side. They cancel each other out, and you don’t have a side force, just torque.” There is a more traditional reduction stage at the output to further reduce the shaft speed to the 6000 rpm output shaft speed. The gearbox – which Unmanned Systems Technology | December/January 2017 Turbine rotor with integral compressor impeller on the back