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

59 UAV Turbines UTP50R 50 hp recuperated gas turbine | Dossier and both are isolated from the hot side, consequently aerodynamic drag- inducing air cooling flows are minimal. The engine, which weighs only 70 lb dry, is just 21 in long by 11.5 in in diameter, making it relatively easy to package. A clear advantage of a turbine is its multi-fuel capability. “In the combuster you are heating the compressed air to more or less the same temperature regardless of the fuel,” notes Frigerio. “The trick is to ensure that you have the conditions in the combuster such that you can burn any fuel, that you can combine it with the air in the correct proportion. “That is a lot easier to do with a continuous combustion than it would be when you are trying to control the combustion events in a piston engine. We don’t have to worry about detonation: so long as we can keep it lit, it will work. With our continuous combustion you can run on almost anything – propane, diesel, jet fuel and so on.” The engine has been designed specifically for UAVs requiring around 50 bhp with a fixed-pitch propeller. But, says Warshaw, “Remember that aircraft don’t fly on horsepower, they fly on thrust, and with the variable-pitch propeller we are very efficient at turning horsepower into thrust. In other words, we need less horsepower for a given amount of thrust. “It is like in the car industry, comparing an engine with a gearbox to one that is direct drive, with only a clutch – using a gearbox is a more efficient means of getting up to speed. Using a variable-pitch propeller is like having a continuously variable transmission. In theory you could put a variable-pitch propeller on a piston or rotary engine but the inherent engine technology is still less efficient. “This engine for the 50 hp class is starting off at 39 bhp, and we intend to grow it from there. With our high-efficiency propeller we can attain 30% more [take- off] thrust than a regular fixed-pitch prop so we don’t really need the extra horsepower. Nevertheless, we intend to grow the power output by about 25% over time. By introducing the engine at 39 bhp we are leaving ourselves room to grow. “Once we fully understand where the life requirements are, where we need to improve, we can push the envelope in terms of temperatures and flows, and first of all get more power from the same airflow. Then we can increase the rpm a bit and increase component efficiency, and that is where that extra power will come from, not from any fundamental change.” Frigerio adds, “This engine is designed for high-altitude use. Most UAVs this engine will power fly at between 9000 and 18,000 ft for most of a mission, and that is what our engine is optimised for. If you look at sea level standard data, we could be more efficient in that condition, but the trade-off would be that we wouldn’t be so efficient at altitude. Or we would be carrying more engine weight. So at take-off we aren’t as efficient as we could be, but that is a small price to pay. “Our gearbox is sized for the engine, and we have left ourselves room to grow in power [using that same box and making only small changes to the gears to handle the additional torque within the same unit]. It’s the same with the Unmanned Systems Technology | December/January 2017 The heart of a single-stage gas turbine engine is an assembly through which the charge air travels, and consists of an upstream compressor that is coupled mechanically to a downstream turbine, with a combustion chamber between those two components. The chamber, which is the stage where the fuel is directly introduced, is often referred to as the combuster. The compressor and turbine wheels rotate on the same shaft, with the turbine driving the compressor. The compressor raises the pressure and the temperature of the incoming air, which it sends into the combuster. There, combustion further raises the temperature of the charge, which continues its passage forward to drive the turbine. In the case of a turbojet engine the expanding gas passing through the turbine is directed through a propelling nozzle, where it is accelerated to high speed to provide thrust. Of the energy in the fuel that is converted to kinetic energy to drive the turbine, only an amount sufficient to drive the compressor is extracted, the balance going towards creating jet thrust. In the case of a turbofan engine, the turbine/compressor shaft directly drives a fan inside the engine, which is the main source of thrust. This is often considered a hybrid in that the exhaust from the turbine contributes some jet thrust to augment the work of the fan. In the case of a turboprop engine, all the work done by the turbine is fed to the shaft driving both the compressor and, via a gearbox, the propeller; the exhaust gas does not contain enough residual energy to create significant thrust. UAV Turbine’s UTP50R is a turboprop engine. The gas turbine engine has constant combustion, and the rotating assembly is designed to operate (as near as possible) at constant speed. The propeller is coupled through reduction gearing that converts the high rpm, low torque of the turbine-powered driveshaft to low rpm and high torque. Normally the gear ratio is not variable; instead the constant-speed propeller exploits the controllable pitch of its blades. The blades are rotated around their long axis to vary pitch, allowing the propeller to be adjusted to different air speeds and levels of thrust without the need to alter its rotational speed. Gas turbine basics