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64 O f the different power plants used by aircraft throughout the world, turbine engines are widely acknowledged as providing the highest power-to-weight ratio, and are the most practical choice where very high power is needed. They also tend to generate less vibration, and deliver higher flight speeds and longer times between overhauls (TBOs) than other engine types. Despite these advantages though, the adoption of turbine engines remains low among commercial UAVs relative to reciprocating and rotary engines. Their large sizes and high operating costs have often limited them to installations on very large MALE-class UAVs, such as those from General Atomics Aeronautical Systems and Piaggio Aerospace. In addition, attempts at smaller turbine designs for producing between 75 and 500 kW have historically brought with them very high rates of fuel consumption compared with reciprocating engines. That is largely because decreasing the turbine and compressor dimensions bring about problematic changes in compression, expansion, mechanical complexity and other factors affecting their thermodynamic cycles. However, Turbotech has developed the 90 kW TP-R90 turboprop engine and 55 kW TG-R55 turbo-generator to overcome these issues, for heavy fixed- wing and multi-rotor unmanned aircraft respectively. They achieve lower rates of fuel consumption than is typically possible with small aircraft turbines by incorporating a heat exchanger section that recovers energy from the exhaust gas. This design approach is sometimes referred to as a ‘recuperated microturbine’. Rory Jackson explains how one company has overcome the drawbacks of small turbine engines for commercial UAVs with this powertrain combination Cut to size April/May 2020 | Unmanned Systems Technology The TP-R90 is a 90 kW turboprop with a heat exchanger mounted on the rear for recuperating waste heat, which greatly improves fuel efficiency (Author’s image)