Thermodynamic and Mechanical Design Concept for Micro-Turbojet to Micro-Turboshaft Engine Conversion

During the last years, the demand of unmanned aerial vehicles (UAVs) for private and commercial use has grown very rapidly. Various technologies can be used to power an UAV: electric motor, internal combustion engine, gas turbine. Each technology has its specific advantages and disadvantages. The gas turbine shows a favourable behaviour of torque over rotational speed as well as lower vibrations in comparison to internal combustion engines for example. Otherwise, the high weight of an electric motor and its battery is a disadvantage for any aircraft application. Because the trend is towards higher maximum take-off weights (MTOW), the required drive power increases. A preliminary market survey has shown, that there are no turboshaft engines available above 15 kW, but jet engines for higher power ranges can be obtained though. Therefore, a design concept is presented, which deals with the conversion of an existing micro – turbojet engine to a micro – turboshaft engine for a specific power output. Furthermore, the simultaneously developed analytical preliminary design of the additional single-stage power turbine is shown. Therefore, a thermodynamic cycle simulation with the commercial software IPSE-Pro has been carried out to obtain the unknown gas generator outlet condition which is similar to the power turbines inlet condition.  Within the cycle calculation, occurring losses due to the small dimensions have also been considered. To minimize losses in the power turbine, the preliminary blade profile has been improved using CFD calculations. It is desirable, that the power turbine´s mean diameter is as big as possible to keep the propeller shaft speed in a low range. For that reason, this work also deals with the question, whether or not a gearbox between the power shaft and the propeller shaft is needed.