Turbine technology offers many advantages, but model aircraft with conventional propeller power systems have never been able to exploit them. Now they can: the Graupner/JetCat SPT5 turbo-prop engine represents a successful combination of high power reserves and High-Tech engineering. In the world of full-size aviation most types of propeller-driven machine have already been converted to turbo-prop power, but the engine“s relentless progress has only just begun in the model arena. As the name indicates, the turbo-prop engine - its full name is a turbo-jet propeller engine - comprises a gas turbine driving an airscrew. The primary advantages of the turbo-prop in full-size aviation lie in its compact shape and its economy and reliability at speeds below 700 km/hr. The basic design and method of working of the new model engine correspond quite closely to those of the full-size power plants.The principle is very easy to understand: it is simply a matter of finding a suitable method of converting the high power of the turbine engine into usable shaft power. However, this is not necessarily a straightforward matter, especially when you consider the turbine“s very high rotational speed. In the engine presented here the rotational speed is reduced in two stages: the first a gas reduction, the second a geared reduction. This means: when the turbine is running, the gas flow from the core engine drives a turbine wheel which is mounted on a second shaft. This second shaft is mechanically completely independent of the rotor of the base engine and not connected to it; it receives its motive power solely from the kinetic energy of the exhaust gas flow. The secondary shaft directly drives a gearbox designed to cope with high rotational speeds, and this in turn reduces the speed to a value suitable for a propeller. The gearbox is fitted with anintegral axial fan which provides the necessary airflow for cooling the components subject to high temperature. Another completely new feature of the engine is the electronic control system, which processes the speed information derived from both shafts: primary and secondary. This simply means that the pilot can concentrate entirely on flying, while the complex engine management processes are carried out fully electronically. The reduction gearbox is a specially developed planetary design, highly efficient and very compact. When ever a new kind of power system concept is introduced, the model flyer is obliged to immerse himself in the subject in order to gain the necessary expertise, and this certainly applies to turbines. However, once the operator has become familiar with the procedure, it is actually simpler to handle a turbine installed in a model aircraft than to operate a piston engine: only a single radio control system channel is required to control the engine, and starting preparations for the engine simply boil down to filling the fueltank and a small auxiliary gas tank required to start the turbine. The engine starts at the press of a button from the transmitter, where upon the entire starting process runs automatically, controlled by the turbine's on-board electronics (ECU). Initially the integral electric motor accelerates the turbine to a speed of around 6000 rpm, then the auxiliary gas supply is opened and the gas is ignited in the turbine's combustor. The turbine then continues to accelerate until the burning gas overtakes the starter motor's speed; the motor then disengages, and the turbine continues to accelerate until it reaches a speed high enough to support running on kerosene. Once the start-up process is completed successfully, the ECU sets a stable idle speed before transferring control to the pilot. After the flight the pilot reduces engine speed in the usual way, then - using the same channel - he initiates the power-down process, which is again entirely automatic, under the control of the on-board electronics: first combustion is halted, then the starter motor is switched on again to push fresh air through the turbine until the internal temperature has fallen to below 100°C. An LED in the model lights up to indicate that this cooling-off phase is complete, at which point the receiving system can safely be switched off.
Operating a model with a jet engine calls for a highly circumspect approach and specialised technical expertise. Please read and observe the safety notes in the operating instructions.
Complete turbo-prop engine with starter mechanism, Jet-tronic ECU (electronic control unit), LED indicator board, miniature fuel pump, electro-magnetic fuel cut-off valve, electro-magnetic gas valve, fuel hoses, quick-release connector set, fueltank fittings including clunk pick-up, refueling fittings, cable set, power supply (6 cells, 1250 mAh), miniature auxiliary gas tank, turbine mounting clamp (RamTec compatible), detailed operating instructions.
SPT5 core gas generator engine
Mass throughput 0.14 kg/sec
Shaft power 6 kW
Exhaust gas temperature 400 - 690°C
Turbine speed range 50,000 - 175,000 rpm
Fuel consumption 65 - 180 ml/min
Output shaft speed 7500 rpm