The introduction of the Graupner/JetCat helicopter mechanics brings to fruition the long-held wish of numerous model helicopter pilots: the dream of operating a scale model helicopter using a scale power plant - a turbine. The turbine mechanics have undergone intensive testing for a full year, and now, installed in the NH 90 fuselage, the system has reached production-ready status as a practical, reliable installation for everyday usage. This has been demonstrated in display events and model flying meetings at home and abroad. The system can now be operated by any experienced model helicopter pilot with the same natural assurance as any model of similar size with a conventional power system. The method of working of the PHT3 shaft turbine is similar to that of the P80 or P120 model jet engines, although the output power is transmitted to the rotors instead of generating pure thrust. Theengine is designed to expend its exhaust gases into the atmosphere via a suitably formed exhaust guide, with as little residual energy as possible. In design terms this is what is known as a single-shaft turbine, i.e. - in contrast to the twin-shaft turbine
layout - the output power for the rotors is derived directly from the (single) turbine shaft, which also drives the compressor. The turbine speed of around 85,000 rpm is initially reduced to a value around that of a piston engine by means of a two-stage toothed belt gearbox, before the power is transmitted via a normal centrifugal clutch with auto-rotation freewheel to the conventional main gearbox, which drives the tail rotor in addition to the main rotor in the usual way. The direction of main rotor rotation can be right or left as preferred by the pilot; the tail rotor is not driven in auto-rotation mode. The all-metal swashplate is actuated directly by four servos which are mounted within the mechanics. The main rotor head used in the GRAUPNER/Heim helicopter system is identical to that of the UNI-MECHANICS 2000, and therefore features a metal centre piece, ballraced mixer levers and ballraced collective pitch compensator. The tail rotor features an all-moving hub and ballraced linkage lever, and is also a standard system taken from the proven GRAUPNER/Heim range. Of course, handling a turbine requires that the model flyer should study the subject intensively and gain as much ex-pertise as possible beforehand. Provided that you have an appropriate level of knowledge, you will actually find itsimpler to handle a turbine engine in a helicopter than to operate a piston engine: the entire engine control system requires only a single radio control channel, while start-up preparations are limited to filling the fueltank and the small auxiliary gas tank for turbine starting. The engine is started simply by pressing a button at the transmitter, and the entire start-up process is automatic in operation, controlled by the turbine's on-board electronics (ECU). Initially the integral electric motor spins the turbine up to about 6,000 rpm, then the auxiliary gas valve is opened, and the gas is ignited in the turbine's combustion chamber (combustor). The engine accelerates further, the burning gas supports and eventually takes over from the starter motor, until the point where the rotational speed is high enough for the turbine to run entirely on kerosene. Once the engine is running and a stable idle speed has become established, control is passed to the pilot. The pilot uses a slider on the transmitter to increase the rotational speed of the turbine slowly until the desired system speed is reached. Any system rotational speed set on the slider is governed by the on-board electronics, and remains constant within broad limits regardless of the load on the system. As a result, mainrotor thrust is controlled exclusively via collective pitch, as the rotational speed is regulated by the turbine electronics. At the end of a flight the turbine's rotational speed is run down to idle again by the pilot in the same way, then the same channel is used to initiate the power-down procedure. This process is again entirely automatic, under the control of the on-board electronics: first combustion is halted, then the starter motor is switched on to force fresh air through the turbine until the internal temperature has fallen below 100°C; an LED on the model indicates the end of the cooling-off phase, and the receiving system can then be switched off. Flying a model helicopter with a turbine power system turns out to be an extremely pleasant experience, as so many of the system's inherent characteristics are in the pilot's favour: the power development of a turbine is always smooth, with no detectable non-linearity in the torque curve - a feature of piston engines which is familiar to all of us, and can lead to irregular and spasmodic movements of the tail boom. For this reason a turbine heli is substantially smoother around the vertical axis than any model powered by a piston or electric motor. Turbines also feature extremely low vibration levels; this ensures that the radio control system components have an easy time and last correspondingly longer, and it also means that it can make sense to add further fine details to scale models. Admittedly the characteristically smooth power development of the turbine does call for a slightly different approach from the pilot to the collective pitch control, i.e. smooth, harmonious control commands are the order of the day. All components, including fuel pump, glowplug, starter and ECU, are powered by a common six-cell battery which is independent of the receiver power supply. The display and programming unit (GSU) supplied in the set features a back-lit two-line alpha-numeric screen as well as 10 operating buttons and 4 LEDs. It can be connected to the engine when it is running in order to display current operating data, and is also used to change set-up parameters and read out flight and statistical data. A hydraulic rotor brake is available as an optional acces-sory; it is servo-operated and has two purposes: it slows down the main rotor quickly after switching the engine off, and it also helps to prevent a rotor blade ending up over the exhaust efflux during the start-up phase.