Fixed turbine control systems
Clearly to meet the RRPM requirements a fixed turbine engine should operate on a constant speeding schedule. This confers the benefit of very fast acceleration times, since there are no inertia problems and massive overfuelling can be tolerated since the low power operating point is typically far from the surge boundary. The basic requirements for the engine control system have already been discussed and, in general, these requirements can be met more simply in a fixed turbine engine. As before the pilot changes the blade pitch by means of the collective lever but in this case, however, a small closed loop system acts to keep RRPM and engine speed constant. The Astazou 3N2 system (as fitted in the Gazelle helicopter) is a good example of such a system. In the governed range constant pressure is maintained across a variable metering valve, so that the fuel flow to the engine depends only on the position of this metering valve. The valve position is adjusted by a servo system controlled by a pilot valve that senses rotor speed. On some systems precise control and adjustment of RRPM may be vested in a speed select lever.
18.104.22.168 Mode of operation
If it is assumed that the engine is running in equilibrium when a collective pitch increase is demanded then it is clear that NR must fall initially. The pilot valve will therefore open and cause the metering valve to increase the fuel supply to the engine. The engine will then accelerate until the datum RRPM is restored. As it does so the pilot valve will gradually approach its null position at which point the metering valve is locked at its new more open setting.
Opening the control loop on the ground is again achieved by using a ‘lowest wins’ system similar in concept to that found on some free turbine engines (see Section 22.214.171.124). At ground idle the rotor speed will be very low (or zero) so the pilot valve and metering valve will be fully open. Under these conditions control of the engine is vested solely in a manually operated fuel valve (throttle). As this valve is opened to accelerate the engine to flight idle, the RRPM will increase until, with the throttle fully open, the pilot valve is nulled and the metering valve is governing the fuel flow. It is interesting to note that with such a system:
• There is no acceleration control fitted since it is not required in the flight range due to the constant speeding nature of the engine. Consequently the manual fuel valve or throttle must be handled very gently during acceleration from ground idle to flight idle.
• There is no static droop since the pilot valve always returns to the same null position. At equilibrium the rotor speed always balances the same spring force irrespective of the metering valve position.
• There will be some transient droop since a RRPM error is required before the pilot valve can move to adjust the metering valve. On some systems the transient droop may be so small that it is not discernible by the pilot. If so it is possible that torque spikes will result as the power output from the engine responds rapidly to the change in rotor speed.
• Since there is no static droop, each engine in a twin engine configuration can be delivering widely different powers and still be running at a common speed. Hence power matching is not usually feasible without the aid of some artificial stability system.