Free turbine governor characteristics

6.3.3.1 Simple governor operation

Most hydro-mechanical rotor governing systems are of the proportional type first developed for steam engines. When the load on the engine is increased (by the application of collective pitch) the rotor speed and consequently the power turbine speed falls. This reduction is used to signal an increased fuel flow to the gas generator. The resultant increase in power produced by the gas generator tends to restore the power turbine RPM and equilibrium is re-established when the torque produced by the power turbine equals the torque required by the rotor system.

In its crudest form the proportional governor contains bob-weights that are thrown out away from the axis of rotation thus generating a force that is proportional to the speed of rotation. This force is then used to close a fuel-metering valve against the action of a spring. Typically the free power turbine drives the bob-weights and their position regulates the amount of fuel fed to the gas generator. Consider a demand for more rotor thrust via an application of collective pitch. Initially the RRPM and hence the power turbine speed (NF) will fall, reducing the force opposing the spring thereby allowing it to force the valve open and allow a greater fuel flow. This increased fuel flow accelerates the gas generator enabling it to deliver more power and arrest the decay in rotor speed. Eventually when the force from the bob-weights once again matches the spring force, NR and NF achieve a new equilibrium condition. At this new equilibrium state a greater fuel flow (FFR) is required to meet the higher power and therefore the fuel valve must be open further than was the case at the lower collective pitch setting. Therefore FFR is inversely proportional to NF and the higher fuel flow rate is achieved at a reduced NF and consequently reduced NR. Clearly the greater the power demand, the further the fuel valve must be opened and the greater the reduction in the rotor RPM. This characteristic, termed static droop, leads to a steady reduction in rotor speed with collective pitch or fuel flow.

If this type of closed loop rotor governing system was functioning when the aircraft was on the ground with the rotors stopped, the system would detect a massive RRPM underspeed and demand maximum fuel flow or gas generator speed. The control loop must be broken, therefore, when the rotors are stopped. This may be achieved either electrically (as on the Rolls-Royce Gnome engine) or hydro-mechanically (as on the Rolls-Royce Gem engine). Some systems feature a single lever moving in a gate. Over the lower portion of the gate from ground idle (GI) to flight idle (FI) the lever controls the gas generator only. At FI a microswitch is made which feeds RRPM signals to the control system so closing the loop. From then on, from FI to maximum, the lever changes its function from one of controlling the gas generator to one of selecting the RRPM datum – hence its name a speed select lever (SSL). This system works well for a single engine installation. In the case of multi-engined helicopters the acceleration of the engine from GI to FI is often achieved via individual engine condition levers (ECLs), a separate SSL being provided for adjustment of the rotor speed datum.

In some hydro-mechanical systems the fuel flow to the engine passes through two variable area orifices in series; one controlled by the free turbine governor, the other by the gas generator governor and/or throttle. Overall system control is determined, therefore, on a lowest wins basis, that is the fuel flow to the gas generator will be determined by which of the orifices has the smaller opening. Alternatively a signal from the proportional governor driven by the free power turbine can be used to change
the gas generator datum speed with the gas generator governor controlling the fuel flow to the engine. In a typical two-orifice system at ground idle (GI) with the rotor stopped, there is no bob-weight force on the free turbine governor and under the influence of the governor spring its orifice is wide open, allowing unrestricted fuel flow. However, the gas generator throttle is in its minimum position and the gas generator governor has the minimum spring loading (corresponding to NG for GI) applied to it. As the ECL is moved, the gas generator throttle opens and at the same time a higher RPM setting is applied to the gas generator governor. The increased fuel flow through the throttle can now accelerate the engine up to the new higher gas generator governor setting. The free turbine governor orifice is still wide open and thus has no controlling effect. At some point in the acceleration process from GI, the rotor brake is released and a RRPM signal is applied to the free turbine governor. By the time the ECL is fully advanced, to a FI gate for example, this RRPM signal will have reached the governed range and a balance achieved with the spring force. The gas generator governor now has the maximum NG setting applied and since in general the engine speed demanded by the rotor governor will be less than that setting, the NG governor acts merely as a maximum gas generator speed limiter. In this way the control function is handed over from the gas generator via the ECL to the rotor via the free turbine governor.

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