V1... a good explanation.
I guess my understanding of the Fuel Topping governor differs a tad. Maybe you can straighten my thingking.
The FTG regulates fuel flow by sampling P3 air through the FCU. When the FTG senses a speed at or above 106% of selected RPM it opens the Py valve (at the FTG) which lowers the pressure of the p3 air being sampled by the FCU. The FCU in turn reduces fuel flow to match a fuel flow consistant with the 'new' P3 pressure created by the gas generator. After fuel flow reduces enough to slow the engine to a speed that will bring the props back below 106% of selected RPM (substancial decrease in N1) the Py valve will once again close. As N1 drops below 92% the P2.5 valve will then start to modulate open, which also effects the p3 being produced, further pronouncing the 'spiking' p3 pressure.
The Py valve only has two positions, open or closed. To modulate P3 pressure to a constant value the Py valve would have to find an intermediate position as P3 air is continuously being sampled by the FCU, which of course is happening whenver there is N1. So this yields an oscilation of P3 pressure, hence an oscilation of fuel flow, torque. RPM, ITT etc as the Py valve opens and closes due to the FTG being satisfied then not, then satisfied etc!
Unlike the pilot valve on the Primary and Overspeed governors, the Py valve does not supply almost instantaneous results. It acts through a long chain of events and is ultimately a very crued governor. When the RPM drops below 106% of selected RPM the Py valve closes at a time when N1 is in rapid decay. It then takes a few moments for P3 pressure to build back up, the FCU to sense this and then start delivering more fuel.
I can see that potentially the oscilations could get smaller and smaller...its just that my understanding is that the chain of events is just to slow and cumbersome for a stable prop speed to exist...its always playing catch up in other words....
Brinda