autojohn
Pr = Pressure regulated
Pg = Pressure governing
The PTG tells the FCU by biasing the Pg pressure relative to the fixed
Pr pressure. It is simply a lever with a face over the end of a hole in a jet.
To change the bleed out of the jet to a different value means that the lever will be in a slightly different position to maintain that value = droop.
The difference in the value
Pr/Pg = fuel flow required to maintain onspeed at the higher load. The onspeed condition where the lever arrives at equilibrium will be different.
Droop stops hunting or instability - imagine if there was no speeder spring - wouldnt work at all.
Droop compensation from the link to the collective adjusts the speeder spring load to remove the effect of droop but the characteristic is still there just masked or compensated for.
Most mechanical systems that I have seen in helicopters are overcompensated. Nr or N2 will rise with the application of collective and vice versa (if correctly rigged) This deals with the rate of change in power setting.
Lookup PID controllers - in a mechanical governor you basically only get P = proportional.
FADEC you get the lot - Proportional Integral and Derivative
Where the Bendix has an issue is the quality of the air it uses - smoke particles corrosion and other detritus accumulate on the lever and on the jet face and wear it out as well as changing the jet size.
Even FADEC's are not immune as they still sense P. On the PW200 series, for example, they put a tee in the P line that feeds a rather cunning setup that uses air flow to the output shaft seal. Where the P is sensed at the tee there is no flow
just pressure. Guess where all the dirt likes to sit!!
I hope this makes sense.