N!Vibes
EPR Command - is the EPR the autothrottle of the a/c requires to maintain an a/c speed
EPR Trimmer - Function to trim the engine so that at a given EPR, at a given OAT at a given Baro that all engines produce the same thrust. It is NOT there to restrict the engine to below test-cell thrust levels.
N1 - On the RB/Trent is used as the alternate, not the primary thrust setting parameter
EPR - is a ratio of the air pressure measured behind the fan OGV's at the front of the engine and in the LP turbine exit at the rear, therefore it has a pretty close relation to thrust. If you cut 2" of the end of all your fan blades or the fan attrition lining was damaged/worn, but the fan was turning at 100% N1 speed, would you still be getting the required thrust? No. If the fan was dirty and therefore less efficient, then it's compression would be less efficient, therefore pressure will be lower, which EPR would show. Since the engine would have to work harder overall to make the same pressure, this would be seen on ECM trending by the engine speeds gradually trending up in relation to EPR.
Hopefully this illustrates.
Good explanation for the RB, but how about
PW the subject title of this thread?