"EPR on the RB211 measures the diff-pressure from in front of the fan to behind the last section of the turbine giving you exactly what EPR is... Engine Pressure Ratio.."
We probably both know what we are talking about, Brenoch, but, hopefully, we can clarify our interpretations of "EPR's" in follow-up postings....
I'm guessing that what you mean by "EPR" is, by definition, the measurement of back/front differential pressure by whatever means, complete with all the inaccuracies of the measuring process (and may not reflect true EPR or what the pilot sees on his EICAS displays/instrumentation). Surely, to measure the true pressure differential between the front and back of the engine with any exactitude would involve the measurement of pressures at an infinite number of points in front of and behind the engine, using perfect sensors?
When I used the expression "reasonable facsimile", I meant a useful cockpit indication. To clarify my point of view....
On the RB211 (modern version) the "P2T2" is a single air intake at one point at the top of the engine, which feeds air pressure to the FAFC (Engine Electronic Control box) via metal tubing. Various techniques are used to improve the reliability of the pressure fed to the FAFC, such placing an accumulator (spherical tank) in the line to help damp out pressure fluctuations. The EEC converts this information to an electrical signal: i.e. the pressure in the plumbing now becomes an "electronic facsimile" of the intake pressure, so to speak.
Air inlets on the "A frames" (which bridge the gap between the engine core and the fan cowls) just aft of the N1 fan, provide coldstream duct pressure. This pressure is known as the "P1.3" pressure. Air inlets on five rakes in the hot exhaust close to the last stage of the turbine provide a "P5.0" pressure. The P1.3 and P5.0 pressures are combined simply by joining the plumbing together in a manifold known as the "multiple connector". This combined pressure is then sent to the FAFC for conversion to an electrical signal. An assumption that the reading taken by the sensors in the hot/cold exhaust streams does not require some kind of processing to improve accuracy of true total exhaust pressure should not be made... Nor should we assume that the resultant EPR value generated by the FAFC (from sensed intake and exhaust pressures ) does not go through some further processing before it reaches the pilots instruments (Actually, on every FAFC, there is an EPR trim plug, which adds an electrical bias to the EPR signals leaving the FAFC (to ensure that the "EPR" displays in the cockpit give a more accurate indication of actual engine thrust).
I have heard rumours that GE owns the copyright to using N1 as the primary engine indication, thus putting the pressure on other engine manufacturers to provide a suitable alternative (such as EPR). There doesn't seem to be too many disadvantages to using this alternative. The pilots want to see a reliable indication of thrust. The EPR values... as seen in the cockpit (after much manipulation), seem to provide a reasonable indication of such.
Hope this clarifies things.
Rgds.
Q.
(Edited to add italics and bold characters)
[ 14 November 2001: Message edited by: QAVION ]