Rob21;
I am not talking about engine performance (density altitude), I am just wondering if EPR shouldn't be a result of a given TLA...
Interstingly, I think you answered your own question. EPR is a ratio, not a rate. Ergo, it varies with conditions beyond those strictly influencing power generation such as RPM, fuel flow etc. A given TLA therefore cannot at the same time be "equated" with EPR and power generated. One cannot easily reverse-engineer EPR and this is why the thread is having some difficulty with the concept of deriving "power" (in terms of force exerted) from EPR. "EPR settings" are always driven by operational circumstance and outside pressure/temperature conditions, (the aforementioned Density Altitude). One would need to know the conditions under which varying EPR readings were recorded before knowing what power was being developed. See my response to Flyingnewbie10 below.
Bernd, enjoying your posts and your approach throughout... Looking forward to the paper. In re
but probably the EPR reading is unreliable when the reverser doors are deployed.
. Reversers may or may not influence EPR readings but probably not, at least in a direct (varying interference = varying readings = unreliable readings) way. That said, the initial EPR "bloom" could have resulted from this - I don't know.
In the last century
, we flew the DC8 (40 series through 63 series) and the DC9 on EPR, (I think the CFM-56 installation on the 71/73 series was N1), and we also set reverse thrust on EPR. There may have been certification corrections for the setting of reverse using EPR but that would be an engineering matter not a pilot matter. The A330 (Trent installation) uses EPR but the CFM56 340 installation uses N1.
Flyingnewbie10, re,
I searched a little bit and found something about EPR and ETR being key to find actual thrust. But it demands some calculation.
Why isn't it recorded in the FDR ? How do pilots know (if they know) what is the actual thrust they get from the engines ?
Not sure what you found but the following link is useful:
http://www.grc.nasa.gov/WWW/K-12/airplane/epr.html .
You are correct in stating that deriving power produced from EPR demands a little calculation.
In response to your questions, EPR, like hundreds of other parameters are indeed recorded on DFDRs as would N1, N2, (N3), fuel flow, EGT, N1/N2/(N3) vibration levels etc. It would be part of a legal (certified/required) parameter set. Flight Data Analysis (FDA) QARs (Quick Access Recorders) generally record far more parameters (in the thousands), at rates (per second) which can exceed regulated data-capture rates for DFDRs. However, QARs are not crash-protected.
In reference to N1, it would have been nice to have had the parameter in the TAM traces so we could see what the N1 was doing during the EPR 1.3 "bloom" as reverse was selected. The fuel flow doesn't change so we could assume neither did the N1 and the "bloom" may mean less than first appears.
Regarding your question about pilots knowing what the actual thrust they are getting from their engines is, this is approaching the issue in reverse, if you will. Pilots do not think in terms of "needing nnnnnn pounds/Kn of thrust" and then determine the EPR to get said thrust. EPR settings for takeoff, climb, cruise, approach and reverse on landing were, before the introduction of FADEC, set by reading a chart, usually a quick-reference card on the flight deck. The altitude and temperature were cross-referenced and an EPR power setting was read for the combination of same. For any given conditions outside the engine, the power developed either stayed the same (roughly) and the EPR changed with altitude, or it sometimes varied, keeping the EPR the same but varying power, (as in the MCT - maximum continuous thrust) case. Power settings using EPR representing a ratio of internal pressures to outside pressures and that is why the setting can vary with the same power being developed.
Appropos the above point regarding thrust developed then, knowing what power the engine(s) is/are developing, is immaterial. Knowing Kn or pounds is not as important as knowing that a referenced and selected setting, be it N1 or EPR, is "sufficient" for the conditions encountered in any one regime of flight. Whether 95% N1 or 1.3EPR is developing 9000lbs of thrust or 95,000lbs, (423 kN) is less important than knowing that the setting is "by the book" and can be expected to deliver on it's design certification "promise"... . After all, do we need to know that the engine under the hood delivers 230hp? We only need to know that it has (or hasn't) sufficient power to do the job in all operational circumstances and that, in the case of aviation, both our training and experience support a high degree of confidence "in the numbers".
PJ2