EPR as Thrust Indicator on Turbo Fans
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EPR as Thrust Indicator on Turbo Fans
A question posed to me recently,
If on a large fan aircraft such as the RB211-524H 75% of the thrust comes from the fan, why is the EPR used as the primary indicator of thrust?
Any ideas
If on a large fan aircraft such as the RB211-524H 75% of the thrust comes from the fan, why is the EPR used as the primary indicator of thrust?
Any ideas
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On an RB211, there is a pressure sensor at the front of the fan ("P2T2 probe") for sensing inlet pressure... and pressure sensors both in the coldstream and hotstream for exit pressure. In proper combination, they provide a reasonable facsimile of EPR.
Rgds.
Q.
Rgds.
Q.
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It always struck me as being a bit stupid having a primary engine indication that is read to four significant figures.
N values - which are percentages - always seemed much more common sense.
But then, having spent 9 years on B737 EFIS and only 0.5 on A320, I would say that.
N values - which are percentages - always seemed much more common sense.
But then, having spent 9 years on B737 EFIS and only 0.5 on A320, I would say that.
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>IMHO, N1 is the gauge to watch.
P probes are susceptible to icing (potomac River), engine speed far more reliable. Comments? <
Not many Air FLorida mishaps, but plenty of FAN FOD events (birds) where the N1 value goes up but the thrust down.
In other words there are some valuable pros and cons out there.
P probes are susceptible to icing (potomac River), engine speed far more reliable. Comments? <
Not many Air FLorida mishaps, but plenty of FAN FOD events (birds) where the N1 value goes up but the thrust down.
In other words there are some valuable pros and cons out there.
Well, there was also the Air Nauru one with phosphate dust over BOTH PT2 tubes. The article makes for interesting (if not sad given what happened to the flight crew subsequently) reading.
A copy is on AVWEB here.
[ 09 November 2001: Message edited by: compressor stall ]
A copy is on AVWEB here.
[ 09 November 2001: Message edited by: compressor stall ]
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I don't have any experience with really modern engines, but as far as I know, the GE CF6 engines are one of the only engines that use N1 as the primary power instrument. The RB-211's, JT3's, most pratt engines, all use EPR. I'm not going to second guess the engine Engineers, but they probably know whats best. AS far as reliability goes, most planes can be dispatched with an EPR gauge in-op, and use the N1 as the back up. During engine monitor coupon research, engine efficiency can be determined by analyzing all the parameters;ie, fuel flow and then engine thrust. Its a way to accureatly determine specific fuel consumption and relate that to engine life! EPR is Good!
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"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 ]
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 ]
The text bellow was posted before on this forum:
The basic thrust setting parameters for modern high bypass engines are Engine Pressure Ratio EPR used by Pratt & Whitney and Rolls Royce, and Fan speed N1, used by GE and CFM. The engine manufacturers have documented the reasons for their choice but in fact neither of the 2 parameters gives a proper indication of engine thrust. Direct thrust management is a continuing research topic.
Early turbojet engines used rpm as the operating parameter to establish thrust, while more modern turbofan low bypass engines like the JT8 use EPR. EPR is the ratio of the total pressure at the front of the compressor to the total pressure at the rear of the turbine, which may be thought as thr product of pressure and cross sectional area. On a hot day, compressor rpm for given thrust will be higher than on a cool day. Furthermore, a dirty or damaged compressor will reduce thrust for a given rpm. Using EPR as the thrust indicator means that on a hot day it is quite possible for the engine rpm to exceed 100%.
On newer high-bypass ratio engines, such as GE CF6, the fan speed N1 is used as the primary method of setting power because most of the total thrust is generated by the fan. Engine control is acomplished by keeping N1 constant for a selected throttle lever position, regardless of the ambient conditions, and (with Autothrust disconneced)let the pilot adapt to the non-linear relationship between rpm and thrust. The pilot is selecting not thrust but a proportion of the maximum thrust that can be developed by the engine at a particular altitude and Mach No.
The advantages of using N1 parameter for control are:
-rotational speed can be measured easily with good accuracy
-measurement is independent of aircraft altitude
The problem with monitoring thrust through rpm is that thrust changes greatly for the last few percent of engine speed before 100% rpm is reached. The system must be very sensitive to small changes in rotational speed.
Although EPR is also a reliable indicator of thrust the main disadvantage is that pressures cannot be measured as exactly as rotational speed. In a high bypass-ratio engine the EPR method would be more complex, because the thrust of both the fan and the core engine streams would have to be calculated separately. On the Rolls-Royce RB211 engine the parameter used to indicate and manage thrust is the (IEPR) integrated engine pressure ratio. This parameter is the integrated average of the fan and core exhaust pressures divided by the inlet total pressure. RR feels that because IEPR is based on both fan and core presssure ratios, provides the most accurate indication of engine thrust compared to other parameters.
On modern FADEC engines thrust control is enhanced. On the PW4000 series engine power is controled by both EPR and N1 rpm. The EPR is the rated or normal mode, while N1 is the alternate or fault mode.
Regards
The basic thrust setting parameters for modern high bypass engines are Engine Pressure Ratio EPR used by Pratt & Whitney and Rolls Royce, and Fan speed N1, used by GE and CFM. The engine manufacturers have documented the reasons for their choice but in fact neither of the 2 parameters gives a proper indication of engine thrust. Direct thrust management is a continuing research topic.
Early turbojet engines used rpm as the operating parameter to establish thrust, while more modern turbofan low bypass engines like the JT8 use EPR. EPR is the ratio of the total pressure at the front of the compressor to the total pressure at the rear of the turbine, which may be thought as thr product of pressure and cross sectional area. On a hot day, compressor rpm for given thrust will be higher than on a cool day. Furthermore, a dirty or damaged compressor will reduce thrust for a given rpm. Using EPR as the thrust indicator means that on a hot day it is quite possible for the engine rpm to exceed 100%.
On newer high-bypass ratio engines, such as GE CF6, the fan speed N1 is used as the primary method of setting power because most of the total thrust is generated by the fan. Engine control is acomplished by keeping N1 constant for a selected throttle lever position, regardless of the ambient conditions, and (with Autothrust disconneced)let the pilot adapt to the non-linear relationship between rpm and thrust. The pilot is selecting not thrust but a proportion of the maximum thrust that can be developed by the engine at a particular altitude and Mach No.
The advantages of using N1 parameter for control are:
-rotational speed can be measured easily with good accuracy
-measurement is independent of aircraft altitude
The problem with monitoring thrust through rpm is that thrust changes greatly for the last few percent of engine speed before 100% rpm is reached. The system must be very sensitive to small changes in rotational speed.
Although EPR is also a reliable indicator of thrust the main disadvantage is that pressures cannot be measured as exactly as rotational speed. In a high bypass-ratio engine the EPR method would be more complex, because the thrust of both the fan and the core engine streams would have to be calculated separately. On the Rolls-Royce RB211 engine the parameter used to indicate and manage thrust is the (IEPR) integrated engine pressure ratio. This parameter is the integrated average of the fan and core exhaust pressures divided by the inlet total pressure. RR feels that because IEPR is based on both fan and core presssure ratios, provides the most accurate indication of engine thrust compared to other parameters.
On modern FADEC engines thrust control is enhanced. On the PW4000 series engine power is controled by both EPR and N1 rpm. The EPR is the rated or normal mode, while N1 is the alternate or fault mode.
Regards
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CFM 56 on a B737-300 onward does not use EPR gauges, so N1 and EGT are used as appropriate (ower nad MCT respectively).
B737 100/200 primarliy use EPR but back up with N speeds.
The Conway on a VC10/B707 did not have EPR but had P7 which registered pressure at a particular point within the engine so N1, N2 and P7 were used with limits on each (min and max) to determine thrust. In essence the P7 gave a direct thrust indication (like EPR does) but was backed up by N1/N2 to avoid misreading with icing (Air Florida style).
So, it varies motor to motor according to engine type and to a lesser degree installation. Bottom line is - don't use to just gauge, the others should be checked too.
B737 100/200 primarliy use EPR but back up with N speeds.
The Conway on a VC10/B707 did not have EPR but had P7 which registered pressure at a particular point within the engine so N1, N2 and P7 were used with limits on each (min and max) to determine thrust. In essence the P7 gave a direct thrust indication (like EPR does) but was backed up by N1/N2 to avoid misreading with icing (Air Florida style).
So, it varies motor to motor according to engine type and to a lesser degree installation. Bottom line is - don't use to just gauge, the others should be checked too.
Moderator
I think this is going around in circles a bit, chaps. A manufacturer provides various means, with appropriate guidance in the use thereof, for the pilot to assess acceptable operational engine performance.
Providing all the bits are working properly, then one ought to do it like the manufacturer suggests, lest one go off at a tangent and be found wanting at the subsequent enquiry.
This is not to suggest that we can't have our own preferences in matters such as the routine use of EPR or N1, where both are provided, so long as we meet all the requirements specified by the manufacturer in the approved flight manual.
I notice that one post referred to the Air Nauru incident. The author of the article is a regular (and very interesting) contributor to Pprune and his expanded tale about the incident makes quite frightening listening. Hence the need for cross-checking those measures which are sensitive to externally caused error.
Providing all the bits are working properly, then one ought to do it like the manufacturer suggests, lest one go off at a tangent and be found wanting at the subsequent enquiry.
This is not to suggest that we can't have our own preferences in matters such as the routine use of EPR or N1, where both are provided, so long as we meet all the requirements specified by the manufacturer in the approved flight manual.
I notice that one post referred to the Air Nauru incident. The author of the article is a regular (and very interesting) contributor to Pprune and his expanded tale about the incident makes quite frightening listening. Hence the need for cross-checking those measures which are sensitive to externally caused error.
