EPR
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EPR
seasons greetings.
ill get down straight to the question.
whats the main benefit of using EPR over N1 for engine indication?
in most emergency situaions ( for eg. emergency electrics or unreliable speed for the bus's) you end up controlling the engines using the N1 mode.
so do we really need EPR?
thanx
ill get down straight to the question.
whats the main benefit of using EPR over N1 for engine indication?
in most emergency situaions ( for eg. emergency electrics or unreliable speed for the bus's) you end up controlling the engines using the N1 mode.
so do we really need EPR?
thanx
Warning Toxic!
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If the manufacturer brings out an engine controlled by EPR, who am I to argue? They do it, presumably, for a good reason that applies to their engine. Rather than discuss the point, I always felt there were more important things to be getting on with.
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Well, EPR is a direct measure of thrust, N1 is not. To get a specific performance, we need to achieve a certain ammount of Thrust.
If you go the N1 route then somewhere in the process you need to account for variations in thrust from engine to engine for a given N1. Obviously poorer than normal performance is a problem, but also better than normal (because of minimum controlability). Obviously limits can be established with runs in a ground cell for confirmation of performance. The only engine I'm familiar with (the details of) works by a correction factor being calculated for each engine that accounts for how much more thrust is created than the minimum spec. On that basis a downwards correction factor is calculated.
e.g. if the engine can produce its rated thrust at 99.5% N1 instead of needing to be at 100%, then a 0.5% fiddle factor is sent to the thrust management system and the engine instruments. The result being that when engine is producing rated thrust and actually doing 99.5 percent both the N1 display and the autothrottle, by virtue of the 0.5% correction, think its doing 100%.
So you can see that getting the correct thrust developed by an N1 based engine requires some steps. Now, I'm not familair with the equivalent inner guts for an EPR based engine but would it not be a bit simpler as EPR is a direct thrust measurement?
pb
If you go the N1 route then somewhere in the process you need to account for variations in thrust from engine to engine for a given N1. Obviously poorer than normal performance is a problem, but also better than normal (because of minimum controlability). Obviously limits can be established with runs in a ground cell for confirmation of performance. The only engine I'm familiar with (the details of) works by a correction factor being calculated for each engine that accounts for how much more thrust is created than the minimum spec. On that basis a downwards correction factor is calculated.
e.g. if the engine can produce its rated thrust at 99.5% N1 instead of needing to be at 100%, then a 0.5% fiddle factor is sent to the thrust management system and the engine instruments. The result being that when engine is producing rated thrust and actually doing 99.5 percent both the N1 display and the autothrottle, by virtue of the 0.5% correction, think its doing 100%.
So you can see that getting the correct thrust developed by an N1 based engine requires some steps. Now, I'm not familair with the equivalent inner guts for an EPR based engine but would it not be a bit simpler as EPR is a direct thrust measurement?
pb
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Probably the most accurate system, assuming it's well maintained, is R-R's integrated EPR. As the engine deteriorates with time, core thrust goes up, and fan thrust goes down. so the IEPR averages the two and stays closely matched to true thrust.
Not so with P&W's EPR, which measures only core EPR. Deterioration will drive up core thrust and EPR, which means that the decrease in fan thrust is not accounted for.
Since engine inlet total pressure is the denominator in EPR computation, icing or other blockage of the inlet probe can cause erroneous readings. This was a primary cause of the Jan. 1982 DCA AF90 accident.
N1 is not susceptible to icing, and as described in the Engine Insight link, tends to be conservative when engine deterioration is considered.
Not so with P&W's EPR, which measures only core EPR. Deterioration will drive up core thrust and EPR, which means that the decrease in fan thrust is not accounted for.
Since engine inlet total pressure is the denominator in EPR computation, icing or other blockage of the inlet probe can cause erroneous readings. This was a primary cause of the Jan. 1982 DCA AF90 accident.
N1 is not susceptible to icing, and as described in the Engine Insight link, tends to be conservative when engine deterioration is considered.
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One way to consider this is to view the modern, high bypass ratio fan engine as nothing more than a large, fixed pitch, ducted turboprop. Some of them even sound like one. As the linked document pointed out, 75 to 80% of the thrust comes from fan air. The total thrust output is somewhat parallel to the turboprop manufacturer citing an "equivalent" horsepower based on shaft horsepower plus the small amount of tailpipe thrust.
That said, EPR is only measuring the core efficiency, unless it is an integrated system like described above. Even then, it gets really hard to accurately measure fan airflow based on pressure differential. However, the RPM of a fixed pitch propeller is a pretty reliable indication of the thrust output, assuming no serious airfoil degradation like that resulting from icing.
How these two relate, I suspect, depends a great deal on the bypass ratio and the number of spools. I also suspect that limiting EPR is an indirect way of insuring that other parameters such as EGT and ITT are appropriately limited. This may explain why some manufacturers default to N1 when EPR fails; it gives accurate thrust, but provides no protection of core limits. On the other hand, some designs may simply be operating at such a derated output that fan speed is sufficient as a primary indication.
In the end, when operating a high bypass engine, you can have all the EPR in the world, but without fan speed you will likely be somewhat dissatisfied with the performance.
That said, EPR is only measuring the core efficiency, unless it is an integrated system like described above. Even then, it gets really hard to accurately measure fan airflow based on pressure differential. However, the RPM of a fixed pitch propeller is a pretty reliable indication of the thrust output, assuming no serious airfoil degradation like that resulting from icing.
How these two relate, I suspect, depends a great deal on the bypass ratio and the number of spools. I also suspect that limiting EPR is an indirect way of insuring that other parameters such as EGT and ITT are appropriately limited. This may explain why some manufacturers default to N1 when EPR fails; it gives accurate thrust, but provides no protection of core limits. On the other hand, some designs may simply be operating at such a derated output that fan speed is sufficient as a primary indication.
In the end, when operating a high bypass engine, you can have all the EPR in the world, but without fan speed you will likely be somewhat dissatisfied with the performance.
Years ago, a Pratt engineer told me a 1% error in N1 equals approximately 3%-4% error in thrust output, while a 1% error in EPR equals a 1% error in thrust. That is, EPR is a measure of thrust, but N1 is a measure of fan speed.
GF
GF
With the RR RB211 series the real thrust was measured by a load cell on the test bed and with all the usual corrections applied a resistor was selected and installed in the IEPR system on the engine so that indicated EPR equalled a measured amount of thrust. There was an tolerance of plus/minus 1% N1.
The result was picky crews occasionally wrote up that N1's disagreed by x amount with EPR's aligned generally inferring an EPR error which was a pain to troubleshoot. Digging into the test bed figures invariably revealed that the engine with low (or high) N1 passed off the test cell like that. An engine with a fully refurbished fan with nice sharp leading edges on the blades might achieve its thrust on the low side of the allowed N1 band whereas one put through test with only minor cleanup to the fan might run on the high side of the band.
The result was picky crews occasionally wrote up that N1's disagreed by x amount with EPR's aligned generally inferring an EPR error which was a pain to troubleshoot. Digging into the test bed figures invariably revealed that the engine with low (or high) N1 passed off the test cell like that. An engine with a fully refurbished fan with nice sharp leading edges on the blades might achieve its thrust on the low side of the allowed N1 band whereas one put through test with only minor cleanup to the fan might run on the high side of the band.
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One thing to consider is that N1 is just RPM. N1 x Density gives thrust.
(i) EPR is thrust.
Some engines may have EPR "bumped" at altitude to compensate, to maintain thrust equivalent to SL; So the thrust/EPR relationship DOES change!
Years ago, a Pratt engineer told me a 1% error in N1 equals approximately 3%-4% error in thrust output, while a 1% error in EPR equals a 1% error in thrust. That is, EPR is a measure of thrust, but N1 is a measure of fan speed.
Thrust is made up of a whole bunch of pressure gradients and mass flow within the engine relative to the outside air. EPR and N1 relationships are simple expressions of relative thrust for a normal functioning engine. When things get damaged, including inlets through exhaust plugs ,these relationships are no longer valid.
Like plugged pitot tubes I would trust whatever the manufacturer says up until damage occurs, after that I would fly the machine by basics (attitude, etc.)
Like plugged pitot tubes I would trust whatever the manufacturer says up until damage occurs, after that I would fly the machine by basics (attitude, etc.)
It suppose it depends on what you're used to. The first jet I flew, a 737-200, had EPR gauges; <1.00 was drag, 1.00 was neutral and anything above that was net thrust. You could work out a ballpark approach setting with a simple formula and if you had an engine failure, you just doubled the bit after the decimal point and everything stayed the same. No problem. Then I flew the other variants which had N1 as the controlling parameter but it didn't take long to get to know the various %age settings to achieve a desired result.
I currently fly the 777, GE (N1) & RR (EPR) and I have to say the RR "EPR" might as well be calibrated in elephants per nano-furlong. I have no real idea what an appropriate value might be as it varies so much with temperature & density; you can fly level at 270kts with EPR values less than unity and as to what's a good takeoff setting... ??? I tend to refer to the N1 indications instead.
Maybe an ideal indicator would use strain gauges and be calibrated in kilo-Newtons? Or possibly like the new Rolls-Royce car and have "power reserve" or just a straight percentage of maximum thrust?
I currently fly the 777, GE (N1) & RR (EPR) and I have to say the RR "EPR" might as well be calibrated in elephants per nano-furlong. I have no real idea what an appropriate value might be as it varies so much with temperature & density; you can fly level at 270kts with EPR values less than unity and as to what's a good takeoff setting... ??? I tend to refer to the N1 indications instead.
Maybe an ideal indicator would use strain gauges and be calibrated in kilo-Newtons? Or possibly like the new Rolls-Royce car and have "power reserve" or just a straight percentage of maximum thrust?
