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speedracer1407
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EPR Vs N1

Mon Jul 17, 2006 7:16 pm

This forum has informed me that most RR and PW engined planes have EPR guages installed as the primary engine performance measurment (and also, that some newer RR engines use N1 now).

My understanding is that earlier turbojet and low bypass engines were more effectively monitored via EPR-that is, of course, pressure in to pressure out. My understanding is also that big fan engines make the majority of their power with their first stage compressor (fan), and thus are, more often, monitored with an N1 guage. It also seems like common sense that reading guages that display a percentange value is more intuitive than reading a ratio.

So why do aircraft installed with somewhat modern engines like the RB211 and PW JT9 (and PW4000 also?) use EPR as their primary performance measurment? What are the advantages/disadvantages of an EPR guage versus an N1 guage?
By the way, where exactly is EPR measured? Are there sensors? Is it a calculated asumption?

O
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Buzz
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RE: EPR Vs N1

Mon Jul 17, 2006 9:49 pm

Hi Speedracer, Buzz here. The second part of your question is easy to answer, I've thumped my head on the PT2 probe hanging in the inlet, about 11 o'clock on a 757 engine (Pratt 2000), similar for the V2500 powered A320. It measures inlet pressure and temperature, a hard line (pipe) sends the data to the EEC.
In the last set of "struts" that support the back end of the last set of turbine blades there are inlet holes that measure the PT7 (to use the JT8D term), the Pratt 2000 term escapes me just now. Anyway the ratio between inlet pressure and tailpipe pressure is calculated for EPR.
But would you believe the 757 doesn't use the inlet pressure... except as a back up. The EEC prefers data from the left, or right Air Data Computer in the E+E rack.

I think there's a "tradition" of using EPR at Pratt+Whitney. I've heard the reason why they prefer it, but have forgotten. I prefer measuring thrust by N1.
g'day
 
kaddyuk
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RE: EPR Vs N1

Mon Jul 17, 2006 10:12 pm

Quoting Speedracer1407 (Thread starter):
So why do aircraft installed with somewhat modern engines like the RB211 and PW JT9 (and PW4000 also?) use EPR as their primary performance measurment? What are the advantages/disadvantages of an EPR guage versus an N1 guage?

I think nearly all Rolls Royce engines use Engine Pressure Ratio...

The entire Trent series certainly does...


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SkydrolBoy
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RE: EPR Vs N1

Tue Jul 18, 2006 5:51 am

From what I have been led to believe, EPR will tell you how well your engine is running (ie: that your compressor is dirty, or is the engine compressing the air the required amount) where as N1 will only tell you how fast the compressor is turning.
 
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HAWK21M
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RE: EPR Vs N1

Tue Jul 18, 2006 9:02 pm

Quoting Speedracer1407 (Thread starter):
By the way, where exactly is EPR measured? Are there sensors? Is it a calculated asumption?

Engine Pressure Ratio.
Total Pressure Ratio between Aft and Fwd stages in the Engine.Indicating Amount of Thrust Developed.
regds
MEL
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mandala499
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RE: EPR Vs N1

Wed Jul 19, 2006 1:20 am

EPR 1.0 = engine not creating drag or thrust...
EPR >1 = Engine creating thrust
EPR <1 = it's creating drag (thrust reversers exempted and if the gauges allow such a display)...

Well, not quite... or is it... anyways..
EPR = pressure of air coming in to the engine vs the pressure of the air coming out the back...

If I was informed correctly, N1 is a quadratic indication to thrust relationship... ie: thrust difference between N1 50% and 51% is not as great as 90% & 91%

Whereas EPR is a more linear relationship... but I could be wrong.

Advantage of N1 is simple... just see how fast it's going, you don't need to know how much thrust it's making. Disadvantage is the non-linear relationship... and the % number can be... err... arbitrary... you can have >100% N1... so what's the N1 based from?

For EPR, I've been told it's easier to use for manual thrust setting. Disadvantage... you need to cross check the take off EPR with the necessary N1... (on the 732 at least) as a method to cross check engine performance. EPR in icing conditions can lead to problems (Air Florida crash) as it could result in higher than actual EPR reading.

OK, I better stop before I write endless rubbish... Feel free to correct...

Mandala499
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Jetlagged
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RE: EPR Vs N1

Wed Jul 26, 2006 12:18 am

Rolls Royce have used a variety of thrust setting systems. Early jets used rpm (i.e. N1 as they were all single shaft). Aircraft such as the VC10 (Conway engines) used exhaust pressure, which is a coarse but acceptable method for pure jets and low bypass turbofans. It is only useful for takeoff (i.e. static) thrust settings, as it's a measure of gross thrust, not net thrust.

Later RR used Thrust Index (e.g. Spey on F.28). This was basically Engine Pressure Difference, exhaust minus intake, scaled in percent. An index number would be calculated from takeoff charts and dialled into the Thrust indicator, which offset the pointer. 100% was always the set value of Thrust Index. The actual thrust then depends on the index setting of course. For in flight use, the index was set to an standard number.

All the different methods may be explained by the fact that RR was a conglomeration of several British engine makers, each with different views as to what was best.

RR seem to have standardised on EPR since the RB-211. The Tay (refanned Spey used on the Fokker 100/70) uses EPR, not Thrust Index as might be expected.

EPR is probably more closely related to net thrust than N1, but N1 is good enough and is a more reliable indication (not liable to probe blockage/icing).
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speedracer1407
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RE: EPR Vs N1

Sun Jul 30, 2006 12:41 pm

Quoting Jetlagged (Reply 6):
Later RR used Thrust Index (e.g. Spey on F.28). This was basically Engine Pressure Difference, exhaust minus intake, scaled in percent. An index number would be calculated from takeoff charts and dialled into the Thrust indicator, which offset the pointer. 100% was always the set value of Thrust Index. The actual thrust then depends on the index setting of course. For in flight use, the index was set to an standard number.

Sorry to bump this thread but I hadn't seen Jetlagged's last post.

I was planning on asking why EPR wasn't scaled in percent for the sake of ease-of-use. Is your above expaination just that, scaling EPR as a percent? Obviously this isn't used anymore, but I wonder why. Is it because various atmospheric pressures change the EPR-to-actual thrust relationship?
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zeke
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RE: EPR Vs N1

Sun Jul 30, 2006 1:17 pm

Quoting Jetlagged (Reply 6):
EPR is probably more closely related to net thrust than N1, but N1 is good enough and is a more reliable indication (not liable to probe blockage/icing).

As an engine ages the target N1 to achieve a certain thrust setting changes, e.g. you may need 89% N1 for takeoff when new, and 91% when 10 years old to get the same thrust.

Without EPR a turbine with damaged blades may rotate at the required N1/N2/N3 however as no direct thrust measurement is made, one does not know if that rotation is absolutely associated with the production of thrust.

However as an engine ages, an EPR of 1.40 gives you the same thrust as an EPR of 1.40 10 years later, N1/N2/N3 is adjusted to meet the target EPR.

EPR can be thought of as the pressure difference across a disc in propeller theory, so it is easier to associate to thrust.

Whilst blockage of the P2T2 probe is a known disadvantage, FADEC monitors the EPR and N1/N2 on newer engines and will alert crew if any discrepancy exists.

When I call power set on the takeoff roll I compare the EPR to the N1 gauge as a backup to the FADEC.
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LTU932
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RE: EPR Vs N1

Mon Jul 31, 2006 1:14 am

Question: why don't GE and CFMI engines not use the EPR for e.g. power settings?
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Jetlagged
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RE: EPR Vs N1

Mon Jul 31, 2006 2:10 am

Quoting Speedracer1407 (Reply 7):
I was planning on asking why EPR wasn't scaled in percent for the sake of ease-of-use. Is your above expaination just that, scaling EPR as a percent? Obviously this isn't used anymore, but I wonder why. Is it because various atmospheric pressures change the EPR-to-actual thrust relationship?

No, Thrust Index was based on Engine Pressure Difference, i.e exhaust pressure minus intake pressure. EPR is the Ratio of the two.

To scale EPR in percent would only introduce the problem of deciding what 100% EPR should be for a given engine. As an engine grows in thrust the simplicity of 100% EPR being nominal max thrust disappears.

Quoting LTU932 (Reply 9):
Question: why don't GE and CFMI engines not use the EPR for e.g. power settings?

Presumably because GE always have used N1. CFMI is half GE, half SNECMA, so GE presumably influenced that choice too. There would be no gain for them to change to EPR, so why do so? It's hardly a big issue for a pilot and has been said above, they will cross check to N1 anyway.
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VC-10
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RE: EPR Vs N1

Mon Jul 31, 2006 7:08 am

Quoting SkydrolBoy (Reply 3):
From what I have been led to believe, EPR will tell you how well your engine is running (ie: that your compressor is dirty, or is the engine compressing the air the required amount) where as N1 will only tell you how fast the compressor is turning.

Rotor speed will also tell you how healthy your engine is. You know the speeds vs fuel flow & EGT when it came off the test bed and as the engine get older you will see the FF & EGT increase for a given speed.

When you carry out a Power Assurance run on such an engine you will take the engine up to a target speed, let it settle at that speed for 5 mins (for the temps, tip clearences etc) to settle and then check the EGT, N2, & FF are with in an acceptable range.

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