EPR vs N1 and Why...

[Arcla]

Hi all!
Can anyone pls help?
I know the definitions of EPR and N1 and how they work etc..
But why does the 747 RB211 use EPR vs the Cfm which uses N1...
It seems like the newer generation engines are using N1. Is this due to the increasing by pass ratios and if so why?
Have checked previous 100 days threads and apologise if i've missed the simple obvious answer...
Thanks in advance!

[john_tullamarine]

.. if you have taken the trouble to do a decent search, none of us mind helping out as much as you need ... I am sure that you will get a number of responses in a very short period of time ...

[Notso Fantastic]

It makes little difference. There is a minor reason that Rollers tend to use EPR (which is a ratio of outlet pressure over inlet pressure). I suppose you could say that EPR was slightly better as a measure of actual power the engine produces rather than just rotational speed of the front fan in engines of varying efficiency-90% N1 can vary in power, EPR is probably slightly more accurate. It's a close call, and it's all just numbers in the end.

Are you trying to call RB211s 'older generation'? You Bounder! That's the way to get a knuckle sandwich if you say that to operators of them!

[Arcla]

Hi , yea i was referring to RB211 on 747-200 not something like newer RB211-524-HT.
So basically you saying not much of difference it really just come's down to the manufarture's design philosophy?
thanks for post!

[404 Titan]

Arcla

What Notso Fantastic may have been trying to say is that N1 is just a percentage of rotational speed. EPR is the ratio of inlet to outlet pressure. As both engines get older the N1 percentage remains the same but the output of the engine deteriorates. This isn’t presented to the crew. An engine that measures its performance with EPR won’t be able to deliver the same performance as a new engine. The EPR gauges will show this to the crew as a lower EPR value.

[lomapaseo]

What Notso Fantastic may have been trying to say is that N1 is just a percentage of rotational speed. EPR is the ratio of inlet to outlet pressure. As both engines get older the N1 percentage remains the same but the output of the engine deteriorates. This isn’t presented to the crew. An engine that measures its performance with EPR won’t be able to deliver the same performance as a new engine. The EPR gauges will show this to the crew as a lower EPR value.

Well this is not necessarily true and while your description of how things work is OK the judgements of cause vs effect is not.

In both cases(N1 vs EPR display in the cockpit) it all depends on how the engine is controlled and what FCOM procedures are trained.

For the high bypass engines the majority of the propelsive force comes out the fan duct, but is rarely displayed as such to the pilot or controlled by the engine control system. The displays to the pilot as well as the contolling effects of the engine vs PLA in the cockpit are mostly synthesized effects based on assumed engine health within limit ranges. Inh other words don't wrack your brains about it.

In the event of damaged engines, i.e. FOD, this synthesization breaks down and the display in the cockpit of either EPR (out the jet exhaust) or FAN N1 do not reflect actual propelsive thrust (it will be less) Under this condition it's best to just fly the aircraft , keeping EGT within limits.

[Old Smokey]

It's basically up to the manufacturer to choose to have EPR or N1 as the primary thrust setting parameter.

EPR is a true measurement of the results, i.e. is my engine producing the increase in pressure commensurate with the thrust required. On the same note, N1 is providing the N1 which in a perfect engine should produce the thrust required, a degraded engine may still produce the required N1, but due to deterioration, NOT be producing the required pressure increase, and is therefore not a good indication of the actual thrust produced.

Operationally, N1 engines are decidedly the simplest to operate. Static Takeoff thrust and In-Flight Go-Around thrust are the same for the N1 engine, whereas for the EPR engine forward flight speed, which introduces the ram pressure factor at the engine intake, must be considered to apply a 'corrected' EPR' to achieve the same thrust.

EPR engines are also subject to severe inaccuracies if any ice accumulates on the Pt2 probe, to wit the B737 accident in icing conditions at Washington.

Having flown EPR and N1 engines of all engine sizes, my personal preference leans strongly towards the N1 engine.

[Young Paul]

Having used both (and being currently condemned to EPR), I have to say that I think EPR is particularly silly. For example, on the approach, it will vary between about 1.00 and 1.07. When variation in the third significant figure / digit of a number covers a range from too little approach power to too much approach power (1.03-1.09), you have a poor means of measurement. This corresponds (on my old type) to a range of N1 between about 50% and 75%. I know which I think is the more useful measure!

[Notso Fantastic]

It's just numbers! I've tried both- it's irrelevant. Your EPR could be N1 for all the difference it makes. I have also worked N1 and EPR, and thousands of rpm. You just remember the important bits. The Classic with Pratt & Whitneys and RR, the 747-400 with RR, the 737- all using EPR. An EPR reading correlates to an N1. As for thinking of EPR readings as over a hundred, they are not- you just ignore the initial unit- the 400 uses 1.14 approach power- on three engines it's about ,07 more. Take-off power should be about 1.56- really heavy and hot and you might have 1.76
If anyone is really concerned about one or the other, then it shows they have very little else to grumble about!

[Arcla]

Thanks for all the posts Gent's has def given me more insight!

[FLYbyWIT]

Hey, Sorry to hijack the thread but on a similar topic and due to the fact I cant find the info on SEARCH can someone please help me with the following. As I have done sim time on the 737-400 with the N1 readings I now find myself with a sim check on the 200 with the EPR readings and a little in the dark on what to look for.
What is the range of the EPR readings and is there a formula for working one to the other.

250kts N1 69% EPR?
210 60% ?
190 60% ?
170 62% ?
150 68% ?

3degree GS 58% ?
T/O setting
Climb setting
Cruise

Anything else I may need to be aware of?????????

Sorry Arcla but it seemed some what similiar to topic.

[411A]

Hmmm, Rollers 'prefer' EPR?

Well yes, with the more modern types.
But, in the distant past, I operated a RR powered B707 (for a short time), and it didn't have any EPR gauges, so you had to use N1 only.

One less 'clock' to look at....

[woftam]

Having experienced both EPR and N1 on a variety of types (B747 P&W,B744 GE & RR,B727,B737,B707) I would have to give the thumbs up to N1.
The major reason being the possibility of ice on the Pt2 probe giving erroneous readings. It is however a great indication that you need engine anti-ice when your EPR goes haywire!
"Old Smokey" mentions this previously.

[Semaphore Sam]

With due care, both are ok for takeoff....but, the Air Florida crash into the 14th St bridge was caused by the use of EPR readings unadjusted for the effects of icing. N1, if accurately set, would have precluded the mis-setting of take-off thrust; I can't see any reason for not using N1 in some form, to preclude the Air Florida problem. Sam

[FullWings]

What's really confusing me is that at certain weights and temperatures (RR Trent 895) you can go along quite happily in STABLE LEVEL FLIGHT @ 250kts with 0.98EPR

I'm pleased to say you can call up N1,2,3... etc. as well and the engines still appear to be running...

[V/S]

Having flow both EPRs and N1s, I have to say that I also give tumbs up to N1s!!! As almost everyone here! EPRs just make your life more dificult, the numbers to memorize for different power settings are easier for N1s and without EPR it`s one less thing to check at take off power! Check EPR versus N1, always had to do that, several incidents happened, due to false EPR readings, that would be easy to detect, just looking to N1!
So one less thing to do and concentrate on the rest... that`s simplyfing your life to release you for also other important things during those phases of flight that you have to be cheking everything...!

[74Freight]

I thought that on a high bypass modern fan jet as the fan produces the majority of power, N1 gives a better indication of the power being produced than EPR would which is why it is used for this type. Unfortunately the ones I fly show torque so I am not really qualified to say.

[Seat1APlease]

In the back of my mind I seem to recall that an accident in the states ,was it Washington? lead to the change on the Boeing 737 from epr to n1.

I think from memory that one of the epr sensors had iced up and they thought they had set take off power when they were in fact considerably down on power, whereas if they had looked at the other instruments such as rpm,tgt, fuel flow or even throttle position they may have realised something was wrong.

[Bally Heck]

As a matter of interest, does anyone know how EPR is measured on a high by-pass engine? Is it the ratio of by-pass air, or the ratio of the hot bit, or an amalgamation of the two?

If it is the former, and given the lack of deterioration of cold engine parts, I would imagine that N1 and EPR measure the same thing in a different way. If it is the 2nd (hot bit), then I would imagine EPR gives a more accurate indication of power, and if it is the third then I don't have a clue

Serious question though.

(And yes...I don't have a clue)

[Pegasus77]

I fly EPR and N1 on the 330, 346 and 343... You can fly with both of them, but I prefer N1 WAY over EPR, mostly due to the fact that EPR is non-linear, and especially at lower powersettings a millimeter of lever-change quite a thrust change. Which, according to Airbus, should be not true. But it is.

Most of my colleages, including me, fly EPR-engines by the still available N1-gauges.

P77