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EPR vs N1

Old 20th Sep 2000, 13:22
  #21 (permalink)  
Posts: n/a

Having seen several EPR over-read situations -doesn't take much take much damage to the fan/core rake to cause this. Famous one was a Tech Log write up, "EPR reading correct but all other parameters low". IMHO EPR indication only has a place if the fuel control uses EPR as the primary control parameter...otherwise N1 is more reliable...but not necessarily as accurate.

Go for N1 indication- control on a modern engine is using a 'model'with inputs of speeds,EGT,ff etc etc and no one controlling parameter anyway....and besides it's super easy to change a speed probe (Rollers exempt as per usual!?!) and difficult to fix a damaged EPR rake.

Ground Checked Satis- Please report futher.
Old 20th Sep 2000, 16:46
  #22 (permalink)  
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In the critical phase of take off wouldnt an early indication of engine problem be useful, therefore monitoring EPR would show a problem earlier than a reduction in fan speed N1 and allow direct action to be taken earlier?

As was mentioned before as long as parameters are set and agreed to before t/o roll then any deviation from the norm is an indication that something is wrong.

Technically I'm not sure if this is why EPR is still used but it would make sense to monitor the core function which at the end of the day is driving the big fan around to get you airborne.

Its life Jim, but not as we know it!!
Old 21st Sep 2000, 08:15
  #23 (permalink)  
bizjet pilot
Posts: n/a

EPR is rather elegant but its a ratio, not a value. As for it being a "truer" measurement of thrust, only half convinced. Seems to me that fuel flow is the best for answering the question: how much thrust are the engines putting out. Fuel flow in lb or kg per hour will give you virtually the same indicated air speed at any indicated altitude, weight and configuration being constant. more reliable than either thrust or EPR. But problem there is fuel flow is never operationally limiting, whereas all kinds of temp or power limits expressed as N1 or EPR.
Old 25th Sep 2000, 17:53
  #24 (permalink)  
Posts: n/a


"In the critical phase of take off wouldnt an early indication of engine problem be useful, therefore monitoring EPR would show a problem earlier than a reduction in fan speed N1 and allow direct action to be taken earlier?"

The best indication (although not the only one) of a potential engine problem would be a sharp rise / spike in engine vibration with a close eye then kept on the other primary engine parameters (noteably EGT).

The fan is the big driver, the one doing the lions share of the work and the reliability of its indication is (IMHO) better than that of an EPR system. On todays N1 system's there are no moving contact parts, with the probe end itself burried inside the engine where its nice and warm and free from ice. As well I'm with Gnd_Chk_Satis, its an easier system to maintain (Roller aside). As an example, the T700 on the A330 with the common nozzle assy has the nozzle EPR trimmed with the engine. If a problem is encountered with the nozzle, you cannot just replace it, it also has to have the correct EPR trim value as well. Makes maintenance all that much more stimulating.....

GE has got the right handle here.
Old 28th Sep 2000, 23:50
  #25 (permalink)  
Posts: n/a

I kind of like this question EPR versus N1 ?

What is EPR actually ?

It’s a pressure ratio, that’s all, nothing else.

It doesn’t measure thrust at all.

This pressure ratio will give an accurate image of the thrust if the physic dimensions of the engine are known in advance, which of course they are.

But suppose your big fan’s efficiency has worn a bit, or there is a bit of icing on the fan blades, or an acoustic panel ….whatever ( for example: freezing fog, we all know that just taxiing in freezing fog gets them iced up a bit, or dents in the fan etc… .

If under those conditions you set your target EPR you might find that you N1 is really revving it up to deliver your target EPR.

If now you turn around the reasoning, and you use N1 as the target value you may not have the required EPR.

On the other end if your EPR is not reading accurately you will see a bit of a strange N1.

So really it’s a bit like taking the heart beat and the blood pressure of your engine at the same time.

I am not a doctor but if blood pressure is low and heart beat high I guess something is really wrong.
I know it’s not 100% physics here, but then again, physics is only a mathematical concept to grasp reality with numbers as accurately as possible and that’s what EPR and N1 are actually doing for us.

They put one of them in the loop for the FADEC and you have a look at the other one. If the first fails you can swap the controlling one with the push of a button.

Smooth Trimmer

[This message has been edited by Streamline (edited 28 September 2000).]

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