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