N1 is a perfect way to present engine thrust.

 

Here is a little formula:

 

F=airmass moved per second (Kg/sec) x speed change the air mass undergoes (m/sec)

F=(Kg/sec) x (m/sec) = Newton

 

The only thing changing the amount of air per second that goes through the engine is RPM (N1).

So N1 relates directly to Thrust :-)

(On a side note: The formula also shows that Thrust decreases as airspeed of the aeroplane increases)

 

I am sure a formula can be found proving EPR works just as well but I dont have that handy, sorry.

 

GE has defended using N1 because they find it more reliable.

Only one RPM pickup sensor required for N1 versus two pressure sensors (pressure in and out) required for the EPR equation.

N1 indicates purely the speed at which the low pressure compressor shaft is rotating, not the movement of airmass nor changes in its speed, so as far as I can see, this argument is invalid. As Kyle's said before, if you were to remove the fan blades, N1 indications would still be valid and accurate, as the shaft would still be rotating, but no air would be moving at all.

What's the difference in getting used to pointing the RPM needle of your Piper at 2500 RPM and the N1 needle of your NG at 95%?

 

Part of the reason people started using N1 to begin with is to make the processing easier on your brain.  The brain picks up on issues on bounded numbers a little more readily with a bounded number (percentages are bounded between 0-100%), as compared to an unbound scalar (RPM).

 

While EPR is not bounded like %-based N speeds, it's giving a scaled pressure ratio between static (1.00) and how much is being developed by the engine.  Higher values = more thrust.  The difference is that N speeds are really rotational speeds for the engine, and EPR values are directly derived from thrust output.  If I removed the fan blades from an engine and spun it at whatever 95% N1 was, the N1 gauge would read 95% even though I wasn't developing thrust at all.  Looking at the EPR gauge, you'd only see 1.00, however, and would realize that you were developing no thrust even at 95% N1.

 

It's just a different way to show overall engine output.

It should be said that N1, N2, N3, etc can be expressed either as a percentage of the nominal maximum rpm or as rpm.  Percentage rpm indication was introduced for the reasons you say because the rpm produced by a turbine can be a meaninglessly large number.

 

If there were no blades on the shaft it you wouldn't have a turbofan.  The fact is there are fan blades and at a given air density the mass flow through them, and hence the thrust of the engine, is directly proportional to fan rpm.  EPR is the ratio of the fan inlet pressure and the core exhaust pressure.  Thus it relates more to the thrust produced by the engine core.  In a high bypass ratio turbofan this is only a fraction of the total thrust produced.  However total thrust does also correlate well to EPR so it is still a useful indication of thrust.

 

N1 indicates purely the speed at which the low pressure compressor shaft is rotating, not the movement of airmass nor changes in its speed, so as far as I can see, this argument is invalid. As Kyle's said before, if you were to remove the fan blades, N1 indications would still be valid and accurate, as the shaft would still be rotating, but no air would be moving at all.

Errm, are you going to tell GE they are wrong to use N1 as a thrust indication parameter or will Kyle?  N1 directly relates to the mass flow through the engine, and hence thrust.

Here is a good read on comparing the differences between N1 and EPR.

 

http://theflyingengineer.com/flightdeck/cockpit-design-epr-vs-n1-indication/

 

Nick

 

 

If there were no blades on the shaft it you wouldn't have a turbofan.  The fact is there are fan blades and at a given air density the mass flow through them, and hence the thrust of the engine, is directly proportional to fan rpm.  EPR is the ratio of the fan inlet pressure and the core exhaust pressure.  Thus it relates more to the thrust produced by the engine core.  In a high bypass ratio turbofan this is only a fraction of the total thrust produced.  However total thrust does also correlate well to EPR so it is still a useful indication of thrust.

 

Please don't take my comments out of context.  If you took one minute to dissect what was being said you'd understand why I said it.

N1/2/3 are indications of the rotational speed, which, in all normal cases of an engine, are going to be attached to blades, sure.  Therefore N1/2/3 are all related in some way to thrust.  I was merely abstracting the concept to take a look at the core of the issue: N1/2/3 are indications of RPM.  Anything thereafter is an inference, no matter how accurate, reliable, or valid.

 

Using EPR is simply a different way to judge engine output, and I never said it was better or worse than anything else.

 

I was really just explaining some of the background of what each reading is meant to be showing.  It was meant as an explanation to help someone understand the concepts (which I believe was effective enough for him), not to be turned into some discussion about the semantics of how I conveyed the message.

 

 

 

Errm, are you going to tell GE they are wrong to use N1 as a thrust indication parameter or will Kyle?

 

I really don't recall ever stating that N1 was not a proper method of deriving thrust.  I do recall, however, stating that N1 is a measure of RPM, and that EPR is a measure of differential pressure.  While EPR is more of a direct measurement of the thrust output, N1 can certainly be used as well.

 

My only point in my posts here was to point out what each is actually measuring.  Their comparative validity in addressing the needs of showing engine output was neither addressed, nor intended to be addressed.

 

 

 

 

So, to be absolutely sure everyone is aware who passes by this, here is what I'm intending to convey:

N1/2/3 are rotational speeds, measured as a percentage value of a max rated RPM.

EPR is a pressure ratio, measured as an unbounded scalar number of differential pressure.

Both can be - and clearly are - used as indications of engine output, with neither being more valid than the other.

 

If you're going to go looking for an argument, go pick at someone else's posts.  Nowhere in any of my posts did I say one was more or less valid, or wholly invalid.

I really don't recall ever stating that N1 was not a proper method of deriving thrust.

 

Neither did I.

 

If you're going to go looking for an argument, go pick at someone else's posts.  Nowhere in any of my posts did I say one was more or less valid, or wholly invalid.

 

Okay, maybe I did say that. :Whistle: And maybe I shouldn't have put it that way, but now I know better.

 

Kyle, you're so good with words. It's impossible to catch you in any argument. You always say everything, but just enought not to compromise your point. I don't know if you understand what I mean, but it's like you always find a way out when someone push you against the wall. You've got to teach me how you do that!  :Praying:  

Here is a good read on comparing the differences between N1 and EPR.

 

http://theflyingengineer.com/flightdeck/cockpit-design-epr-vs-n1-indication/

 

Nick

 

Very good read, thank you Nick.

N1 indicates purely the speed at which the low pressure compressor shaft is rotating, not the movement of airmass nor changes in its speed, so as far as I can see, this argument is invalid. As Kyle's said before, if you were to remove the fan blades, N1 indications would still be valid and accurate, as the shaft would still be rotating, but no air would be moving at all.

Eeeeh I think we were all assuming a working engine here.

When all the fanblades are gone you have what is called an engine failure :-)

 

Or you have a turbojet engine (militairy type engines that are more effective at speeds around 1500km/hr) rather than a turbofan engine that is very effective at lower speeds.

 

But even then, the formula still holds!

At 100% N1 the engine delivers its maximum thrust. When all fan blades intact this is for instance 90.000lbs of thrust. When all blades are removed the engine still delivers maximum thrust at 100% N1 but it is not going to be 90.000lbs obviously

 

F=m.a is not an argument.

 

It is one of Newtonn laws, and it hold for engines with fanblades and without fanblades!

I'm just going to throw a spanner in the works here and ask: if both methods are just as good as each other, why do the PW and RR engines display both N1 and EPR, while GE only has N1?

 

This seems a little confusing to me, if RR ans PW use EPR as the main way of measuring how much thrust is being produced(and base de-rates on it as well) why do they feel it is necessary to display N1 as well?

 

Quick edit:

 

 

At 100% N1 the engine delivers its maximum thrust.

IIRC, the N1 can go above 100%, and hence generate more thrust, however it can't be used for extended periods of time.

I really don't recall ever stating that N1 was not a proper method of deriving thrust.  I do recall, however, stating that N1 is a measure of RPM, and that EPR is a measure of differential pressure.  While EPR is more of a direct measurement of the thrust output, N1 can certainly be used as well.

 

My only point in my posts here was to point out what each is actually measuring.  Their comparative validity in addressing the needs of showing engine output was neither addressed, nor intended to be addressed.

Maybe so, but Matheus seems to take your argument about removing blades as showing N1 was only an engine speed and that it was not related to engine airflow (and therefore thrust).  He took that further, but used your argument as a starting point. It was that response that led me to reply to your post. You may not have actually stated N1 wasn't a valid measure of thrust, but that is how it appears to have been taken by at least one person.

 

My second comment, in reply to Matheus about telling GE, included you in a tongue in cheek way. It wasn't meant to be an attack, sorry.

 

And all this in response to Luke's aside about getting used to using EPR instead of N1 for thrust limits.  Was that really worth the lecture you gave in response?

 

So, to be absolutely sure everyone is aware who passes by this, here is what I'm intending to convey:

N1/2/3 are rotational speeds, measured as a percentage value of a max rated RPM.

EPR is a pressure ratio, measured as an unbounded scalar number of differential pressure.

Both can be - and clearly are - used as indications of engine output, with neither being more valid than the other.

If EPR was a differential pressure it would not be a scalar, it would have dimensions of pressure.

 

 

If you're going to go looking for an argument, go pick at someone else's posts.  Nowhere in any of my posts did I say one was more or less valid, or wholly invalid.

I'm not looking for an argument with you, but if I see something incorrect or ambiguous I think it's only right to clarify it. If that happens to be in one of your posts I'll certainly "pick at it".

I'm just going to throw a spanner in the works here and ask: if both methods are just as good as each other, why do the PW and RR engines display both N1 and EPR, while GE only has N1?

 

This seems a little confusing to me, if RR ans PW use EPR as the main way of measuring how much thrust is being produced(and base de-rates on it as well) why do they feel it is necessary to display N1 as well?

EPR is only there as a measure of thrust. N1 is a measure of engine rpm which is also a good indication of thrust. N1 must be displayed, EPR is an optional parameter giving additional information.

 

IIRC, the N1 can go above 100%, and hence generate more thrust, however it can't be used for extended periods of time.

%N1 is relative to the nominal maximum N1 rpm. The actual maximum N1 rpm limit may well be higher than nominal and for unlimited amounts of time. Or it may be less than nominal maximum, it depends on the engine in question. For example the GE CF6-50 has a takeoff thrust rating of approx 112% N1 at ISA sea level. Climb N1 is still well above 100%. The engine can run at that thrust for an unlimited time. Whereas on the CFM56 engine, as used on the A320 and 737, maximum thrust setting is usually below 100% N1.

Too much picking going on here in general!

 

I thought we were trying to explain and understand how N1 and EPR work and how they can be used as a representation of thrust.

Instead of saying "you are all wrong, that cant be, I know it all better" why dont we ask "I dont understand what you are saying....I thought it was like this or that..please explain"?

 

There is nothing strange or confusing about one manufacturer doing things this way and the other one doing it that way.

We could just as well say it is confusing that RR has N1/N2/N3 and GE only N1/N2!

And we could say that one is wrong and one is right.

There is not just one way to Rome is there!?

Or another one, why does Boeing make a fly by wire aircraft with a conventional yoke and Airbus one with a stick?

 

If we start doubting EPR or N1 is a proper indication to use to set Thrust, then I guess we all better get a degree in thermodynamics and physics before we tell them at GE and RR they are doing it all wrong!

 

 

%N1 is relative to the nominal maximum N1 rpm. The actual maximum N1 rpm limit may well be higher than nominal and for unlimited amounts of time. Or it may be less than nominal maximum, it depends on the engine in question. ...

 

Thanks Kevin, thats what I thought. I was just trying to clear up with 777Simmer that Max thrust is not always produced at 100% N1

The GE engines on the 777 have a different indicator on the N1 dial for RPM limit and rated thrust limit.

N1 RPM limit is the redline on the outside of the dial.

Maximum rated thrust (a parameter the engine will not exceed as long as its protection features are functioning) is the amber line on the inside of the N1 dial

 

And it shows that they are not the same,(as cva1077 said)

The amber line is depending on environmental conditions. The RPM limit is not.

Lets see if I can manage to add a picture of it.

 

Cruise flight with GE90s

 

Ha it worked :-)