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

N1 100% is a nominal RPM given by the manufacturer based upon structural limits and engine wear.

 

N1 100% may be exceeded for specified periods but specific exceedences must be recorded for engineering inspection. Very rarely do big Gas Turbines operate at their max rated thrust! Gas turbines perform far better at high altitude than at sea level but the fan doesn't. N1 100% is a figure given by the manufacturer which, I would assume not being an engineer, gives the best performance to fuel burn for a specified averaged cruising block of altitudes.

 

For example, the CFM 56 normal operation is:

100% N1= 5175 RPM

 

for exceedences:

 

N1 : 106.1-109.0 --- Normal operation to next landing

N1 : Over 109.0 --- Precautionary shut down

 

So, I would dare to suggest that max continuous operations would be governed to approx 100% N1 for the sake of safety margin, performance and wear. In the event of engine malfunctions and the requirement to run on a single donk then the margins may be reduced and the N1 increased.

 

So, max thrust is generally (dependant on the manufacturer and type of engine) not attainable at 100% N1 there is a safety margin built in.

 

 

For example, the CFM 56 normal operation is:
100% N1= 5175 RPM

for exceedences:

N1 : 106.1-109.0 --- Normal operation to next landing
N1 : Over 109.0 --- Precautionary shut down

 

Thanks for the clarification Greg. This means that the max thrust on the CFM56 is indeed greater than 100% N1, however it is not normally exceeded on regular operations.

 

 

 

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

 

Well, it is quite obvious that the max N1 limit is greater than 100% N1, as per your picture, so I think that clears it up nicely.

N1 100% is a nominal RPM given by the manufacturer based upon structural limits and engine wear.

 

N1 100% may be exceeded for specified periods but specific exceedences must be recorded for engineering inspection. Very rarely do big Gas Turbines operate at their max rated thrust!

Greg,

 

You've totally ignored my example of the CF6-50 (and CF6-80 also) where the engine does operate at well above 100% N1 continuously during takeoff and climb to altitude.  I'm talking real world engines here, not FSX, by the way.  It is as you say a nominal maximum RPM, possibly set at an early stage of design.  As an engine grows in thrust as it is developed due to materials advances, better blade cooling, etc maximum N1 will increase yet the nominal max RPM remains the same.

 

100% N1 is not the red line limit, or even the amber limit, on every engine type.  It's very misleading to say 100% N1 is always a limit you can only break for a limited time.

It's very misleading to say 100% N1 is always a limit you can only break for a limited time.

Hmm, I seem to remember saying:

 

specific exceedences must be recorded for engineering inspection

Followed by the reported N1% that the CFM56 requires.

 

100% N1 is a baseline, nothing more. I fail to see where I made it a limit? Specific exceedences include manufacturers and operates requirements to operate the engine within a specific governed range.

 

The GE 75B for example has a 'Denver bump' profile which allows extra thrust for departure. The CFM powered Airbus A320 also had this feature. Two buttons behind the thrust levers.

 

EPR will give you the specific thrust of the engine based on mass flow through the FAN. N1 is the rotation speed of the fan. In an 'ideal' world where the fan produces perfect thrust then N1 will equate to thrust, the world isn't perfect and fans aerodynamically degrade over time. EPR will always give you the actual thrust irrespective of the fan wear or other age related deficiencies, N1 will not.

 

Real world.

 

 

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.

 

That's a fair assessment.  Re-reading the post, I can see how he may have taken it.

 

 

 

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?

 

It certainly wasn't a lecture.  If it were, I doubt I would've gotten a 'thanks' for it, and would've gotten the response that has become rather commonplace for me.  I could've just lumped it in with the PMs that he and I have been exchanging, but the post had potential to educate others as well, so I just lumped it in with this discussion.

 

 

 

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

 

Perhaps we're not on the same page.  Last time I checked, pressure was a scalar quantity.

 

 

 

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".

 

As that line was below the "to be absolutely sure everyone is aware," I'd assumed that it would be understood that it wasn't aimed at anyone in particular.

Hmm, I seem to remember saying:

 

 

Followed by the reported N1% that the CFM56 requires.

 

100% N1 is a baseline, nothing more. I fail to see where I made it a limit? Specific exceedences include manufacturers and operates requirements to operate the engine within a specific governed range.

 

The GE 75B for example has a 'Denver bump' profile which allows extra thrust for departure. The CFM powered Airbus A320 also had this feature. Two buttons behind the thrust levers.

 

EPR will give you the specific thrust of the engine based on mass flow through the FAN. N1 is the rotation speed of the fan. In an 'ideal' world where the fan produces perfect thrust then N1 will equate to thrust, the world isn't perfect and fans aerodynamically degrade over time. EPR will always give you the actual thrust irrespective of the fan wear or other age related deficiencies, N1 will not.

 

Real world.

Please note, I specifically stated I was talking about real world engines too.

 

You suggested that in normal operation engines are governed to 100% N1.  This simply is not true.  With the CF6 engine you regularly exceed 100% N1 in normal operation while being well within reportable exceedance limits.  You simply can't generalise and say 100% is always the maximum as  the limits vary from engine to engine.

 

For example

 

CF6-50E2

The N1 limit varies between 116.5% and 118.5% depending on the service bulletin implemented

The N2 limit similarly is between 108% and 109.5%

 

JT9D-7Q

N1 limit is 108%

N2 limit is 102.5%

 

JT9D-7J

N1 limit is 104.2%

N2 limit is 102.5%

 

Timed engine limits in all these cases are related to EGT, not RPM.

 

EPR is the ratio of core exhaust pressure to fan inlet pressure.  This doesn't tell you anything about fan mass flow or thrust.  EPR is much more directly related to core massflow and therefore thrust.  It is of course a good indication of engine net thrust however.  Yes, fans degrade over time, turbines much more so (which won't affect the relationship between N1 and thrust).  But not so much as to put aircraft performance at risk.  If it did GE could not use N1 for rating purposes.  The N1 indication in a GE engine is not there to tell you what your thrust is, it's the parameter you set to give you the rated thrust for a given flight condition.  As such it correlates closely to the thrust produced by an average engine, not a brand new one nor a severely degraded engine.  EPR may well be slightly better but it's also less reliable as it requires twice as many sensors, half of which are in the hot section of the engine.

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

 

Kyle said differential pressure ratio, which is dimensionless.

It certainly wasn't a lecture.  If it were, I doubt I would've gotten a 'thanks' for it, and would've gotten the response that has become rather commonplace for me.  I could've just lumped it in with the PMs that he and I have been exchanging, but the post had potential to educate others as well, so I just lumped it in with this discussion.

As you say yourself, the post had potential to educate others. So you were intending to teach the forum something. So "lecture" is correct.

 

Perhaps we're not on the same page.  Last time I checked, pressure was a scalar quantity.

My mistake, your use of the term scalar threw me. I'm not even sure why you used it here. EPR being scalar isn't really relevant to the debate, is it? Yes pressure is scalar. I was thinking of dimensionless quantities, such as EPR, and dimensioned quantities, such as engine pressure difference, and forgot my physics education momentarily.

 

As that line was below the "to be absolutely sure everyone is aware," I'd assumed that it would be understood that it wasn't aimed at anyone in particular.

As I was the only person you were accusing of taking your post the wrong way, and you were quoting my post, I took it as aimed at me.  The preceding paragraph didn't seem to relate to it.

Kyle said differential pressure ratio, which is dimensionless.

No, he said EPR was measured by engine pressure differential, which has units of pressure.  Engine pressure ratio is of course dimensionless, but it is not a differential pressure ratio.

 

EPD = P_exhaust - P_inlet

 

EPR = P_exhaust / P_inlet

Huh?

 

Here I thought my little formula would be too much to post here, lol

 

For all practical purposes I would like to give a resume of some of the limits on the GE90:

N1 rpm limit = 109%

N2 rpm limit = 117%

A purely structural (rotational) limit as far as I know.

The protection features on the 777 will try to prevent an N1 or N2 overspeed condition by reducing fuel flow automatically if required.

 

There was talk about a limit that can be exceeded for a coulple of minutes during T.O.

That limit is actually the maximum continuous thrust EGT limit!

Its an EGT limit, not an RPM limit.

 

For the engine I am talking about here, maximum contionous thrust EGT limit is 1015C.

So that means you can set thrust and as long as EGT does not reach 1015C you can use that thrust setting all day long.

EGT should not go beyond 1015C, if it does you get an amber indication (the digits change from white to amber) which mean limit is exceeded, no good, perform checklist which will say retard throttle.

The exception is that during T.O. and Go Around this limit may be exceeded for up to 5 minutes (10 minutes in an engine failure case) up to 1030C.

During T.O. and G.A. it is ok if the EGT goes into the amber band (1015 - 1030C) and the systems are smart and know this. Which is why the EGT digits do not turn amber during those 5 min (or 10). But they will after those 5 (10) minutes.

At 1030C the EGT digits turn red in all cases and you are back in the checklist.

 

And finally the amber maximum N1 line on the inside of the N1 dial.

Although this line is called Maximum N1 it has nothing to do with the RPM limit (the red line on the outside of the dial).

The Maximum N1 amber line is that RPM at which the engine will produce its maximum rated (90.000lbs) thrust. This line is not at a fixed position on the N1 dial.

It moves as ambient conditions change and thus maximum rated thrust is reach at a different RPM.

Again the systems are smart and try to prevent a thrust limit exceedance (also called overboost).

 

Even when setting thrust manually these protection features work and will try to prevent an N1/N2 RPM limit or thrust overboost.

 

So, you can not set an N1 RPM at which you produce more than rated thrust (on this engine on this aircraft. I dont know about other aircraft/engines !)

The protection features will not allow it.

You might be able to set 105% N1 but you will still be at or below maximum rated thrust (90.000lbs)

If you turn those protection features off (EEC switches to alternate mode) then yes, then you can overboost the engine (to prevent for instance a ground collision).

 

As you can see one must be very carefull with wording like "maximum thrust" and "limit" and "100% N1 is the limit", etc because one needs to define if we are talking about maximum thrust for RPM limit or EGT limit or Thrust (in lbs) limit purposes!

I was not carefull enough either before (when I said that 100% N1 is maximum thrust) but I did not realise at that point we were going to be so exact with wording.

I will try to be more carefull in the future.

I guess I'm a victim of 'keep it simple'. The thrust of the engine is primarily derived from the fan. Therefore to get the thrust you have to move mass through it.

 

I never stated that EPR was derived from it? I did stated that it was based upon it. The primary job of the core is to turn the fan. Many years ago, in large helicopters, the engines performance was calculated using power performance index figures based upon the power turbine inlet temperature and the Ng of the gas generator. The difference between these two gave you the overall performance of the engine based upon generator wear and convergent duct erosion to the power turbines. A very similar thing to EPR measuring the pressure across the core before driving the fan.

 

If I set a specific EPR then I can be sure, irrespective of any wear in the engine, that the speed I am driving the fan is constant. I cannot guarantee that with N1, a constant EPR on a new engine will probably deliver a lower N1 than on an older engine. Such is the nature of these thing. My personal opinion is that if I have to go to a pitch power scenario (unreliable airspeed) then EPR is a more stable baseline for me to work with.

 

There are many ways to skin a cat and many of my colleagues prefer to use N1. Horses for courses.

 

We could 'google' and 'wikipedia' this all day but, to be honest, it's totally irrelevant and very pedantic.

We could 'google' and 'wikipedia' this all day but, to be honest, it's totally irrelevant and very pedantic.

So who did you think was doing that? I certainly wasn't and hinting people were isn't helpful. You say you were trying to keep things simple. That's all very well as long as you are accurate too. I think people in this forum are well enough informed to be able to understand detail.

And here we go again with the picking and bickering.

Oh boy - I'm sorry I asked haha. Maybe I'll PM Greg next time...

The simple fact is that I use these aircraft every week and I'm not in the slightest bit fussed if I have a GE or a RR. As I'm not an engineer I am not particularly bothered exactly how specific sensors, knobs, wheels and widgets work in the engine either. I know how to use them. I have enough to do with managing the flight, passengers, destination and ATC not to get bogged down in trivia hence I look for a simplistic approach.

 

Many years ago, in the late 80's, I flew a display at Fairford. Afterwards we had to stand on the line and answer questions. I had one gentleman who, honestly, knew far more about my aircraft, where and when it was build, the engines, lift, thrust, weapons etc. etc. etc. What he didn't know was how to fly it and fight it. :-D

 

Please feel free to dig all you want, I don't want to get involved in a nit picking discussion from a thread that was posted for a couple of picces.

 

Enough said.

I agree with you Greg,

 

At first I thought N1 vs EPR was an interesting discussion.

Quite deep cause like you said, you dont need that info to operate the engines properly but still, interesting.

 

But in the end it became one of those typical for AVSIM "I know better and you are wrong" discussions :-(

 

Which is why I tried (but failed) to get the discussion back to more practical knowledge with my post on the GE90 limits.