Gas turbine rpms - sea level & cruise?

[Mechta]

Can anyone tell me for a given throttle setting how the rpm of the different stages in a turbofan vary between sea level (airport height will do) and cruise (say 35,000ft)?

It would seem reasonable that as the air is thinner at altitude that the fan could turn faster. Is this the case, and what about the other stage or stages in the engine?

[HazelNuts39]

It would seem reasonable that as the air is thinner at altitude that the fan could turn faster. In general, that is not true. The rpm's of the different spools are 'tied together' by the thermodynamic relationships between the engine components. In other words, changing density affects all spools in the same way. The parameters changing with altitude are pressure, temperature and flight Mach number. How the spool speeds change with these parameters for given throttle setting is governed by the fuel control system (FCU, EEC or FADEC). Older control systems would maintain constant N2, newer systems might for example maintain constant N1 or constant N1/SQRT(T1).

[Turbine D]

Mechta

HN39's response is correct. On most two spool engines, fan speed (N1) is controlled setting the power (thrust) to achieve desired speed at any altitude. Thrust is more accurately controlled because all the air passes through the fan with only a portion passing into the core. For various reasons, core speed is not an optimum manipulated variable to control fan speed. However, in three spool engines, the control for thrust seems to be N2, or the speed of the IP spool. Either way, since the spools are coupled, the speed of the fan remains within the desired power/thrust settings.

[Mechta]

HazelNuts39 and Turbine D, Thanks for the replies. As I understand it, from what you are saying, the three stages stay at pretty much the same ratio of RPMs to each other, but I'm still not sure whether the RPM of N1 changes significantly with altitude, all other things being equal?

since posting the original question, I've since found this thread: http://www.pprune.org/tech-log/312480-actual-rb211-rpm.html

This suggests that air density makes a difference to rpm, I'm curious by how much though. Does the fan (and the rest of the engine in proportion) not unload and increase rpm when at cruise altitude and speed to a significantly higher level compared to when the aircraft is static and near sea level?

[HazelNuts39]

Mechta;
Sorry, but you have to explain more specifically what question you have. Your OP question was about rpm variation "for a given throttle setting", and Turbine D and I replied to that question. If you are asking about rpm's for the maximum thrust available within a given thrust rating (e.g. Max CLB), that's another story. If you are talking about rpm's for the same thrust (e.g. EPR) at various temperatures at a given altitude, that's yet another story.
regards,
HN39

P.S. For a given thrust, altitude and Mach, rpm's of all spools vary in direct proportion to square root of ambient temperature (absolute, e.g. Kelvin). For given ambient temperature, rpm and Mach, thrust varies proportional to ambient pressure.

[Jane-DoH]

When did fuel-control systems change from mechanical to partially-electronic, to full-electronic?

[Mr.Vortex]

This thread is very interesting.
I'm often having a question for the factor that effect the compressor speed vs
altitude.

Is it possible that :
-the higher it go, the lower temp it is, and hence the more
EGT margin that the engine could archive with out burning the turbine.
[I'm heard some old engine like JT8D use the EGT as the factor to determine
the maximum thrust output]
-The higher we go, the more ram air pressure [for a typical flight], and hence
the compressor can produce more pressure to turn the turbine and hence
higher compressor speed.
- Or the Fuel Control Unit always work to keep the Compressor ratio constant?

Best Regards

[HazelNuts39]

Jane-DoH;

Don't remember. I suggest you go to Wikipedia and search for "FADEC".

regards,
HN39

[HazelNuts39]

I'm heard some old engine like JT8D use the EGT as the factor to determine the maximum thrust outputAll turbine engines have limits on spool speeds, gas temperature, and possibly compressor delivery pressure to protect engine integrity. Gas temperature depends on where it measured: EGT aft of the last turbine stage, and TGT is measured between turbine stages. EGT/TGT and rpm limits together serve to limit turbine entry temperature TET which is too hot to be measured directly. EDIT: The maximum thrust output is defined in the rating structure selected (somewhat arbitrarily) by the engine manufacturer, and must comply with the limits described above.

-The higher we go, the more ram air pressure [for a typical flight], and hence
the compressor can produce more pressure to turn the turbine Ram air pressure ratio is a function of Mach, which may vary with altitude.

Or the Fuel Control Unit always work to keep the Compressor ratio constant?As the name implies, the FCU controls the fuel flow in response to throttle setting and other parameters. AFAIK it doesn't control compressor (pressure) ratio directly.

regards,
HN39

[Escape Path]

-the higher it go, the lower temp it is, and hence the more
EGT margin that the engine could archive with out burning the turbine.

But the higher you go, the more thrust you need to maintain your airspeed thus making the engine hotter or at least not colder, methinks.

How wrong am I? :O

Best regards

Escape Path

[HazelNuts39]

-the higher it go, the lower temp it is, and hence the more EGT margin that the engine could archive with out burning the turbine. That is essentially correct. When operating at the limiting EGT, thrust decreases with increasing ambient temperature and vice versa, at any altitude.

regards,
HN39

P.S. By "EGT margin" I understand you mean the difference between the limiting EGT and that corresponding to the thrust rating. As said earlier, the rating selection is somewhat arbitrary. There is no particular reason for the 'margin' to increase with altitude, on the contrary perhaps.

[minimumunstick]

I am confused. Has someone really answered the OPs post?

I am curious to know the answer to his question, whether the RPM of the fan stays the same or increases for a given power setting as you go up in altitude. I will ask some questions to try to clarify this matter.

Let's say you use N1 (fan rpm) as reference. N1 will be shown as a percentage of the fan maximum RPM.

If I set a certain N1 setting for take off and do not move the thrust levers as I climb, what will the N1 display show (assuming no autothrottle etc.)?

If I am not mistaken it will decrease as we go up in altitude, meaning that the fan RPM will goes down, which means that we have to move the thrust levers forward to keep our maximum fan speed or N1 setting. However, when we reach that same N1 value (if throttle lever movement is sufficient to do so), since we are higher up, the engine will not be able to produce the same amount of thrust due to the thinner air.

So does that not mean that to maintain a certain power setting at higher altitude, RPM will have to go up? And also, if we are able to keep the same amount of thrust from the ground and up, RPM will have to increase?

I am no expert on this so please correct me if I am wrong.

[HazelNuts39]

minimumunstick;

N1 will be shown as a percentage of the fan maximum RPM.Not true. RPM's are displayed as a percentage of some arbitrary reference RPM which, in general, is not the maximum RPM. The maximum RPM varies with the rating, e.g.: TOGA, Max. Continuous, Max Climb, Max Cruise.

If I set a certain N1 setting for take off and do not move the thrust levers as I climb, what will the N1 display show (assuming no autothrottle etc.)?That depends on the display. In most cases it will show actual N1 increasing as the engine spools up, settling at the N1 that corresponds with the thrust lever position (assuming that your engine is N1-controlled).

If I am not mistaken it will decrease as we go up in altitude, meaning that the fan RPM will goes down), which means that we have to move the thrust levers forward to keep our maximum fan speed or N1 setting. That depends on what you have selected and how the engine is controlled (i.e. engine control system design). If you selected CLB and that rating is implemented in the EEC, then the RPM's will be whatever is necessary to provide the rated thrust schedule as you climb. If you selected a throttle setting below max, then some engines will maintain N1 and other engines will maintain N2 as you climb, until you hit some engine limit, and then (typically) the control stays on that limit. If you are climbing with constant N2, then N1 may increase or decrease, depending on the engine's thermodynamic characteristics. For a given engine, the relation between N1 and N2 depends on ambient temperature, not density. EDIT: It may also be affected by change of Mach number as you climb.

the engine will not be able to produce the same amount of thrust due to the thinner air.See previous posts.

regards,
HN39

P.S. Density depends on pressure and temperature. It is usual to treat these parameters separately, since they have distinctly different effects on engine performance, as explained in earlier posts.

[minimumunstick]

Thanks for your post HazelNuts39


If I set a certain N1 setting for take off and do not move the thrust levers as I climb, what will the N1 display show (assuming no autothrottle etc.)?

That depends on the display. In most cases it will show actual N1 increasing as the engine spools up, settling at the N1 that you have set (assuming that your engine is N1-controlled).

I understand what you are saying, and I realize it is correct, however it is not really the answer I am looking for. I think you might have misunderstood my post just a little. The thing is that I am trying to understand the basics of how engine thrust and fan speed is affected by increased density altitude in a pure aerodynamic sense, disregarding any autothrottle or FADEC systems or specific technicalities.

I know some things can't be oversimplified, but yet..

Also I might have worded myself poorly. When I said "set a certain N1 setting" I didn't mean set the N1 as a permanent setting or in a computer or whatever, I meant move the throttle levers so that we have a specific N1 power setting at sea level on take off (again disregarding any FMS etc to control performance on T/O or climb or whatever)

Let me try again to make myself more clear:

We are on the runway and move the thrust levers 3/4 forward. We take off and climb and do not move the thrust levers from their position. Will the N1 that we had on the take-off roll at sea level increase or decrease?

As I said in my previous post, if I understand correctly the N1 percentage displayed will decrease, correct? So for a given throttle lever setting RPM will decrease with altitude, which means that we need to move the throttle levers forward to keep the same N1 setting as we climb.

If we again disregard any FMS or FADEC etc., this will mean that as we climb, engine performance will deteriorate due to higher density altitude / lower density? Does this not always apply? (you wrote see previous posts, but I couldn't really find an answer to this, probably because some of the replies given might have been a little too complicated for me to understand, so if you care to point me in the right direction on this I would appreciate it)


P.S. Density depends on pressure and temperature. It is usual to separate these parameters, since they have distinctly different effects on engine performance, as explained in earlier posts.

This to me is very interesting. When you say this, are you referring to the issue where some engines are EGT limited, and with a lower temperature can increase power output? In that case I understand what you are saying, but just to confirm, is it not still correct to say that as long as density altitude increases, if all other factors stay constant, engine performance will decrease?


Thanks

[John Farley]

If the pilot sets an RPM on any jet engine the function of the fuel control unit is to maintain that RPM regardless of any change in airspeed or altitude UNLESS changes are needed to keep the engine inside any of its operating limits.

[minimumunstick]

If the pilot sets an RPM on any jet engine the function of the fuel control unit is to maintain that RPM regardless of any change in airspeed or altitude UNLESS changes are needed to keep the engine inside any of its operating limits.

So even if I set the RPM (N1 setting) using the throttle levers only and not any FMC or any computer, any fuel control unit will still maintain the N1? Are you sure this applies even in small jets etc.?

[barit1]

So-called PMC's (Power Management Controls) - electronic controls performing fine trim on a basic hydromechanical system - came into the commercial engine market about 1982. The first I recall were on the CF6-80A on 767s and A310s.

FADEC - Full Authority Digital Electronics - were a few years later, mid or late 80s

[HazelNuts39]

the basics of how engine thrust and fan speed is affected by increased density altitude in a pure aerodynamic sense, disregarding any autothrottle or FADEC systems or specific technicalities.
The first turbofan engine I became aquainted with was low bypass-ratio, two-spool, and had a hydromechanical fuel control unit featuring an overspeed governor for N2, an N1-limiter, a P3 limiter and variable-geometry feature in the form of a bleed valve that changed the relation between N1 and N2 in the low speed regime to prevent surge during rapid engine spool-up from idle. As long as you stayed clear of these features, a thrust lever position commanded a fixed N2. The point is that in the type of questions you are posing, you must try to distinguish what the control system does (no engine can do without it) from the "pure aerodynamics" of the turbomachinery of fixed (or variable) geometry. Regarding the latter, the best I can offer is: P.S. For a given thrust, altitude and Mach, rpm's of all spools vary in direct proportion to square root of ambient temperature (absolute, e.g. Kelvin). For given ambient temperature, rpm and Mach, thrust varies proportional to ambient pressure.
Which brings me to: as we climb, engine performance will deteriorate due to higher density altitude / lower density? That may be true, but it's not how I would describe it. What you describe is a matter of scale. Performance people reduce the number of variables by thinking in terms of 'non-dimensional' or 'referred' engine parameters, which are basically ratio's independent of 'scale'. For example: Exhaust pressure ratio EPR, or the ratio of thrust to ambient pressure, or the ratio of spool speed to the speed of molecules or the speed of sound (air temperature, sorry if I digress too far here). In those terms, there is no 'deterioration': at high altitude the engine processes a small mass of low-density air with the same efficiency as it does a larger mass at low altitude (at same OAT, Mach and EPR).

Perhaps I should stop here and await further questions, rather than repeating what I wrote earlier.

regards,
HN39

[GarageYears]

Not sure I understand or want to understand some of the replies in this thread, but I have absolute data for the A320 in front of me, and the reality is that for a given N1 RPM% value the thrust produced by the engine will change with altitude....

See here:

N1% = 78%, FL50, thrust = 29387N, CAS = 259kts
N1% = 78.2% FL370, thrust = 14130N, CAS = 260kts/M0.802

Therefore, it is my understanding that the FADEC (or whatever control system is in place) adjusts the fuel delivery to maintain the demanded RPM, with thrust changing as a function of altitude. This makes sense since at lower altitude the air is 'thicker' and hence you need more push to achieve a given forward speed, with the air density decreasing the higher you get, hence you need less push...

The point is that 78% N1 RPM is always the same irrespective of altitude (i.e. the blade passing frequency will be maintained), what varies is the fuel burn needed to maintain this RPM, as a consequence the thrust delivered by the engine.

- GY

[lomapaseo]

Garageyears

The point is that 78% N1 RPM is always the same irrespective of altitude

Best you define percent of what??

and just what is or how is 100% defined.

Unless the definitions are defined it might turn out that even the 100% number varies with altitude.

[GarageYears]

Best you define percent of what??

and just what is or how is 100% defined.

Unless the definitions are defined it might turn out that even the 100% number varies with altitude. The percent RPM is design value specified by the engine manufacturer. Let's consider this - the fan stage for example has a mechanical limitation on the maximum RPM it can withstand - that is NOT affected by altitude/aircraft speed/moon-phase... it is a function of physical strength. Beyond some RPM the thing will come unhinged and pieces fly all over the place. Now that of course is not to say 100%N1 is the fail speed, but it is related.

My specialism is in aircraft sound simulation - therefore I am very aware of the audio signature of engines, which not surprisingly is mostly related to RPM (engine whine tones), and I can assure you an engine turning at any given N1% RPM will turn at the same RPM irrespective of altitude. At least on all the modern era aircraft I have worked on (737NG/A320/lots of military jets...).

- GY :8

[lomapaseo]

Garageyears

The percent RPM is design value specified by the engine manufacturer. Let's consider this - the fan stage for example has a mechanical limitation on the maximum RPM it can withstand - that is NOT affected by altitude/aircraft speed/moon-phase... it is a function of physical strength. Beyond some RPM the thing will come unhinged and pieces fly all over the place. Now that of course is not to say 100%N1 is the fail speed, but it is related.



Well the percent RPM may be a design value specified by the engine manufacturer but it is not directly related to "redline speed" which is directly related to physical strength. Thus the same mechanical design used in multiple engine models may have different 100 % RPMs on their data sheets

Maybe we can get barit1 to cite some GE CF6 examples like how do you justify operating at 105-110% N1 on takeoff etc.

[HazelNuts39]

lomapaseo;

These are fixed numbers for a particular engine type and model, different per spool, for example (see link posted in Mechta's post#4):

RB211-535E4 on 757

100% N1 = 4500 LP RPM
100% N2 = 7000 IP RPM
100% N3 = 10611 HP RPM

But, as I wrote above, these numbers should not be confused with the 'red line' limits, which are also fixed but generally not 100%, and differ per rating.

regards,
HN39

[barit1]

lomapaseo digs me in the ribs:
Maybe we can get barit1 to cite some GE CF6 examples like how do you justify operating at 105-110% N1 on takeoff

Well - there's some history here. The CF6-50 fan is physically the same as the CF6-6, and GE in their wisdom did not change the arbitrary 100% N1 gage calibration point. Thus the fan spins faster to create more airflow => more thrust.

Also note that the CF6 (like other airline engines) is flat-rated up to a "corner point" OAT. This means that on cold days, the fan doesn't turn as fast, because the cold dense air doesn't need as much pumping to create the same thrust. Conversely on a hot day - up to that corner point OAT - the fan must spin faster. (Beyond that corner point temp - typically ISA+15 - fan speed cuts back to provide EGT protection). So at that sea level corner point OAT, fan speed reaches a peak around 110% IIRC.

NOW THEN - let's do a takeoff at DEN or MEX or a similar field elevation. At a given N1, less thrust is available. The airline doesn't want to take a huge payload penalty in the less dense air, but the engine manufacturer knows that maintaining thrust takes more N1. So some compromise is reached, the engine gets run a bit faster, but probably not all the airline would like. IIRC the CF6-50 mught run up the 118% N1 in this case. :uhoh:

[John Farley]

In a word yes. The size or type of the jet engine is not a factor in this statement.

I trust there is no hint of ambiguity in what I wrote.

One of the engine limits most likely to cause a reduction in RPM with increasing altitude is the engine RPM divided by the square root of the absolute temperature or N/ √ Θ as it is known in the trade. This is more likely to happen at very high throttle settings which if not reduced could cause the engine to approach the steady running surge boundary.

If you are looking for a simple intuitive explanation of why this (and other aspects of jet engines) should be so then I am afraid there is not one. The only way to understand what is behind the paragraph above for example (which is the PRL or pressure ratio limiter function of the engine fuel system) is to have a basic understanding of the aerodynamics of the compressor components.

I am not trying to blind you with science, the basics behind many engine issues are not difficult but they are complex in their interaction. People spend years studying the stuff.

[HazelNuts39]

GarageYears;

Please note that between the two conditions you gave in your post #19, thrust changes because atmospheric pressure and temperature, and Mach are different. They don't show that the FADEC maintains constant N1 in climb, do they?

EDIT:: Because of the difference in SAT and Mach the thrust values are not directly comparable. If you want to verify the 'aerodynamic similarity' rules I gave, then you must find two conditions with the same Mach, and either the same SAT, or with two RPM's that give the same N/ √ Θ (RPM corrected for temperature).

regards,
HN39

[barit1]

BTW: There is a gross overspeed criterion that must be demonstrated during certification testing. The case I'm familiar with (and the criteria may have changed over the decades) used the FAR propeller overspeed demo of 141% of the max normal operating RPM.

To get the fan to turn that fast, the fan nozzle will be butchered so it doesn't create much back pressure; thus the normal N1:N2 relationship is highly distorted. :eek:

[minimumunstick]

John Farley

Thanks, I guess you are right.

Anyway reading the replies in this thread helps me grasp it better. I have also just re-opened my ATPL books and relearning stuff there which helps too.

[HazelNuts39]

This sketch (https://docs.google.com/leaf?id=0B0CqniOzW0rjNDg3ZGEyMDktMDBiMS00Y2E0LTg5YjgtOGU1NTY 1OTJjNmQy&hl=en_GB&authkey=CPGRmb8B) compares schematically the thrust rating schedule to the engine never-exceed limits for N1, N2, EGT and P3. The full line shows the max. thrust setting recommended by the manufacturer that should not be exceeded intentionally and is normally implemented in the EEC of modern engines. It is selected so that it does not exceed any of the physical limits of the engine that are shown by the the dashed lines. If the rating is ignored and the engine is run 'on the limit', it would be P3-limited at ambient temperature up to 'A', N1-limited between temperatures 'A' and 'B', N2-limited between 'B' and 'C', and EGT-limited above temperature 'C'.

regards,
HN39

[Jane-DoH]

John Farley,

One of the engine limits most likely to cause a reduction in RPM with increasing altitude is the engine RPM divided by the square root of the absolute temperature or N/ √ Θ as it is known in the trade. This is more likely to happen at very high throttle settings which if not reduced could cause the engine to approach the steady running surge boundary.

And I guess this has to do with the mach number changing with temperature?

[barit1]

No, it's air density changing with temperature. Since a fan or compressor tends to be a constant-volume pump at a given speed, it implies that as temperature changes at e.g. 100% rpm, the mass pumped is larger at low temperature, and lesser at high temperature.

By trimming rpm according to N/ √ Θ we hold mass airflow constant.

[HazelNuts39]

In my view it's not air density either. The surge boundary occurs at a certain N/ √Θ for any air density. I think the N/ √ Θ of a compressor is somewhat analogous to the AoA of an airfoil. Just as an airfoil stalls when driven to a too high AoA, a compressor stalls when driven to a too high N/ √ Θ. Isn't 'surge' the stalling of compressor airfoils?

As John Farley writes, high N/ √ Θ is associated with high throttle settings. It is also associated with low ambient temperature, because at higher temperature N1, N2 and EGT limits will prevail, as illustrated in my sketch (at a given altitude and Mach, constant thrust is constant N/ √ Θ, so the 'P3' limit shown in the sketch is almost constant N/ √ Θ).

I agree with barit1 that mach number has little to do with it.

regards,
HN39

[barit1]

N/ √ Θ isn't greatly involved with stall margin on a single-stage fan, which after all is pumping into a fixed-area fan exit nozzle at a moderate pressure ratio.

But N/ √ Θ has a great deal to do with stall margin in a multi-stage axial compressor. If you design the compressor so each stage has about the same alpha at design rpm, then each stage is sharing the pumping equally and all is lovely.

The problem occurs at off-design rpm - starting, idle, and especially acceleration. Here the front end is trying to pump more than the rear stages can handle, and there's a large alpha mismatch (too high in front, too low or negative in back). The problem is aggravated with HIGH inlet temperature (LOW N/ √ Θ). Thus the front stages are stall-prone.

Historically three remedies are available:
1) Bleed off the excess air (but quite inefficient - wasted pumping energy)
2) Multiple independent compressor spools which can change speed to re-match pumping.
3) Variable stator vanes to directly control alpha.

Each system had its advocates in history, but now all OEMs use all these devices.

[Turbine D]

HazelNuts39
In my view it's not air density either.

After giving this some thought, I think barit1 is correct. For example, in the past, some jet engines used water injection to gain added thrust:

The maximum power a turbine engine can output depends largely upon the density or weight of the flow of the gases through the engine. Therefore, when the atmospheric pressure decreases or ambient air temperature increases, there is a loss in thrust. The power output can be boosted or restored by cooling the airflow with water or coolant.

Turbine D

[HazelNuts39]

Turbine D;

Yes, at a given RPM, but ... I thought we were discussing surge at this point.

Prior to that, we were discussing what causes RPM to change at constant throttle setting (essentially the OP).

regards,
HN39

[DERG]

"People spend years studying the stuff."

You know when you think of all the variables it becomes mind numbing. In some of the supersonic jets the guys must have a hell of a faith to push that lever forward.

Imagine the stress say at 5k feet on the turbine blades when you ask it to GO.

If anyone has a link how a supersonic jet fighter performs over M1 I would be grateful.

[barit1]

If anyone has a link how a supersonic jet fighter performs over M1 I would be grateful.

In simple terms - the gas turbine becomes less and less important, the afterburner (reheat) takes over. However - with improved aerodynamics, so-called "supercruise" (> m1.0 sans burner) is practical.

[HazelNuts39]

Returning to OP: - In my graph I sketched the limiting parameters quite arbitrarily. It so happens that the way they are drawn implies that when decreasing OAT at constant N2 (as occurs in climb), N1 increases, and EGT decreases.

Any ideas if that is generally so?

regards,
HN39

P.S. In any case, the changes of N1 and EGT are strictly due to the change of ambient temperature, whereas ambient pressure has nothing to with it. Therefore it would be wrong to attribute the changes to 'density' effects.

[John Farley]

In answer to your question - no.

If it was like that I would have said so.

Please read all of my post.