Altitude and engine efficiency
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Altitude and engine efficiency
I’ve done a search for the information I’m looking for, read many fascinating postings (as well as some ill-informed and bad-tempered ones), but still can’t find the information I’m looking for.
As a non-pilot, I believe I am in the right forum to ask a question of this nature:
Why exactly do turbofan and indeed turboprop engines burn less fuel at higher altitudes?
Intuition (which can clearly be wrong), suggests to me at least that the less dense the air, the less efficient the engine. If this is not the case, I’m sure I’ll be told...
Or is the air in fact more dense, due to lower temperatures? This is one of several suggestions made by pilots and engineers whom I have asked over the years - but most have given contradictory answers.
I’m always grateful to those who take the time to make an informed comment, so thanks in advance...
Shamrock 602
As a non-pilot, I believe I am in the right forum to ask a question of this nature:
Why exactly do turbofan and indeed turboprop engines burn less fuel at higher altitudes?
Intuition (which can clearly be wrong), suggests to me at least that the less dense the air, the less efficient the engine. If this is not the case, I’m sure I’ll be told...
Or is the air in fact more dense, due to lower temperatures? This is one of several suggestions made by pilots and engineers whom I have asked over the years - but most have given contradictory answers.
I’m always grateful to those who take the time to make an informed comment, so thanks in advance...
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It's far more technical then this and I tip my hat to members here more knowledgeable then myself.
It is actually a function of operating temperature of the jet engine core . All jet engines work best between a small temperature range (which remains pretty constant whatever the altitude). As you go higher and the air gets less dense it also gets colder therefore the the EGT (exhaust gas temp) remains much the same but the N1 (fan speed goes up) which inproves the operating efficency of the engine. CosmosSchwartz answear also plays it's part too.
Well that was how it was explained to me anyways.
It is actually a function of operating temperature of the jet engine core . All jet engines work best between a small temperature range (which remains pretty constant whatever the altitude). As you go higher and the air gets less dense it also gets colder therefore the the EGT (exhaust gas temp) remains much the same but the N1 (fan speed goes up) which inproves the operating efficency of the engine. CosmosSchwartz answear also plays it's part too.
Well that was how it was explained to me anyways.
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Think I recall someone telling me in a class that if there is a greater difference in temperature between the core and the outside air(ie, at altitude), the greater the pressure difference (remembering the relationship between temperature and pressure) thus thermal efficiency overcomes the lack of volumetric (air) efficiency.
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Why does a turbine engine burn less fuel at altitude?
Because it produces less thrust. In a jet engine, thrust is proportional to fuel flow.
Why are turbine engines more efficient at altitude?
THEY'RE NOT! Efficiency, when measured as the TSFC (ratio of fuel burned to thrust produced), goes down as altitude increases (TSFC goes up).
This means that at altitude you are burning MORE fuel to create the same amount of thrust.
In other words, engines are LESS efficient at altitude. Engines would LOVE to stay at sea level all the time.
NOTE: For those who look at the cold temperatures aloft, remember this: what you need to look at is not just OAT, but Potential Temperature. At altitude, potential temperature is always higher than sea level (we'll ignore the ABL on extreme hot days). As you compress the air, it warms up. The air at 30,000' actually has more energy than sea level air, despite being pretty cold. Thats why its up there... because it has more energy.
Thats why any time to take bleed air for the packs, you need to cool it before feeding it to the cabin.
So why do we bother flying at 30,000ft if the engines are less efficient?
Beacuse the air is thinner, creating less drag, reducing the amount of thrust we need to produce to push the airplane.
So the aircraft, as a whole system, operates more efficiently at altitude. But the engines are more efficient at sea level.
Regards,
palgia
Because it produces less thrust. In a jet engine, thrust is proportional to fuel flow.
Why are turbine engines more efficient at altitude?
THEY'RE NOT! Efficiency, when measured as the TSFC (ratio of fuel burned to thrust produced), goes down as altitude increases (TSFC goes up).
This means that at altitude you are burning MORE fuel to create the same amount of thrust.
In other words, engines are LESS efficient at altitude. Engines would LOVE to stay at sea level all the time.
NOTE: For those who look at the cold temperatures aloft, remember this: what you need to look at is not just OAT, but Potential Temperature. At altitude, potential temperature is always higher than sea level (we'll ignore the ABL on extreme hot days). As you compress the air, it warms up. The air at 30,000' actually has more energy than sea level air, despite being pretty cold. Thats why its up there... because it has more energy.
Thats why any time to take bleed air for the packs, you need to cool it before feeding it to the cabin.
So why do we bother flying at 30,000ft if the engines are less efficient?
Beacuse the air is thinner, creating less drag, reducing the amount of thrust we need to produce to push the airplane.
So the aircraft, as a whole system, operates more efficiently at altitude. But the engines are more efficient at sea level.
Regards,
palgia
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Jet engines run at max efficency at around 90-95% N1 at low level with dense air they would produce so much thrust that they would cause the aircraft to fly at greater than VMO (ie not allowed!). Thus the engines have to be throttled back and are less efficient and thus have a higher SFC.
At high level with the less dense air the engines are able to operate at their design N1 without exceeding VMo or MMo, they thus run more efficently and thus have a lower SFC.
As for less air 'resistance' we operate IAS (up to approx 26000ft them fly Mach no) ie we keep the difference between the static pressure and the dynamic pressure the same for the same IAS thus the 'resistance' is the same.The TAS at altitude is very high compared with the IAS and thus mile for mile the aircraft uses less fuel. (assuming still air)
Remember that as we climb we keep the same IAS, the TAS and the Mach No increase. When we reach our cruise mach no we then fly mach no and as we climb the IAS then decreases.
The decrease in density due to increase in altitude by far outweights the would be increase in density due to lower temperatures ie density decreases with altitude.
As for having to cool the bleed air at altitude because it has mor energy WTF?? It is cooled because it is taken from (in the B737) the 5th stage Low pressure compressors bleed and the 9th stage high pressure compressor bleed ie the air has just been compressed and thus heated. Thats why its HOT and needs to be cooled! Exactly the same at low altitude.
Probably oversimplified but K.I.S.S
At high level with the less dense air the engines are able to operate at their design N1 without exceeding VMo or MMo, they thus run more efficently and thus have a lower SFC.
As for less air 'resistance' we operate IAS (up to approx 26000ft them fly Mach no) ie we keep the difference between the static pressure and the dynamic pressure the same for the same IAS thus the 'resistance' is the same.The TAS at altitude is very high compared with the IAS and thus mile for mile the aircraft uses less fuel. (assuming still air)
Remember that as we climb we keep the same IAS, the TAS and the Mach No increase. When we reach our cruise mach no we then fly mach no and as we climb the IAS then decreases.
The decrease in density due to increase in altitude by far outweights the would be increase in density due to lower temperatures ie density decreases with altitude.
As for having to cool the bleed air at altitude because it has mor energy WTF?? It is cooled because it is taken from (in the B737) the 5th stage Low pressure compressors bleed and the 9th stage high pressure compressor bleed ie the air has just been compressed and thus heated. Thats why its HOT and needs to be cooled! Exactly the same at low altitude.
Probably oversimplified but K.I.S.S
Last edited by ifleeplanes; 15th Mar 2005 at 19:53.
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I think you will find that due to the need to keep passengers alive, the air is bled from the compressor, prior to any combustion. In fact, there are very stringent rules applied to bleed air cleanliness by the certification authorities and I know from experience that any leakage of oil through seals and into the bleed system (normally giving "smoke" odors) is definately frowned upon by Joe Operator and his SLF
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As for having to cool the bleed air at altitude because it has mor energy WTF?? It is cooled because it is taken from (in the B737) the 5th stage Low pressure compressors bleed and the 9th stage high pressure compressor bleed ie the air has just been compressed and thus heated. Thats why its HOT and needs to be cooled! Exactly the same at low altitude..Probably oversimplified but K.I.S.S
Before bashing on my statement that air at altitude has more energy than sea level air, why don't you do your homework?
A quick review of thermodynamics might help... (some elementary-school spelling review wouldn't hurt either
)Please re-read Shamrock's last sentence....
I repeat: air at altitude has a greater potential temperature than air at sea level. This means that when it is adiabatically compressed to a higher pressure, it will be warmer than pre-existing air at that pressure level. For this reason, when you compress the outside air to a pressure required for cabin pressurization, the temperature will be excessively high and will require cooling.
If the air at altitude had a lower potential temperature than sea-level air, then even AFTER compressing the air, it would still require additional heating.
Reagrds,
palgia
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Geez are we going along the old 'Your spelling is rubbish route' I hope (2 mistakes and a typo big deal, we will ignore your typo
). The air is cooled because it is heated because its been HIGHLY compressed. We are not removing any of the potential energy the air at altitude holds because we are leaving at altitude. There is no net gain due to the change of potential (ie altitude) the net gain is due to the compression.Thats the basic level of thermodynamics we need to understand.
We shall ignore the rest of your basic aerodynamic errors then shall we..
As for not having INFORMED comment....if you read my post it is highly informed! I guess an ATPL and 8000hours jet plus 3000 turboprop counts for little informed comment.
). The air is cooled because it is heated because its been HIGHLY compressed. We are not removing any of the potential energy the air at altitude holds because we are leaving at altitude. There is no net gain due to the change of potential (ie altitude) the net gain is due to the compression.Thats the basic level of thermodynamics we need to understand. We shall ignore the rest of your basic aerodynamic errors then shall we..
As for not having INFORMED comment....if you read my post it is highly informed! I guess an ATPL and 8000hours jet plus 3000 turboprop counts for little informed comment.
Last edited by ifleeplanes; 16th Mar 2005 at 08:24.
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I recall from years back something to do with the gas laws; Charles Law and Boyles law. (compression and volumes of gases and the effects of density and temperature; altitude) Search for those on the internet and I think you'll go a long way to finding your answer.
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Hope this makes sense!!!
Take a piece of graph paper and draw two graph axes on it. Label the vertical Pressure and the Horizontal Temperature.
Now I am going to keep this SIMPLE: -
IGNORE temperature increase due to pressure.
Put a point on the graph which would indicate the state just before it enters the engine. Assuming standard atmosphere this would be around 1013.2 millibars, and 21 Celsius…
The first thing that happens is the air is compressed; this increases the pressure and so would generate a vertical line on the graph stopping at the appropriate value of pressure.
The next stage is the fuel insertion; it burns and increases the temperature. This places a horizontal line on our graph, moving away from the TOP of the vertical line.
This completes the engines part. The next bit happens anyway, and should be dotted in. It is an open system, but can technically be thought of as a closed system; as the air goes into the atmosphere and can get sucked back into the engine.
There are two things that happen to the air when it leaves the engine. It expands and cools.
It will expand, and another vertical line will move down from the top horizontal line. Will bring our pressure back to ambient, and then a final horizontal line to close the box that should be on the graph paper.
Now… the techie bit…
The engine will operate at the same internal pressure and temperature. Therefore the top horizontal line is fixed. The distance between the to horizontal line and the bottom horizontal line is an indication of how much energy is being released from the fuel. The greater the distance the more energy; thus the more efficient it operates.
As the altitude increases the ambient pressure drops, but the top temperature remains the same, therefore the distance between the top and bottom horizontal line grows with height, and thus the efficiency of the engine.
It is true the engine produces less real thrust (measured by a force metre) at altitude than at sea level. But the required thrust reduces too, less dense air means less air resistance. However there is an equivalent required value of thrust at altitude. Say to move an aircraft at sea level requires 1000kgf, and ignoring air-resistance you still need 1000kgf at 30,000ft, due to the changes in pressure you will be able to reduce the throttle by 20% because the energy is removed from the fuel more efficiently. In reality, because the thrust required at height reduces you can often throttle back even further.
Take a piece of graph paper and draw two graph axes on it. Label the vertical Pressure and the Horizontal Temperature.
Now I am going to keep this SIMPLE: -
IGNORE temperature increase due to pressure.
Put a point on the graph which would indicate the state just before it enters the engine. Assuming standard atmosphere this would be around 1013.2 millibars, and 21 Celsius…
The first thing that happens is the air is compressed; this increases the pressure and so would generate a vertical line on the graph stopping at the appropriate value of pressure.
The next stage is the fuel insertion; it burns and increases the temperature. This places a horizontal line on our graph, moving away from the TOP of the vertical line.
This completes the engines part. The next bit happens anyway, and should be dotted in. It is an open system, but can technically be thought of as a closed system; as the air goes into the atmosphere and can get sucked back into the engine.
There are two things that happen to the air when it leaves the engine. It expands and cools.
It will expand, and another vertical line will move down from the top horizontal line. Will bring our pressure back to ambient, and then a final horizontal line to close the box that should be on the graph paper.
Now… the techie bit…
The engine will operate at the same internal pressure and temperature. Therefore the top horizontal line is fixed. The distance between the to horizontal line and the bottom horizontal line is an indication of how much energy is being released from the fuel. The greater the distance the more energy; thus the more efficient it operates.
As the altitude increases the ambient pressure drops, but the top temperature remains the same, therefore the distance between the top and bottom horizontal line grows with height, and thus the efficiency of the engine.
It is true the engine produces less real thrust (measured by a force metre) at altitude than at sea level. But the required thrust reduces too, less dense air means less air resistance. However there is an equivalent required value of thrust at altitude. Say to move an aircraft at sea level requires 1000kgf, and ignoring air-resistance you still need 1000kgf at 30,000ft, due to the changes in pressure you will be able to reduce the throttle by 20% because the energy is removed from the fuel more efficiently. In reality, because the thrust required at height reduces you can often throttle back even further.
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Thanks all for the replies.
I see that my impression that there are different answers to this question (or at least different ways of approaching the question) was not mistaken. But the thread remains informative and enjoyable.
Can we establish the things we agree on? I am assuming we are all happy with this:
(1) Greater altitude = less dense air = less drag (as described in terms of density, IAS vs TAS, etc)?
(2) The reduction in drag outweighs any (disputed) reduction in engine efficiency, or enhances any increase in efficiency.
The disagreement seems to centre on whether turbines are more efficient at higher altitudes, and if so, why. For “efficiency”, I think we all agree this is the rate at which fuel is burnt for a given amount of thrust, described as TSFC.
I hope I'm correct when I say I am reading that:
(a) The ideal air/fuel ratio is 15:1, so less air higher up means less fuel;
(b) Engines are more efficient at higher N1 speeds (ideally 90-95%). Some have added that this fan speed would produce too much thrust at lower altitudes, but produces the right amount of thrust for the cruise higher up;
(c) The pressure and temperature inside the engine remain constant, but the lower outside pressure at altitude means that more energy is derived from the fuel burnt, even if there is less real thrust. (Italianjon’s graph is elegant and satisfying, which makes me want to believe it!)
(d) Engines are in fact less efficient at altitude, but that this is more than compensated for by reduced drag.
Scenarios (a) to (c) do not appear to me to contradict each other, but they cannot live very comfortably with (d).
Can’t help another question coming into my head: is 100% of N1 measured in absolute revolutions (RPM), regardless of context? Or does the theoretical 100% vary for temperature and/or altitude? (Hope this is just a small worm rather than a whole can of them.)
The conversations which have prompted all this include:
- A Learjet pilot years ago saying the fuel burn while taxiing was similar to that used in the cruise; and
- A turboprop pilot (flying the Czech-built Let 410) saying last year that the engines “used more fuel” at lower altitudes. I assumed at the time he meant for the same throttle setting, but maybe he was referring to a higher throttle setting due to greater drag?
One gas turbine engineer, whose machines are designed to generate electricity on the ground rather than make things fly, told me somewhere over the Bay of Biscay recently that they look for maximum air density when selecting sites for the power stations. That includes not just the height of the place, but siting it near bodies of water to benefit from onshore breezes. So the turbines didn’t just prefer sea level, they liked to live beside it. But then they’re attached to a generator rather than a big fan. (We speculated about the density/pressure/temperature trade-off with greater altitude for the engines on the A320 we were in, but I see now we were wrong.)
I think the speling is oK... I understand that not ending a sentence with a preposition, and beginning one with a conjunction, is a convention rather than a grammatical rule. I do seem to have strayed away from the informed comment I was seeking, but hope that the speculative attempt at synthesis might lead us somewhere!
Thanks again for the replies.
Shamrock 602
I see that my impression that there are different answers to this question (or at least different ways of approaching the question) was not mistaken. But the thread remains informative and enjoyable.
Can we establish the things we agree on? I am assuming we are all happy with this:
(1) Greater altitude = less dense air = less drag (as described in terms of density, IAS vs TAS, etc)?
(2) The reduction in drag outweighs any (disputed) reduction in engine efficiency, or enhances any increase in efficiency.
The disagreement seems to centre on whether turbines are more efficient at higher altitudes, and if so, why. For “efficiency”, I think we all agree this is the rate at which fuel is burnt for a given amount of thrust, described as TSFC.
I hope I'm correct when I say I am reading that:
(a) The ideal air/fuel ratio is 15:1, so less air higher up means less fuel;
(b) Engines are more efficient at higher N1 speeds (ideally 90-95%). Some have added that this fan speed would produce too much thrust at lower altitudes, but produces the right amount of thrust for the cruise higher up;
(c) The pressure and temperature inside the engine remain constant, but the lower outside pressure at altitude means that more energy is derived from the fuel burnt, even if there is less real thrust. (Italianjon’s graph is elegant and satisfying, which makes me want to believe it!)
(d) Engines are in fact less efficient at altitude, but that this is more than compensated for by reduced drag.
Scenarios (a) to (c) do not appear to me to contradict each other, but they cannot live very comfortably with (d).
Can’t help another question coming into my head: is 100% of N1 measured in absolute revolutions (RPM), regardless of context? Or does the theoretical 100% vary for temperature and/or altitude? (Hope this is just a small worm rather than a whole can of them.)
The conversations which have prompted all this include:
- A Learjet pilot years ago saying the fuel burn while taxiing was similar to that used in the cruise; and
- A turboprop pilot (flying the Czech-built Let 410) saying last year that the engines “used more fuel” at lower altitudes. I assumed at the time he meant for the same throttle setting, but maybe he was referring to a higher throttle setting due to greater drag?
One gas turbine engineer, whose machines are designed to generate electricity on the ground rather than make things fly, told me somewhere over the Bay of Biscay recently that they look for maximum air density when selecting sites for the power stations. That includes not just the height of the place, but siting it near bodies of water to benefit from onshore breezes. So the turbines didn’t just prefer sea level, they liked to live beside it.
But then they’re attached to a generator rather than a big fan. (We speculated about the density/pressure/temperature trade-off with greater altitude for the engines on the A320 we were in, but I see now we were wrong.)I think the speling is oK... I understand that not ending a sentence with a preposition, and beginning one with a conjunction, is a convention rather than a grammatical rule. I do seem to have strayed away from the informed comment I was seeking, but hope that the speculative attempt at synthesis might lead us somewhere!
Thanks again for the replies.
Shamrock 602
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The theoretical N1 limits are absolute i.e they are set by the mechanical design of the rotating components which allows for some over-speed margin before you get a hub failure and some serious battle damage to the airplane.
Now, the N1 speed attainable for a specific set of conditions i.e. density etc will vary hence "N1 for the day" tables and now (in this FADEC world) "bugs" on the displays
Now, the N1 speed attainable for a specific set of conditions i.e. density etc will vary hence "N1 for the day" tables and now (in this FADEC world) "bugs" on the displays
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Shamrock,
The comments about the Gas Turbine generating electricity, would be because the gas turbine is connected to a generator. It is probably similar to an alternator in a car, so the voltages produced are always similar... anyway, for that you need maximum force, hence maximum air density.
You can see the effect of force required by try a little experiment... next time you are in a car, leave it idleing, and try to turn on as many lights as you can at the same time... Fog Lights, Main Beams, Brake lights, hazards everything... if you can combine this with spinning the steering wheel too (power assisted steering required) you will see a substantial drop in rpm, as the engine has to produce more force to spin. This is due to increased resistance from the alternator, the rpm should return to normal when all is switched off.
Of course with aircraft you want maximum fuel efficiency. Less cost to airline, less cost to passenger.
I was re-reading some university notes, and I believe the speed of the air from the turbine remians constant, but as the air density drops the mass flow rate drops, and so by Conservation of Momentum the total force drops. But as the air is thinner, the result is greater total net force.
Remeber NetForce=GrossForce - Resistance
so as the aircraft goes higher resistance gets less, and so does gross force, but as engine becomes more efficient there is a relativly higher gross force than there should be. So you'll probably find the Net FOrce is higher at altitude, but the ACTUAL force the engine is producing is less...
Probably confused the hell out of you.
(Thanks for the mention in you post though! )
The comments about the Gas Turbine generating electricity, would be because the gas turbine is connected to a generator. It is probably similar to an alternator in a car, so the voltages produced are always similar... anyway, for that you need maximum force, hence maximum air density.
You can see the effect of force required by try a little experiment... next time you are in a car, leave it idleing, and try to turn on as many lights as you can at the same time... Fog Lights, Main Beams, Brake lights, hazards everything... if you can combine this with spinning the steering wheel too (power assisted steering required) you will see a substantial drop in rpm, as the engine has to produce more force to spin. This is due to increased resistance from the alternator, the rpm should return to normal when all is switched off.
Of course with aircraft you want maximum fuel efficiency. Less cost to airline, less cost to passenger.
I was re-reading some university notes, and I believe the speed of the air from the turbine remians constant, but as the air density drops the mass flow rate drops, and so by Conservation of Momentum the total force drops. But as the air is thinner, the result is greater total net force.
Remeber NetForce=GrossForce - Resistance
so as the aircraft goes higher resistance gets less, and so does gross force, but as engine becomes more efficient there is a relativly higher gross force than there should be. So you'll probably find the Net FOrce is higher at altitude, but the ACTUAL force the engine is producing is less...
Probably confused the hell out of you.
(Thanks for the mention in you post though!
)
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Can you explain how resistance gets less since we dont fly a constant TAS we fly a constant IAS ( ignoring transition to Mach Nos.) Thus if the IAS is constant the resistance or Q dynamic pressure remains constant regardless of altitude.
If we flew TAS then I would agree but we dont!
Jet engines produce more thrust at sea level than at altitude but this is not a measure of their efficency. TSFC is approx constant for a Jet where SFC is approx constant for a turboprop or piston.
If we flew TAS then I would agree but we dont!
Jet engines produce more thrust at sea level than at altitude but this is not a measure of their efficency. TSFC is approx constant for a Jet where SFC is approx constant for a turboprop or piston.
Last edited by ifleeplanes; 16th Mar 2005 at 23:26.
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ifleeplanes
You are perfectly right in saying that the reason the temperature rises is that the air is compressed. I did not write this in my original post because I assumed anyone with a GCSE would know this.
We are both describing the same effect, in different ways.
I agree that your way to look at it is probably easier to understand for most pilots.
You say air has to be cooled because it was warmed through compression.
I look at why would this air end up being warmer than "normal" air at 750mb (normal cabin alt) after it is compressed. (by normal I mean either ISA temp at 750mb or standard cabin temperature, which would obviously be higher than ISA. Either way, the temperature of the compressed air ends up being higher than either one.)
A more pragmatic way to look at it would be this: the reason the air ends up being too warm is that, at 35,000ft, the air has a temperature that, when adiabatically compressed to 750mb, will result in a temperature that is too warm to be fed to the cabin.
Beacuse potential temperature always increases with height, the above will be true any time you take air above you and compress it.
CosmosSchwartz
Please re-read my first post, especially the last line. I think it sums it up.
"So the aircraft, as a whole system, operates more efficiently at altitude. But the engines are more efficient at sea level."
(btw, from what I read around pprune, the reason the BA crew did not get high enough was not due to lack of performance on 3 eng but rather ATC restrictions...why they let ATC fly their airplane all the way into emergency fuel without being more assertive is beyond me, but I don't know all the facts)
Shamrock
From what I was taught, TSFC increases with height. In fact, I have several texts in front of me that also show this.
I am not going into the reasons why this occurs, because I don't have the technical knowledge. (I will speculate that it also might have to do with my good-old potential temperature being higher up there
)
ifleeplanes
Commercial aircraft spend most of their time in cruise. In cruise you fly a mach number. You cannot ignore the transition to mach...that's what you are flying 90% of the time!
By flying a mach number we are "closer" to flying TAS than IAS.
When you are cruising at 35,000ft, because of the reduced air density, you have a reduced drag for a given TAS or even for a given Mach than at a lower altitude. If you flew a constant IAS, you would be right, the dynamic pressure would not change (actually if we really want to be pedantic, its EAS, not IAS...
)
but airliners cruise at a constant mach, not IAS.
Even when you are climbing at constant IAS, your TAS in increasing as you go up... meaning that you are travelling faster for the same amount of drag. (drag is proportional to FF) This means you are getting further down the road using less fuel. To me this means more efficiency.
Again, I think it depends on how you look at it.
I look at this way: the higher you go, the less dynamic pressure you get for a given TAS. (after all, TAS is what will determine how fast you get to destination, ignoring winds) This is where the whole intuitive thing of "less resistance for the same speed" comes into play.
You look at it this way: for the same dynamic pressure I am getting more TAS. Once again we are saying the same thing
Regards,
palgia
You are perfectly right in saying that the reason the temperature rises is that the air is compressed. I did not write this in my original post because I assumed anyone with a GCSE would know this.
We are both describing the same effect, in different ways.
I agree that your way to look at it is probably easier to understand for most pilots.
You say air has to be cooled because it was warmed through compression.
I look at why would this air end up being warmer than "normal" air at 750mb (normal cabin alt) after it is compressed. (by normal I mean either ISA temp at 750mb or standard cabin temperature, which would obviously be higher than ISA. Either way, the temperature of the compressed air ends up being higher than either one.)
A more pragmatic way to look at it would be this: the reason the air ends up being too warm is that, at 35,000ft, the air has a temperature that, when adiabatically compressed to 750mb, will result in a temperature that is too warm to be fed to the cabin.
Beacuse potential temperature always increases with height, the above will be true any time you take air above you and compress it.
CosmosSchwartz
Please re-read my first post, especially the last line. I think it sums it up.
"So the aircraft, as a whole system, operates more efficiently at altitude. But the engines are more efficient at sea level."
(btw, from what I read around pprune, the reason the BA crew did not get high enough was not due to lack of performance on 3 eng but rather ATC restrictions...why they let ATC fly their airplane all the way into emergency fuel without being more assertive is beyond me, but I don't know all the facts)
Shamrock
(d) Engines are in fact less efficient at altitude, but that this is more than compensated for by reduced drag.
I am not going into the reasons why this occurs, because I don't have the technical knowledge. (I will speculate that it also might have to do with my good-old potential temperature being higher up there
)ifleeplanes
Can you explain how resistance gets less since we dont fly a constant TAS we fly a constant IAS ( ignoring transition to Mach Nos.) Thus if the IAS is constant the resistance or Q dynamic pressure remains constant regardless of altitude.
Commercial aircraft spend most of their time in cruise. In cruise you fly a mach number. You cannot ignore the transition to mach...that's what you are flying 90% of the time!
By flying a mach number we are "closer" to flying TAS than IAS.
When you are cruising at 35,000ft, because of the reduced air density, you have a reduced drag for a given TAS or even for a given Mach than at a lower altitude. If you flew a constant IAS, you would be right, the dynamic pressure would not change (actually if we really want to be pedantic, its EAS, not IAS...
)but airliners cruise at a constant mach, not IAS.
Even when you are climbing at constant IAS, your TAS in increasing as you go up... meaning that you are travelling faster for the same amount of drag. (drag is proportional to FF) This means you are getting further down the road using less fuel. To me this means more efficiency.
Again, I think it depends on how you look at it.
I look at this way: the higher you go, the less dynamic pressure you get for a given TAS. (after all, TAS is what will determine how fast you get to destination, ignoring winds) This is where the whole intuitive thing of "less resistance for the same speed" comes into play.
You look at it this way: for the same dynamic pressure I am getting more TAS. Once again we are saying the same thing
Regards,
palgia
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Cosmos I agree with you wholeheartedly, you can lead a horse to water but you cant make it drink!
( Ultimatly a thirsty horse wont be very efficent whatever altitude its at. )
Jets are more efficent at altitude...full stop!
( Ultimatly a thirsty horse wont be very efficent whatever altitude its at.
)Jets are more efficent at altitude...full stop!
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CosmosSchwartz
I appreciate the time you took to write your post. However I can guarantee you I perfectly undestand the difference between the amount of thrust and the EFFICIENCY of an engine. (and they BOTH go down as altitude increases)
What I think you don't undserstand is that TSFC INCREASES WITH HEIGHT. It does NOT stay the same.
I repeat, TSFC increases with height, making the engine more inefficient with altitude.
However, the aircraft as a whole will be more efficient at altitude, due to the aerodynamic effects we discussed.
(so really the BA example you took has nothing to do in our discussion, I never argued that aircraft were more efficient at lower altitudes. It's no secret that range increases with altitude.)
Yup, I agree. I am sure you'll keep your opinion and I'll keep mine.
But just for fun I decided to dig up some performance numbers for the CF-6 from my uni text book...
Data comes from a table, so the formatting here appears weird.
Here is goes:
GROUND PERFORMANCE, ISA STANDARD DAY
Thrust Setting - Gross Thrust (lbs) - TSFC (lb/hr/lbt) - Fuel Flow (lbs/hr)
Takeoff (S.L) -------- 50,200 --------- 0.394 ----------- 19,779
Max continuous ------ 46,200 --------- 0.385 ----------- 17,787
75% Takeoff --------- 37,600 --------- 0.371 ----------- 13,579
Flight Idle ------------- 5,190 --------- 0.450 ----------- 2,320
Ground Idle ------------ 1,740 --------- 0.850 ----------- 1,490
ALTITUDE PERFORMANCE (Mach 0.85) ISA TEMPERATURE
Thrust Setting - Gross Thrust (lbs) - TSFC (lb/hr/lbt) - Fuel Flow (lbs/hr)
Max Climb -------- 11,500 ----------- 0.664 ----------- 7,636 (max)
Cruise ------------ 10,800 ----------- 0.654 ----------- 7,063
Data comes from page 7-4. "Aircraft Gas Turbine Powerplants" by Charles E Otis and Peter A Vosbury. 2002. Jeppesen Sanderson Training Products. ISBN 0-88487-311-0
Okay, now some things to ponder while having this data in front of you...
1. If we consider that max continuous thrust on the ground and maximum cruise power in flight are the same power setting, note that even though TSFC is higher in flight, actual fuel consuption is only 40% of fuel used on the ground. This shows that despite TSFC INCREASES at altitude, an airline will save fuel by cruising higher. (but the engines stay more inefficient at altitude)
2. The reason TSFC is higher at altitude than on the ground is that , in order to keep the engine power up when inlet density is dropping, more RPM (by the way of increased fuel flow) is needed to maintain correct mass airflow. The increase in fuel flow per pound of thrust results in a higher TSFC.
Now ifleeplanes and CosmosSchwartz, you guys were probably incorrectly taught that TSFC stays constant with altitude. This might be due to fact that early "pure turbojets" or low bypass turbofans had very small increases in TSFC with altitude. To give you an example of this, lets take the TFE-731 (a business jet sized engine). The TFE-731 has a sea-level TSFC of 0.51 lb./hr./lbt. The CF-6 (which if you don't know is a large high-bypass engine) has a sea-level TSFC of 0.38 lb./hr./lbt. You notice right away that the smaller engine has a much worse fuel efficiency... no surprise here. The larger engine is more efficient at sea level because it has a large diameter fan (hence greater propulsive efficiency) but at altitude the fan, just like a fixed pitch propeller, losses efficiency. The favorable angle of attack of the fan blades at sea-level is diminished at altitude so the core portion of the engine has to make up for it with an increase in fuel flow. In this way, TSFC increases more on a high bypass ratio engine going to altitude than it does on a smaller engine.
Here are some of the numbers:
On the CF-6, TSFC increases from 0.38 at sea level max cont thrust to 0.654 at altitude. Change in TSFC = 75% INCREASE.
On the TFE-731, TSFC increases from 0.51 at sea-level max cont thrust to 0.80 at altitude. Change in TSFC= 56% INCREASE.
Either way, even on a small engine like the TFE-731, a 56% in TSFC is very significant!
Not to mention that for the CF-6 its 75% higher! THAT IS A LOT! It means you are almost burning TWICE the amount of fuel for the same amount of thrust. This is what I would call less efficiency.
Good thing that at those altitudes most jets require only a fraction of the thrust they would need at sea-level to reach M0.80 cruising speeds. THATS WHAT MAKES JET AIRCRAFT MORE EFFICIENT AT ALTITUDE: THE REDUCTION OF THRUST REQUIRED FOR A GIVEN TAS OR MACH NUMBER. NOT THE ENGINES, which are actually LESS efficient at altitude!
So, to sum it up... repeat after me: TSFC INCREASES WITH INCREASING ALTITUDE (in all airliners). The higher the bypass ratio, the greater the TSFC increase will be.
This makes engines LESS efficient at altitude than at sea-level. However, the aicraft as a whole is more efficient at altitude.
Hope this makes things a little clearer Shamrock.
Regards,
palgia
I appreciate the time you took to write your post. However I can guarantee you I perfectly undestand the difference between the amount of thrust and the EFFICIENCY of an engine. (and they BOTH go down as altitude increases)
What I think you don't undserstand is that TSFC INCREASES WITH HEIGHT. It does NOT stay the same.
I repeat, TSFC increases with height, making the engine more inefficient with altitude.
However, the aircraft as a whole will be more efficient at altitude, due to the aerodynamic effects we discussed.
(so really the BA example you took has nothing to do in our discussion, I never argued that aircraft were more efficient at lower altitudes. It's no secret that range increases with altitude.)
you can lead a horse to water but you cant make it drink!
But just for fun I decided to dig up some performance numbers for the CF-6 from my uni text book...
Data comes from a table, so the formatting here appears weird.
Here is goes:
GROUND PERFORMANCE, ISA STANDARD DAY
Thrust Setting - Gross Thrust (lbs) - TSFC (lb/hr/lbt) - Fuel Flow (lbs/hr)
Takeoff (S.L) -------- 50,200 --------- 0.394 ----------- 19,779
Max continuous ------ 46,200 --------- 0.385 ----------- 17,787
75% Takeoff --------- 37,600 --------- 0.371 ----------- 13,579
Flight Idle ------------- 5,190 --------- 0.450 ----------- 2,320
Ground Idle ------------ 1,740 --------- 0.850 ----------- 1,490
ALTITUDE PERFORMANCE (Mach 0.85) ISA TEMPERATURE
Thrust Setting - Gross Thrust (lbs) - TSFC (lb/hr/lbt) - Fuel Flow (lbs/hr)
Max Climb -------- 11,500 ----------- 0.664 ----------- 7,636 (max)
Cruise ------------ 10,800 ----------- 0.654 ----------- 7,063
Data comes from page 7-4. "Aircraft Gas Turbine Powerplants" by Charles E Otis and Peter A Vosbury. 2002. Jeppesen Sanderson Training Products. ISBN 0-88487-311-0
Okay, now some things to ponder while having this data in front of you...
1. If we consider that max continuous thrust on the ground and maximum cruise power in flight are the same power setting, note that even though TSFC is higher in flight, actual fuel consuption is only 40% of fuel used on the ground. This shows that despite TSFC INCREASES at altitude, an airline will save fuel by cruising higher. (but the engines stay more inefficient at altitude
)2. The reason TSFC is higher at altitude than on the ground is that , in order to keep the engine power up when inlet density is dropping, more RPM (by the way of increased fuel flow) is needed to maintain correct mass airflow. The increase in fuel flow per pound of thrust results in a higher TSFC.
Now ifleeplanes and CosmosSchwartz, you guys were probably incorrectly taught that TSFC stays constant with altitude. This might be due to fact that early "pure turbojets" or low bypass turbofans had very small increases in TSFC with altitude. To give you an example of this, lets take the TFE-731 (a business jet sized engine). The TFE-731 has a sea-level TSFC of 0.51 lb./hr./lbt. The CF-6 (which if you don't know is a large high-bypass engine) has a sea-level TSFC of 0.38 lb./hr./lbt. You notice right away that the smaller engine has a much worse fuel efficiency... no surprise here. The larger engine is more efficient at sea level because it has a large diameter fan (hence greater propulsive efficiency) but at altitude the fan, just like a fixed pitch propeller, losses efficiency. The favorable angle of attack of the fan blades at sea-level is diminished at altitude so the core portion of the engine has to make up for it with an increase in fuel flow. In this way, TSFC increases more on a high bypass ratio engine going to altitude than it does on a smaller engine.
Here are some of the numbers:
On the CF-6, TSFC increases from 0.38 at sea level max cont thrust to 0.654 at altitude. Change in TSFC = 75% INCREASE.
On the TFE-731, TSFC increases from 0.51 at sea-level max cont thrust to 0.80 at altitude. Change in TSFC= 56% INCREASE.
Either way, even on a small engine like the TFE-731, a 56% in TSFC is very significant!
Not to mention that for the CF-6 its 75% higher! THAT IS A LOT! It means you are almost burning TWICE the amount of fuel for the same amount of thrust. This is what I would call less efficiency.
Good thing that at those altitudes most jets require only a fraction of the thrust they would need at sea-level to reach M0.80 cruising speeds. THATS WHAT MAKES JET AIRCRAFT MORE EFFICIENT AT ALTITUDE: THE REDUCTION OF THRUST REQUIRED FOR A GIVEN TAS OR MACH NUMBER. NOT THE ENGINES, which are actually LESS efficient at altitude!
So, to sum it up... repeat after me: TSFC INCREASES WITH INCREASING ALTITUDE (in all airliners). The higher the bypass ratio, the greater the TSFC increase will be.
This makes engines LESS efficient at altitude than at sea-level. However, the aicraft as a whole is more efficient at altitude.
Hope this makes things a little clearer Shamrock.
Regards,
palgia