Helicopter Urban Myths
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Paul,
Try this, which plots the volume and pressure (mass) of air vs the temperature. It shows the area under that curve as the work the engine produces. Thus endith the first lesson:
http://www.lerc.nasa.gov/WWW/K-12/ai...ges/carnot.gif
Try this, which plots the volume and pressure (mass) of air vs the temperature. It shows the area under that curve as the work the engine produces. Thus endith the first lesson:
http://www.lerc.nasa.gov/WWW/K-12/ai...ges/carnot.gif
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Okay, if you won't go with my explaination of the theory then I have kindly added a couple of links. You'll like the first one, done for kids with lots of pretty pictures and, oddly enough, talks about velocity??? With heat only being the primer required for gaseous expansion, well theres a surprise.
http://science.howstuffworks.com/turbine.html
A little history, but then Nick with your patents you won't need that will you. You've written it.
http://www.aircav.com/histturb.html
Ironic that all of them talk about gaseous expansion and velocity. Uncanny to think that they use these engines with thrust in aircraft, whatever will they think of next. I know HEAT CONVERTORS, need one of those on a cold day.
Cheers
Oh, and thanks for the piccie but where is velocity?????
p.s. From your beginners website have alook at this:-
http://www.grc.nasa.gov/WWW/K-12/airplane/bgp.html
Good website, by the way have alook at this:-
http://www.grc.nasa.gov/WWW/K-12/air...btyp/ettp.html
and notice where the highest pressure is, ie where the gas flow with the most energy, doing the most work is. Oh, its at the face of the power turbine, providing the energy to turn the compressor, whats left is THRUST my dear.
Sorry Nick, you went to the wrong page for your ideas
Have a look at the Brayton Cycle, thats the one that equates to Gas turbines by the way, which shows the pressure against volume. Sorry but your work done by transfer of heat energy equation that is at the end of the Carnot cycle, for reciprocation engine I believe but then I\'m not sure,
http://www.lerc.nasa.gov/WWW/K-12/airplane/brayton.html
Questions?
Sorry one last one, a quote from the NASA website:
The temperature increase depends on the type of fuel used and the fuel-air ratio. The hot exhaust is then passed through the power turbine in which work is done by the flow from station 4.
Suck, Squeeze, Bang, Blow
QPQ
http://www.lerc.nasa.gov/WWW/K-12/ai.../braytonpv.gif
http://science.howstuffworks.com/turbine.html
A little history, but then Nick with your patents you won't need that will you. You've written it.
http://www.aircav.com/histturb.html
Ironic that all of them talk about gaseous expansion and velocity. Uncanny to think that they use these engines with thrust in aircraft, whatever will they think of next. I know HEAT CONVERTORS, need one of those on a cold day.
Cheers
Oh, and thanks for the piccie but where is velocity?????
p.s. From your beginners website have alook at this:-
http://www.grc.nasa.gov/WWW/K-12/airplane/bgp.html
Good website, by the way have alook at this:-
http://www.grc.nasa.gov/WWW/K-12/air...btyp/ettp.html
and notice where the highest pressure is, ie where the gas flow with the most energy, doing the most work is. Oh, its at the face of the power turbine, providing the energy to turn the compressor, whats left is THRUST my dear.
Sorry Nick, you went to the wrong page for your ideas
Have a look at the Brayton Cycle, thats the one that equates to Gas turbines by the way, which shows the pressure against volume. Sorry but your work done by transfer of heat energy equation that is at the end of the Carnot cycle, for reciprocation engine I believe but then I\'m not sure,
http://www.lerc.nasa.gov/WWW/K-12/airplane/brayton.html
Questions?
Sorry one last one, a quote from the NASA website:
The temperature increase depends on the type of fuel used and the fuel-air ratio. The hot exhaust is then passed through the power turbine in which work is done by the flow from station 4.
Suck, Squeeze, Bang, Blow
QPQ
http://www.lerc.nasa.gov/WWW/K-12/ai.../braytonpv.gif
Last edited by Paul McKeksdown; 11th Jan 2005 at 19:51.
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The Brayton Cycle. Thanks Paul for the reminder. So can you see now that a gas turbine engine depends on temperature more than anything else?
Mass flow rates are what goes into the engine and comes out the other end, but they dont explain what is going on inside the engine. Mass flow rate depends on average gas velocity. Average exhaust gas velocity depends on the temperature (Tout rises when lever is pulled). Therefore the mass flow rates depend on temperature. Increase gas temperature, increase gas velocity, hence afterburners. Afterburners only increase the amount of mass emmitted by the engine by the amount of extra fuel burnt. Insignificant. No extra air is going through the engine when the afterburners are on. Extra thrust is provided by extra heat.
Mass flow rates are what goes into the engine and comes out the other end, but they dont explain what is going on inside the engine. Mass flow rate depends on average gas velocity. Average exhaust gas velocity depends on the temperature (Tout rises when lever is pulled). Therefore the mass flow rates depend on temperature. Increase gas temperature, increase gas velocity, hence afterburners. Afterburners only increase the amount of mass emmitted by the engine by the amount of extra fuel burnt. Insignificant. No extra air is going through the engine when the afterburners are on. Extra thrust is provided by extra heat.
Now I know what a dog watching television feels like!
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So, does extra heat push the aircraft faster through the air???
Have a closer look at the theory, the more fuel you add the greater and more rapid the expansion of gases and therefore the greater the velocity. Temperature does indeed increase however this is not the route cause but more a problem. If temperature were not a problem then we could pump the afterburner fuel load into the combustion chamber instead and let the compressor sbenefit from the extra ooomph. Why do we burn the fuel in an after burner?? and why do we have to protect the aircraft from the heat with a convering shround like the F16 for example?
The heat is a LIMITING factor, without it we could dump all the fuel we wanted into the engine and enjoy unlimited power. Gas turbines work well in cold temperatures because the Torque limitations are reached first.
Look at start limitations eg. S61 start temp allowed 840 degrees momentairily (hate that word). Normal operating is 680, why is that? It's because during the start cycle the engine can handle a short hot blast. The overtemp checks are to look for small molten bits of the combustion chamber on the turbines and exhaust casing. Heat kills engines full stop. More fuel, more heat but more importantly a greater gaseous expansion.
p.s. on the afterburner thingy look at the back of most military aircraft and you'll see a moveable shroud. This reduces the cross section of the afterburning exhaust output to increase the available thrust. (Newtons laws) The reason being a converging duct produces an increase in velocity for a given reduction in pressure. As we are looking for pure thrust at this point a duct is introduced.
Adding to that look at the construction of the compressor and you'll find convergent ducts used to impart anergy and velocity and in the power turbine divergent ducts to extract energy. Moving air is stored energy pure and simple it doesn't matter how hot or cold it is.
Adding to the other comments, have alook again at the wording Diethelm.
In the S-61N the Free power turbine is effectively a windmill sitting in the exhaust outflow. It turns high speed shaft connected to the MRGB. There are two Power Turbines that are placed directly behind the combustion chamber and fed from the chamber through a convergent duct. These two are then connected DIRECTLY to the Gas generator or compressor if you will, how else is it going to spin. The biggest problem is that the airflow is split into tertiary which provides the combustion chamber cooling, if you reduce this you reach your temperature limitation quicker.
Sorry should have said that that comment is for a fixed lever/throttle position!!
The Bang comes when you light blue touch paper in the combustion chamber!!!!
Any more???
Have a closer look at the theory, the more fuel you add the greater and more rapid the expansion of gases and therefore the greater the velocity. Temperature does indeed increase however this is not the route cause but more a problem. If temperature were not a problem then we could pump the afterburner fuel load into the combustion chamber instead and let the compressor sbenefit from the extra ooomph. Why do we burn the fuel in an after burner?? and why do we have to protect the aircraft from the heat with a convering shround like the F16 for example?
The heat is a LIMITING factor, without it we could dump all the fuel we wanted into the engine and enjoy unlimited power. Gas turbines work well in cold temperatures because the Torque limitations are reached first.
Look at start limitations eg. S61 start temp allowed 840 degrees momentairily (hate that word). Normal operating is 680, why is that? It's because during the start cycle the engine can handle a short hot blast. The overtemp checks are to look for small molten bits of the combustion chamber on the turbines and exhaust casing. Heat kills engines full stop. More fuel, more heat but more importantly a greater gaseous expansion.
p.s. on the afterburner thingy look at the back of most military aircraft and you'll see a moveable shroud. This reduces the cross section of the afterburning exhaust output to increase the available thrust. (Newtons laws) The reason being a converging duct produces an increase in velocity for a given reduction in pressure. As we are looking for pure thrust at this point a duct is introduced.
Adding to that look at the construction of the compressor and you'll find convergent ducts used to impart anergy and velocity and in the power turbine divergent ducts to extract energy. Moving air is stored energy pure and simple it doesn't matter how hot or cold it is.
Adding to the other comments, have alook again at the wording Diethelm.
In the S-61N the Free power turbine is effectively a windmill sitting in the exhaust outflow. It turns high speed shaft connected to the MRGB. There are two Power Turbines that are placed directly behind the combustion chamber and fed from the chamber through a convergent duct. These two are then connected DIRECTLY to the Gas generator or compressor if you will, how else is it going to spin. The biggest problem is that the airflow is split into tertiary which provides the combustion chamber cooling, if you reduce this you reach your temperature limitation quicker.
Sorry should have said that that comment is for a fixed lever/throttle position!!
The Bang comes when you light blue touch paper in the combustion chamber!!!!
Any more???
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The story of any heat engine is the story of the flow of energy as the working fluid is heated by fuel, and then the heat is exchanged for work by some physical means. There is nothing wrong with thinking about the velocity of the gas stream as doing work, but it is not the origin of that work, only an intermediate step.
Paul, the way you are analyzing a turbine engine is as if you looked at a light bulb as the start and the electric generator as the end of a system that converted darkness into fuel.
I do suggest that you take a peek at a thermodynamics text, you are far and away over your head on this subject, although I am sure your intentions are good.
SASless, if I put up a picture of a mailman, will you bark?
Paul, the way you are analyzing a turbine engine is as if you looked at a light bulb as the start and the electric generator as the end of a system that converted darkness into fuel.
I do suggest that you take a peek at a thermodynamics text, you are far and away over your head on this subject, although I am sure your intentions are good.
SASless, if I put up a picture of a mailman, will you bark?
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Pull My Keks Down has tossed a bait into the forum and hooked a couple of serious fish.(Nick et al). He has played the catch with a lot of energy and got a lot of fun out of it.
But he doesn't know when to cut the line before the fish climbs into the boat and eats him.
Is this another appearance of Flare Dammit / Pprune Fan #1??
But he doesn't know when to cut the line before the fish climbs into the boat and eats him.
Is this another appearance of Flare Dammit / Pprune Fan #1??
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I'm not in over my head I was just trying to answer a simple question of which limits a gas turbine. The thermal expansion dynamics of air whilst in the combustion chamber is of no great interest but I have looked into it and the dispersal of fuel particles from the injectors using laser spectro annomymetry but who cares.
The simple answer is that if the makers could build the thing to accelerate the air without wasting the stored energy of the fuel in heat we would all sing hallelugha or however its spelt. Please tell me I'm wrong.
Are you really taking this down to the transfer of stored energy within the fuel to the air mass???? How deep do we wish to go? We could start talking about the relative properties of different aviation fuel but where will that take us? We wish to extract the energy held within a fluid medium and apply that, within our turbine, to and air flow to produce thrust. The stored energy is transferred by combustion causing explosive expansion. This produces a variety of energies. 1 Velocity through expansion, Perfect. 2. Heat, waste of stored energy unfortunately necessary in energy transfer 3 sound, see point 2.
Combustion is a means to accelerate the medium to produce the motion from which the work is extracted.
Newtons law that any force has an equal and opposite force.
Heat pushes nothing it is a means to an end. It is our limitation. The more heat you produce during the combustion the more rapid the expansion and transfer of heat and the graeter the pressure and when guided the velocity. Lets pull till the thing melts shall we.
You talk about the conversion of heat to work and energy but you provide no physical evidence of how this is acheived within the engine. My theory is corroborated by just about every internet site that explains jet engines, fuel is used to accelerate the air, how it does it is, at the end of the day, irrelevant.
P.s. Light bulbs are wonderful things but I aint interested how they work just where the spares are! Ah there under the undercarriage light indicator.
The simple answer is that if the makers could build the thing to accelerate the air without wasting the stored energy of the fuel in heat we would all sing hallelugha or however its spelt. Please tell me I'm wrong.
Are you really taking this down to the transfer of stored energy within the fuel to the air mass???? How deep do we wish to go? We could start talking about the relative properties of different aviation fuel but where will that take us? We wish to extract the energy held within a fluid medium and apply that, within our turbine, to and air flow to produce thrust. The stored energy is transferred by combustion causing explosive expansion. This produces a variety of energies. 1 Velocity through expansion, Perfect. 2. Heat, waste of stored energy unfortunately necessary in energy transfer 3 sound, see point 2.
Combustion is a means to accelerate the medium to produce the motion from which the work is extracted.
Newtons law that any force has an equal and opposite force.
Heat pushes nothing it is a means to an end. It is our limitation. The more heat you produce during the combustion the more rapid the expansion and transfer of heat and the graeter the pressure and when guided the velocity. Lets pull till the thing melts shall we.
You talk about the conversion of heat to work and energy but you provide no physical evidence of how this is acheived within the engine. My theory is corroborated by just about every internet site that explains jet engines, fuel is used to accelerate the air, how it does it is, at the end of the day, irrelevant.
P.s. Light bulbs are wonderful things but I aint interested how they work just where the spares are! Ah there under the undercarriage light indicator.
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Urban Myth No. 56b
You don't need to be a rocket scientist to fly a helicopter.
Urban Truth No 1
You do need to be a rocket scientist to explain how it all works.
Paul,
Look once again at the Brayton Cycle that you referenced. It is a plot of temperature against entropy. Entropy is a measure of the energy in a system that is unavailble to further processes. Gas turbine theory has NOTHING TO DO WITH MASS FLOW. You have given us all a link that unequivocally shows this.
Yes temperature is a limiting factor because of material technology - if a metal could withstand 1200 deg C, the engines would have an amazing power to weight ratio. The graph of T against S shows that the bigger the temperature change, the more work is produced, ie the more power you get. That is exactly what the Brayton cycle says. If you cannot see this then enrol on a first year engineering degree. This is first year Thermodynamics.
You don't need to be a rocket scientist to fly a helicopter.
Urban Truth No 1
You do need to be a rocket scientist to explain how it all works.
Paul,
Look once again at the Brayton Cycle that you referenced. It is a plot of temperature against entropy. Entropy is a measure of the energy in a system that is unavailble to further processes. Gas turbine theory has NOTHING TO DO WITH MASS FLOW. You have given us all a link that unequivocally shows this.
Yes temperature is a limiting factor because of material technology - if a metal could withstand 1200 deg C, the engines would have an amazing power to weight ratio. The graph of T against S shows that the bigger the temperature change, the more work is produced, ie the more power you get. That is exactly what the Brayton cycle says. If you cannot see this then enrol on a first year engineering degree. This is first year Thermodynamics.
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Who is right, who is wrong and, who cares.
To: Hughesy
Mainly because Lu does not give a Sh!t. My knowledge of a jet engine and application of that knowledge is the function of every part of a jet engine, the interrelation of the parts within that engine and the effect of the failure of said parts on the engine and ultimately on the craft that the engine is installed in.
All of this without the understanding of mass flow, heat input, heat rejection, entropy, enthalpy, turbine airfoils and power extraction as well as many other things jet engine related.
However I do know what a convergent divergent duct is (De Laval Nozzle).
I find it interesting that each of you with similar engineering background have come up with your own interpretation of how a jet engine works and none of you agree with the other participants on this thread.
And no, I am not Paul McKeksdown.
This is all quite interesting, but with Mr Lappos posting here there is no sign of good ole Lu.
All of this without the understanding of mass flow, heat input, heat rejection, entropy, enthalpy, turbine airfoils and power extraction as well as many other things jet engine related.
However I do know what a convergent divergent duct is (De Laval Nozzle).
I find it interesting that each of you with similar engineering background have come up with your own interpretation of how a jet engine works and none of you agree with the other participants on this thread.
Unless Paul McKeksdown is Lu
Last edited by Lu Zuckerman; 12th Jan 2005 at 00:52.
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third language needed
Paul, Nick, et.al.
can a no-mind join in ??
seems to me like you are both saying the same thing from different angles
your talking different languages but tell much the same story
in my small space, heat is an agent to interact with something to get a result
bear with me as we reverse engineer this to the basics, ability to turn a power wheel, but lets use baking soda and vinegar instead of fuel and air
keg of vinegar, shovel full of baking soda, uncontained, gas goes everywhere, contain it in a baloon, baloon gets big, contain it by a steel vessell, pressure builds, open a port, gas is expelled against a wheel with slanted veins, wheel goes around, no heat, no outside air, just vinegar and soda which produces a gas.
now lets substitute vinegar with air and baking soda with jet-b (match reuired), same results as with vin and soda
the HEAT creates the expansion of air, the expansion of air drives the wheel.. so WHAT DOES THE WORK, THE HEAT ACTING ON THE AIR, OR THE AIR EXPANDED BY THE HEAT ???
depends on your slant i guess
open the other end of the vessel and continually put in vin and soda OR fuel and air and get same result as a turbine (much less power, granted, but i'm operating with a 25 watt bulb, not 500)
or should i learn how to knit ????
can a no-mind join in ??
seems to me like you are both saying the same thing from different angles
your talking different languages but tell much the same story
in my small space, heat is an agent to interact with something to get a result
bear with me as we reverse engineer this to the basics, ability to turn a power wheel, but lets use baking soda and vinegar instead of fuel and air
keg of vinegar, shovel full of baking soda, uncontained, gas goes everywhere, contain it in a baloon, baloon gets big, contain it by a steel vessell, pressure builds, open a port, gas is expelled against a wheel with slanted veins, wheel goes around, no heat, no outside air, just vinegar and soda which produces a gas.
now lets substitute vinegar with air and baking soda with jet-b (match reuired), same results as with vin and soda
the HEAT creates the expansion of air, the expansion of air drives the wheel.. so WHAT DOES THE WORK, THE HEAT ACTING ON THE AIR, OR THE AIR EXPANDED BY THE HEAT ???
depends on your slant i guess
open the other end of the vessel and continually put in vin and soda OR fuel and air and get same result as a turbine (much less power, granted, but i'm operating with a 25 watt bulb, not 500)
or should i learn how to knit ????
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407 too,
You do really understand it! The point is the heat from the burning fuel is the source of the energy, it is used to push against a wheel and convert the heat energy into motion which creates the velocity, then it pushes against another wheel and actually cools the airflow down (T5 is always hotter than T7 because the work has been taken out of the flow). Heat is the thing that we are tracking, it is pure energy.
You do really understand it! The point is the heat from the burning fuel is the source of the energy, it is used to push against a wheel and convert the heat energy into motion which creates the velocity, then it pushes against another wheel and actually cools the airflow down (T5 is always hotter than T7 because the work has been taken out of the flow). Heat is the thing that we are tracking, it is pure energy.
407....very akin to our Nick thinking of Nicole Kidman.
Heat and energy!
Heat and energy!
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interjected myth
as for the Hp or Hd myth, i say - - YES
if you correct (making number up) say 5,000 ft at 10C you may get 5600 ft **AT WHAT TEMP ?? **
if you correct 4,000 ft at 20C you may get 5600 ft **AT WHAT TEMP ?? **
is it ISO or ISO corrected for std. lapse rate, either way it is an altiutde which is a temp corrected Hp !! (it can't be the starting temp or you are not corrected)
the aircraft will act as if flown at that Hp
SO, if you say Hp is the one, I think it needs to be qualified by saying it has been corrected for temp
if you correct (making number up) say 5,000 ft at 10C you may get 5600 ft **AT WHAT TEMP ?? **
if you correct 4,000 ft at 20C you may get 5600 ft **AT WHAT TEMP ?? **
is it ISO or ISO corrected for std. lapse rate, either way it is an altiutde which is a temp corrected Hp !! (it can't be the starting temp or you are not corrected)
the aircraft will act as if flown at that Hp
SO, if you say Hp is the one, I think it needs to be qualified by saying it has been corrected for temp
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re: SASLESS cogent summation
. . . one wonders whether a smart dog can at least make sense of the scoreboard. At least much on telly HAS a scoreboard . . .
how 'bout if we form an umpire committee to keep score on the hot air debate and advise-at-end-of-thread? Then my occasional lapses into somnolence will interfere less with comprehension of the bottom line.
Poor Paul, to so ruthlessly take on Nick, and presumably thereafter retire forever from the forum in shame!
how 'bout if we form an umpire committee to keep score on the hot air debate and advise-at-end-of-thread? Then my occasional lapses into somnolence will interfere less with comprehension of the bottom line.
Poor Paul, to so ruthlessly take on Nick, and presumably thereafter retire forever from the forum in shame!
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"The simple answer is that if the makers could build the thing to accelerate the air without wasting the stored energy of the fuel in heat we would all sing hallelugha or however its spelt. Please tell me I'm wrong."
"Heat pushes nothing it is a means to an end. It is our limitation. The more heat you produce during the combustion the more rapid the expansion and transfer of heat and the graeter the pressure and when guided the velocity. Lets pull till the thing melts shall we."
Paul,
It seems to me in these two simple sentences you've succinctly outlined why you're failing to grasp such a simple concept. Let off some steam, have a good long look at what you've written here and you may start to see where you've become so completely unhinged!
Best of Luck
"Heat pushes nothing it is a means to an end. It is our limitation. The more heat you produce during the combustion the more rapid the expansion and transfer of heat and the graeter the pressure and when guided the velocity. Lets pull till the thing melts shall we."
Paul,
It seems to me in these two simple sentences you've succinctly outlined why you're failing to grasp such a simple concept. Let off some steam, have a good long look at what you've written here and you may start to see where you've become so completely unhinged!
Best of Luck
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Hi all, sorry the moon came up and I had to get some sleep 
So, after taking the advice I had a good look back through the posts and came to an interesting conclusion.
Nick, I can see what you are getting at when we look at a pinpoint section of the gas turbine combustion chamber. However, it sounds like you are an engineer who is having a problem putting across an overview. Correct me if I'm wrong, I'm sure you will.
You are stating that the thermodynamic heat and energy transfer given by the combustion of the fuel within the combustion chamber will be less due to the increased starting temperature of the combustible mass. I.e. The overall range of thermodynamic movement is restricted by the temperature limit of the engine. Start at 0 limit at 100 = 100% energy transfer, start at 10 limit at 100 = 90% energy transfer. Does that squeeze your physics into a bean can for mere mortals???
Sorry but you took a damn long time to come to a useable point
If that is so I can finally see what your aiming at and I think we are looking at this from two differing perspectives. I don't know what the teaching is over in the good 'ol US of A.
My standpoint is of what we, as helo drivers, can understand within the turbine. My point to answer the question was a point of keeping the combustion chamber within its physical design limitations. If the core of the primary air is burning at 1500 degrees plus then a hike of 20 degrees in the OAT is going to make a change but will that change be significant enough to limit the engine? With the combustion chamber walls glowing at 800 degrees as the secondary flow attempts to contain the flame the hike in OAT will, probably, cause a more noticeable rise in engine operating temperature. Cool the whole thing with hotter air and you hit your engine limitation far quicker. We are talking about the MODULE here, not just the fact that your fuel burn is slightly more inefficent.
I think that both factors play a role here, the question is which is more limiting?
I have contacted an engineer in the aero engine section of Rolls Royce to ask his opinion and will post his reply here when I have it.
I must apologise to Nick for a previous, not so gentlemanly, post but it did take you a long time to get to a useable argument
R1Tamer, the two things are mutually inclusive. If we could extract the locked in energy in the fuel without wasting it in heat transfer the world would be a better place. Take your car engine after a long drive, touch the engine and think how much petrol you have wasted getting that big lump of metal to heat up. If we could develop useful ceramic combustion chambers then okay, lets pull till we die, the problem comes in stopping the stuff melting. Unhinged? don't think so look at a video from Rolls Royce doing a PPI check on a gas turbine and watch it glow white!
So, after taking the advice I had a good look back through the posts and came to an interesting conclusion.
Nick, I can see what you are getting at when we look at a pinpoint section of the gas turbine combustion chamber. However, it sounds like you are an engineer who is having a problem putting across an overview. Correct me if I'm wrong, I'm sure you will.
You are stating that the thermodynamic heat and energy transfer given by the combustion of the fuel within the combustion chamber will be less due to the increased starting temperature of the combustible mass. I.e. The overall range of thermodynamic movement is restricted by the temperature limit of the engine. Start at 0 limit at 100 = 100% energy transfer, start at 10 limit at 100 = 90% energy transfer. Does that squeeze your physics into a bean can for mere mortals???
Sorry but you took a damn long time to come to a useable point
If that is so I can finally see what your aiming at and I think we are looking at this from two differing perspectives. I don't know what the teaching is over in the good 'ol US of A.
My standpoint is of what we, as helo drivers, can understand within the turbine. My point to answer the question was a point of keeping the combustion chamber within its physical design limitations. If the core of the primary air is burning at 1500 degrees plus then a hike of 20 degrees in the OAT is going to make a change but will that change be significant enough to limit the engine? With the combustion chamber walls glowing at 800 degrees as the secondary flow attempts to contain the flame the hike in OAT will, probably, cause a more noticeable rise in engine operating temperature. Cool the whole thing with hotter air and you hit your engine limitation far quicker. We are talking about the MODULE here, not just the fact that your fuel burn is slightly more inefficent.
I think that both factors play a role here, the question is which is more limiting?
I have contacted an engineer in the aero engine section of Rolls Royce to ask his opinion and will post his reply here when I have it.
I must apologise to Nick for a previous, not so gentlemanly, post but it did take you a long time to get to a useable argument
R1Tamer, the two things are mutually inclusive. If we could extract the locked in energy in the fuel without wasting it in heat transfer the world would be a better place. Take your car engine after a long drive, touch the engine and think how much petrol you have wasted getting that big lump of metal to heat up. If we could develop useful ceramic combustion chambers then okay, lets pull till we die, the problem comes in stopping the stuff melting. Unhinged? don't think so look at a video from Rolls Royce doing a PPI check on a gas turbine and watch it glow white!