The relationship between thrust and tempreture?
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The relationship between thrust and tempreture?
Why is thrust inversly proprotional to tempreture?
I used common sense to explain this but im not sure wether my analyzis is correct or not .
What i thought is , since air density is inversely proprtional to tempreture ,then when so much tempeture corresponds to very little air density which means less air drag ( corrrect me if im wrong about this one ) i assumed that drag is directly proprtional to density , so if we have so much drag we need more thrust to maintaine a particular IAS than the thrust that we would need to maintaine the same IAW with less drag ( which occurs as a cosequence of more tempreture and less density ).
Is my analyzis correct ? and if it is then is it what we consider to determine the optimum flexible tempreture when we configure the FMC before take off .
And if my analyzis is wrong , then please correct me and explain the correct relationship between thrust and tempreture and how do we use it to configure the FMC's flexible tempreture .
Thanks in advance
I used common sense to explain this but im not sure wether my analyzis is correct or not .
What i thought is , since air density is inversely proprtional to tempreture ,then when so much tempeture corresponds to very little air density which means less air drag ( corrrect me if im wrong about this one ) i assumed that drag is directly proprtional to density , so if we have so much drag we need more thrust to maintaine a particular IAS than the thrust that we would need to maintaine the same IAW with less drag ( which occurs as a cosequence of more tempreture and less density ).
Is my analyzis correct ? and if it is then is it what we consider to determine the optimum flexible tempreture when we configure the FMC before take off .
And if my analyzis is wrong , then please correct me and explain the correct relationship between thrust and tempreture and how do we use it to configure the FMC's flexible tempreture .
Thanks in advance
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more density = more thrust
less temp = more density = more air to move and burn = more thrust
high temp = less density = less air to move and burn = less thrust
high altitude = less air pressure = less thrust
low altitude = more air pressure = more thrust
Density's effect to thrust is more than air pressure.
those are thrust basics..
less temp = more density = more air to move and burn = more thrust
high temp = less density = less air to move and burn = less thrust
high altitude = less air pressure = less thrust
low altitude = more air pressure = more thrust
Density's effect to thrust is more than air pressure.
those are thrust basics..
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Hey thanks alot that cleared all the ambiguity 
however i got a few more questions, how do you apply these principles to set the flexible tempreture on the FMC ?
And hows cruising at high altitudes conserving fuel ?
however i got a few more questions, how do you apply these principles to set the flexible tempreture on the FMC ?
And hows cruising at high altitudes conserving fuel ?
My rationale behind Density/Temp is this:
The colder the air - the more Oxygen molecules for the same volume! The more Oxygen to burn - the less fuel required for the same thrust.
Water injection cools the air, making it more dense, AND adds more "O's" in the h2o. Nitrous Oxide does the same for Car engines.
Cruising? - Constant fuel flows at the most economic setting for the Outside Air Temperature and the range/speed required. Not too difficult really.
FMC? - ask someone else!
The colder the air - the more Oxygen molecules for the same volume! The more Oxygen to burn - the less fuel required for the same thrust.
Water injection cools the air, making it more dense, AND adds more "O's" in the h2o. Nitrous Oxide does the same for Car engines.
Cruising? - Constant fuel flows at the most economic setting for the Outside Air Temperature and the range/speed required. Not too difficult really.
FMC? - ask someone else!
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But at high altitudes the air is less dense and by applying your logic you get less oxygen to burn so ,more fuel is needed :S im confused now
And by the FMC i meant the flight mamangment computer when the pilots configure it for the flexible tempreture before take off .
And by the FMC i meant the flight mamangment computer when the pilots configure it for the flexible tempreture before take off .
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High alt = low temp = high density = more thrust
high alt = less atmospheric pressure = less thrust
But density's effect to thrust is > atmospheric pressure's effect.
high alt = less atmospheric pressure = less thrust
But density's effect to thrust is > atmospheric pressure's effect.
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Higher altitude = lower pressure = lower air density = less thrust.
Higher temperature = lower air density = less thrust.
That' s what is referred to as hot & high.
But flex take-off has nothing to do with hot & high.
Outside air temperature affects the TIT (turbine inlet temperature).
For a given setting, the higher the temperature of the air that comes into the engine, the higher the temperature at the entrance of the turbine.
The turbine has to bear with alot of heat from the combustion chamber.
The material it is made of has a given life limit with exposure to these high temps but also very differing life limits for different temperatures.
To give you an approximative example, a turbine exposed to a 1600°C could do 2000 cycles when a turbine exposed to a 1500°C could do 2500 cycles.
By reducing only a few % on the take-off thrust setting you can save alot on cycles.
By flexing a take-off, you can create a margin that helps you save the turbines from being exposed to higher temperatures, significantly increasing its life expectancy.
If you have hot and high conditions, you don' t flex.
You can' t because this is where you need the additional thrust that is going to eat up on your turbine cycles.
If it' s cold and you have some margin to play with, you can flex and save cycles on your turbine.
That simple.
Higher temperature = lower air density = less thrust.
That' s what is referred to as hot & high.
But flex take-off has nothing to do with hot & high.
Outside air temperature affects the TIT (turbine inlet temperature).
For a given setting, the higher the temperature of the air that comes into the engine, the higher the temperature at the entrance of the turbine.
The turbine has to bear with alot of heat from the combustion chamber.
The material it is made of has a given life limit with exposure to these high temps but also very differing life limits for different temperatures.
To give you an approximative example, a turbine exposed to a 1600°C could do 2000 cycles when a turbine exposed to a 1500°C could do 2500 cycles.
By reducing only a few % on the take-off thrust setting you can save alot on cycles.
By flexing a take-off, you can create a margin that helps you save the turbines from being exposed to higher temperatures, significantly increasing its life expectancy.
If you have hot and high conditions, you don' t flex.
You can' t because this is where you need the additional thrust that is going to eat up on your turbine cycles.
If it' s cold and you have some margin to play with, you can flex and save cycles on your turbine.
That simple.
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Thanks its all clear now
so flexing is basically for saving the engine from wear and extending its service life limit and its all got to do with inlet tempreture.
so flexing is basically for saving the engine from wear and extending its service life limit and its all got to do with inlet tempreture.
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so flexing is basically for saving the engine from wear and extending its service life
By entering an assumed temperature into the FMC you are fooling the engines into thinking the OAT is hotter than it actually is. The system responds by calculating a reduced power setting to keep the engine parameters within limits.
As a point of interest, the B757 fitted with RB211 motors has such a high power to weight ratio that the majority of take offs are conducted using a de-rate, even at very high AUM. It is a requirement within our company that a full power take-off is carried out every so often to ensure the engines will meet target EPR and produce the required performance.
In hot and high conditions you want the maximum power available. It is not uncommon to see spool speeds approaching the red line or even EGT exceed specified limits. The later can often be attributed to temperture inversions whereby the OAT increases with altitude as the aircraft is established in the initial climb. Quite often, the drivers will pull the power back to keep the EGT just inside the limit.
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And hows cruising at high altitudes conserving fuel ?
This is not directly related to conserving fuel, however once in the cruise the throttles can be backed off as additional power is not required for the climb.
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Hows speed of sound higher at higher altitudes im not quite convinced with this , as far as i understand as per my school notes mach number decreases at higher altitudes consequentlly speed of sound reduces .:S
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I have to correct myself on one thing.
Actually the temperature of the air entering the engine is not much of significance. More of significance is the density (or density altitude) and the required thrust setting.
About high speed flight:
The higher the speed of sound, the lower your airplane' s machnumber.
You and dodo are saying the same thing, it' s just a matter of interpretation.
If M1 = 1500km/h and your speed is 1000km/h, you are flying Mach 0.83
If M1 = 1200km/h and your speed is the same, you are flying Mach 0.67
The simple explanation for high altitude cruise is that the air being less dense, you have less parasite drag and less total drag.
You can fly faster for the same amount of drag.
For high altitude transonic flight you will add the lower machnumber which means that you will suffer less from the effect of the compressibility of air, shockwave-induced drag.
You can fly faster for the same amount of drag.
By flying faster for the same amount of drag, you fly faster for the same amount of thrust.
There are other things but I need to add this one to get you thinking.
Gravity decreases by a bit over 0.3% at 10km from sealevel.
The apparent mass of a 500 ton A380 will decrease by 1.5 tons.
Actually the temperature of the air entering the engine is not much of significance. More of significance is the density (or density altitude) and the required thrust setting.
About high speed flight:
The higher the speed of sound, the lower your airplane' s machnumber.
You and dodo are saying the same thing, it' s just a matter of interpretation.
If M1 = 1500km/h and your speed is 1000km/h, you are flying Mach 0.83
If M1 = 1200km/h and your speed is the same, you are flying Mach 0.67
The simple explanation for high altitude cruise is that the air being less dense, you have less parasite drag and less total drag.
You can fly faster for the same amount of drag.
For high altitude transonic flight you will add the lower machnumber which means that you will suffer less from the effect of the compressibility of air, shockwave-induced drag.
You can fly faster for the same amount of drag.
By flying faster for the same amount of drag, you fly faster for the same amount of thrust.
There are other things but I need to add this one to get you thinking.
Gravity decreases by a bit over 0.3% at 10km from sealevel.
The apparent mass of a 500 ton A380 will decrease by 1.5 tons.
Last edited by fly_antonov; 5th Mar 2010 at 12:54.
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fly_antonov
First i'd have to express my praise for you again for all the help
and im pleased with your answer since it disambiguated everything with respect to high altitude flights and fuel economization .
However i'd have to disagree with one thing you said.
''If M1 = 1500km/h and your speed is 1000km/h, you are flying Mach 0.83
If M1 = 1200km/h and your speed is the same, you are flying Mach 0.67''
I reckon it should be the other way around .
1000/1500 =0.67
1000/1200 =0.83
Thanks everyone for all the attempts to help
First i'd have to express my praise for you again for all the help
and im pleased with your answer since it disambiguated everything with respect to high altitude flights and fuel economization .
However i'd have to disagree with one thing you said.
''If M1 = 1500km/h and your speed is 1000km/h, you are flying Mach 0.83
If M1 = 1200km/h and your speed is the same, you are flying Mach 0.67''
I reckon it should be the other way around .
1000/1500 =0.67
1000/1200 =0.83
Thanks everyone for all the attempts to help
