ATPL max thrust
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ATPL max thrust
hey guys this might sound a stupid question but I generally am not sure. Where would high by-pass trubo fan jet engine produce max thrust? so far if the test gives me the option I would pick extreamly fast at extreamly low level on an extreamly cold day but I don't think that option would be there.
Would the affect of altitude be enough to limit ram recovery to a max thrust below that which you would get sitting on the runway with brakes holding still?
Would the affect of altitude be enough to limit ram recovery to a max thrust below that which you would get sitting on the runway with brakes holding still?
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Hi Fhead.
Not sure if this will help at all, but have a play anyway.
http://www.grc.nasa.gov/WWW/K-12/airplane/ngnsim.html
Hope it helps!
Not sure if this will help at all, but have a play anyway.
http://www.grc.nasa.gov/WWW/K-12/airplane/ngnsim.html
Hope it helps!
Hi Fhead,
If I recall my basic jet theory correctly, and I think I do, all jet engines, regardless of type, that is turboet or turbofan produce their maximum possible thrust under static conditions.
Since thrust is a force,this statement comes from the basic physics formula--
F=MA, that is force equals mass times acceleration. Acceleration is defined as a change in velocity, and therefore the formula becomes--
Thrust=F=M(V2-V1) where V2 is the jet exhaust velocity and V1 is the intake air velocity.
As you can see from the above formula in order to produce the maximum thrust, the engine has to give a certain mass of air the greatest possible acceleration.
Therefore, for A to be at a maximum value,V1 must be zero, that is the aircraft is not moving, and therefore the intake air velocity is zero, hence the difference between the intake and exhaust velocity is at a maximum.
Obviously, as you are probably aware,the engine performance criteria, in this case thrust rating will be calculated under MSL ISA-conditions.
As a general guide, compared to its sea level, static thrust rating, a jet engine at altitude, say at approximately 35 to 40,000 feet, produces approximately one quarter of its static sea level thrust.
This is generally true of either a turbojet or turbofan engine and is due to the large reduction in air density at such altitudes, therefore, the mass flow into the engine is reduced compared to sea level.
You have probably heard someone say, when referring to a jet engine that, for eg- "Model ABC produces 6000 pounds static thrust". They say "static thrust" in this case, because they are referring to the maximum that the engine can produce.
I hope that answers your question in basic terms, after all they are the only terms I know. haha
If I recall my basic jet theory correctly, and I think I do, all jet engines, regardless of type, that is turboet or turbofan produce their maximum possible thrust under static conditions.
Since thrust is a force,this statement comes from the basic physics formula--
F=MA, that is force equals mass times acceleration. Acceleration is defined as a change in velocity, and therefore the formula becomes--
Thrust=F=M(V2-V1) where V2 is the jet exhaust velocity and V1 is the intake air velocity.
As you can see from the above formula in order to produce the maximum thrust, the engine has to give a certain mass of air the greatest possible acceleration.
Therefore, for A to be at a maximum value,V1 must be zero, that is the aircraft is not moving, and therefore the intake air velocity is zero, hence the difference between the intake and exhaust velocity is at a maximum.
Obviously, as you are probably aware,the engine performance criteria, in this case thrust rating will be calculated under MSL ISA-conditions.
As a general guide, compared to its sea level, static thrust rating, a jet engine at altitude, say at approximately 35 to 40,000 feet, produces approximately one quarter of its static sea level thrust.
This is generally true of either a turbojet or turbofan engine and is due to the large reduction in air density at such altitudes, therefore, the mass flow into the engine is reduced compared to sea level.
You have probably heard someone say, when referring to a jet engine that, for eg- "Model ABC produces 6000 pounds static thrust". They say "static thrust" in this case, because they are referring to the maximum that the engine can produce.
I hope that answers your question in basic terms, after all they are the only terms I know. haha
Last edited by aussie027; 11th Jun 2007 at 03:10.
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cheers guys
I wasn't sure how effective the ram recovery was because none of these graphs from 3 different books have any values on them. That link but my mind at rest as max thrust is at static unless you can get about about 1000knots at FL330
P.S. I edited this just after the post after this one
I wasn't sure how effective the ram recovery was because none of these graphs from 3 different books have any values on them. That link but my mind at rest as max thrust is at static unless you can get about about 1000knots at FL330
P.S. I edited this just after the post after this one
Last edited by Fhead; 11th Jun 2007 at 04:57.
Aussie, acceleration's a rate of change of velocity, ie change in velocity / the time it takes for the change to happen.
Also there's things like ram effect to take into account - fast moving engine gets air stuffed down its throat which compresses it a bit, adding to the mass flow through the engine per unit volume.
I can't remember all the whys and wherefores on the subject off the top of my head, but I think it can get a bit more complex than what you said.
Also there's things like ram effect to take into account - fast moving engine gets air stuffed down its throat which compresses it a bit, adding to the mass flow through the engine per unit volume.
I can't remember all the whys and wherefores on the subject off the top of my head, but I think it can get a bit more complex than what you said.
Hi arm out the window,
Yes I am aware it is a subject that has many different factors involved.
Yes Ram effect does make a difference to the mass flow due to compression but this is only significant at extremely high speeds, I cannot remember the exact values but from memory it tends to be more significant at supersonic speeds as opposed to sub sonic and trans-sonic.
Either way this has no effect on the absolute maximum thrust output of the engine which can only occur at static conditions for the reasons I have stated above. Certainly this ram effect adds to the general available thrust level at very high speeds due to the compression and increase in mass flow.
Cheers, Aussie 027
Yes I am aware it is a subject that has many different factors involved.
Yes Ram effect does make a difference to the mass flow due to compression but this is only significant at extremely high speeds, I cannot remember the exact values but from memory it tends to be more significant at supersonic speeds as opposed to sub sonic and trans-sonic.
Either way this has no effect on the absolute maximum thrust output of the engine which can only occur at static conditions for the reasons I have stated above. Certainly this ram effect adds to the general available thrust level at very high speeds due to the compression and increase in mass flow.
Cheers, Aussie 027
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Ram effect most certainly is significant at passenger jet speeds according to my ATPL notes and you just need to look at a TO distance graph from systems and performance to see what effect temperature has. Lower temp gives higher density and better compressor efficiency from cooling so my guess would be that best thrust would be produced at some altitude above sea level and some speed above zero..
Hi Mattyj.
Sorry, I didn't cx my notes but, no that is not correct. the increase in mass flow from ram effect and compression does not offset the reduction in density at high altitudes enough to give you a thrust increase compared to ISA MSL, plus the fact that the max thrust is produced at static conditions as the acceleration of the airflow is at a max when V1, intake air velocity is zero.
As I said earlier--
Ram effect most certainly is significant at passenger jet speeds according to my ATPL notes and you just need to look at a TO distance graph from systems and performance to see what effect temperature has. Lower temp gives higher density and better compressor efficiency from cooling so my guess would be that best thrust would be produced at some altitude above sea level and some speed above zero..
Sorry, I didn't cx my notes but, no that is not correct. the increase in mass flow from ram effect and compression does not offset the reduction in density at high altitudes enough to give you a thrust increase compared to ISA MSL, plus the fact that the max thrust is produced at static conditions as the acceleration of the airflow is at a max when V1, intake air velocity is zero.
As I said earlier--
As a general guide, compared to its sea level, static thrust rating, a jet engine at altitude, say at approximately 35 to 40,000 feet, produces approximately one quarter of its static sea level thrust.
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check out the link it shows it really well and don't trust the looks of those graphs. They don't show thrust through all stages of flight so you have to compare servial drifferent ones and none of them (that I have seen) have any vaules written on them, which makes them impossible to compare anyway.
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If Mr Higgins taught me anything, it would be worth looking at the relationship between THRUST and PROPULSIVE EFFICIENCY.
Cheers
Cheers
Last edited by flyingfrenchman; 13th Jun 2007 at 14:39. Reason: The punctuation nazis
