Trust Question
#1
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Trust Question
what i dont inderstand is:
why does hot air produce more trust than normal air (talking abour full scale aircraft).
so why did the 707 engines work less eficently than 747.
and what about after burners????
why does hot air produce more trust than normal air (talking abour full scale aircraft).
so why did the 707 engines work less eficently than 747.
and what about after burners????
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RE: Trust Question
very easy answer to the 707 vs. the 747 engine...straight turbojet versus a high bypass engine...plus I think advancements in technology had a lot to play in it.
As far as hot vs. cold air , cooler air is more dense therefore it produces more thrust. However your question is a bit vague. hot air from a jet engine is from the combustion inside the engine Im sure you are aware of that. So Ireally dont understand your question.
Pete
As far as hot vs. cold air , cooler air is more dense therefore it produces more thrust. However your question is a bit vague. hot air from a jet engine is from the combustion inside the engine Im sure you are aware of that. So Ireally dont understand your question.
Pete
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RE: Trust Question
HOT AIR EXPAND SO MORE VELOCITY, MORE THRUST. 707 IS A TURBOJET SO IT USES MORE FUEL AT SHORTER DISTANCE . THE TURBO FAN PRODUCE ALMOST THE SAME POWER WITH LOWER FUEL USE AND LONGER DISTANCES.
FOR EXAMPLE A DYNMAX DUCTED FAN PRODUCE 12 TO 13 STATIC THRUST AT 20K RPM. A MODEL 12 # TURBINE AT IDLE PRODUCE 2 # OF THRUST AT 33.000 OR 50.000 RPM. IF YOU COULD INSTALL THE DYMINMAX FAN UNIT ONTO A MODEL JET ENGINE YOU COULD GET MORE ALMOST TO SAME POWER AT A LOWER RPM AND USE LESS FUEL.
FOR EXAMPLE A DYNMAX DUCTED FAN PRODUCE 12 TO 13 STATIC THRUST AT 20K RPM. A MODEL 12 # TURBINE AT IDLE PRODUCE 2 # OF THRUST AT 33.000 OR 50.000 RPM. IF YOU COULD INSTALL THE DYMINMAX FAN UNIT ONTO A MODEL JET ENGINE YOU COULD GET MORE ALMOST TO SAME POWER AT A LOWER RPM AND USE LESS FUEL.
#5
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RE: Trust Question
ORIGINAL: PJFaller
As far as hot vs. cold air , cooler air is more dense therefore it produces more thrust.
Pete
As far as hot vs. cold air , cooler air is more dense therefore it produces more thrust.
Pete
But then again a model jet engine does not have the capacity of changing compression rates.
Relative humidity is more of a factor than temperature for a jet engine. the more O2 you can get to the expansion stage, the more thrust. Viceversa, more H20 means less O2 meaning less Thrust~equivalent to the leaning-reachening of a reciprocal engine when climbing/descending.
So: cool air + high humidity thrust can be lower than hot dry air thrust. Just a thought on Thermo I.
David
#8
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RE: Trust Question
Re: cold air vs. hot air
The energy of a given mass of air is a function of the specific heat of the gas X its temperature. The higher the temperature, the more enery ity has (which can be expanded through a nozzle to generate THRUST). An afterburner is a classic example of this physical relationship.
Peter
The energy of a given mass of air is a function of the specific heat of the gas X its temperature. The higher the temperature, the more enery ity has (which can be expanded through a nozzle to generate THRUST). An afterburner is a classic example of this physical relationship.
Peter
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RE: Trust Question
and then there was water injection (the JT9's comes to mind).
At times the degradation in performance was a better option than dropping the payload to carry the water..............
At times the degradation in performance was a better option than dropping the payload to carry the water..............
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RE: Trust Question
ORIGINAL: MiragePilot
Re: cold air vs. hot air
The energy of a given mass of air is a function of the specific heat of the gas X its temperature. The higher the temperature, the more enery ity has (which can be expanded through a nozzle to generate THRUST). An afterburner is a classic example of this physical relationship.
Peter
Re: cold air vs. hot air
The energy of a given mass of air is a function of the specific heat of the gas X its temperature. The higher the temperature, the more enery ity has (which can be expanded through a nozzle to generate THRUST). An afterburner is a classic example of this physical relationship.
Peter
#11
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RE: Trust Question
John,
You need to read my previous posting with a little more attention to detail.
I said that the increased energy due to the heat addition is converted to thrust by EXPANDING/accelerating it through a nozzle (i.e. the convergent-divergent nozzle that is typically seen at the back end of an afterburner. The large increase in thrust that is obtained by means of an afterburner is a result of the heat addition to the gas, not a "pressure rise". In fact, the combustion process in an afterburner results in a net LOSS of stagnation pressure of the gas. Don't confuse constant volume combustion (that occurs in a CI engine, which does lead to a pressure rise) with constant pressure combustion (that occurs in a gas turbine engine) that leads to an increase of the enthalpy/energy increase of the working gas.
Peter
You need to read my previous posting with a little more attention to detail.
I said that the increased energy due to the heat addition is converted to thrust by EXPANDING/accelerating it through a nozzle (i.e. the convergent-divergent nozzle that is typically seen at the back end of an afterburner. The large increase in thrust that is obtained by means of an afterburner is a result of the heat addition to the gas, not a "pressure rise". In fact, the combustion process in an afterburner results in a net LOSS of stagnation pressure of the gas. Don't confuse constant volume combustion (that occurs in a CI engine, which does lead to a pressure rise) with constant pressure combustion (that occurs in a gas turbine engine) that leads to an increase of the enthalpy/energy increase of the working gas.
Peter