Why are jet engines more fuel efficient at high altitude?
Guest
Posts: n/a
You are all obviously well educated gentlemen and pilots, I use the term loosely for some of you 
One question that has not been answered is what actually moves the aircraft.
For props, is it the vacuum ahead of the aircraft that is pulling, or the flow in the back pushing??
What about jets or fans?? Are the turbine blades or fans pulling the aircraft, and the resultant reaction is the thrust that we measure.
One question that has not been answered is what actually moves the aircraft.
For props, is it the vacuum ahead of the aircraft that is pulling, or the flow in the back pushing??
What about jets or fans?? Are the turbine blades or fans pulling the aircraft, and the resultant reaction is the thrust that we measure.
Guest
Posts: n/a
Has nothing to do with 'vacuum' or 'sucking'.
For every action there's an equal and opposite reaction. So, if you throw a mass in one direction YOU will be pushed in the opposite direction. The more momentum (amount of mass or the additional velocity) that you impart then the greater the opposite reaction.
This is also true if the mass that is being thrown happens to be air. The engine uses the chemical energy in fuel to raise the temperature (adding heat energy) of the air flowing through it then converts that heat enercy into velocity (kinetic energy).
It effectively shoves the air out of the back a lot harder than the air came in. The net result is the engine experiences a reaction in the opposite direction.
Even in space where the vacuum is all around (apart from the immediate vicinity of the efflux) this is the case. If it weren't, all those satellites & astronauts up there are in for a very rude surprise - quite apart from the ones waiting to go.
For every action there's an equal and opposite reaction. So, if you throw a mass in one direction YOU will be pushed in the opposite direction. The more momentum (amount of mass or the additional velocity) that you impart then the greater the opposite reaction.
This is also true if the mass that is being thrown happens to be air. The engine uses the chemical energy in fuel to raise the temperature (adding heat energy) of the air flowing through it then converts that heat enercy into velocity (kinetic energy).
It effectively shoves the air out of the back a lot harder than the air came in. The net result is the engine experiences a reaction in the opposite direction.
Even in space where the vacuum is all around (apart from the immediate vicinity of the efflux) this is the case. If it weren't, all those satellites & astronauts up there are in for a very rude surprise - quite apart from the ones waiting to go.
Join Date: Dec 2003
Location: Dubai
Posts: 212
Likes: 0
Received 0 Likes
on
0 Posts
N2
Or does RAM effect at high altitudes lead to saturation of the exhaust pipe leading to an increse of thruse that will patially compensate for the loss due to the same increase of TAS ?
Does ram effect now provide the bulk of compressive efficiency at altitude and crz speed?
Or does RAM effect at high altitudes lead to saturation of the exhaust pipe leading to an increse of thruse that will patially compensate for the loss due to the same increase of TAS ?
Join Date: Oct 2003
Location: Canberra Australia
Posts: 1,300
Likes: 0
Received 0 Likes
on
0 Posts
Sometimes too much power available for cruise.
Some aircraft which operate at lower altitudes have their engines operating inefficiently at low power.
In these circumstances it is a technique to close 1 of 4, or maybe even 2 of 4, down so that the operating engines will be at much higher power up near their best efficiency.
The P3 maritime patrol aircraft is one where this is done often to extend patrol time.
Probably done too with turbojet powered maritime aircraft.
Any others do this routinely?
Some aircraft which operate at lower altitudes have their engines operating inefficiently at low power.
In these circumstances it is a technique to close 1 of 4, or maybe even 2 of 4, down so that the operating engines will be at much higher power up near their best efficiency.
The P3 maritime patrol aircraft is one where this is done often to extend patrol time.
Probably done too with turbojet powered maritime aircraft.
Any others do this routinely?
Turbofans
Turbofans belong in this discussion, especially as the vast majority (if not all) of current civil jet transports are powered by turbofans.
Here the jet core exists primarily to power the fan which supplies the majority of the thrust; so we find ourselves moving back towards the propeller case where a large mass of air is accelerated a small amount compared to the jet core.
How this all works out against altitude I shall have to leave to the more erudite. My guess is that the engine designers have optimised overall cruise SFC by balancing fan parameters against jet core parameters. And as others have mentioned, TAS gains at altitude are quite useful.
Now who can offer an explanation for why turbojets are more effective in the takeoff case than turbofans -- or at least were when compared against early turbofans?
Here the jet core exists primarily to power the fan which supplies the majority of the thrust; so we find ourselves moving back towards the propeller case where a large mass of air is accelerated a small amount compared to the jet core.
How this all works out against altitude I shall have to leave to the more erudite. My guess is that the engine designers have optimised overall cruise SFC by balancing fan parameters against jet core parameters. And as others have mentioned, TAS gains at altitude are quite useful.
Now who can offer an explanation for why turbojets are more effective in the takeoff case than turbofans -- or at least were when compared against early turbofans?
Super-Friendly Aviator
Join Date: Jun 2004
Location: Reigate, UK
Age: 42
Posts: 424
Likes: 0
Received 0 Likes
on
0 Posts
I suspect the answer to the question "why are jet engines more efficient at altitude?" involves a combination of more than one factor.
Looking at it in a simplistic way - the higher up you are, the less dense the atmosphere i.e. atmospheric pressure is lower (yeah yeah, we know!). Assuming the pressure of the exhaust gas is constant throughtout the altitude regime, then this exhaust gas will be allowed to expands more at higher altitude than at lower altitude, as it always expand to ambient pressure. This means more work is done by the gas and, as long as the expansion occurs in the engine nozzle, more thrust is imparted to the aircraft.
This applies very well to rocket engines (which work best in vacuum) and the principle should be equally appicable to the exhaust from a jet engine.
A simple jet engine is more complicated than a simple rocket engine so I'm sure other factors (no doubt mentioned above) come into play...and may be more important.
Regards,
V1R
Looking at it in a simplistic way - the higher up you are, the less dense the atmosphere i.e. atmospheric pressure is lower (yeah yeah, we know!). Assuming the pressure of the exhaust gas is constant throughtout the altitude regime, then this exhaust gas will be allowed to expands more at higher altitude than at lower altitude, as it always expand to ambient pressure. This means more work is done by the gas and, as long as the expansion occurs in the engine nozzle, more thrust is imparted to the aircraft.
This applies very well to rocket engines (which work best in vacuum) and the principle should be equally appicable to the exhaust from a jet engine.
A simple jet engine is more complicated than a simple rocket engine so I'm sure other factors (no doubt mentioned above) come into play...and may be more important.
Regards,
V1R
