Altitude and efficiency
Guest
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Altitude and efficiency
All,
Probably one you have all been asked at some point and an old favourite for selection interviews - the question of why do aircraft, especially jets, fly so high.
I am currently embarking on a number of selection interviews, and have researched the topic, but would like to see what some of you experienced guys make of my reasoning.
Basically, I gather that as an aircraft climbs into the rarefied atmosphere holding a constant airspeed, the true airspeed of the aircraft increases. Thus, by flying at FL370 (or whatever), you are effectivly travelling a lot faster than you would at a lower altitude for a given thrust setting.
Also, the amount of thrust required to hold a given IAS increases with altitude. Thus, this allows the EPR's to be set higher, and at higher RPM, the turbines operate more efficiently.
Am I heading in the right direction or will I make a fool of myself?? Thanks for taking the time to read this (if anyoine has!!)
Cheers
Cuban
Probably one you have all been asked at some point and an old favourite for selection interviews - the question of why do aircraft, especially jets, fly so high.
I am currently embarking on a number of selection interviews, and have researched the topic, but would like to see what some of you experienced guys make of my reasoning.
Basically, I gather that as an aircraft climbs into the rarefied atmosphere holding a constant airspeed, the true airspeed of the aircraft increases. Thus, by flying at FL370 (or whatever), you are effectivly travelling a lot faster than you would at a lower altitude for a given thrust setting.
Also, the amount of thrust required to hold a given IAS increases with altitude. Thus, this allows the EPR's to be set higher, and at higher RPM, the turbines operate more efficiently.
Am I heading in the right direction or will I make a fool of myself?? Thanks for taking the time to read this (if anyoine has!!)
Cheers
Cuban
Guest
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Flying at a high altitude has numerous advantages over flying at lower levels. At higher flight levels there will be a great reduction in Total Drag and therefore a reduction in required thrust. From sea level to 40,000 ft a reduction of approximately 75% is achieved. Also the higher you go the higher your TAS and GS. So by increasing your speed you get extra benefits too-
(1) you get an increase in propulsive efficiency resulting from the higher velocity
(2) a greater "ram" effect at the entrance to the compressor, reducing slightly the work done to drive this component. In summary it can be said that the reason we fly jet a/c at high Flt. Levels is due to SFC being lower due to-(i)lower inlet temp(ii)improved propulsive efficiency(iii)"Ram" effect at higher velocity
Hope this helps!
(1) you get an increase in propulsive efficiency resulting from the higher velocity
(2) a greater "ram" effect at the entrance to the compressor, reducing slightly the work done to drive this component. In summary it can be said that the reason we fly jet a/c at high Flt. Levels is due to SFC being lower due to-(i)lower inlet temp(ii)improved propulsive efficiency(iii)"Ram" effect at higher velocity
Hope this helps!
Guest
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Well you might not be reading in the right direction.
Many factors affect the power requirements of a jet engine with regards to efficiency.
The power required from a jet engine is proportional to the acceleration required, drag developed, and the designed cruising speeds.
Before going into great details, one should consider a basic law.
1)Force is equal to mass x acceleration.
(Mass being a quantity that is constant)
If you apply this to a stationary aircraft at high power settings, W the weight of air will be accelerated from rest to V2Ft/Sec.
The thrust developed will be WV2Lbs./G. Because the aircraft is not moving, the "propulsive efficiency" will be zero.
If you allow the aircraft to move, it will attain a certain velocity, call it V1Ft/sec. the thrust formula changes to W(V2-V1)Lbs./G
The air that now passes into the engine has a velocity. (Actually, it is the forward movement of the aircraft that give the static airmass a velocity)Assuming the resultant velocity of this air relative to the aircraft is still V2Ft/sec. the acceleration of the airmass will be
V2-V1Ft./sec. If the "mass" flow remains the same, the thrust will be W(V2-V1)Lb./G
Developed Thrust will decrease as altitude increases, but efficiency increases as altitude increases, Why?
At sea level, the power available from one pound of air is increased if the temperature is lowered. The cubic volume of measured air will have more weight due to the increased density.
2) As stated by Mr. lovell, Ram effect maybe the most important single effect concerning the efficiency of a Jet engine.
Pressure increases significantly above 250 kts. This increases the "Mass" flow through the engine per unit of fuel.
Again, Mass X acceleration = Thrust.
As altitude is increased, Pressure, Density, and temperature decrease. At a constant RPM (Or N1) the pressure ratio of the compressor and the temperature rise within the engine remains constant irrespective of altitude. Due to the reduced density, fuel will have to be reduced because high EGT's are possible due to the fuel/air mixture. As the OAT decreases with altitude, more fuel can be burned without exceeding the max EGT's. Sound confused? The increase in temperature results from an increase in change of momentum of the gases passing through the engine. One pound of air now produces more thrust at altitude because there is an improved expansion ratio across the turbine which increases the efficiency of the engine at altitude!!! Don't forget, one pound of air at altitude will have a much larger volume.
(The FCU again detects this and reduces the FF accordingly, increasing efficiency)
As the aircraft gains altitude,the compressor loads will decrease due to the loss of density and the engine could overspeed if f. f. is not adjusted.
(This is part of the FCU function again)
An increase of temperature at altitude would would cause a loss of power due to the reduced air density (Less weight per volume)and more fuel would have to be added to develope the same power.
At altitude, the over all thrust is less mainly due to the reduced air density. Since drag is relative to density, the TOTAL drag will be also reduced and less power will be required for the same TAS.
If thrust decreases with an increase in altitude at the same rate as the decrease in total drag, then the TAS will remain constant at all altitudes. But as altitude is gained, the drag curve reduces and less power is required for the same TAS.
All this increases the compressor efficiency and it therefore absorbs less "work" (defined as heat) from the turbines!
I'm probably not making sense, but it works!
3) More power to maintain the same IAS.
You will definately require an increase of power to maintain the same IAS at altitude. But remember that TAS and not IAS is the important factor for fuel efficiency. Remember the formula,
SAR=TAS/FF= NM per Lb. Fuel.
Most large transport aircraft could not attain the higher IAS at altitude due to the limiting Vmo and Mmo.
Many factors affect the power requirements of a jet engine with regards to efficiency.
The power required from a jet engine is proportional to the acceleration required, drag developed, and the designed cruising speeds.
Before going into great details, one should consider a basic law.
1)Force is equal to mass x acceleration.
(Mass being a quantity that is constant)
If you apply this to a stationary aircraft at high power settings, W the weight of air will be accelerated from rest to V2Ft/Sec.
The thrust developed will be WV2Lbs./G. Because the aircraft is not moving, the "propulsive efficiency" will be zero.
If you allow the aircraft to move, it will attain a certain velocity, call it V1Ft/sec. the thrust formula changes to W(V2-V1)Lbs./G
The air that now passes into the engine has a velocity. (Actually, it is the forward movement of the aircraft that give the static airmass a velocity)Assuming the resultant velocity of this air relative to the aircraft is still V2Ft/sec. the acceleration of the airmass will be
V2-V1Ft./sec. If the "mass" flow remains the same, the thrust will be W(V2-V1)Lb./G
Developed Thrust will decrease as altitude increases, but efficiency increases as altitude increases, Why?
At sea level, the power available from one pound of air is increased if the temperature is lowered. The cubic volume of measured air will have more weight due to the increased density.
2) As stated by Mr. lovell, Ram effect maybe the most important single effect concerning the efficiency of a Jet engine.
Pressure increases significantly above 250 kts. This increases the "Mass" flow through the engine per unit of fuel.
Again, Mass X acceleration = Thrust.
As altitude is increased, Pressure, Density, and temperature decrease. At a constant RPM (Or N1) the pressure ratio of the compressor and the temperature rise within the engine remains constant irrespective of altitude. Due to the reduced density, fuel will have to be reduced because high EGT's are possible due to the fuel/air mixture. As the OAT decreases with altitude, more fuel can be burned without exceeding the max EGT's. Sound confused? The increase in temperature results from an increase in change of momentum of the gases passing through the engine. One pound of air now produces more thrust at altitude because there is an improved expansion ratio across the turbine which increases the efficiency of the engine at altitude!!! Don't forget, one pound of air at altitude will have a much larger volume.
(The FCU again detects this and reduces the FF accordingly, increasing efficiency)
As the aircraft gains altitude,the compressor loads will decrease due to the loss of density and the engine could overspeed if f. f. is not adjusted.
(This is part of the FCU function again)
An increase of temperature at altitude would would cause a loss of power due to the reduced air density (Less weight per volume)and more fuel would have to be added to develope the same power.
At altitude, the over all thrust is less mainly due to the reduced air density. Since drag is relative to density, the TOTAL drag will be also reduced and less power will be required for the same TAS.
If thrust decreases with an increase in altitude at the same rate as the decrease in total drag, then the TAS will remain constant at all altitudes. But as altitude is gained, the drag curve reduces and less power is required for the same TAS.
All this increases the compressor efficiency and it therefore absorbs less "work" (defined as heat) from the turbines!
I'm probably not making sense, but it works!
3) More power to maintain the same IAS.
You will definately require an increase of power to maintain the same IAS at altitude. But remember that TAS and not IAS is the important factor for fuel efficiency. Remember the formula,
SAR=TAS/FF= NM per Lb. Fuel.
Most large transport aircraft could not attain the higher IAS at altitude due to the limiting Vmo and Mmo.
Guest
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Cuban-
Just to clear up one thing-
In the real world, most carriers fly a constant Mach cruise schedule. For any given indicated M (.82 or .84 for example), your TAS will actually DECREASE as you climb.
TAS is dependent upon temperature, not altitude. So when you climb, the OAT is colder, so for any given indicated Mach, the TAS is lower. For example, TAS at FL 370 is about 20 kts. slower than at FL 270 at the same indicated M.
Just to clear up one thing-
In the real world, most carriers fly a constant Mach cruise schedule. For any given indicated M (.82 or .84 for example), your TAS will actually DECREASE as you climb.
TAS is dependent upon temperature, not altitude. So when you climb, the OAT is colder, so for any given indicated Mach, the TAS is lower. For example, TAS at FL 370 is about 20 kts. slower than at FL 270 at the same indicated M.
Guest
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I think it's this.
Flying high is efficient because of vastly reduced drag and its corollary - increased speed. Increased speed attracts passengers, makes better use of your capital assets and your crews. So you go as high as you reasonably can. Jets can generate the power required so design them for high altitude.
Against that however you have complications such as exposure to sudden decompression and falling speed of sound.
These factors combine to give you an optimal flight zone of 30-40000 feet.
Note that if you change certain parameters eg the cost of fuel rises much more than the cost of crew or maintenance, then this optimal level may change radically eg see the return of propellors or engines optimised for lower altitudes.
Flying high is efficient because of vastly reduced drag and its corollary - increased speed. Increased speed attracts passengers, makes better use of your capital assets and your crews. So you go as high as you reasonably can. Jets can generate the power required so design them for high altitude.
Against that however you have complications such as exposure to sudden decompression and falling speed of sound.
These factors combine to give you an optimal flight zone of 30-40000 feet.
Note that if you change certain parameters eg the cost of fuel rises much more than the cost of crew or maintenance, then this optimal level may change radically eg see the return of propellors or engines optimised for lower altitudes.
Guest
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The jet range formula beloved by CFS is
SAR=TAS/EAS x EAS/Drag x 1/SFC
The nice thing about this is that it splits up the 3 important factors which each need to be maximised.
TAS/EAS is maximised by flying as high as possible.
EAS/Drag is maximised by flying at 1.32 x Vmd.
1/SFC is maximised by flying at design RPM (normally around 95%).
1.32 Vmd is a very low IAS and, at low altitude, you'd be using a lot less than 95% RPM to hold it so you need to climb to an altitude where 95% RPM gives you the right speed and this will also maximise TAS/EAS.
There's more to it than this because, at high altitude, transonic effects come in and these will reduce the best range speed.
As far as climbing is concerned, you need to keep the maximum excess power for the best climb rate. This requires a decreasing TAS and one way to achieve this is to fly constant Mach (speed of sound decreases with altitude as far as the Tropopause).
SAR=TAS/EAS x EAS/Drag x 1/SFC
The nice thing about this is that it splits up the 3 important factors which each need to be maximised.
TAS/EAS is maximised by flying as high as possible.
EAS/Drag is maximised by flying at 1.32 x Vmd.
1/SFC is maximised by flying at design RPM (normally around 95%).
1.32 Vmd is a very low IAS and, at low altitude, you'd be using a lot less than 95% RPM to hold it so you need to climb to an altitude where 95% RPM gives you the right speed and this will also maximise TAS/EAS.
There's more to it than this because, at high altitude, transonic effects come in and these will reduce the best range speed.
As far as climbing is concerned, you need to keep the maximum excess power for the best climb rate. This requires a decreasing TAS and one way to achieve this is to fly constant Mach (speed of sound decreases with altitude as far as the Tropopause).
