Why are jet engines more effective higher ?
Guest
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Simplistic answer:
Jets don't like being throttled back, i.e. they are most efficient at approx 100% design rpm.
At low level 100% rpm produces way too much thrust, fine for take off, too much to crz without pulling wings off. (Higher mass flow of air means higher mass flow of fuel)
As alt increased, amount of thrust goes down until you don't have to throttle back too much to crz comfortably so engine more efficient. (Mainly due to optimised gas flows internally).
So - higher means engine operates nearer optimal regime.
Of course there are other advantages - increased TAS being the obvious one. The optimum level is a compromise between what the airframe likes and what the engine likes.
If you want a jet a/c optimised for low level build it so it requires four engines to take off, then shut two down once you're airborne. (I believe that's what the RAF do with Nimrods if loitering low level for long periods)
Jets don't like being throttled back, i.e. they are most efficient at approx 100% design rpm.
At low level 100% rpm produces way too much thrust, fine for take off, too much to crz without pulling wings off. (Higher mass flow of air means higher mass flow of fuel)
As alt increased, amount of thrust goes down until you don't have to throttle back too much to crz comfortably so engine more efficient. (Mainly due to optimised gas flows internally).
So - higher means engine operates nearer optimal regime.
Of course there are other advantages - increased TAS being the obvious one. The optimum level is a compromise between what the airframe likes and what the engine likes.
If you want a jet a/c optimised for low level build it so it requires four engines to take off, then shut two down once you're airborne. (I believe that's what the RAF do with Nimrods if loitering low level for long periods)
Guest
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While its true that engine components are designed to optimize their efficiency for the cruise condition to give best overall SFC, SAR etc. The over-riding reason that the efficiency is best at altitude is down to the fact that the cycle efficiency for gas turbines is a function of overall temperature ratio.
Maximum cycle temperature is fixed by HP turbine and NGV material and cooling limits, but the minimum is governed by ambient temperature and thus altitude (upto the tropopause at least).
So High Altitude = High Temperature Ratio
Maximum cycle temperature is fixed by HP turbine and NGV material and cooling limits, but the minimum is governed by ambient temperature and thus altitude (upto the tropopause at least).
So High Altitude = High Temperature Ratio
Guest
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Conclusion.
If you're higher, you can run the engine hotter. If the engine runs hotter, it produces more thrust for a given liter of kerosene. So it goes either faster or farther, all else being equal.
Of course jet engines are, inconveniently for this explanation, bolted to airframes, and airframes have wings. The altitude best for the wing isn't usually the altitude best for the engine. But we'll let that one go.
If you're higher, you can run the engine hotter. If the engine runs hotter, it produces more thrust for a given liter of kerosene. So it goes either faster or farther, all else being equal.
Of course jet engines are, inconveniently for this explanation, bolted to airframes, and airframes have wings. The altitude best for the wing isn't usually the altitude best for the engine. But we'll let that one go.
Guest
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Maybe I didn't explain it well enough before.
Cold air requires less work energy to compress a given mass than hotter air.
Because it is colder, you can put more heat energy into it without melting the HP turbine.
Because of the lower compressor requirement, less turbine work and expansion is required to power the compressor.
Hence proportionally more gas energy is available to turn the fan or expand through the primary nozzle (= greater thrust per weight of fuel = efficiency)
Cold air requires less work energy to compress a given mass than hotter air.
Because it is colder, you can put more heat energy into it without melting the HP turbine.
Because of the lower compressor requirement, less turbine work and expansion is required to power the compressor.
Hence proportionally more gas energy is available to turn the fan or expand through the primary nozzle (= greater thrust per weight of fuel = efficiency)
