Timeline for How does fuel economy quantitatively depend on altitude for a jet?
Current License: CC BY-SA 4.0
10 events
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| Oct 14, 2020 at 14:20 | comment | added | Bianfable | @JanHudec Something like that is plotted in the question: TAS and Mach as a function of altitude, where IAS is from the climb profile 250/280 and then later fixed Mach. | |
| Oct 14, 2020 at 12:35 | comment | added | Jan Hudec | it would certainly be better to have some theory or at least empirical formula for it, but it is enough to show just how much jet engine efficiency increases with speed. Would be useful to accompany with plot/formula for the $\frac{\rho}{\rho_0}$ that gives the relationship between velocity and indicated airspeed (equivalent airspeed, actually) to see how quickly the velocity increases with altitude for given indicated airpspeed. | |
| Oct 14, 2020 at 10:02 | comment | added | Bianfable | @JanHudec I added propulsive efficiency now, but I'm not sure if the graph I found is useful for this comparison. | |
| Oct 14, 2020 at 9:59 | history | edited | Bianfable | CC BY-SA 4.0 |
add propulsive efficiency
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| Jun 6, 2020 at 18:58 | comment | added | Jan Hudec | the problem is that good data is difficult to find. Long ago I found some NASA lectures with a Java applet that is supposed to calculate this. The four links I wrote down are: <grc.nasa.gov/WWW/k-12/airplane/specth.html>, <grc.nasa.gov/WWW/k-12/airplane/turbfan.html>, <grc.nasa.gov/WWW/k-12/airplane/ngnsim.html> and <grc.nasa.gov/WWW/K-12/airplane/EngineTheory.pdf>. Back then I didn't see sources of the applet and I didn't get around to read through the paper, but sources seem to be there now. | |
| Jun 6, 2020 at 14:11 | comment | added | Bianfable | @JanHudec Thanks for your comments. I knew I was missing at least one term. I will have a closer look at the jet propulsive efficiency... | |
| Jun 6, 2020 at 12:20 | comment | added | Jan Hudec | Your conclusion is actually correct for propeller-driven aircraft. With decent constant speed propeller the propulsive efficiency, that is fuel consumed for unit of energy actually given to the aircraft is fairly flat. And because constant drag means constant energy for travelling given distance, the fuel consumption varies little with altitude. That is, however, not the case with turbojet and turbofan engines. Their propulsive efficiency increases quite significantly with forward speed. Unfortunately I didn't find good quantification anywhere. | |
| Jun 6, 2020 at 12:04 | comment | added | Jan Hudec | Watch out. The “drag is proportional to dynamic pressure” is sloppy wording from Peter. It is a function of dynamic pressure, but there is a dynamic pressure where drag is minimal. And there is another indicated airspeed a little above that where the power of drag (drag times speed is the rate of energy loss due to drag) and these only depend on the dynamic pressure, which means indicated airspeed, and not altitude (much; see that Peter's answer). But the conclusion that drag is basically constant is correct. | |
| S May 7, 2020 at 9:25 | history | answered | Bianfable | CC BY-SA 4.0 | |
| S May 7, 2020 at 9:25 | history | made wiki | Post Made Community Wiki by Bianfable |