Register Forms FAQ Wikiposts Calendar Search Today's Posts Mark Forums Read

 Tech Log The very best in practical technical discussion on the web

 18th Jun 2000, 20:13 #1 (permalink) DECU Guest   Posts: n/a Why are jet engines more fuel efficient at high altitude? Hi all, Can anyone please tell me why jeyt engines are more fuel efficient at high altitude? Thanks, DECU
 19th Jun 2000, 02:48 #2 (permalink) Tinstaafl Guest   Posts: n/a My, admittedly simple, understanding: Jet engines are most efficient operating at max permitted RPM, subject to operating limits. Unfortunately, at lower altitudes Temp & pressure limits would be exceeded so, to produce a given amount of thrust eg maximum or some cruise setting, the RPM must be reduced from maximum. As altitude increases air density reduces so thrust output for a given RPM reduces. To maintain this thrust RPM must be increased. As RPM is increased (but not beyond limits) efficiency rises. Couple with this, the airframe is designed to be most efficient at a particular Mach No. This will cause a certain amount of drag which in turn will require a certain fixed amount of thrust (all other factors being equal). As altitude increases with the airframe maintaining an efficient airspeed, the thrust is produced by an engine that operates closer & closer to it's max RPM. Hence overall efficiency rises. NB: Rate of change of gain in thrust efficiency would have to be greater than rate of change of TAS reduction, with gain in altitude at this 'efficient' Mach No. Someone please jump in and correct my errors!!
 19th Jun 2000, 06:12 #3 (permalink) flite idol Guest   Posts: n/a Ok for starters TAS is Equivalent airspeed corrected for density and it is related inversly, in other words a decrease in density will give an increase in TAS. Its relationship to altitude requires no furthur explanation.
 19th Jun 2000, 22:54 #4 (permalink) Tinstaafl Guest   Posts: n/a Flite Idol, My comment regarding the reduction in TAS with increasing altitude was related to maintaining a constant Mach No. As temp reduces with altitude so does the TAS associated with a constant Mach No. I was trying to refer to flight where Mach No. is the limiting factor and not the relationship between EAS & TAS with changing density. T ------- Grammar! Do we really need it?
 19th Jun 2000, 23:55 #5 (permalink) mik Guest   Posts: n/a D. P. Davies's "Handling the Big Jets" has a section on why higher is better for jet transport aircraft. Its a book that is well worth getting hold of, although it is now a bit old. Mik p.s. the reason I'm not going to quote Mr. Davies's explanation is that a) it's fairly long, and b) it includes graphs. [This message has been edited by mik (edited 19 June 2000).]
 20th Jun 2000, 00:11 #6 (permalink) flite idol Guest   Posts: n/a Okeydokey T!!
 20th Jun 2000, 03:13 #7 (permalink) Tinstaafl Guest   Posts: n/a sigh I loved my copy of 'Handling the Big Jets' - but it disappeared years ago. sniff
 20th Jun 2000, 03:41 #8 (permalink) CaptainSquelch Guest   Posts: n/a Handling the big jets is nice but if you want an answer you just might start with "Gas turbine theory" by Cohen, Rogers and Saravanamuttoo ISBN 0 582 44927 8 If you have any questions after that there is a nice load of literature mentioned in it. BTW it has something to do with cold air and thin air. hence the optimum around the tropopause..... good luck and a lot of reading pleasure.
 20th Jun 2000, 13:35 #9 (permalink) Atlas Guest   Posts: n/a Hmmm. Thought one of the real engineering types would venture in here. In the meantime, just the musings of another pilot who tries to retain some of his 6th form physics, so take with a grain of salt (KISS as always) . As Tinstaafl and others points out, the jets' guts are optimized for operation at high altitude where they spend most of their time, therefore they will have somewhat poorer sfc lower down, but where the efficiency of the aircraft is concerned, that isn't the dramatic difference. Jets do their work by throwing masses of air out the pipe at a velocity, at a rate, which is pretty much force=massxacceleration. The forward velocity (TAS) of the A/C is largely secondary. Consider that the thrust (=drag) to maintain 250-300 CAS (about what jet airliners cruise at) will be similar at MSL as it is at FL310 and above. Thus, the fuel burn at 250-300 (CAS=TAS) at MSL is similar to that at about 440-480 TAS at the FL's (250-300 IAS!), actually a little greater due to the inefficiencies to which Mr. T. et al referred. I know I don't wish to read several long tomes to answer every little curiosity I have, but if you do want to read up a bit, in addition to the other books referred to, you might look at MECHANICS of FLIGHT by A.C. Kermode. If its in your nearest library, the index will direct you to the dozen or so pages that will explain it all quite nicely. Cheers, Atlas
 22nd Jun 2000, 17:10 #12 (permalink) Mice Guest   Posts: n/a Jeeesus Checkboard! I think I might just give my engineer licence back to the authorities, I am obviously not worthy! As you said, complex answers for a simple question. As the aircraft frame is encountering less air resistance at higher altitude, there is less requirement for power to propell it at constant speed through the air. Therefore, as the air is less dense, the optimum fuel/air ratio is maintained by reducing the fuel input. Modern turbofans are gas path optimised for best SFC in CRZ also. The advantage they have over a piston engine at high altitude is the high mass airflow the turbine engine design can process, whereas the piston engine mass flow is a limit of the swept volume, and the ability of other equipment to compress intake air into the cylinder at pressures that allow the engine to still function. Remember, the turbine engine mass airflow capacity is variable, whereas this is not exactly so for the piston design.
 22nd Jun 2000, 20:10 #13 (permalink) DECU Guest   Posts: n/a Hi all, Wow! Thanks for all your inputs. It is very much appreciated. I asked this question because it was a question which I was asked during an interview recently, and without going into too much detail, I just told them the basics, which I myself do not know if it was the correct answer. In fact, I cant even really remember what I told the interviewers, except that it was due to a higher TAS and also the air being less dense. Once again, I really appreciate the comments and in depth explanations. I guess I'll know what to tell the interviewers the next time round. Cheers, DECU
25th Jun 2000, 15:44   #14 (permalink)

Join Date: Aug 1998
Location: Ex-pat Aussie in the UK
Posts: 3,629

Quote:
 then the momentum given to the air per second is mv, so Thrust = mv.
F=ma, a is in m/s/s, v = m/s, for momentum per second = ma = mv/s

[This message has been edited by Checkboard (edited 04 July 2000).]

 25th Jun 2000, 18:51 #15 (permalink) N2 Guest   Posts: n/a Checkboard, In an effort to reduce ripping through dusty boxes of college notes, some stub of my brain seems to recall the natural limiting fwd speed the engine may attain once the active (thrust) and reactive forces are in balance. Our savior here now being ram effect. Does ram effect now provide the bulk of compressive efficiency at altitude and crz speed?
 28th Jun 2000, 20:35 #16 (permalink) Elevation Guest   Posts: n/a If I remember correctly, it has something to do with the exhaust gas. The cooler the outside air temp, the more the exhaust gas expands as it exits the chamber hence creating more forward "push". Then again I better go back and dig for my old textbook on turbine
 1st Jul 2000, 10:06 #17 (permalink) twistedenginestarter Guest   Posts: n/a I've just found this at http://www.faa.gov/avr/afs/acs/61-107.txt (1) The efficiency of the jet engine at high altitudes is the primary reason for operating in the high-altitude environment. The specific fuel consumption of jet engines decreases as the outside air temperature decreases for constant revolutions per minute (RPM) and TAS. Thus, by flying at a high altitude, the pilot is able to operate at flight levels where fuel economy is best and with the most advantageous cruise speed. For efficiency, jet aircraft are typically operated at high altitudes where cruise is usually very close to RPM or exhaust gas temperature limits.
 1st Jul 2000, 13:32 #18 (permalink) ironbutt57 Guest   Posts: n/a so all the above gobbledygook translates to a simple answer... jet-powered AIRCRAFT operate more efficiently at optimum altitudes which tend to be higher than other aircraft types for many many reasons
4th Jul 2000, 11:19   #20 (permalink)

Join Date: Aug 1998
Location: Ex-pat Aussie in the UK
Posts: 3,629

N2,
Does ram effect now provide the bulk of compressive efficiency at altitude and crz speed?

At usual operating thrust the engine exhaust achieves supersonic velocity, referred to as being "choked". Thrust levels above this increase the mass flow through the engine, but the exhaust speed is fixed. As in my original explaination this means that the faster the engine is going (in terms of TAS), the less thrust it is producing (but at a greater efficiency).

There is also a loss through momentum drag - if you are an airmolecule hanging around in the sky, and are scooped up by an engine rushing past, then the engine has to accelerate you up to its speed as you pass into the engine, and this is known as momentum drag, which has a net effect of reducing the thrust.

On the upside is the "ram ratio effect". The higher dynamic pressure into the front of the engine increases the mass flow (Force = mass x acceleration, so the more mass the engine processes, the more thrust it produces). The higher mass flow requires a higher fuel flow to meet it, the ram effect covers the losses from momentum drag, but doesn't cover the losses associated with the higher airspeed.

Elevation:
Quote:
 it has something to do with the exhaust gas. The cooler the outside air temp, the more the exhaust gas expands as it exits the chamber hence creating more forward "push".
The jet engine is a reaction engine - it produces thrust by throwing mass out the back, what happens to the exhasut after it has passes the jet nozzle has no effect on thrust produced. See below for more on temperature, though.

This common misconception also has people thinking that the jet blast deflectors seen on carriers increase the takeoff thrust of the jets on a cat launch - they don't, they just stop people & equipment behind the jet being toasted

twistedenginestarter:
Quote:
 The specific fuel consumption of jet engines decreases as the outside air temperature decreases for constant revolutions per minute (RPM) and TAS.
The reason the statement refers to a constant RPM is that the RPM controls the pressure ratio of the engine, which is the dominating factor in engine efficiency.

Having said that, when the inlet air temperature is lowered, a given heat addition can provide greater changes in pressure or volume & greater efficiency. If the RPM was limited due to EGT considerations a higher RPM can be achieved as well.