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Effect Of Temperature On Fuel Consumption & Height  
User currently offlineJulesmusician From , joined Dec 1969, posts, RR:
Posted (8 years 12 months 15 hours ago) and read 6689 times:

Supposing two 747-400's are cruising at the same speed at the same height in different parts of the world. One is in a True Air Temperature of -28C and the other is in -38C. Which would be using the most fuel? (assuming almost every other variable is the same)

[Edited 2005-12-24 16:49:09]

9 replies: All unread, jump to last
 
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2577 posts, RR: 25
Reply 1, posted (8 years 12 months 14 hours ago) and read 6683 times:

The one at the lower Total Air Temperature. Jet engines are more efficient in cold air. Also the speed of sound is lower at a colder temperature, so assuming they have the same cruise Mach number, the one flying in -38 degree air is actually flying slower, so creating less drag.


The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineJulesmusician From , joined Dec 1969, posts, RR:
Reply 2, posted (8 years 12 months 14 hours ago) and read 6682 times:

therefore is it better to conduct a flight in the coldest air at night compared to a day flight?

User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2577 posts, RR: 25
Reply 3, posted (8 years 12 months 14 hours ago) and read 6677 times:

I think schedules might have something to do with flying in whatever temperature air there is  Smile


The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineGlom From United Kingdom, joined Apr 2005, 2821 posts, RR: 10
Reply 4, posted (8 years 12 months 13 hours ago) and read 6670 times:

Quoting Jetlagged (Reply 1):
Also the speed of sound is lower at a colder temperature, so assuming they have the same cruise Mach number, the one flying in -38 degree air is actually flying slower, so creating less drag.

But colder air is denser, and denser air has a higher speed of sound, doesn't it?


User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2577 posts, RR: 25
Reply 5, posted (8 years 12 months 13 hours ago) and read 6668 times:

Quoting Glom (Reply 4):
But colder air is denser, and denser air has a higher speed of sound, doesn't it?

No, the speed of sound varies with the square root of static temperature. Pressure, and so density, is not involved.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineGlom From United Kingdom, joined Apr 2005, 2821 posts, RR: 10
Reply 6, posted (8 years 12 months 12 hours ago) and read 6661 times:

Quoting Jetlagged (Reply 5):
No, the speed of sound varies with the square root of static temperature. Pressure, and so density, is not involved.

Ah yes. I remember now.


User currently offline3201 From , joined Dec 1969, posts, RR:
Reply 7, posted (8 years 12 months 11 hours ago) and read 6652 times:

Cruise temperatures might not factor into schedule decisions, but departure times for some long-hauls from very hot places (Mombassa comes to mind) are restricted to certain times of day (for certain aircraft types) due to takeoff performance. Those UK charter operators with 593.5 people on a 767-300 (in a comfortable 2-4-2 seating arrangement) have to leave at 5am so they can get off the ground.

User currently offlineXXXX10 From United Kingdom, joined Jan 2000, 777 posts, RR: 0
Reply 8, posted (8 years 12 months 7 hours ago) and read 6632 times:

I would imagine that colder air would improve fuel economy but the slower mach number and therefore ground speed would mean a longer flight time,although there would be many other variables

User currently offline777236ER From , joined Dec 1969, posts, RR:
Reply 9, posted (8 years 12 months 7 hours ago) and read 6629 times:

Quoting XXXX10 (Reply 8):
I would imagine that colder air would improve fuel economy but the slower mach number and therefore ground speed would mean a longer flight time,although there would be many other variables

Mach Number varies with the root of the static temperature, the compressor pressure ratio varies with inversely but proportionally with the temperature. So despite the lower Mach Number, it''ll still be more efficient cruising at a lower temperature.


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