Julesmusician From , joined Dec 1969, posts, RR: Posted (9 years 5 months 1 day 2 hours ago) and read 7406 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)
Jetlagged From United Kingdom, joined Jan 2005, 2602 posts, RR: 25
Reply 1, posted (9 years 5 months 1 day 1 hour ago) and read 7400 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.
Glom From United Kingdom, joined Apr 2005, 2823 posts, RR: 9
Reply 4, posted (9 years 5 months 1 day ago) and read 7387 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?
3201 From , joined Dec 1969, posts, RR:
Reply 7, posted (9 years 5 months 22 hours ago) and read 7369 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.
777236ER From , joined Dec 1969, posts, RR:
Reply 9, posted (9 years 5 months 18 hours ago) and read 7346 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.