julesmusician
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Effect Of Temperature On Fuel Consumption & Height

Sun Dec 25, 2005 12:48 am

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]
African Civil Aviation Commission president "You don't want to fly out as a passenger and come back as cargo."
 
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Jetlagged
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RE: Effect Of Temperature On Fuel Consumption & Height

Sun Dec 25, 2005 1:35 am

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.
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julesmusician
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RE: Effect Of Temperature On Fuel Consumption & Height

Sun Dec 25, 2005 1:44 am

therefore is it better to conduct a flight in the coldest air at night compared to a day flight?
African Civil Aviation Commission president "You don't want to fly out as a passenger and come back as cargo."
 
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Jetlagged
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RE: Effect Of Temperature On Fuel Consumption & Height

Sun Dec 25, 2005 1:49 am

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.
 
Glom
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RE: Effect Of Temperature On Fuel Consumption & Height

Sun Dec 25, 2005 2:25 am

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?
 
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Jetlagged
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RE: Effect Of Temperature On Fuel Consumption & Height

Sun Dec 25, 2005 2:35 am

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.
 
Glom
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RE: Effect Of Temperature On Fuel Consumption & Height

Sun Dec 25, 2005 3:12 am

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.
 
3201
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RE: Effect Of Temperature On Fuel Consumption & Height

Sun Dec 25, 2005 4:43 am

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.
7 hours aint long-haul
 
XXXX10
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RE: Effect Of Temperature On Fuel Consumption & Height

Sun Dec 25, 2005 8:12 am

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
 
777236ER
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RE: Effect Of Temperature On Fuel Consumption & Height

Sun Dec 25, 2005 8:31 am

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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