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Fuel Consumption Question

Aak777 From Qatar, joined Apr 2001, 284 posts, RR: 0
Posted (10 years 1 month 1 week 4 days 12 hours ago) and read 1489 times:

My question is why do jet engines consume less fuel with higher altetude? is it the thin air or the less drag force exerted on the plane?
thanks.

4 replies: All unread, jump to last

PanAm747 From United States of America, joined Feb 2004, 4242 posts, RR: 11
Reply 1, posted (10 years 1 month 1 week 4 days 12 hours ago) and read 1453 times:

I really don't know a lot about jet engines, but for a prop at high cruising altitudes, the mixture is leaned and the throttle is reduced. So you have less gas (and more air) going into the engine.

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Bragi From Iceland, joined May 2001, 218 posts, RR: 0
Reply 2, posted (10 years 1 month 1 week 3 days 22 hours ago) and read 1424 times:

It´s actually a combination of both. Less drag means less power for a given airspeed, than for a similar a.s. at lower altitude.
It´s more practical, even on shorter flights, to climb to let´s say 32.000 and begin descending soon than to stay at 25.000 feet for a longer period of time.
For best range you should go high & slow!

Muhammad Ali: "Superman don’t need no seat belt." Flight Attendant: "Superman don’t need no airplane, either."

FDXmech From United States of America, joined Mar 2000, 3251 posts, RR: 44
Reply 3, posted (10 years 1 month 1 week 3 days 19 hours ago) and read 1395 times:

A large reason for fuel efficiency at high altitudes is, basically, true airspeed (TAS) increases as altitude increases.

Though the engines are consuming large amounts of fuel, this is greatly offset by the very high TAS the aircraft is traveling (at high altitudes). This translates to huge amounts of distance traveled per unit of fuel.

You're only as good as your last departure.

Pmk From United States of America, joined May 1999, 664 posts, RR: 2
Reply 4, posted (10 years 1 month 1 week 3 days 5 hours ago) and read 1357 times:

FDXmech is right, as always. But to avoid a mistake, a piston engine gets less fuel efficient and less effective at high altitudes. This is because there is less oxygen in the air, therefore more air must be taken to complete the necessary stochemeric reaction. In short more air must be taken in to burn the fuel completely. Also a prop becomes less effective as there is less air mass.

Peter

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