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Fuel consumption at high altitude in a jet

Postby HJ1AN » Tue Jul 28, 2009 1:22 am

Hi guys, just wanted to ask the aviation experts on this.

In MSFS and various other flight sims games I had before, I noticed that fuel consumption at high altitudes is lower even at full throttle. In FG (as far as I know) the fuel consumption stays the same unless you throttle down.

Which is correct? All this time I've been led to the belief by those games that at high altitudes the fuel consumption of the plane will be much lower on the same throttle setting as it would be at a lower altitude.

I did some basic research into this and found this :
Because the air is less dense at higher altitudes, drag reduces, but for the same reason thrust reduces. The balance of these two effects in any particular case determines the altitude at which the maximum envelope airspeed will be reached. - Source


So can anyone explain, in not-too-technical-terms, how it actually is? Is it that when thrust reduces the engine will have a lower thrust automatically and therefore - lower fuel consumption even when the throttle is pushed to 100% (for example)?

I find that with FlightGear I can "cruise" at high altitudes (or any altitudes) with the throttle at about 30% with no problems. The only effect that high altitudes seem to have is that the Mach speed is higher than indicated airspeed.
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Re: Fuel consumption at high altitude in a jet

Postby MD-Terp » Tue Jul 28, 2009 6:05 am

HJ1AN wrote:In FG (as far as I know) the fuel consumption stays the same unless you throttle down.

Nope, this effect is modeled in FG too. If there are any models for which it doesn't work, they're not modeled correctly. But at least in the Citation Bravo and CRJ-200, I've personally verified that the fuel efficiency is improved the higher up one flies, and I can only assume that the other jets are modeled roughly the same way.

Now, I'm foggy on the physics as to why, other than the reduced drag. Maybe someone can explain it better.
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Re: Fuel consumption at high altitude in a jet

Postby HJ1AN » Tue Jul 28, 2009 8:11 am

MD-Terp wrote:
HJ1AN wrote:In FG (as far as I know) the fuel consumption stays the same unless you throttle down.

Nope, this effect is modeled in FG too. If there are any models for which it doesn't work, they're not modeled correctly. But at least in the Citation Bravo and CRJ-200, I've personally verified that the fuel efficiency is improved the higher up one flies, and I can only assume that the other jets are modeled roughly the same way.

Now, I'm foggy on the physics as to why, other than the reduced drag. Maybe someone can explain it better.


OK, thanks, didn't know that. Maybe I just never noticed, especially some instruments don't have a "fuel flow" indicator. ( i chcked through the "equipment>fuel& payload" menu).
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Re: Fuel consumption at high altitude in a jet

Postby NicQ » Tue Jul 28, 2009 2:29 pm

HJ1AN wrote:OK, thanks, didn't know that. Maybe I just never noticed, especially some instruments don't have a "fuel flow" indicator. ( i chcked through the "equipment>fuel& payload" menu).


The "/" key is your friend : it opens up the property tree, which is a graph of all the FGFS properties (which in turn are tied to runtime variables), some* of which can even be edited in the tree. You'll find fuel flow under engines/engine[number]
You can display any property as an overlay on screen by shift-clicking on it (thanks jentron for showing me the the light ! :)), as well as toggle booleans by control clicking on them.
Nifty stuff.

*some properties can't be edited directly.
Last edited by NicQ on Wed Jul 29, 2009 2:12 am, edited 1 time in total.
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Re: Fuel consumption at high altitude in a jet

Postby HJ1AN » Wed Jul 29, 2009 1:33 am

NicQ wrote:
HJ1AN wrote:OK, thanks, didn't know that. Maybe I just never noticed, especially some instruments don't have a "fuel flow" indicator. ( i chcked through the "equipment>fuel& payload" menu).


The "/" key is your friend : it opens up the property tree, which is a graph of all the FGFS properties (which in turn are tied to runtime variables), most of which can even be edited in the tree. You'll find fuel flow under engines/engine[number]
You can display any property as an overlay on screen by shift-clicking on it (thanks jentron for showing me the the light ! :)), as well as toggle booleans by control clicking on them.
Nifty stuff.



Never knew that !! Thanks for the tips!
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