Fuel Flow and temperature

[Mintraman]

Hi guys, I am little confused about the relationship between FF and Temperature. On jet engines do you consume more or less fuel on a cold day?

Thanks

[oggers]

More. With the denser air you will get more thrust but also require more power to maintain a given compressor RPM. According to my ATPL notes.

[tdracer]

Actually, for the same amount of thrust, there is very little difference in fuel required vs. ambient temperature. The compressor should be slightly more efficient on a cold day and hence require less fuel, but were talking a very small number here - basically in the mud.

[Mintraman]

Thanks oggers and tdracer for your answer, I really appreciate.

A friend of mine flying the Q400 showed me on his the FCOM the FF at different temperatures for the same weight and FL, and obviously you will consume more on a cold atmosphere, but is it the same with jet engines?

[Derfred]

I would think that at the same weight and FL you will consume more on a cold day, because the air you are flying through is denser (more drag). But on a cold day you should be able to climb higher to achieve optimum altitude, and at that higher level consume much the same fuel as you would have consumed on a warmer day at a lower level.

In other words, if you just think about density, the engine doesn't really care about temperature (within practical limits).

I have no figures for this, just my gut feeling.

[imriozer]

It depends, are you looking for Mach or TAS?

[Old Fella]

The simple answer to the original question is:

Cold air is more dense, so for a given engine RPM, more fuel is required to maintain that RPM on a cold day (and thrust at that RPM will be greater on a cold day than on a hotter day).

In summary, if an engine produces X RPM on an ISA Standard day at a given fuel flow at sea level, that engine will require more fuel and will produce more thrust at X RPM on an ISA -10 day than the fuel it would require to maintain X RPM on an ISA +10 day (with a lower thrust produced).

BTW Mintraman, the Q400 is powered by a jet engine, albeit one which drives a propeller.

[Mintraman]

Excellent :ok::ok::ok:

[Willit Run]

The Old Fella is right!

Hope i'm that sharp at 75!:O.