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Shendal
24th Mar 2006, 18:25
I would like to know how wind affects required thrust and fuel flow on the cruise phase.

Keith.Williams.
25th Mar 2006, 08:59

For any constant airspeed, the winds will not affect the required thrust or fuel flow.

But for any constant groundspeed the situation will be different. A headwind reduces groundspeed at any given airspeed. So to maintain constant ground speed you will need more airspeed. Assuming that you are above Vmd this will require more thrust and more fuel flow.

Shendal
25th Mar 2006, 11:12
I mean,

If the airplane fly at 300 Kt True Air Speed and there is 20 Kt of headwind, it means it will be flying at 280 Kt ground speed.

On the other hand, it the same aircraft has a 20 kt of tailwind, it will be flying at 320 Kt ground speed.

Anyways, all of two true air speeds will be 300 Kt, but in the first case the airplane engines will produce less thrust, isnīt it ?

enicalyth
25th Mar 2006, 12:38

Macgyver
25th Mar 2006, 19:37
Here is an exert from "See How It Flies," section 7.5.4:

2 exact same a/c at the same weight are flying at different altitudes but are both at the exact same AOA. At the higher altitude:
- the lift is the same (since lift= weight)
- the L/D ratio is the same (since it depends on AOA, and both are equal)
- the drag is the same (since L/D are the same)
- the thrust is the same (since thrust=drag)
- the IAS is the same (the produce the same lift at the same AOA)
- the TAS is greater (since the density is lower at higher altitudes)
- the power required is greater (since power= TAS*drag, as TAS is higher above).

So, any a/c requires more power required to maintain a given IAS or AOA at higher altitudes. Wind does not directly affect the power, but it does affect the groundspeed, which effects the total fuel burned (i.e. higher g/s= reach destination sooner, less fuel burned, even if this a/c is flying at the exact same power setting with the same fuel burn as an a/c with a low g/s).

barit1
26th Mar 2006, 01:30
The key issue is that the aircraft doesn't know or care whether it's in a headwind or tailwind.

Airspeed (Mach), gross weight, temperature and altitude determine the thrust required and thus the fuel burned per hour. The wind simply means you're flying for a longer or short duration to cover the same ground track, thus requiring more or less total fuel. :cool:

Shendal
27th Mar 2006, 17:04
Thanks all of you, I understand what I wanted to know >:)