View Full Version : How wind affects required thrust ?

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

25th Mar 2006, 08:59
You need to give more infomation about what the aircraft is attempting to do before an answer can be given.

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.

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 ?

25th Mar 2006, 12:38
Not trying to be mean shendal but you seem to be asking basic questions answered in any amount of basic text books. As a result of blasting an anonymous bulletin board you may not be able to distinguish between good answers and bad answers or even, god forbid, when someone is winding you up. (For example an answerer to an earlier question did not understand "C.I." and I don't mean Chief Instructor). Either that was a classic wind-up or someone was about to give you advice from a standpoint not much clearer than your own. So at the risk of upsetting the apple cart instead of seeking instant answers from unknown sources that may convey a false impression why not invest in a really good book? And study? No one in the business is going to put you down but if you are going to win the hearts and minds of a lot of professional people with 1000's of hours you are going to have to come up with better questions than that. And if you are learning to fly, ask your instructor?

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).

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:

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