why do a/c takeoff and land into wind?

[purple haze]

hi all

just a quick q, being the title.

i do know that wind should move over the moving parts of the a/c as this makes them more responsive and therefore more efficient.

but how does that help fuel consumption and does it make a big difference?

. .cheers

. .ph

[tom775257]

From my PPl training so far:. .Aircraft take off into the wind...because say you have a 10 kts wind down the runway (as you are stationary on the runway) towards your nose, you therefore have an relative airspeed of 10kts, while stationary with a groundspeed of 0.....say you rotate (take-off) at 65 kts, with a 10 kts headwind, you can rotate at a ground speed of 55 kts. hence use less runway on the takeoff roll....always a good thing. Saves worrying about crashing into stuff and other such issues.. .Landing....with a headwind (i.e. flying into wind) your ground speed is slower. Gives you more time to think, lower speed at landing, plus any gust down the runway gives you a relatively higher air speed. Get a strong wind up the arse end of the aircraft and you might just stop flying!. .Cheers.... .tom (with a little help from stella...)

[ 03 February 2002: Message edited by: tom775257 ]</p>

[Tinstaafl]

There seems to be several questions that you're asking, Purple Haze.

Some background:

A/c fly in relation to the body of air in which they're contained. They require a certain amount of airspeed to produce the force ('Lift') that is used to counteract their weight.

Producing this Lift has a penalty in the form of a retarding force ('Drag'). You will have experienced/felt a form of drag if you stick your hand out of a moving vehicle. Drag is the force that tries to move your hand backwards.

To overcome this unwanted Drag, and to accelerate the aircraft, it needs some form of engine to produce a force in the direction of travel. This force is called 'Thrust'.

To take-off ie become airborne, an aircraft must be accelerated until sufficient airflow is occurring to enable the wings to produce enough Lift to counteract the a/c's Weight.

By taking off into a headwind the a/c gets a bit of a headstart. As far as the wings are concerned they are already getting some of the airspeed needed before the a/c has even started its take-off roll. The end result is that the speed over the ground that the a/c will be doing when it has sufficient airspeed to get airborne will be lower than in no wind at all.

This means the a/c will require a shorter distance to take-off. A similar case exists for landing. The headwind means that for a given speed through the air, the a/c will be doing a lower speed over the ground.

A lower groundspeed gives a shorter landing distance.

By choosing a level during the cruise that has a tailwind the a/c gets a bit of a free ride to its destination. Exactly the same effect as walking along a moving sidewalk.

Even while maintaining a normal walking pace to the end you will, at the same time, be getting transported to the end. Your walking speed hasn't changed but your speed over the (fixed) ground is faster.

The reverse, of course, if you walk the wrong way along the moving sidewalk. You have your normal walking speed but your speed over the (fixed) ground is much slower, taking you longer to get to the end.

Since a tailwind improves the speed of the a/c over the ground, resulting in a reduced flight time the a/c will use less fuel for the trip.

A headwind will result in an increased journey time and an increased amount of fuel burnt for the trip.

Interestingly, or relatively constant winds, a journey that eventually terminates back at the departure point - no matter how many intermediate landings are made - no wind at all will result in the lowest trip time for the whole trip.

[747FOCAL]

Jet engines don't like wind blowing up their tailpipe. Not to bad if your rolling but standing stationary at high thrust with a tailwind can cause a compresor stall and those can be expensive.