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Why does an aircraft take off?

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Why does an aircraft take off?

Old 29th Oct 2012, 09:59
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Why does an aircraft take off?

Hi folks,

I'll go straight to the point:

an aircraft lifts off the ground because the lift produced is greater than its weight or because the thrust vector rotates upward?

Last edited by AirGek; 29th Oct 2012 at 10:00.
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Old 29th Oct 2012, 10:01
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Lift being generated due to forward speed is enough to support the weight component and the Thrust vector becomes greater than drag when the aircraft is in a nose up attitude, causing the aircraft to climb.
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Old 29th Oct 2012, 10:16
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it takes off for the same reason it climbs
excess power
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Old 29th Oct 2012, 10:16
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it takes off for the same reason it climbs
excess power over drag
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Old 29th Oct 2012, 10:16
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Everybody agree with that?
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Old 29th Oct 2012, 10:18
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I'd say both. Rotating the thrust vector will generate a lift component which can be easily computed and then itself compounded with with the aerodynamic lift component.
On aircraft where thrust can easily exceed the MTOW (mostly military hardware) the thrust component can be extremely relevant, making near-vertical climbs (more a lift-off than a take off) possible. On your average single engine Cessna the contribution is on the other hand probably pretty minuscule and completely dwarfed by the aerodynamic component.

Last edited by Dg800; 29th Oct 2012 at 10:23.
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Old 29th Oct 2012, 10:23
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Old 29th Oct 2012, 10:28
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You'll never get me up in one of those!!
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Old 29th Oct 2012, 10:40
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Just to make myself clear, I'm not saying that lift has nothing to do with lift off but personally I don't think it pulls the plane up because it overcomes its weight.

Does someone think that a plane takes off because lift overcomes weight and if so why?

Last edited by AirGek; 29th Oct 2012 at 10:43.
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Old 29th Oct 2012, 10:49
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Your last question, made me think about a glider.

When you pull a glider with another airplane, there is no "thrust" as such, nor vertical component of the same (it is pulled forward and normally it lifts off befor the pulling airplane)
In that case, YES the lift overcomes the weight of the same

On the same principle, on your A320, you use the thrust to accelerate along the runway... imagine an infinite runway and no speed limitation, your aicraft will be lifting off in a flat attitude just because at that speed, the lift is bigger than the weight.
What you do at "rotation" is changing your Cp (and thus your lift, considering the same speed), not rotating the thrust vector upwards

The thrust vector is there, indeed, but its effect is playing a negligible effect on the vertical axis

Last edited by Tiennetti; 29th Oct 2012 at 10:51.
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Old 29th Oct 2012, 10:56
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If you think that the lift produced by the wing as its angle of attack is increased at rotation, is not the main reason that an airplane gets airborne then take a look at planes taking off and watch the wing flexing. Its a cleat indication of how hard the wing is working at lift off.

There's no way that the jet thrust being forced downward is the main reason for the airplanes initial lift off. It plays a part, but it isn't the main reason. Its the lift from the wing. Even if you remove the engines so long as you can somehow accelerate the aircraft to around rotation speed it will still fly, jet thrust or not, take a look at gliders.
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Old 29th Oct 2012, 11:05
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In the case of the glider I don't know if my reasoning is correct but I would interpret the pulling force of the tug as the thurst of the glider.

It doesn't matter to me who is providing the force but the force is there beacause without it it won't move.

Besides that when you talk about an endless runway, I could disagree with that because if we have a symmetric airfoil the airplane will never climb no matter the speed it's rolling at.

Even if we have a symmetric airfoil, accelerating to the point that the aircraft needs an AoA of 0.5 degrees to lift off doesn't seem to me the usual take off technique so please lets stick to the real world.

Last edited by AirGek; 29th Oct 2012 at 11:42.
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Old 29th Oct 2012, 11:18
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For a transport airplane taking off at maximum weight, one engine inoperative, lift-off attitude about 10 degrees, the vertical component of thrust is about 2% of the weight.
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Old 29th Oct 2012, 11:21
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There's no way that the jet thrust being forced downward is the main reason for the airplanes initial lift off. It plays a part, but it isn't the main reason. Its the lift from the wing.
Ok, I see your point and the example of the wing flexing during take off is quite good.

I came across this yesterday when reading "Ace The Techincal Pilot Interview". The query was "During what phase of flight is lift the greatest?" and the answer was "In general, the takeoff".
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Old 29th Oct 2012, 11:25
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In the case of the glider I don't know if my reasoning is correct but I would interpret the pulling force of the tug as the thurst of the glider.
Yes, but in this case the "thrust" is never directed on the vertical axis, as the cable is pulling only horizontally (the glider normally lifts off before the puller... so the thrust in that case would have a little vertical component... towards the ground!)

It doesn't matter to me who is providing the force but the force is there beacause without it it won't move.
True

Besides that when you talk about an endless runway, I could disagree with that because if we have a symmetric airfoil the airplane will never climb no matter the speed it's rolling at.

Even if we have an asymmetric airfoil, accelerating to the point that the aircraft needs an AoA of 0.5 degrees to lift off doesn't seem to me the usual take off technique so please lets stick to the real world.
Ahahah Good catch! but if you don't consider a simmetric airfoil, then the "theoretical case" can work out.

Ok then let's work on the usual take off technique....
Engine failure after V1, you loose one engine, but you keep on accelerating to the same speed (v2) and normally you have a shallower pitch angle
It is that way, because you have now less thrust to counteract the same (2 engine case) drag.
If you follow your tought, then in this case we should pitch more, so to have a bigger thrust vector towards the sky, right?
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Old 29th Oct 2012, 11:55
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4 forces act on the aircraft relative to the earth. Lets call them weight, lift, drag and thrust. When one of the opposing forces exceeds the other an acceleration is produced. The engines are normally thought to provide thrust to equal or overcome drag, but they may (or may not) also provide an element of lift. The wings provide lift and various forms of drag. Weight can contribute to drag or to thrust. And so it goes on. The greatest lift would be produced when the product of mass and vertical acceleration is greatest (probably on rotation in a normal airliner flight).

Simples!

Except that these forces are usually referred to relative to aircraft datums, so lift is at its maximum during a max-g avoiding action turn shortly after acceleration after take-off?
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Old 29th Oct 2012, 11:59
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As I already said I'm not saying that we can ignore lift in a climb as it's unimaginable to think that thrust is the only way of climbing considering the attitudes we fly at.

This confirms it
For a transport airplane taking off at maximum weight, one engine inoperative, lift-off attitude about 10 degrees, the vertical component of thrust is about 2% of the weight.
From the bottom of my ignorance the first thing I did was to revert to the immage in my mind of an MD82 taking off, quite steep attitude, I think 20. What will be the attitude at lift off, half that value?

With that picture in my mind, I found hard to think that lift, during lift off, would be greater than in level flight during cruise because of the negligible, as much as you want, help of the thrust vector.

Someone says that the reason for that answer is that at lift off is the moment of the flight where the aircraft weights more so this automtically leads to the conslusion that the lift is the gratest of all the flight.

It was the fact that the thrust vector was there, helping the "upward force" component that leads me to believe that this answer is not correct.
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Old 29th Oct 2012, 12:16
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MY friend, you don't seem to understand the relationship between, speed, and angle of attack over an airfoil. Given that the weight of the aircraft is generally the same no matter what the altitude, then yes, during lift off the lift is the greatest. But not because of engine thrust: BECAUSE THE ANGLE OF ATTACK OF THE WING IS AT ITS HIGHEST during Take Off. During cruise weight=lift, therefore no change, angle of attack is at around 2-3 degrees. During descent the weight overcomes lift, but some lift is still required to stop the aircraft just plummeting out of the sky.

The main part that the engines play is to create thrust which creates airflow over the wings, this creates enough of a pressure difference over the top and bottom of the wing to allow for lift. The two main factors in a wing producing lift is speed and angle of attack (up to alpha max) Increase either one and you get more lift.

The thrust of the engines produces very little lift force. In fact props have more of a lift advantage because the airflow from the prop flows over the wing.

What is your level of education vis a vis theory of flight, you are asking some pretty strange questions and based upon your comments you sound pretty confused. The question you are asking is quite straight forward. Like I said I don't think you understand the basic principles of flight.

Last edited by Airmann; 29th Oct 2012 at 12:32.
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Old 29th Oct 2012, 12:32
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I think I made something very simple way tooo complicated.

It happens to me sometimes, apologize.

I simply tought about when I used to play with Flight Simulator and putting winds like 150kts. No need for thrust to lift off...

Here the same principle. https://www.youtube.com/watch?v=pqL9S48xtak

Sorry if I wasted your time, thanks for participating and thank you Airmann, it was your example about the wing flexing during take off that leaded me to the light.
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Old 29th Oct 2012, 12:36
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If take off is not achieved because lift overcomes weight, due to airfoil angle of attack, then how do helicopters take off?

Last edited by Avionker; 29th Oct 2012 at 12:37.
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