Why does an aircraft take off?

[AirGek]

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?

[Advs]

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.

[sevenstrokeroll]

it takes off for the same reason it climbs
excess power

[sevenstrokeroll]

it takes off for the same reason it climbs
excess power over drag

[AirGek]

Everybody agree with that?

[Dg800]

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.

[DaveReidUK]

http://cdn-www.airliners.net/aviation-photos/middle/6/6/9/0287966.jpg

[BOAC]

You'll never get me up in one of those!!

[AirGek]

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?

[Tiennetti]

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

[Airmann]

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.

[AirGek]

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.

[HazelNuts39]

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.

[AirGek]

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".

[Tiennetti]

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 :ok:

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! :p 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? :8

[Dan Dare]

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? :eek:

[AirGek]

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.

[Airmann]

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.

[AirGek]

I think I made something very simple way tooo complicated. :ugh:

It happens to me sometimes, apologize. :O

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. :ok:

[Avionker]

If take off is not achieved because lift overcomes weight, due to airfoil angle of attack, then how do helicopters take off?

[Airmann]

Ya well there you go, if you have 150kt wind then you've achieved enough airflow over the wing to produce lift. The only thing left to do is to achieve the right angle of attack. That's why even with 150kt winds you still need to PULL BACK ON THE CONTROLS to get the right angle of attack before the plane lifts off.

If you happen to live on the east coast of the US right now go to any flight school or GA airport and you'll see them tying down all their airplanes due to the impending hurricane. Thats because with the high winds there can be enough speed over the wings to have the planes lift off, no thrust required.

[darkroomsource]

The real reason an airplane takes off is that if it didn't, it wouldn't be an airplane.

[Capn Bloggs]

The query was "During what phase of flight is lift the greatest?" and the answer was "In general, the takeoff".
Correct. The aircraft is heaviest at takeoff, and so the lift must the greatest then. As weight reduces (fuel burn off), so does the lift required to hold the thing up in the air.

All thrust does is provide speed/airflow over the wing to create the lift.

[Lightning Mate]

4 forces act on the aircraft relative to the earth.

Sorry, but it's five.

[Denti]

Helicopters don't take off, earth just repels them because they're so ugly...

[glum]

Don't forget aircraft use high lift devices at take off / landing too - flaps and slats to increase the lift available at lower speeds. Consequently, the greatest lift is available during approach, when all the high lift devices are deployed to their maximum settings.

If you think of it this way:

An L1011 can weigh about 225tons at take off.
The engines can only produce about 70 tons of thrust.

To make the 225 tons go upwards, the aircraft structure would have to generate 155 tons of lift even if the engines pointed straight down as they couldn't overcome the weight of the aircraft.

[Clandestino]

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".

In general? Not a chance. Generally lift is greatest in rapid pull-ups or very steep turns. To achieve 60° bank coordinated turn one needs 100% more lift than weight while at typical (that is: not VMCA limited) take-off safety speed highest possible achievable lift exceeds weight by mere 44%. Even moderate turbulence will give you (briefly) greater lift than just taking off. Methinks the author of this pamphlet masquerading as a serious book here had a case of extreme narrow-mindedness and focused on the case of ideal airliner flying in ideal atmosphere - now that would make his statement true.

I wonder why in the world people keep insisting on reading this faulty book?

Why does aircraft take off?

Depends on the type of the aircraft but there always has to be force opposing and greater than weight to accelerate said aircraft upwards. Airliners climb at (relatively) shallow angle and have low thrust-to-weight so while their engines contribute to total upwards force, aerodynamic lift is main culprit.

As weight reduces (fuel burn off), so does the lift required to hold the thing up in the air.We have more uses for lift than simply keeping the aroplane in the air. E.g. turning.

[Lightning Mate]

The engines can only produce about 70,000 tons of thrust.

Now that really is some thrust!!!

[RAT 5]

Once there was a very large a/c and someone filled it full of freight to try and make a big profit on 1 flight. They found a very long runway (perhaps that's why the USAF go to the Nevada desert. You can have a rwy as long as you want in any direction.) They then nitro'ed the engines and blasted off, very slowly. I think curvature of the earth came to their rescue.

[Lord Spandex Masher]

An L1011 can weigh about 225tons at take off.
The engines can only produce about 70,000 tons of thrust.

A fair amount of excess thrust then?!

[Lightning Mate]

Sure is!!!

I wonder why in the world people keep insisting on reading this faulty book?

That's not the only one around either.

[A2QFI]

How is the 100 tons (say) needed by your aircraft on approach "More" than the 225 tons needed to get it off the ground? The wing may be producing lift more efficiently (Lbs per Sq ft of wing area) but it is isn't producing more lift, mathematically speakng

[Lightning Mate]

A2,

I can see this thread getting very silly. :uhoh:

[AirGek]

In general? Not a chance. Generally lift is greatest in rapid pull-ups or very steep turns.The question asks in what phase not in what manoeuver. As long as I know the phases are take off, climb, cruise, descent, approach and landing.

And the reason because I read this book is to have a quick summary of the various subjects.

[Clandestino]

As long as I know the phases are take off, climb, cruise, descent, approach and landing.
All in a straight line, eh?

And the reason because I read this book is to have a quick summary of the various subjects. You don't mind the summary being wrong as long as it's quick?

I can see this thread getting very silly.Which might turn out to be quite an improvement.

Anyway, to discount the turns: lift produced by the wings of typical airliner doing V2+10 climbout is lesser, both in total terms and as a fraction of weight, than the lift produced by the same aeroplane during low level-off (say 5000ft as the inbound traffic to LHR jumps over you) a few minutes later. So statement "lift is generally highest at takeoff" is quite silly and most of the time false.

People struggling and failing to comprehend Newtonian mechanics shouldn't be allowed to write flying handbooks. Too bad this ban would interfere with the free market or some PC let's-respect-everyone crap.

[Lonewolf_50]

To answer the OP in as neutral a manner as possible:

Differential pressure applied over an area results in differential force. When forces are unequal, an object tends to accelerate in the direction of the lesser of two forces. The airfoil can been as the boundary/contact point/contact plane, of two different forces.

In the case of the airplane .. when the force acting in the "up" direction exceeds the force acting in the "down" direction, aircraft (anything else for that matter) will tend to go up unless otherwise constrained (in otherwords, being acted upon by yet another force).

Depending upon how much runway you have, and how much thrust, and what the shape of your airfoil is, it is conceivable that you could start rolling down the runway and eventually, with no further manipulation of the controls, achieve liftoff due to the differential described above as your speed increased above an airspeed that procuded a differential large enough to do so. (also depends upon the shape and location and neutral position of the horizontal stab ... )

[Chu Chu]

Maybe the book meant that the coefficient of lift is greatest at takeoff? That sounds like it could be right.

[Clandestino]

No. Highest Cl is at alpha crit and we definitively don't want that at take-off.

EDIT: Well, taken in the contest of flight phases, could be, but is as relevant to general discussion about flying as is the difference between airspeeds of common and African swallow.

[AirGek]

You don't mind the summary being wrong as long as it's quick?

Absolutely yes, that's why I opened this thread. Because according to what I know (or let's say what I belive to know :E) this is wrong.

It's not the only thing I think it's wrong. I saw also that best lift/drag ratio is obtained at Vmd which according to what I belive to know it's obtained at 1.32 Vmd for a jet, the tangent at the drag curve not the lower part, equal to best endurance...

[ShyTorque]

If take off is not achieved because lift overcomes weight, due to airfoil angle of attack, then how do helicopters take off?

Many fixed wing pilots say because helicopters are so ugly the earth just repels them.

[pilotmike]

So far in this thread we've seen 'experts' declaring that "During descent the weight overcomes lift, but some lift is still required to stop the aircraft just plummeting out of the sky." whilst admonishing others for not understanding the "relationship between, speed, and angle of attack over an airfoil." amongst other such muddled misconceptions.

We've also had a L1011 whose engines "can only produce about 70,000 tons of thrust."

All we need to add to the mix now is a converor belt, followed by a downwind turn and we'll all be falling out of the sky, or more likely, falling out of our chairs laughing.

Keep up the good work, and please, please keep the gems of humour coming!

[Pelikal]

Well, perhaps I'm missing something obvious here but I thought an aircraft takes off because it (or its inhabitants) want to go somewhere....:\

[Lord Spandex Masher]

Lift and Thrust (http://www.messybeast.com/dragonqueen/liftdemon.htm)

It's a long read but will aid your understanding.

[mike-wsm]

Lord Spandex - Many thanks for your kind reference to this erudite work. The final reference in connection with kite-flying is apposite and I would comment that the better behaved lift demons are bred at sea and arrive here on the west wind, as today when I enjoyed particularly good flying. :ok:

[barit1]

Max lift implies max G-load - for example in maneuvering, like a tight turn, pullup, or stall recovery.

At takeoff you're only pulling maybe 1.1 G.

BUT - because of the low airspeed, CL is probably highest (or close) at takeoff.

[airline man]

Airmann :D

I couldn't beleive how many posts it took to get to the right answer on this thread. :ugh:

[PPRuNeUser0171]

Along with several other enthusiasts posts your are deleted and most
banned from the forum. You are the worst type - an accident
junkie/ghoul

Rob PPRuNe Admin

[john_tullamarine]

The Definitive Answer

.. now that's let the cat out of the bag ...

[Lightning Mate]

or more likely, falling out of our chairs laughing.

I already am!!!

[dixi188]

How about the B52 take off.

It's the curvature of the earth that gets it airborne at heavy weights. ;)

[aerobat77]

is it not a sweety to see all the mental effusions for a question where the basic answer is quite obvious ?

aicrafts have (on good days) wings attached to the hull- and there is a reason for it.

wikipedia helps a lot :ok:

Aircraft - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Aircraft)

cheers !

[cwatters]

What is the thrust angle relative to the horizontal at take off?

Google suggests that the A380 engines produce a total thrust of 130,000 kg roughly. If the thrust angle was 20 degrees the vertical component would be

130,000 * sin(20) = 44,000Kg

The aircraft max TOW is around 560,000kg so the vertical component of thrust is about 10% of the total force required to lift off.

[glum]

That's the point I made - albeit with a mixup of units for the engines!

[barit1]

If you have big enough flaps, rotation is unnecessary: http://www.youtube.com/watch?v=ZMkF6aPMMrg

[overstress]

In an airliner's flight profile, lift is greatest at the moment of level-off in the cruise. At all other times in the climb, there is a proportion of the thrust vector acting vertically, despite what Capt Bloggs may say ;)

Of course, if you start pulling 2g turns then that's a different matter :)

[pattern_is_full]

Google suggests that the A380 engines produce a total thrust of 130,000 kg roughly. If the thrust angle was 20 degrees the vertical component would be

130,000 * sin(20) = 44,000Kg

Not even that much, since most planes don't rotate much beyond 10 degrees, and certainly not airliners unless they have cast-iron tails (to handle the ensuing tailstrike). Go ahead and try rotating directly to a 20-degree pitchup on the AH the next time you take off and see what happens.... ;)

The typical takeoff and climb angles of all Boeing planes - Bangalore Aviation (http://www.bangaloreaviation.com/2009/05/typical-takeoff-and-climb-angles-of-all.html)

More like 10 degrees, which gives 22,100 lbs of "vertical" thrust vector in that calculation.
____________

To me, it just isn't that mysterious or complicated:

First - wings don't just suddenly begin producing lift at V1 or Vr - they are producing some lift at any forward airspeed. The faster you go, the more lift they produce. At just below Vr, they may be producing lift equal to, say, 80% of the aircraft weight, with the remaining 20% keeping the wheels on the ground.

At take-off, you rotate the plane, which increases AoA. Increasing AoA from 0 to 10 degrees increases a generic wing's lift coefficient nearly 3x: File:Lift curve.svg - Wikipedia, the free encyclopedia (http://en.wikipedia.org/w/index.php?title=File:Lift_curve.svg&page=1)

3x increase in lift means lift increases quickly from 80% to 240% of aircraft weight - and up you go.

Even a 2° increase in AoA increases lift nearly 50% (and the Buff in that video does rotate about 2° - the nose wheel comes up at about 0:15) - which is enough to increase lift from our assumed 80% of weight, to 120% of weight.

or put even more simply:

Condition 1 - airplane weight > than lift = airplane stays on ground

Condition 2 - airplane weight < than lift = airplane climbs

A plane takes off when the increased AoA of rotation, and subsequent increase in lift, moves the plane from condition 1 to condition 2.

[Chu Chu]

Lift at 240% of weight would mean pulling 2.4gs at liftoff. I'm only SLF, but I'd think I'd have noticed that.

[Burnie5204]

The description from Pattern is full is also evidenced by the way wings flex upwards more, the faster that an aircraft goes on its take-off run.

[overstress]

Lift at 240% of weight would mean pulling 2.4gs at liftoff.

Would you care to explain that statement?

[Capt Pit Bull]

Quote:
Lift at 240% of weight would mean pulling 2.4gs at liftoff.
Would you care to explain that statement?


Shouldn't be a surprise.

N = L / W

We clearly do not pull 2.4 g at take off therefore Pattern's analysis is wrong.

Pb

[Capt Pit Bull]

Not to mention that if we are rotating at what.. 1.3 Vs? (sorry been a while since perf, too much desk flying) then we could not achieve more lift than 1.69 x weight anyway before we stalled the wing.

Damn that pesky lift equation!

[Sydy]

Guys,

Sorry, but you all are wrong... Forget the Lift formula, drag and so on...

The airplanes take-off because it has passenger, cargo or both to pay the bills...

:E

All the best,

Sydy

[cockney steve]

Chu Chu...you seem to have forgotten that 1x weight needs (lift=1xweight) to cancel it out....at that point the aircraft is "weightless" therefore, the only thing the pax will feel is the effect of the SURPLUS lift n' thrust.

Just another "outsider" as well but trying to use logic.

If you tie down any "normal" aircraft on a windy day, facing into wind, -

1- it will be blown backward until the pickets restrain it
2- it will lift because the airfoil is assymetric and approximately horizontal wrt the airflow and the ground.....
watch a soaring bird...it often "floats" upward whilst it's body remains horizontal.

Some of us as kids did aeromodelling and Meccano and steam-engines and stuff like that- wa learned a lot of practical skills....today's idiot-box viewers miss out, big style, no matter how fast they can also twiddle their fingers and thumbs with a controller/phone/remote.

[Chu Chu]

Steve,

You're certainly right that the aircraft would be accelerating upwards at 1.4 times the acceleration due to gravity.

I've always thought the convention was that an aircraft in level flight "pulls" 1 g (and the vomit comet zero g). But I could easily be wrong about that.

[Capt Claret]

If one subscribes to the attitude controls airspeed and power controls rate of descent school of thought, then an aeroplane gets airborne because after the pilot lines up, s/he lowers the nose to accelerate down the runway, and upon reaching rotate speed, applies power to lift off.

[RatherBeFlying]

Here's a slight variation on the question:

You are in a glider on the downside of a lazy eight at 70-80 kt. heading downhill.

Why does the glider level off and gain altitude when you pull back on the stick?

Note that while the longitudinal component of gravity was substituting for thrust on the way down, that component has joined forces with drag on the way up.

[barit1]

Why does the glider level off and gain altitude when you pull back on the stick?

A glider has two forms of energy to trade off against each other: Kinetic, and potential. In other words, velocity and altitude.

Stick the nose down, you lose potential energy, and gain kinetic energy.
Pull the nose up, reverse the process.

BTW, by clever seeking of a thermal of rising air, you can steal a bit more potential energy and stay aloft much longer. A powered airplane gets extra energy from its fuel tanks, otherwise it's like a glider.

Bob Hoover used to demonstrate this really well - the tradeoff between potential and kinetic energy in unpowered flight.

[Pelikal]

"Guys,

Sorry, but you all are wrong... Forget the Lift formula, drag and so on...

The airplanes take-off because it has passenger, cargo or both to pay the bills..."

Spot on mate.

The original question was WHY?

[barit1]

Forget the Lift formula, drag and so on...

And the pax / cargo formulae do not apply to military jetfighters, and are thus discredited.

No - the real answer (at least for aircraft with one extra seat) lies in the ARMREST THEORY - which goes like this:

The pilot(s) taxi the aircraft onto the runway, and then push the "LOUD NOISE" lever(s) forward to make the motors much more noisy. Passengers, upon hearing this fearsome noise, grip their armrests more tightly and pull upwards. This upward force on the armrests is transferred through the structure to every molecule of the aeroplane, and thus the vehicle begins to fly.

(source: The book of Lift, chapter 29.92)

[greenslopes]

The aircraft takes off in my case cos it's on the roster.

[Lyman]

The WHY of anythin belongs in a religious thread.

The correct question is HOW?

[Crabman]

Q: Why?
A: Why not.