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!

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

The Definitive Answer
 

Lift and Thrust

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

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:

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.

Airmann :D

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

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

The Definitive Answer

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

I already am!!!

How about the B52 take off.

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

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

cheers !

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.

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

If you have big enough flaps, rotation is unnecessary:

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 :)

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

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

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.

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.

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.

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

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!

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

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.

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.

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.

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

"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?

ARMREST THEORY
 

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)

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

The WHY of anythin belongs in a religious thread.

The correct question is HOW?

Q: Why?
A: Why not.