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.