How Does a Wing Lift a 70-Tonne Airplane? Lift Explained with an A320 Takeoff
A modern airliner weighs as much as dozens of cars, yet a pair of thin wings holds it in the air. The secret is not one trick but a simple relationship: lift grows with the square of the speed.
The animation below follows an A320-size airliner through takeoff. You can change the takeoff mass and switch the flaps on and off, and watch the speed, the angle of attack, the lift and the weight on the gauges.
The short answer
A wing makes lift by turning the oncoming air downward. By Newton's third law the air pushes the wing up. Air flowing over the curved top speeds up and its pressure drops, while the air under the wing is pushed and slowed, so the pressure there is higher. Both descriptions, the pressure difference and the downward turning of air, are two views of the same effect.
The lift equation puts numbers on it:
L = ½ · ρ · v² · S · C_L
Here ρ is air density, v is speed, S is wing area (122.6 m² in the model) and C_L is the lift coefficient, which depends on the wing's shape and angle. For the 70 t case in the animation the model gives rotation at about 134 knots and liftoff after 24.8 seconds at about 148 knots.
What the animation shows, stage by stage
Rolling. The engines push the plane along the runway. The wing already makes some lift, but far less than the weight.
Faster air above. Air going over the curved top speeds up and its pressure drops. The air below is slowed and pushed up.
Rotate. At the rotation speed the pilot raises the nose by 8°. A bigger angle bends the airflow down more and the lift coefficient jumps (from about 1.05 to about 1.85 in the model).
Liftoff. Lift beats the weight (about 687 kN for 70 t) and the wheels leave the ground. The wing throws roughly 30 tonnes of air downward every second in this case.
The answer. Double the speed and you get four times the lift.
The physics: why speed matters so much
Lift scales with v². At liftoff the lift must equal the weight, W = m·g. Solving the lift equation for the speed gives the speed at which a given wing can carry a given weight:
v = √( 2·m·g / (ρ · S · C_L) )
Heavier planes need a higher speed, and a wing with a higher lift coefficient needs a lower one. That is exactly what flaps are for: they increase the wing's camber and area, raising C_L so the plane can take off and land at lower speeds.
What the numbers say: mass and flaps
These values come from running the animation's model:
Takeoff mass | Rotate at | Liftoff after |
50 t | 113 kn | 15.3 s |
64.5 t | 128 kn | 22.0 s |
70 t (default) | 134 kn | 24.8 s |
79 t | 142 kn | 29.6 s |
And the flaps, at 70 t:
Wing | Rotate at | Liftoff after |
Takeoff flaps | 134 kn | 24.8 s |
Clean wing | 173 kn | 31.4 s |
A clean wing needs far more speed to carry the same weight, and therefore a longer run. Weight and rotation speed do not scale one-to-one: carrying 58 percent more mass (50 t to 79 t) raises the rotation speed by only about 26 percent, because speed depends on the square root of the weight.
Honest limits of the model
The animation is a simplified takeoff: a fixed wing area, a standard-density air model, a smooth thrust and drag and a stepped lift coefficient. It is not an aircraft performance calculator, and real takeoff speeds depend on the flap setting, runway, temperature, altitude and the operator's procedures. A clean-wing takeoff is shown only to illustrate what flaps do, not because airliners do it.
Frequently asked questions
What creates lift on a wing? The wing turns the airflow downward, and the air pushes back up on the wing. Pressure is lower above the wing and higher below it. These are two descriptions of the same effect.
Do the two air streams have to meet at the trailing edge in the same time? No. The popular "equal transit time" explanation is wrong: air going over the top arrives earlier than air going under. The lift comes from the change in the airflow and the pressure field, not from a race.
Why does doubling the speed quadruple the lift? Because lift depends on v². Both the mass of air meeting the wing per second and the speed change given to it grow with v.
What do flaps do? They increase the wing's camber and sometimes its area, which raises the lift coefficient so the plane can fly at lower speed.
What is the angle of attack? The angle between the wing and the oncoming air. A bigger angle bends the air down more and raises lift, up to the stall angle, where the flow separates.
Why does a heavier plane take off at a higher speed? It needs more lift, and for a given wing that takes more speed. In the model the rotation speed rises with the square root of the mass.
Related
Figures in this article come from the animation's simplified model: A320-size wing of 122.6 m², takeoff mass 50 to 79 t (70 t default), rotation at 8° and a lift coefficient from about 1.05 to about 1.85. They illustrate the mechanism and are not aircraft performance data.
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