Watching a passenger aircraft take off can feel like watching a building change its mind about gravity. The aircraft has not become lighter, and gravity has not taken a break. Its wings are generating an aerodynamic force large enough to support it.
That force is lift. Understanding it means following the air around the aircraft, rather than imagining the wing resting on an invisible cushion or the engines simply pulling the whole machine upward.
A wing changes the airflow
As an aircraft moves through air, its wings influence the air's speed, direction and pressure. The pressure is not the same everywhere on the wing. Add those pressures over the surface and they produce a net aerodynamic force.
Lift is the component of that force perpendicular to the incoming airflow. In ordinary level flight, it acts mainly upward. The wing also gives surrounding air a net downward change in momentum, with a corresponding force on the aircraft.
These are compatible ways of describing the same event. A pressure explanation associated with Bernoulli and a momentum explanation associated with Newton do not have to compete. A good account respects both the detailed flow and the forces it produces.
The air does not have a meeting appointment
A familiar story says air travelling over the curved upper surface must move faster because it has farther to go, and must meet air travelling underneath at the trailing edge. That supposed meeting requirement is false.
There is no physical rule that two neighbouring parcels separated at the front of a wing must reunite at the back. The actual speed distribution depends on the airflow, wing shape and orientation. Explaining it using a fictional deadline produces a memorable but misleading story.
Wing shape matters, but so does the angle between the wing and the approaching air. Symmetrical wings can generate lift at suitable angles. A curved upper surface alone is therefore not a complete explanation of flight.
Four forces, one moving aircraft
Weight pulls downward. Lift helps oppose weight. Drag resists motion through the air, while thrust from an engine or propeller helps overcome drag. In steady, straight, level flight, the opposing forces balance.
Engines are important because they keep the aircraft moving through air despite drag. But an engine is not a requirement for every moment of flight: gliders generate lift too, trading height for continued motion unless rising air helps them gain altitude.
The aircraft needs the right combination of speed, wing area, air density and aerodynamic behaviour. Carrying more weight changes the demands placed on that combination.
Why pulling upward has a limit
Increasing a wing's angle to the airflow can increase lift, but only up to a point. Beyond a critical angle, airflow can separate substantially from the wing and lift drops. This is an aerodynamic stall, not necessarily an engine stopping.
Flight is therefore a controlled interaction with moving air. The aircraft stays up because its shape and motion continuously create the required forces—not because its weight has somehow disappeared.
Sources and further reading
NASA Glenn: Bernoulli and Newton
NASA Glenn: Four forces on an airplane