Aircraft are kept in the air by the forward thrust of the wings or aerofoils, through the air. The thrust driving the wing forward is provided by an external source, in this case by propellers or jet engines.
The result of the movement of the wing through stationary air is a lift force perpendicular to the motion of the wing, which is greater than the downwards gravitational force on the wing and so keeps the aircraft airborne. The lift is accompanied by drag which represents the air resistance against the wing as it forces its way through the air. The drag is dependent on the effective area of the wing facing directly into the airflow as well as the shape of the aerofoil.
The magnitudes of the lift and drag are dependent on the angle of attack between the direction of the motion of the wing through the air and the chord line of the wing.
See more about the angle of attack and the theories of aerodynamic lift below.
