The four forces that are acting on an airplane in flight are: Lift, Gravity,
Thrust, and Drag. Lift is needed to overcome gravity and is produced by
airflow of the wings.
Because of the special curved shape of the wings, a lower pressure area
is created on the smooth upper suface as it slices through the air. This
pulls the wing up against the downforce of gravity.
Thrust is produced by the engines to overcome the drag caused by the wind
resistance. The bigger and heavier the airplane, the more thrust is required
for large engines.
Airplanes can stay in level flight when all four forces are balanced at
the same time.
Editor's note: Many learned people have responded to this page saying that it is the angle of attack (not an upward curved wing) which produces lift. If it was just the curveture of the wings, how can airplanes fly upside down? Nothing Carl has said is in error. He just didn't give the complete explanation. Please, no more emails explaining angle of attack.