There are four main forces acting upon a plane which controls the way it flies. Lift is when is moves upward, as opposed to the gravity which pulls it downward. Thrust is when is moves in a forward direction, which acts against the force of drag (which is when air resistance pulls it backward.) In order for a plane to become airborne, its force of life must be greater than the force of gravity (9 m/s^2 on Earth.) Also, the propellers must thrust it in the direction it wants to go with a greater force than that of air resistance. This is how a plane flies.