The 4 Forces of Flight
The four forces that act upon an aircraft are Thrust, Drag, Lift, and weight. Thrust is the force that propels the airplane forward through the air, while drag is the force pushing backwards limiting the speed of the aircraft. Lift is the force pushing the aircraft up as airflow passes over and under the wing, while weight is the downward force pulling the plane down, caused by gravity.
In unaccelerated flight, thrust = drag and lift = weight. This means that the plane is neither accelerating or decelerating. It is keeping the same speed.
Creating Lift
Bernoulli's Principle states that as the velocity of a fluid, such as air, increases, its pressure decreases.
Image 1
Lets try to visualize this. Taking a look at Image 1, you see a Venturi. As the air enters the Venturi, it is traveling at a set velocity. However, when the air enters the narrower section of the Venturi, that set velocity speeds up and the pressure decreases. Then as the air continues through the tube, it returns to its original velocity and pressure.