Why is it that a 650,000 pound Lockheed C-5 Galaxy is capable of staying in the air when everyday objects, like our sofa, would just fall to the ground? An aircraft can stay in the air by generating enough lift (upward force) to balance out its weight (downward force).
Lift is created by manipulating the air around the plane, particularly the wing. Air flows in paths that we call streamlines, and a certain amount of air passes between two streamlines with a specific speed. As air flows over the wing the distance between the streamlines above the airfoil (which is the cross sectional shape of the wing) is decreased. However, the mass flowing through these squished streamlines must remain the same by conservation of mass (density of the fluid x Area x velocity = constant). This results in the air above the airfoil moving faster similar to how water comes out of a gardening hose much quicker than if you were pour the water out of a cup since the gardening hose has a smaller opening for the water to exit out of.
In 1738, a mathematician by the name of Daniel Bernoulli told us that the pressure from the non-moving surrounding air (static pressure) plus the pressure resulting from fluid motion (dynamic pressure) must remain constant. When the air began moving faster above the airfoil, the dynamic pressure on top was increased. By Bernoulli’s principle the static pressure above the wing must decrease. The dynamic pressure remains unchanged below the airfoil so the static pressure is also unchanged. The end result is a greater static pressure below the wing, creating a net upward pressure.
Summing up the pressure all along the surface area of the wing gives us a net upward force. If the upward force is equal to the downward force the plane will stay in the air. If the upward force is greater than the weight of the plane, the aircraft will begin to rise higher into the air. In the case of our sofa, the shape of the couch does not allow for the previously described phenomenon, which means the sofa cannot generate enough lift to balance its weight. As a result, the sofa falls to the ground and stays there.