Drag can be thought of as aerodynamic friction, and one of the sources of drag is the skin friction between the molecules of the air and the solid surface of the aircraft. Drag can also be thought of as aerodynamic resistance to the motion of the object through the fluid. This source of drag depends on the shape of the aircraft and is called form drag. As air flows around a body, the local velocity and pressure are changed. Since pressure is a measure of the momentum of the gas molecules and a change in momentum produces a force, a varying pressure distribution will produce a force on the body.
This causes pressure drag. As an aircraft approaches the speed of sound, shock waves are generated along the surface. There is a drag penalty, known as wave drag that is associated with the formation of the shock waves. The magnitude of the wave drag depends on the Mach number of the flow. Ram drag is associated with slowing down the free stream air as air is brought inside the aircraft. Jet engines and cooling inlets on the aircraft are sources of ram drag. (Benson, 2006) There is an additional drag component caused by the generation of lift, known as induced drag, is the drag due to lift.
It is also called “drag due to lift” because it only occurs on finite, lifting wings. This drag occurs because the flow near the wing tips is distorted span wise as a result of the pressure difference from the top to the bottom of the wing. Swirling vortices are formed at the wing tips, which produce a downwash of air behind the wing which is very strong near the wing tips and decreases toward the wing root. The local angle of attack of the wing is increased by the induced flow of the down wash, giving this, downstream-facing, component to the aerodynamic force acting over the entire wing.