Drag is the force that resists the movement of an aircraft through the air. There are two basic types of drag: parasite drag and induced drag.
Parasite drag consists of all the forces that work to slow an aircraft’s movement. It includes the displacement of air by aircraft, turbulence generated in the air stream, or the hindrance of air moving over the surface of an aircraft and airfoil. There are three types of parasite drag:
Form drag is generated by the aircraft due to its shape and airflow around it. Examples include the engine cowlings, antennas, and the aerodynamic shapes of other components.
Interference drag comes from the intersection of air streams that creates eddy currents, turbulence, or restricts smooth airflow. For example, the intersection of the wing and the fuselage at the wing root has significant interference drag.
Skin friction drag is the aerodynamic resistance caused by moving air colliding with an aircraft's surface. Every surface has a rough, ragged surface when viewed under a microscope.
Induced drag is inherent whenever an airfoil is producing lift, and in fact, this type of drag is inseparable from the production of lift. It is always present if the lift is produced. The amount of induced drag varies with the square of the airspeed. Conversely, parasite drag increases as the square of the airspeed. As the airspeed decreases to near the stalling speed, the total drag becomes greater, mainly due to the sharp rise in induced drag. As the airspeed reaches the terminal velocity of the aircraft, the total drag again increases rapidly due to the sharp increase of parasite drag.