Drag is the force that resists movement of an aircraft through the air. There are two basic types of drag: parasite drag and induced drag.
Parasite Drag is compromised of all the forces that work to slow an aircraft’s movement. It includes the displacement of the air by aircraft, turbulence generated in the airstream, or a hindrance of air moving over the surface of aircraft and airfoil. There are three types of parasite drag:
- Form Drag, generated by the aircraft due to its shape and airflow around it. Examples include the engine cowlings, antennas, and the aerodynamic shape of other components.
- Interference Drag comes from the intersection of airstreams that creates eddy currents, turbulence or restricts smooth airflows. For example, the intersection of the wing and the fuselage at the wing root has significance interference drag.
- Skin fraction Drag, is the aerodynamics resistance due to contact of moving air with the surface of an aircraft. 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 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, due mainly 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.