thumb|x250px|right|[[Drag curve for a lifting body in steady flight]]
Parasitic drag, also known as profile drag, is a type of aerodynamic drag that acts on any object when the object is moving through a fluid. Parasitic drag is defined as the combination of form drag and skin friction drag.
It is named as such because it is not useful, in contrast with lift-induced drag which is created when an airfoil generates lift. All objects experience parasitic drag, regardless of whether they generate lift. Parasitic drag comprises all types of drag except lift-induced drag, and the total drag on an aircraft or other object which generates lift is the sum of parasitic drag and lift-induced drag.
Form drag
Form drag arises because of the shape of the object. The general size and shape of the body are the most important factors in form drag; bodies with a larger presented cross-section will have a higher drag than thinner bodies; sleek ("streamlined") objects have lower form drag. Form drag follows the drag equation, meaning that it increases with the square of the velocity, and thus becomes more important for high-speed aircraft.
Form drag depends on the shape of the longitudinal section of the body (along its long axis). A prudent choice of body profile is essential for a low drag coefficient. Streamlines should be continuous, and separation of the boundary layer with its attendant vortices should be avoided.
Form drag includes interference drag, caused by the mixing of airflow streams. For example, where the wing and fuselage meet at the wing root, two airstreams merge into one. This mixing can cause eddy currents, turbulence, or restrict smooth airflow. Interference drag is greater when two surfaces meet at perpendicular angles, and can be minimised by the use of fairings. For boundary layers without a pressure gradient in the x direction, it is related to the momentum thickness as
:<math>C_f = 2 \frac{d \theta}{d x}.</math>
For comparison, the turbulent empirical relation known as the One-seventh Power Law (derived by Theodore von Kármán) is:
:<math>C_{f,tur} = \frac{0.074}{Re^{0.2} },</math>
where <math>Re</math> is the Reynolds number.
:<math>C_{f,lam} = \frac{1.328}{\sqrt{Re</math>
See also
- NACA duct
- Jet engine ram drag
- Skin friction line