Editing Parasitic drag (section)

==Skin friction drag==
{{main|Skin friction drag}}
'''Skin friction drag''' arises from the friction of the fluid against the "skin" of the object that is moving through it. Skin friction arises from the interaction between the fluid and the skin of the body, and is directly related to the wetted surface, the area of the surface of the body that is in contact with the fluid. Air in contact with a body will stick to the body's surface and that layer will tend to stick to the next layer of air and that in turn to further layers, hence the body is dragging some amount of air with it. The force required to drag an "attached" layer of air with the body is called skin friction drag. Skin friction drag imparts some momentum to a mass of air as it passes through it and that air applies a retarding force on the body. As with other components of parasitic drag, skin friction follows the [[drag equation]] and rises with the square of the [[velocity]].

Skin friction is caused by [[viscosity|viscous drag]] in the [[boundary layer]] around the object. The boundary layer at the front of the object is usually laminar and relatively thin, but becomes turbulent and thicker towards the rear. The position of the [[transition point]] from laminar to turbulent flow depends on the shape of the object. There are two ways to decrease friction drag: the first is to shape the moving body so that laminar flow is possible. The second method is to increase the length and decrease the cross-section of the moving object as much as practicable. To do so, a designer can consider the [[fineness ratio]], which is the length of the aircraft divided by its diameter at the widest point (L/D). It is mostly kept  6:1 for subsonic flows. Increase in length increases [[Reynolds number]] (<math>Re</math>). With <math>Re</math> in the denominator for skin friction coefficient's relation, as its value is increased (in laminar range), total friction drag is reduced. While decrease in cross-sectional area decreases drag force on the body as the disturbance in air flow is less.

The skin friction coefficient, <math>C_f</math>, is defined by
:<math>C_f \equiv \frac{\tau_w}{q},</math>
where <math>\tau_w</math> is the local [[wall shear stress]], and q is the free-stream [[dynamic pressure]].<ref>{{cite web|url=http://www.cfd-online.com/Wiki/Skin_friction_coefficient|title=Skin friction coefficient -- CFD-Wiki, the free CFD reference|website=www.cfd-online.com|access-date=22 April 2018}}</ref> 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 flow|turbulent]] empirical relation known as the ''One-seventh Power ''Law'' (derived by [[von Karman|Theodore von Kármán]]) is:
:<math>C_{f,tur} = \frac{0.074}{Re^{0.2} },</math>

where <math>Re</math> is the Reynolds number.<ref name="Anderson Introduction"/>{{rp|Formula 4.101}}

For a laminar flow over a plate, the skin friction coefficient can be determined using the formula:<ref>{{Cite web|last=tec-science|date=2020-05-31|title=Drag coefficient (friction and pressure drag)|url=https://www.tec-science.com/mechanics/gases-and-liquids/drag-coefficient-friction-and-pressure-drag/|access-date=2020-06-25|website=tec-science|language=en-US}}</ref>

:<math>C_{f,lam} = \frac{1.328}{\sqrt{Re}}</math>