Editing Parasitic drag

{{Short description|Aerodynamic resistance against the motion of an object}}
[[Image:Drag curves for aircraft in flight.svg|thumb|x250px|right|[[Drag curve]] for a lifting body in steady flight]]
'''Parasitic drag''', also known as '''profile drag''',<ref name="Anderson">{{cite book |last1=Anderson |first1=John D. Jr. |title=Fundamentals of aerodynamics |date=1991 |publisher=McGraw-Hill |location=New York |isbn=0-07-001679-8 |edition=2nd}}</ref>{{rp|254}}<ref name="Anderson Introduction">{{cite book |last1=Anderson |first1=John D. Jr. |title=Introduction to flight |date=2016 |publisher=McGraw Hill Education |location=New York, NY |isbn=978-0-07-802767-3 |page=242 |edition=Eighth}}</ref>{{rp|256}} is a type of [[Drag (physics)#Aerodynamics|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|form drag]]'' and ''[[skin friction drag]]''.<ref>Clancy, L.J. (1975). ''Aerodynamics'', Sub-section 5.9. Pitman Publishing. {{ISBN|0 273 01120 0}}</ref><ref name="Anderson"/>{{rp|641-642}}<ref name="Gowree">{{cite thesis |last1=Gowree |first1=Erwin Ricky |title=Influence of Attachment Line Flow on Form Drag |date=20 May 2014 |url=https://openaccess.city.ac.uk/id/eprint/12239/ |access-date=22 March 2022|type=doctoral }}</ref>{{rp|19}}

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 (force)|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]].<ref name="PHAK">{{cite book |url=https://www.faa.gov/sites/faa.gov/files/07_phak_ch5_0.pdf |title=Pilot's Handbook of Aeronautical Knowledge |publisher=FAA |page=Chapter 5, Aerodynamics of flight |quote=The first is called parasite because it in no way functions to aid flight, while the second, induced drag, is a result of an airfoil developing lift.}}</ref> 

==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 longitudinal section{{what|date=November 2022}} of the body. A prudent choice of body profile is essential for a low [[drag coefficient]]. [[Streamlines, streaklines, and pathlines|Streamlines]] should be continuous, and [[Flow separation|separation of the boundary layer]] with its attendant [[vortex|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 [[Aircraft fairing|fairings]].<ref>{{Cite web|url=https://www.skybrary.aero/index.php/Interference_Drag|title = Interference Drag - SKYbrary Aviation Safety| date=25 May 2021 }}</ref><ref>{{Cite web|url=https://www.boldmethod.com/learn-to-fly/aerodynamics/how-interference-drag-affects-your-airplane-performance-and-decreases-performance/|title=How Interference Drag Affects Your Plane's Performance}}</ref><ref name="PHAK"/>

[[Wave drag]], also known as supersonic wave drag or compressibility drag, is a component of form drag caused by [[shock wave]]s generated when an aircraft is moving at [[transonic]] and [[supersonic]] speeds.<ref name="Anderson"/>{{rp|25, 492, 573}}

Form drag is a type of pressure drag,<ref name="Anderson"/>{{rp|254}} a term which also includes lift-induced drag.<ref name="Anderson"/>{{rp|65, 319}} Form drag is pressure drag due to separation.<ref name="Anderson" />{{rp|641-642}}<ref name="Anderson Introduction"/>{{rp|256}}

==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>

==See also==
* [[NACA duct]]
* [[Jet engine]] [[Jet_engine_performance#Ram_drag|ram drag]]
* [[Skin friction line]]

==References==
{{reflist}}

[[Category:Drag (physics)]]