Editing Slipstream

{{Short description|Fluid dynamics phenomenon}}
{{about|the physical phenomenon||Slipstream (disambiguation)}}
{{More footnotes|date=June 2011}}
[[File:Human-Mediated-Dispersal-of-Seeds-by-the-Airflow-of-Vehicles-pone.0052733.s001.ogv|thumb|thumbtime=8|250px|[[Seed dispersal]] by the slipstream of a passing car.]]
A '''slipstream''' is a region behind a moving object in which a [[Wake (physics)|wake]] of [[fluid]] (typically air or water) is moving at [[velocities]] comparable to that of the moving object, relative to the ambient fluid through which the object is moving.<ref>[https://books.google.com/books?id=UzJqTX4LPOsC&dq=slipstream+aerodynamics&pg=PA415 Recent studies of Train Slipstreams by Johnson, Dalley, and Temple]</ref> The term slipstream also applies to the similar region adjacent to an object with a fluid moving around it. "Slipstreaming" or "[[drafting (aerodynamics)|drafting]]" works because of the relative motion of the fluid in the slipstream.

==Overview==
A slipstream created by [[turbulent flow]] has a slightly lower pressure than the ambient fluid around the object. When the flow is [[laminar flow|laminar]], the pressure behind the object is higher than the surrounding fluid. The shape of an object determines how strong the effect is. In general, the more [[aerodynamic]] an object is, the smaller and weaker its slipstream will be. For example, a box-like front (relative to the object's motion) will collide with the medium's particles at a high rate, transferring more momentum from the object to the fluid than a more aerodynamic object. A bullet-like profile will cause less [[turbulence]] and create a more [[laminar flow]].

A tapered rear will permit the particles of the medium to rejoin more easily and quickly than a truncated rear. This reduces lower-pressure effect in the slipstream, but also increases [[skin friction]] (in engineering designs, these effects must be balanced).

==Slipstreaming==
{{refimprove|date=June 2011}}
The term "slipstreaming" describes an object travelling inside the slipstream of another object (most often objects moving through the air though not necessarily flying).  If an object is following another object, moving at the same speed, the rear object will require less power to maintain its speed than if it were moving independently.  This technique, also called [[Drafting (aerodynamics)|drafting]], can be used by bicyclists.
*Following in the slipstream of another motor vehicle, or "drafting", allows for significantly improved [[fuel efficiency]] due to reduced atmospheric [[Drag (physics)|drag]]. [[Truck]] convoys are a common example, travelling highways in a single-file queue several vehicles long. In tests, this has been shown to produce significant fuel savings.<ref>[http://www.ika.rwth-aachen.de/pdf_eb/gb6-24e_konvoi.pdf Konvoi – Development and examination of the application of electronically coupled truck convoys on highways Aachen University study 2012] {{webarchive|url=https://web.archive.org/web/20140414031219/http://www.ika.rwth-aachen.de/pdf_eb/gb6-24e_konvoi.pdf |date=2014-04-14 }}</ref> [[Auto racing]] drivers also draft in order to conserve fuel, the better to gain competitive advantage by reducing the frequency of fuel stops or, more often, to reach a higher speed before pulling out to attempt to overtake another driver  for example, a driver tries to overtake the leading driver so he follows the rear of the leading driver, the rear driver will gain slipstream causing the whole vehicle to gain more speed than the leading driver.
*A related effect used for [[Lift (force)|lift]] rather than drag reduction is [[vortex surfing]] for airborne objects.  The extended formations ([[V formation]]) or "[[wikt:skein|skeins]]" in which many [[bird migration|migratory]] birds (especially [[goose|geese]]) fly enable the birds (except, of course, the [[bird]] at the front) to use vortex surfing to take advantage of one another's [[vortex|vortices]].<ref>[https://web.archive.org/web/20090521201002/http://stinet.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA401264 Drag Reduction from Formation Flight. Flying Aircraft in Bird-Like Formations Could Significantly Increase Range]; Defense Technical Information Center; April 2002; Retrieved February 27, 2008</ref> Other birds (for example [[cormorant]]s) that typically fly in close formation, even on short journeys, are probably also exploiting this effect. Using [[wingtip devices]] to reduce [[induced drag]] caused by [[wingtip vortices]] has been tested for aircraft, and could save 10%–29% fuel.<ref>[http://www.nasa.gov/centers/dryden/news/NewsReleases/2003/03-42.html/ NASA SKY SURFING FOR FUEL ECONOMY] {{webarchive|url=https://web.archive.org/web/20110418012236/http://www.nasa.gov/centers/dryden/news/NewsReleases/2003/03-42.html |date=2011-04-18 }}</ref><ref>Cooney, Michael (October 11, 2012). [https://www.networkworld.com/article/742942/software-air-force-lab-tests-out-aircraft-surfing-technique-to-save-fuel.html "Air Force lab tests out "aircraft surfing" technique to save fuel"]. Network World.</ref><ref>{{cite web|last=Drinnon|first=Roger|title='Vortex surfing' could be revolutionary|url=https://www.af.mil/News/story/id/123321609/|publisher=[[US Air Force]]|access-date=23 November 2012|date=11 October 2012|archive-url=https://archive.today/20121212223542/http://www.af.mil/news/story.asp?id=123321609|archive-date=12 December 2012|url-status=live}}</ref>

== Spiral slipstream <!-- Other articles link here. --> ==

'''Spiral slipstream''', also known as '''propwash''', ''prop wash'', or ''spiraling slipstream'', is a [[spiral]]-shaped slipstream formed behind a rotating [[Propeller (aircraft)|propeller]] on an [[aircraft]]. The most noticeable effect resulting from the formation of a spiral slipstream is the tendency to [[Roll, pitch, and yaw|yaw]] nose-left at low speed and full [[throttle]] (in centerline [[Tractor configuration|tractor]] aircraft with a clockwise-rotating propeller.) This effect is caused by the slipstream acting upon the tail [[vertical stabilizer|fin]] of the aircraft: The slipstream causes the air to rotate around the longitudinal axis of the aircraft, and this air flow exerts a force on the tail fin, pushing it to the right. To counteract this, some aircraft have the front of the fin (vertical stabilizer) slightly offset from the centreline so as to provide an opposing force that cancels out the one produced by the slipstream, albeit only at one particular (usually cruising) speed, an example being the [[Hawker Hurricane]] fighter from [[World War II]].

Propeller slipstream causes increased lift by increasing the airspeed over part of the wings. It also reduces the [[Stall (fluid dynamics)#Stall speeds|stall speed]] of the aircraft by energizing the flow over the wings.<ref name="Davies">{{cite book |last1=Davies |first1=David P. |title=Handling the Big Jets: An Explanation of the Significant Differences in Flying Qualities Between Jet Transport Aeroplanes and Piston Engined Transport Aeroplanes, Together with Some Other Aspects of Jet Transport Handling |date=1971 |publisher=Air Registration Board |url=https://books.google.com/books?id=TKZTAAAAMAAJ |language=en |isbn=0903083019 |edition=3rd}}</ref>{{rp|61}}

==See also==
{{Wiktionary}}
* [[Drafting (aerodynamics)|Drafting]] or slipstreaming as used in sports such as cycling and motor racing
* [[Peloton]]

==References==
;Specific references:
{{Reflist}}
;General references:
{{refbegin}}
*[https://web.archive.org/web/20080214110039/http://www.centennialofflight.gov/essay/Theories_of_Flight/Stability_II/TH27G7.htm Centennial of Flight Commission: diagram of the spiral slipstream]
*[https://web.archive.org/web/20080601035948/http://www.horizonhobby.com/Articles/Article.aspx?ArticleID=1101&Page=2 Forces and Moments: Spiral Slipstream]
{{refend}}

[[Category:Aerodynamics]]
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