Slipstream
Encyclopedia
A slipstream is a region behind a moving object in which a wake
of fluid
(typically air or water) is moving at velocities comparable to the moving object (in comparison to the ambient fluid through which the object is moving). The term slipstream also applies to the similar region adjacent to an object with a fluid moving around it. The reason that "slipstreaming" or "drafting
" works is because of the relative motion of the fluid in the slipstream.
A slipstream created by turbulent flow has a slightly lower pressure than the ambient fluid around the object. When the flow is 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).
The term "slipstreaming" describes an object traveling inside the slipstream of another object (most often objects moving through the air though not necessarily flying). If an object is inside the slipstream behind another object, moving at the same speed, the rear object will require less power to maintain its speed than if it were moving independently. In addition, the leading object will be able to move faster than it could independently because the rear object reduces the effect of the low-pressure region on the leading object.
is important in a number of contexts, including:
-shaped slipstream formed behind a rotating propeller
on an aircraft
. The most noticeable effect resulting from the formation of a spiral slipstream is the tendency to yaw nose-left at low speed and full throttle
. This effect is caused by the slipstream acting upon the tail fin
of the aircraft: the slipstream causes the air to rotate around the forward-aft
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
.
Wake
A wake is the region of recirculating flow immediately behind a moving or stationary solid body, caused by the flow of surrounding fluid around the body.-Fluid dynamics:...
of fluid
Fluid
In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....
(typically air or water) is moving at velocities comparable to the moving object (in comparison to the ambient fluid through which the object is moving). The term slipstream also applies to the similar region adjacent to an object with a fluid moving around it. The reason that "slipstreaming" or "drafting
Drafting (racing)
Drafting or slipstreaming is a technique where two vehicles or other moving objects are caused to align in a close group reducing the overall effect of drag due to exploiting the lead object's slipstream...
" works is because of the relative motion of the fluid in the slipstream.
A slipstream created by turbulent flow has a slightly lower pressure than the ambient fluid around the object. When the flow is laminar
Laminar flow
Laminar flow, sometimes known as streamline flow, occurs when a fluid flows in parallel layers, with no disruption between the layers. At low velocities the fluid tends to flow without lateral mixing, and adjacent layers slide past one another like playing cards. There are no cross currents...
, 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
Turbulence
In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time...
and create a more laminar flow
Laminar flow
Laminar flow, sometimes known as streamline flow, occurs when a fluid flows in parallel layers, with no disruption between the layers. At low velocities the fluid tends to flow without lateral mixing, and adjacent layers slide past one another like playing cards. There are no cross currents...
.
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).
The term "slipstreaming" describes an object traveling inside the slipstream of another object (most often objects moving through the air though not necessarily flying). If an object is inside the slipstream behind another object, moving at the same speed, the rear object will require less power to maintain its speed than if it were moving independently. In addition, the leading object will be able to move faster than it could independently because the rear object reduces the effect of the low-pressure region on the leading object.
Drafting
Slipstreaming, also known as draftingDrafting (racing)
Drafting or slipstreaming is a technique where two vehicles or other moving objects are caused to align in a close group reducing the overall effect of drag due to exploiting the lead object's slipstream...
is important in a number of contexts, including:
- In fast bicycle racesBicycle racingBicycle racing is a competition sport in which various types of bicycles are used. There are several categories of bicycle racing including road bicycle racing, cyclo-cross, mountain bike racing, track cycling, BMX, bike trials, and cycle speedway. Bicycle racing is recognised as an Olympic sport...
, competitors attempt to use one another's slipstream, or "draft", breaking out to overtake the leader only at the last possible moment. In recreational cycling, on the other hand, members of a group can take turns at the leading position, enabling one another to rest a little. In a group of cooperative cyclists with sufficient group-riding skill, stronger members can spend more time leading to give weaker riders more rest, enabling riders of different strengths to ride together, at least on relatively flat routes. On inclines, however, the benefit of drafting is relatively less, as speeds are slower and the cyclist's primary effort is working against gravity. The flat or hilly nature of a route has consequences for both racing and recreational cyclingCyclingCycling, also called bicycling or biking, is the use of bicycles for transport, recreation, or for sport. Persons engaged in cycling are cyclists or bicyclists...
, with different types of routes favouring different types of cyclists. - The extended formations (V formation) or "skeinSkeinSkein may refer to:* A flock of geese or ducks in V formation flight* A wound ball of yarn with a center pull strand, see hank* TV Series, used part of Variety's slanguage.* Skein dubh, a Scottish knife* Skein module, a mathematical concept...
s" in which many migratoryBird migrationBird migration is the regular seasonal journey undertaken by many species of birds. Bird movements include those made in response to changes in food availability, habitat or weather. Sometimes, journeys are not termed "true migration" because they are irregular or in only one direction...
birds (especially geeseGooseThe word goose is the English name for a group of waterfowl, belonging to the family Anatidae. This family also includes swans, most of which are larger than true geese, and ducks, which are smaller....
) fly enable the birds (except, of course, the birdBirdBirds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
at the front) to take advantage of one another's slipstream. Other birds (for example cormorantCormorantThe bird family Phalacrocoracidae is represented by some 40 species of cormorants and shags. Several different classifications of the family have been proposed recently, and the number of genera is disputed.- Names :...
s) that typically fly in close formation even on short journeys are probably also exploiting the slipstream effect. - Following in the slipstream of another motor vehicle, or "drafting", allows for significantly improved fuel efficiency due to reduced atmospheric dragDrag (physics)In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...
. TruckTruckA truck or lorry is a motor vehicle designed to transport cargo. Trucks vary greatly in size, power, and configuration, with the smallest being mechanically similar to an automobile...
convoys are a common example, travelling highways in a single-file queue several vehicles long. Auto racingAuto racingAuto racing is a motorsport involving the racing of cars for competition. It is one of the world's most watched televised sports.-The beginning of 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.
Spiral slipstream
Spiral slipstream (also known as spiraling slipstream, propwash in the US, or just slipstream in the UK) is a spiralSpiral
In mathematics, a spiral is a curve which emanates from a central point, getting progressively farther away as it revolves around the point.-Spiral or helix:...
-shaped slipstream formed behind a rotating propeller
Propeller (aircraft)
Aircraft propellers or airscrews convert rotary motion from piston engines or turboprops to provide propulsive force. They may be fixed or variable pitch. Early aircraft propellers were carved by hand from solid or laminated wood with later propellers being constructed from metal...
on an aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...
. The most noticeable effect resulting from the formation of a spiral slipstream is the tendency to yaw nose-left at low speed and full throttle
Throttle
A throttle is the mechanism by which the flow of a fluid is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases , but usually decreased. The term throttle has come to refer, informally and incorrectly, to any mechanism by which...
. This effect is caused by the slipstream acting upon the tail fin
Vertical stabilizer
The vertical stabilizers, vertical stabilisers, or fins, of aircraft, missiles or bombs are typically found on the aft end of the fuselage or body, and are intended to reduce aerodynamic side slip. It is analogical to a skeg on boats and ships.On aircraft, vertical stabilizers generally point upwards...
of the aircraft: the slipstream causes the air to rotate around the forward-aft
Aft
Aft, in naval terminology, is an adjective or adverb meaning, towards the stern of the ship, when the frame of reference is within the ship. Example: "Able Seaman Smith; lay aft!". Or; "What's happening aft?"...
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
Hawker Hurricane
The Hawker Hurricane is a British single-seat fighter aircraft that was designed and predominantly built by Hawker Aircraft Ltd for the Royal Air Force...
fighter from World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
.
See also
- Drafting or slipstreaming as used in sports such as cycling and motor racing, including Formula 1
- PelotonPelotonThe peloton , field, bunch or pack is the large main group of riders in a road bicycle race. Riders in a group save energy by riding close near other riders...