A slipstream is a region behind a moving object in which a
wake of
fluid
In physics, a fluid is a liquid, gas, or other material that may continuously motion, move and Deformation (physics), deform (''flow'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are M ...
(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. The term slipstream also applies to the similar region adjacent to an object with a fluid moving around it. "Slipstreaming" or "
drafting" works because of the relative motion of the fluid in the slipstream.
Overview
A slipstream created by
turbulent flow
In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by Chaos theory, chaotic changes in pressure and flow velocity. It is in contrast to laminar flow, which occurs when a fluid flows in parallel layers with no disrupt ...
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
Aerodynamics () is the study of the motion of atmosphere of Earth, air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dynamics and its subfield of gas dynamics, and is an ...
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
In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to laminar flow, which occurs when a fluid flows in parallel layers with no disruption between ...
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
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, can be used by bicyclists.
*Following in the slipstream of another motor vehicle, or "drafting", allows for significantly improved
fuel efficiency
Fuel efficiency (or fuel economy) is a form of thermal efficiency, meaning the ratio of effort to result of a process that converts chemical energy, chemical potential energy contained in a carrier (fuel) into kinetic energy or Mechanical work, w ...
due to reduced atmospheric
drag.
Truck
A truck or lorry is a motor vehicle designed to transport freight, carry specialized payloads, or perform other utilitarian work. Trucks vary greatly in size, power, and configuration, but the vast majority feature body-on-frame construct ...
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.
Auto racing
Auto racing (also known as car racing, motor racing, or automobile racing) is a motorsport involving the racing of automobiles for competition. In North America, the term is commonly used to describe all forms of automobile sport including non ...
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 rather than drag reduction is
vortex surfing for airborne objects. The extended formations (
V formation) or "
skeins" in which many
migratory birds (especially
geese) fly enable the birds (except, of course, the
bird
Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...
at the front) to use vortex surfing to take advantage of one another's
vortices. Other birds (for example
cormorants) that typically fly in close formation, even on short journeys, are probably also exploiting this effect. Using
wingtip devices to reduce
induced drag
Lift-induced drag, induced drag, vortex drag, or sometimes drag due to lift, in aerodynamics, is an aerodynamic drag force that occurs whenever a moving object redirects the airflow coming at it. This drag force occurs in airplanes due to wings or ...
caused by
wingtip vortices
Wingtip vortices are circular patterns of rotating air left behind a wing as it generates Lift (force), lift. The name is a misnomer because the cores of the vortex, vortices are slightly inboard of the wing tips. Wingtip vortices are sometimes ...
has been tested for aircraft, and could save 10%ā29% fuel.
Spiral slipstream
Spiral slipstream, also known as propwash, ''prop wash'', or ''spiraling slipstream'', is a
spiral-shaped slipstream formed behind a rotating
propeller
A propeller (often called a screw if on a ship or an airscrew if on an aircraft) is a device with a rotating hub and radiating blades that are set at a pitch to form a helical spiral which, when rotated, exerts linear thrust upon a working flu ...
on an
aircraft
An aircraft ( aircraft) is a vehicle that is able to flight, fly by gaining support from the Atmosphere of Earth, air. It counters the force of gravity by using either Buoyancy, static lift or the Lift (force), dynamic lift of an airfoil, or, i ...
. 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 (in centerline
tractor
A tractor is an engineering vehicle specifically designed to deliver a high tractive effort (or torque) at slow speeds, for the purposes of hauling a Trailer (vehicle), trailer or machinery such as that used in agriculture, mining or constructio ...
aircraft with a clockwise-rotating propeller.) This effect is caused by the slipstream acting upon the tail
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
The Hawker Hurricane is a British single-seat fighter aircraft of the 1930sā40s which was designed and predominantly built by Hawker Aircraft Ltd. for service with the Royal Air Force (RAF). It was overshadowed in the public consciousness by ...
fighter from
World War II
World War II or the Second World War (1 September 1939 ā 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...
.
Propeller slipstream causes increased lift by increasing the airspeed over part of the wings. It also reduces the
stall speed of the aircraft by energizing the flow over the wings.
See also
*
Drafting or slipstreaming as used in sports such as cycling and motor racing
*
Peloton
References
;Specific references:
;General references:
Centennial of Flight Commission: diagram of the spiral slipstreamForces and Moments: Spiral Slipstream{{refend
Aerodynamics
Articles containing video clips