slipstream

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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).^[1] 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, 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.

Slipstreaming/Drafting

Slipstreaming is important in a number of contexts, including:

• Cycling: In fast bicycle races, 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 cycling, with different types of routes favoring different types of cyclists. See also: drafting and peloton.
• Bird flight, especially during migration: the extended formations or "skeins" in which many migratory birds (especially geese) fly enable the birds (except, of course, the bird at the front) to take advantage of one another's slipstream. Other birds (for example cormorants) that typically fly in close formation even on short journeys are probably also exploiting the slipstream effect.
• Automobile transport: Following in the slipstream of another motor vehicle, or "drafting", allows for significantly improved fuel efficiency due to reduced atmospheric drag. Truck convoys are a common example, traveling highways in a single-file queue several vehicles long. 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.

Spiral slipstream

Spiral slipstream (also known as spiraling slipstream, propwash in the US, or just slipstream in the UK) is a spiral-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.

The spiral slipstream can also roll the aircraft to the right. This is also caused by the air hitting the fin of the aircraft. This effect is negligible.

References

1. Centennial of Flight Commission: diagram of the spiral slipstream
2. Forces and Moments: Spiral Slipstream