Drafting (aerodynamics)
From Wikipedia, the free encyclopedia
Drafting or slipstreaming is a technique where two vehicles or objects align in a close group reducing the overall effect of drag due to exploiting the lead object's slipstream. Especially when high speeds are involved, drafting can significantly reduce the echelon's average energy expenditure required to maintain a certain speed and can also slightly reduce the energy expenditure of the lead vehicle or object.
Drafting is used to reduce wind resistance and is seen most commonly in bicycle racing, car racing, and speedskating, though drafting is occasionally used even in cross-country skiing and running. Some forms of triathlon allow drafting. Drafting occurs in swimming as well, both in open-water races (occurring in natural bodies of water) and in traditional races in competition pools. In a competition pool, a swimmer may hug the lane line that separates him/her from a swimmer of whom s/he is abaft, thereby taking advantage of the liquid slipstream in the other swimmer's wake.
In cycling, the main (largest) group of tightly packed cyclists in a race is called a peloton, where cyclists riding in a long shaped formation. Each (but the first ones) drafting behind the others before them.
When cyclists ride fast, they form a paceline. Each cyclist (but the first) is drafting behind another one.
In order to ride very fast, a team of some skilled cyclists may form the belgian tourniquet. Successively, each cyclist lead the group.
Drafting can be cooperative, in which several competitors take turns in the lead position (which requires the most effort and energy consumption). Or, it can be competitive or tactical, where one competitor will try to stay closely behind another leaving him or her more energy for a break-away push to the finish line.[1]
Contents |
[edit] Drafting in nature
Cooperative fluid dynamics techniques like drafting are found in nature, as well. Flocks of geese and some other birds fly in a V formation because the wingtip vortices generated by the front bird will create up-wash circulations. The birds flying behind will receive lift force from these up-wash vortices. Thus, the other birds in the pack won't have to work as hard since studies show they place themselves roughly at the optimum distance predicted by simple aerodynamic theory.[2] The theory behind this is the same as the one for the wings of an airplane in general.[citation needed] However, it does not work quite the same way as drafting in racing. Birds fly in a flock for better lifting force, whereas the race cars stay close for less total aerodynamic drag force.
Goose flocking is not fully understood, but probably involves the recovery of energy from vortices created by the leading birds' wingtips (see wingtip vortices). (For this reason, it may not meet some people's definition of drafting).
Other animals have been observed to use true drafting behavior reminiscent of auto racing or cycling. Lobsters, for example, are known to migrate in close single-file formation "lobster trains", sometimes for hundreds of miles.
[edit] Drafting in motorsport
[edit] Open-wheel racing
In single seater, open wheel racing series such as Formula One and the IndyCar Series, a technique known as slipstreaming is used. Along a long straight, a car following close behind another uses the slipstream created by the lead car to close the gap between them, hoping to be able to overtake the leader under braking for the next corner, or, if he has a straightline speed advantage, to pass on the straight. However, it is very difficult for cars to follow each other in close proximity in fast corners as the "dirty" (turbulent) air that comes off the lead car unbalances the trailing car as its aerodynamic devices provide less grip. However, on the straight this effect is much less of a detriment due to the lower levels of grip required.
[edit] Stock car racing
On the faster speedways and superspeedways used by NASCAR, ARCA, and the IROC series, two or more vehicles can race faster when lined up front-to-rear than a single car can race alone. The low-pressure wake behind a group's leading car reduces the aerodynamic resistance on the front of the trailing car, allowing the second car to pull closer. As the second car nears the first, it pushes high-pressure air forward so less fast-moving air hits the lead car's spoiler. The result is less drag for both cars, allowing faster speeds. [3]
Handling in corners is affected by balance changes caused by the draft: The leading car has normal front downforce but less rear downforce. The trailing car has less front downforce but normal rear downforce. A car with drafting partners both ahead and behind will lose downforce at both ends. [3]
The "slingshot pass" is the most dramatic and widely noted maneuver associated with drafting. A trailing car (perhaps pushed by a line of drafting cars) uses the lead car's wake to pull up with maximum momentum at the end of a straightaway, enters a turn high, and turns down across the lead car's wake. The combination of running downhill and running across the zone of lowest aerodynamic drag allows the trailing car to carry extra speed and pass on the inside of leader. [3]
Drafting was discovered by stock car racers in the 1959 Daytona 500. Drivers found they picked up speed running closely behind other cars, and as they experimented they found that a line of cars could sustain higher speeds than a single car running by itself. [3]
Drafting is most important at Talladega Superspeedway and Daytona International Speedway where a restrictor plate is used, making much less power available to push the large bodies through the air. Race cars reach their highest speeds on these superspeedways, so the aerodynamic forces are highest, and the effects of drafting are strongest. Since restrictor plates were first used as a safety device, their effect has changed the nature of drafting. Vehicles no longer have sufficient horsepower to maintain their drafting speeds upon exiting the draft. This negates the slingshot maneuver. As a result, passing is often the result of cooperation between two or more drivers.
[edit] Bump drafting
Bump drafting is a tactic used in NASCAR races at Talladega Superspeedway and Daytona International Speedway. The technique was initially popularized by the Archer Brothers in the SCCA Sportruck series during the late 1980s.[4] It begins as normal drafting, but the following car pulls up behind the lead car and bumps into the rear of it, pushing the lead car ahead, to maintain momentum.
If done roughly or in the wrong position (e.g. close to the entry of the turn), this tactic can destabilize the handling of the lead car sometimes causing a crash. Use of the tactic in this manner is known as slam drafting. Due to the danger, NASCAR has attempted to limit the bracing on bumpers on cars,disallowed bump drafting in turns, introduced "no bump zones" on certain portions of speedways where this practice is prevalent, and penalized drivers who are too rough in bump drafting.[5][6]
[edit] Drafting for efficiency on the road
Some drivers have been known to draft behind other vehicles, particularly tailgating larger vehicles, to save gasoline. For example, hypermilers using this technique can achieve 75 mpg or more [7]. Some sources say that the most common tailgating does not save gasoline even at freeway speeds because one is likely to accelerate and brake so frequently that any aerodynamic savings are lost through the brakes [8]. On the show Mythbusters, drafting behind an 18-wheeler truck was tested and results showed that traveling 100 feet (30 m) behind the truck increased overall mpg efficiency by 11%. Traveling 10 feet (3.0 m) behind the truck produced a 39% gain in efficiency. Of course, they warn that this type of driving "is insane" because the truck's blindspot is in that area, and if the truck stops quickly there is much less time to react [9].
[edit] Simulation of drafting
Computer simulation (computational fluid dynamics or CFD) is increasingly being used to analyse drafting. It is important to understand the aerodynamic behaviour of a motor vehicle when drafting, for example if the rear car is too close to the front car, the air supply to its radiatior will be reduced and there's a possibility of the engine overheating. Most motor sport aerodynamic analysis is performed using wind tunnel testing. This becomes difficult for drafting cases, if only because you need a very large wind tunnel. CFD, a kind of virtual wind tunnel, is used by race teams to understand the car's performance while drafting.
[edit] References
- ^ YouTube - Car Crash compilation
- ^ 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
- ^ a b c d Lemasters Jr., Ron, "One Wild Wind", Stock Car Racing (ISSN 0734-7340), Volume 36, Number 1, January 2001.
- ^ Glick, Shav, "Motor Racing", Los Angeles Times, p. 10, Dec 3, 1987.
- ^ Racing Car Crash - Video
- ^ YouTube - Race car crash
- ^ ‘Hypermilers’ wring out every last bit of mpg, msnbc.com May 29, 2007
- ^ Tailgate for Mother Earth! ecogeek.org, July 16, 2007
- ^ Mythbusters study drafting on the road
