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Drafting (aerodynamics)

From Wikipedia, the free encyclopedia

A paceline of drafting cyclists

Drafting or slipstreaming is an aerodynamic technique where two moving objects are aligning in a close group to exploit the lead object's slipstream and thus reduce the overall effect of drag. Especially when high speeds are involved, as in motor racing and cycling, drafting can significantly reduce the paceline's average energy expenditure and can even slightly reduce the energy expenditure of the lead vehicle.

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Transcription

NARRATOR: Daytona International Speedway is a 2.5 mile track. So why are all the cars only inches apart? And if you got hit from behind while going 200 miles an hour, would you thank the driver who did it? If you understood aerodynamics, you probably would. The roar of the engines, the squeal of the tires, then the race to victory lane. It all says, NASCAR. A race car is much more than steel, gas, rubber and speed. A race car is a science experiment on wheels. DIANDRA LESLIE-PELECKY: It's hard to imagine that air matters much to a speeding race car. But the billions and billions of air molecules hitting the car really do make a big difference. Sometimes air helps the car by pushing the tires into the track and creating grip. But other times, air is the enemy. And that can be a real drag. PATRICK CANUPP: Drag is a bad thing because it basically acts against the motor, and it hurts you as far as fuel efficiency goes, and it hurts you in terms of speed, top-end speed. STEVE LETARTE: Roll your window down and stick your hand out. That force on your hand trying to pull it back towards the back of the car, that's drag. That's how much physical drag you have on your hand. JOHN PROBST: You got lower pressure on the back side, higher pressure on the front side, so it just wants to push your hand back. Drag is essentially that back pressure, if you will, on the car. DIANDRA LESLIE-PELECKY: Compare the force you feel when you hold your hand perpendicular to the ground, to the force you feel when you hold your hand parallel to the ground. When your hand is parallel, it has to push fewer air molecules out of the way, so there's less drag. Less drag is a good thing. Rough surfaces and anything that sticks out from the car increase drag. That's why there are no side-view mirrors like there are on passenger cars. The mirrors are placed as far inside the car as possible. And rough surfaces, like grills, are taped. JOSH BROWNE: We're no longer passing air through a grill, through a radiator, which has a lot of resistance and friction. We put the tape on the grill, most of the air just goes up and over the car. From an aerodynamic standpoint it's the best thing we can do. Reduces drag. The tradeoff is that it heats up the engine, and eventually that's bad. DIANDRA LESLIE-PELECKY: But four times a year teams pay much more attention to drag than they do the rest of the season. That's when NASCAR visits the two longest tracks on the circuit. Daytona and Talladega. PATRICK CANUPP: At those race tracks, the banking is tremendous. You can get around the track very quickly at a very high speed. DIANDRA LESLIE-PELECKY: To keep speeds in the safe range, NASCAR requires cars to use an engine restrictor plate at Daytona and Talladega. PATRICK CANUPP: If we took our normal Charlotte car to Daytona it would probably run 215 miles an hour. DIANDRA LESLIE-PELECKY: The team brings a car with as little drag as possible. The drivers have invented their own way of compensating for the limited horsepower. Drafting. BRIAN VICKERS: I love drafting. To be at 200 miles an hour with 43 guys around you, and you're just beating each other down the straightaways. And sometimes in the corners. DIANDRA LESLIE-PELECKY: When two cars are far apart, each car has to push air molecules out of its way, and each car is pulled backward by the low pressure region created by its wake. Now imagine what happens when the two cars get close to each other. By close, I don't mean a few feet. I mean a few inches. When two cars get close enough, air flows around them as if they were a single car. This decreases the total amount of drag on the two cars because one car is pushing the air molecules out of the way for both cars. The first car doesn't generate as much of a wake either, because the second car is so close behind it. The end result is that the second car gets pulled along with the first car, and they both go faster. BRIAN VICKERS: If you could put two engines in one car, that's two cars drafting. You know, because one car is still only pushing the air. DIANDRA LESLIE-PELECKY: Two cars drafting can go three to five miles an hour faster than either car can go on its own. Drafting is most beneficial at Talladega and Daytona because of the restricted engine power. But you'll see drafting at high-speed tracks like Indianapolis and Michigan, too. Driving a few inches from each other, going 200 miles an hour, just wasn't enough for some drivers. The desire for speed quickly turned drafting into bump-drafting. BRIAN VICKERS: Well it feels like bumping at 200 miles an hour. I mean, I know that's a pretty dulled down version. But it's pretty exhilarating. JEFF GORDON: There's some times I look up in my mirror and I go, oh boy, this one, I'm gonna feel this one and I hope it doesn't spin me out. Because sometimes it can be a really abrupt impact. DIANDRA LESLIE-PELECKY: When two cars are drafting, the second car doesn't have to break its own hole in the air, which means the second car has a little unused engine power. When the trailing driver steps on the gas he speeds up and hits the rear bumper of the first car, transferring speed to the car in front. The whole point of racing is being the fastest. So why would a driver help another car go faster? BRIAN VICKERS: Self benefit. Only and surely self benefit. If two cars are faster than one, it doesn't matter which one of the two I am. I wanna be one of the two, and we're gonna go forward. And then once you get to the front, then you pass for the lead. JEFF GORDON: And as the race gets closer and closer to the end, taps become bumps and become hits and become slams. And it's just, bam-bam-bam! And you really feel it. DIANDRA LESLIE-PELECKY: People say that Dale Earnhardt Senior was so good at drafting because he could see the air. Dale Junior is pretty good, too, with seven of his eighteen wins coming at Daytona or Talladega. But even if you don't have any Earnhardt genes, you can still see the air too, if you know the science of drafting.

Drafting in sport

Drafting is used to reduce wind resistance and is seen most commonly in bicycle racing, motorcycle racing, car racing, and speedskating, though drafting is occasionally used even in cross-country skiing, downhill skateboarding, 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 them from the swimmer they are abaft of thereby taking advantage of the liquid slipstream in the other swimmer's wake. Drafting also occurs in competitive longboarding.

It is believed, but not yet conclusively proven, that thoroughbred racing horses draft each other, especially in longer races.[1]

Belgian tourniquet (Belgischer kreisel)

Cycling

In cycling, any time one bicyclist is riding behind another, energy is conserved, especially at higher speeds.

In road bicycle racing, the main (largest) group of tightly packed cyclists in a race is called a peloton where cyclists ride in a long formation with each (but not the first rider) drafting behind the others before them.

When cyclists ride fast they form a paceline. Each cyclist, except 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 leads the group. Drafting can be cooperative: several competitors take turns in the lead position (which requires the most effort and energy consumption). It can also be competitive or tactical: one competitor will try to stay closely behind another, leaving the follower with more energy for a break-away push to the finish line.

Running

Drafting behind another runner can conserve energy when facing a headwind. Generally the effect is much less pronounced than in cycling due to lower speeds.

Nike worked with the aerodynamics expert Robby Ketchell at the University of New Hampshire to experiment with and select a formation of pacemakers that would best minimize drag on the professionals it sponsored in the project it termed Breaking2. A Wired magazine report that interviewed various experts affiliated and unaffiliated with Nike found they universally expected more coordinated pacing efforts to occur in running after Breaking2, with two of the quoted experts predicting that behavior like "cooperative drafting", or races that incentivize cycling-peloton-like behavior could improve running times.[2]

Motorsport

Road racing

In single seater, open wheel racing series such as Formula One and the IndyCar Series, as well as to a lesser extent in sports car racing, 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 they have a straightline speed advantage, to pass on the straight. However it is very difficult for cars to follow each other close together 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.

Stock car racing

A line of cars at Talladega Superspeedway utilizing the slipstream of leader Kyle Busch.

On the faster speedways and superspeedways used by NASCAR and ARCA, 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]

Similar to the "Belgian tourniquet" in cycling, 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 the leader.[3]

Drafting was discovered by stock car racers in the 1960 Daytona 500, when Junior Johnson found that he could use drafting as a strategy that helped him overcome the fact that his Chevrolet could not keep up with other cars, allowing him to win the race.[4] Like Johnson, other 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 and/or use less fuel (resulting in fewer pit-stops) than a single car running by itself.[3]

In recent years, as aerodynamics have become increasingly critical to the performance of stock cars on "intermediate" oval tracks (between 1.33 and 2 mi) and superspeedways not requiring restrictor plates (such as the Indianapolis Motor Speedway), the effect of turbulent, or "dirty", air when following closely behind another car has become much more akin to that described above in open-wheel racing (a situation described in NASCAR circles as aero push), and is often cited as a main reason for a decrease in the amount of overtakes.[5]

Bump drafting

Drafting is most important at NASCAR's restrictor plate tracks, Talladega Superspeedway and Daytona International Speedway, where the plates mean that much less power is 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 or throttle response to maintain their drafting speeds upon exiting the draft; they can pull out and squeeze ahead but lack the response to clear the car being passed. This negates the slingshot maneuver. As a result, passing is often the result of cooperation between two or more drivers or is achieved by sucking air off the side of the car being passed, a technique called side-drafting.[6]

Bump drafting is a tactic used at Talladega and Daytona. The technique was initially popularized by the Archer Brothers in the SCCA Sportruck series during the late 1980s.[7] 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. The 2010 NASCAR season allowed drivers more freedom; bump drafting was allowed anywhere, including turns.

Kyle Busch is largely responsible for a different type of bump drafting, which is now referred to as "two-car drafting"[citation needed] and "tandem drafting".[8] At a 2007 test session in Talladega, he asked Ryan Newman to push him from behind, and was stunned to realize he was two seconds faster with Newman's help. At the newly paved Daytona International Speedway in 2011, Busch was the first to realize that the corners were smooth enough to allow a two-car draft for the complete length of the track. During test sessions on the track, when Busch was pushed by his brother Kurt's Penske Racing teammate Brad Keselowski, they ran 15 mph faster than single cars. Other drivers quickly picked up on Busch's strategy, and the two-car draft dominated the 2011 Daytona 500 and Budweiser Shootout.[9] This strategy had also been very prominent at Talladega. In 2011, two-car tandem drafting was used for the extent of the Aaron's 499, with many drivers drafting their own teammates (e.g., Jimmie Johnson and Dale Earnhardt Jr. drafted together, as did Jeff Gordon and Mark Martin). For the 2012 season, the Sprint Cup series cars were modified in a way that made the tandem impossible, in order to return to pack racing. In 2014, bump drafting was banned by NASCAR in the Nationwide Series and Camping World Truck Series. Tandem Drafting made a return when NASCAR removed the restrictor plate and replaced them with Tapered Spacers, and with the flat noses and bumpers of the modern Gen 6 cars, drivers could more easily tandem and gain speed, much like the early 2010s. After Ryan Newman's scary crash in the 2020 Daytona 500, NASCAR made efforts to change drafting at superspeedways, where less horsepower was used: the removal of aero ducts to eliminate tandem drafting and decrease closing rates, and a smaller throttle body to lower the amount of air into a racecar.[8]

Tailgating and hypermiling

Some drivers have been known to draft behind other vehicles, particularly tailgating larger vehicles, to save fuel. For example, hypermilers using this technique can achieve 75 mpg or more (a 10% increase in efficiency of certain hybrid vehicles).[10] 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.[11]

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%.[12] Traveling 10 feet (3.0 m) behind the truck produced a 39% gain in efficiency. Additionally, on the same episode, MythBusters demonstrated that it can be very dangerous for the following car if one of the truck's tires (or their recaps) delaminate, as the chunks of ejected rubber can be large enough to cause serious harm, even death, to a driver following too closely.[13]

Drafters also face the danger that, if the vehicle in front stops suddenly, there is little time to react. Platooning is under study as a means to get the benefit while also increasing safety.

Computer 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 radiator will be reduced and there is 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 a very large wind tunnel is needed. CFD, a kind of virtual wind tunnel, is used by race teams to understand the car's performance while drafting.

Racing games, such as most in the Mario Kart series, simulate drafting by giving the player a speed boost.

Drafting in nature

Animals have been observed to use true drafting behavior reminiscent of auto racing or cycling. Caribbean spiny lobsters for example are known to migrate in close single-file formation "lobster trains".[14][15]

Vortex surfing

Vortex surfing is a related phenomenon that is currently being investigated by the US Air Force to save fuel on long-distance flights. The idea is to fly aircraft in the upward part of the wingtip vortex of a leading aircraft.[16] In 2003, NASA said one of its F/A-18 test aircraft had a 29% fuel savings by flying in the wingtip vortex of a DC-8. The DC-8/F-18 flight was an exploratory investigation of large aircraft vortex-induced performance benefits on a fighter-type aircraft. The aircraft flew at 25,000 feet with a separation of about 200 feet nose-to-tail. The F/A-18 slowly moved in laterally to explore the vortex effects, NASA said at the time.[17][18] The Air Force has also tested vortex surfing with C-17s using auto pilot in 2012, and indicated a 10% fuel saving.[16] Tests in 2013 produced even greater fuel savings.[19]

Vortex surfing in nature

Canada geese flying in a V formation

Cooperative fluid dynamics techniques like drafting are also found in nature. 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 other birds in the flock do not need to work as hard to achieve lift. Studies show that birds in a V formation place themselves roughly at the optimum distance predicted by simple aerodynamic theory.[20]

References

  1. ^ Spence, Andrew J.; Thurman, Andrew; Maher, Michael; Wilson, Alan M. (2009). "Speed, strategy, drag and drafting in thoroughbred horse racing". Comparative Biochemistry and Physiology A. 153 (2): S127. doi:10.1016/j.cbpa.2009.04.216.
  2. ^ Robbie Gonzalez (Narrator/Senior Writer, Wired Magazine), Rodger Kram (Biomechanics Expert, CU Boulder), Michael Joyner, M.D. (Exercise Physiology Researcher, Mayo Clinic), Chris Beves (Senior Engineer, Siemens PLM) (June 29, 2017). The Science of How Nike Nearly Cracked the Two-Hour Marathon. Wired (online video). Event occurs at 6:35-9:12. Retrieved September 25, 2018.
  3. ^ a b c d Lemasters, Ron Jr. (January 2001) "One Wild Wind", Stock Car Racing (ISSN 0734-7340), Volume 36, Number 1.
  4. ^ "How NASCAR Drafting Works: 4 Important Questions About This Technique-centric Racing - Nascar Chronicle". nascarchronicle.com. October 1, 2021.
  5. ^ "Aero push problem Nascar", Nascar sprint cup, Auro week, June 9, 2015.
  6. ^ Ingram, Jonathan (March 6, 2010). "CUP: Atlanta Is Side Drafting Heaven". Speed Channel. Archived from the original on May 23, 2010. Retrieved May 27, 2010.
  7. ^ Glick, Shav (December 3, 1987), "Motor Racing", Los Angeles Times, p. 10.
  8. ^ a b Bruce, Kenny (January 11, 2014). "NASCAR to Penalize Tandem Drafting". NASCAR. Archived from the original on January 12, 2014. Retrieved January 12, 2014.
  9. ^ Anderson, Lars. "The Kid Wins a Wild One", Sports Illustrated, February 28, 2011.
  10. ^ "'Hypermilers' wring out every last bit of mpg , NBC News May 29, 2007
  11. ^ "Tailgate for Mother Earth!" Archived September 5, 2008, at the Wayback Machine ecogeek.org, July 16, 2007
  12. ^ Mythbusters study drafting on the road Archived October 9, 2008, at the Wayback Machine. Autobloggreen.com. Retrieved on March 28, 2013.
  13. ^ "Episode 80: Big Rig Myths Archived January 16, 2013, at the Wayback Machine. MythBusters Results. Retrieved on March 28, 2013.
  14. ^ "Finding the Way". The Trials of Life. November 1, 1990. BBC.
  15. ^ Kanciruk, Paul; Herrnkind, William (October 1978). "Mass Migration of Spiny Lobster, Panulirus Argus (Crustacea: Palinuridae): Behavior and Environmental Correlates". Bulletin of Marine Science. 28 (4): 601–623. Archived from the original on November 18, 2011.
  16. ^ a b Drinnon, Roger (October 11, 2012). "'Vortex surfing' could be revolutionary". U.S. Air Force. Archived from the original on December 12, 2012. Retrieved November 23, 2012.
  17. ^ NASA Sky Surfing for Fuel Economy. NASA (July 21, 2003)
  18. ^ "Air Force lab tests out 'aircraft surfing' technique to save fuel" Archived April 12, 2013, at the Wayback Machine. Networkworld.com (October 11, 2012). Retrieved on March 28, 2013.
  19. ^ "USAF touts flying technique for fuel savings" Archived October 30, 2013, at the Wayback Machine.
  20. ^ "Drag Reduction from Formation Flight. Flying Aircraft in Bird-Like Formations Could Significantly Increase Range" Archived May 21, 2009, at archive.today; Defense Technical Information Center; April 2002; Retrieved February 27, 2008

External links

This page was last edited on 11 December 2023, at 22:49
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