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Aerodynamics and Race Cars | |
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Introduction
The importance of aerodynamics has been known throughout most of automobile racing history. In the early days of the Indianapolis 500 (Indy 500) cars were built with streamlined bodies. However, engine, suspension, and tire technologies were more important, at that time. Automobile aerodynamics was not studied closely until the early 1960's. Drag reduction is still important, but a new concept (idea) has been introduced: production of an aerodynamic downforce (negative lift), which is thought to be more important than drag reduction. From the beginning of automobile racing, race cars have become faster and faster. By the early 1960's speeds had increased to a dangerous level. In order to decrease speed and increase safety, regulations (rules) were enacted (made) to limit engine power and tire size. Since vehicle and tire drag had already been reduced designers needed to find something else to give their race cars an advantage. Most automobiles produce lift. As the speed increases, the lift force increases and the car becomes unstable. The car must be able to stay on the race track and make the almost constant turns. To counteract the problem of lift, modern race cars are designed to produce negative lift.. This means that devices are added to the race car that cause the race car to press closer to the ground. These devices either overcome lift or actually create negative lift. There are a variety of methods used to reduce lift or create downforce. These devices range from spoilers to ground effects. The type of device used depends on the type of racing and the restrictions (rules) imposed (required). The simplest devices available are front air dams and rear spoilers. These devices actually have several beneficial (good) effects. By reducing the airflow under the vehicle, a front air dam reduces the drag of the vehicle. Also, the pressure immediately behind the air dam is reduced, which aids the cooling flow across the radiator. At the same time, lift is reduced at the front of the car. The rear spoiler can reduce flow separation at the rear window, which reduces drag. It also increases air flow under the body, which promotes downforce at the rear of the car. Actual wings are used on Formula One, Indy and Group C type race cars. These wings are inverted (turned over) to produce a downforce instead of an upward "lift". By mounting the wings close to the ground, larger amounts of downforce can be produced. This is due to the increase of flow speed between the wing and the ground. The increased velocity (speed) means a lower pressure on the lower surface and, therefore, a larger downforce.
The end plates on the wings, as shown in the pictures, are used to increase the amount of downforce produced. These end plates are similar to winglets found on many aircraft. The end plates reduce drag and maintain or increase downforce. Another device used is called a "strake". These are commonly found on high performance aircraft. The strake produces lift on an aircraft. Most often a strake, on a race car, is used in conjunction with a rear-mounted wing to increase the downforce at the back of the car. Strakes mounted on the front of cars are known as dive plates. These can be adjusted to balance the downforce between the front and back strakes. They are used on cars that do not use front wings. Another device was used to increase downforce. Side skirts were mounted close to the ground. The closer the side skirt was to the ground the more downforce produced. But, if the side skirt was suddenly knocked off a tremendous loss of downforce would occur. This could easily lead to the driver losing control of the car. For this reason, skirts were banned in most forms of racing. The ban on skirts lead to the development of underbody channels. These channels go from the front to the rear. As the air increases in speed through the channels, pressure decreases. If air is allowed to come in from the sides a strong vortex will form. This vortex will help stabilize the flow beneath the entire vehicle. Therefore, these channels increase the downforce and decrease the drag of the vehicle. In Formula One and NASCAR racing, underbody channels are not allowed. Therefore a rear slant is added behind the rear axle. This slant generates the same effect as the underbody channels, only to a much smaller degree. One must remember, in automobile racing every advantage counts. Downforce has to be balanced between the front and rear of the car. If the car has a larger load in the front than the rear it will be unstable. A larger load at the rear end stabilizes the car. A balance is still important, for if the car is too stable, it will be difficult to turn. Adding a front wing to an Indy car will cause problems. Adding a wing will cause air flow to be diverted (turned) from the cooling inlets. This can be overcome by cutting out a section of the wing near the root (base of the wing). This can lead to a much better distribution of downforce while maintaining (keeping) the cooling action. For prototype race cars, the use of a front wing creates different problems. The wing can divert air flow over the car and away from the underbody channels. Downforce is reduced at the rear axle. By substituting a concave (indented) upper surface the effects of a front wing can be mimicked (copied) without diverting flow from underneath the car. This can lead to much better distribution of downforce between the front and rear axle. The placement of the rear wing can present problems. The main role of a single rear wing is to aid the flow coming from beneath the car. Because of its placement downforce is reduced. To recover some of the downforce a second wing is placed higher, above the first wing. Racing rules limit the height of the second wing in prototype and Forumula One race cars, but Indy rules allow only one wing at the rear of the car. The tires also create drag for open-wheel race cars. This is due to the separation of flow behind the tires. Several tricks have been used to decrease this drag. Usually, a simple plate is used to divert the air around the tire, limiting the amount of flow separation. The drivers use aerodynamics to their advantage on race day. NASCAR drivers, in particular, take advantage of drafting as much as possible. By following as close as possible behind another car, the drafting car can reduce drag and consume less fuel. The drag on the lead car is also reduced because the flow separation at the rear is less because of the following car. As the cars draw closer together lift and downforce ratios are changed. This causes a problem of less stability for both cars. Prototype and Indy race cars use underbody channels, thereby making drafting an undesireable (not wanted) option. Because of these channels the air flow from the lead car is highly disturbed (turbulent). This, in turn, disturbs the aerodynamic devices of the drafting vehicle. Therefore, in these types of races a lead driver might use his vehicle's aerodynamics to slow down the trailing cars by forcing them to drive in his wake (turbulence). Because of aerodynamic downforce speeds have continued to rise. Changes are constantly being made both in the regulations (rules) on vehicle devices and the attempts by designers to develop new devices to increase speed. As the aerodynamics of race cars change, so do the techniques of the race car driver. A new dimension (area) to automobile aerodynamics may soon be opened as attempts are currently under way to break the sound barrier in an automobile!
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