Aerodynamics and Automobile Racing page 1
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Introduction

Indy

The importance of aerodynamics to automobile racing has been known throughout most of the sports history. In particular, the significance of aerodynamic drag has been known since the early days of the Indianapolis 500 when streamlined shapes were commonplace for race cars. However, aerodynamic effects were a secondary concern to engine, suspension, and tire technologies. The effect of aerodynamic lift on a race car was not examined in detail until the early 1960's. Today, the production of an aerodynamic downforce (negative lift) is considered to be more important than drag reduction.

At 62 mph (100 kph), the aerodynamic drag is the dominant factor of the overall vehicle drag. For the typical sedan at this speed, the aerodynamic drag is about twice the drag due to rolling resistance. The rolling resistance increases slightly with speed while the aerodynamic drag increases with the square of the velocity. When one considers that the one-lap record speed for the Indianapolis 500 has been above 80 mph (~129 kph) since before 1920, it is little wonder that drag reduction was a concern of the race car designer.

By the early 1960's speeds were increasing rapidly. In an attempt to decrease speed and, therefore, increase safety, regulations were enacted to limit engine power and tire size. Car designers were forced to look elsewhere to give their team an advantage. The advantage was found in aerodynamics. Not in the reduction of drag, but in the production of aerodynamic downforce.

Most automobiles produce lift. As the speed increases, the lift force increases and the car becomes unstable. In order to counteract this problem, modern race cars are designed to produce negative lift. The typical family sedan has a lift coefficient of about 0.3, while a race car can have a lift coefficient of -3.00. One can easily see the significant amount of downforce that a race car can produce.

There are a variety of methods used to reduce lift and, ultimately, produce downforce. These devices range from spoilers to ground effects. Their use depends on the type of racing and the restrictions imposed.

The simplest devices available are front air dams and rear spoilers. The flow under a vehicle is disturbed by the various drive train and plumbing components. 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, the lift is reduced at the front of the car.

*Illustration of a race car with front air dam and rear deck spoiler*

The rear deck spoiler is designed to raise the rear stagnation line. In other words, the rear spoiler increases the flow under the body by ensuring that the flow from the upper surface doesn't wrap around the rear of the vehicle (need an illustration). This promotes the production of downforce at the rear of the car. Depending on the car geometry, a rear spoiler can decrease drag by reducing flow separation at the rear window.

*Illustration depicting how rear spoiler alters flow at back of vehicle.*

Actual wings are used on formula one and Indy type race cars as well as Group C prototype racers. These wings are inverted to produce a downforce instead of 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 means a lower pressure on the lower surface and, therefore, a larger downforce.

*F1 Race Car *Indy Race Car *Group C Race Car


Dragster

The end plates found on the wings, as shown in the pictures, are used to increase the amount of downforce produced. These end plates are quite similar to winglets found on many aircraft. The purpose of such a device is to reduce the loss of lift that occurs at the wing tips. Flow from the high pressure region flows around the wing tip to the low pressure region. This results in a decrease of downforce at the wing tips as well as an increase in the induced drag. The endplates reduce the flow around the wing tips thereby maintaining lift while reducing the induced drag.

As an alternative to multi-element airfoils, a device known as a Gurney flap can be added to the trailing edge of the wing. The Gurney flap is a short (<5% of the chord length) flat plate mounted at 90 degrees to the trailing edge. This simple device increases the effective camber of the airfoil. The result is greater downforce with only a small increase in drag. The Gurney flap was first used on race cars but is now finding its way onto aircraft.

Following the more traditional route from aircraft to race car is a device known as a strake. The strake is commonly found on high performance aircraft such as the F-18. The strake produces lift at high angles of attack by producing vortices over the upper surface of the wing. This imposes a low pressure region on the upper surface of the wing which leads to the development of lift. On a race car, a rear strake is used to achieve the same affect only in reverse. Most often a strake is used in conjunction with a rear-mounted wing to increase the downforce at the back of the car.

*Illustration of strakes and dive plates (with labels).*

Strakes mounted at the front of the car are known as dive plates. Dive plates are typically used to trim the front/rear downforce ratio for cars without front wings. A similar effect can be achieved by mounting a horizontal plate at the front of the car. When the stagnation point occurs above the the plate, the high pressure above the plate will produce a downforce.

A significant amount of downforce can also be achieved through the use of ground effects. This was already touched upon with the idea of mounting the wings close to the ground. However, by enclosing the wings with side skirts, larger amounts of downforce will be produced. The smaller the gap between the skirt and the road, the larger the downforce. In order to eliminate the gap altogether, flexible or sliding skirts were used. The amount of downforce produced diminishes rapidly with increasing gap distance. This is due to the air that flows through the gap and into the low pressure region under the wings. If a skirt should fail or suddenly pop up, 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.

*Picture of car with side skirts.

The ban on skirts lead to the development of underbody channels. These channels are a type of diffuser and are sometimes referred to as venturi channels. As the flow accelerates under the car, the pressure decreases as dictated by Bernoulli's relation. This low pressure air then slows down through the channels. At the same time, air enters the channels from the side of the car. If the outer walls are close to vertical, a strong vortex will form in the channel. This vortex will keep the flow attached within the channel while helping to stabilize the flow beneath the entire vehicle. Therefore, these channels increase the downforce and decrease the drag of the vehicle.

*Illustration of underbody channels.

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.

*Illustration of slant behind the rear axle.

While discussing the various methods utilized to produce aerodynamic downforce, the downforce was occasionally differentiated between the front and the rear of the car. The load distribution between the front and the rear axle is an important parameter in determining the car's stability. If the car has a larger load (the combination of weight and downforce) at the front axle than the rear axle, the car will be unstable. On the other hand, a larger the load carried at the rear end stabilizes the car. Therefore, the designer wants to generate more downforce at the rear of the car. A balance must still be maintained, for if the car is too stable, it will be difficult to turn.

*Diagram depicting loads on the front & rear axle.

While the aerodynamics of these individual components are fairly well understood, their interactions with the entire car is not. For example, simply adding a front wing to an Indy car will cause several problems. With the addition of the wing, the flow will be diverted from the cooling inlets. Therefore, the wing section near the root is cut out as shown in the figure. Care must also be exercised to ensure that the front brakes receive undisturbed free stream air for effective cooling.

*Illustration of effect of front wing to the aerodynamics of an open­wheeled race car.

For prototype race cars, the use of a front wing creates different problems. When an easily identifiable front wing is used, the flow is diverted upwards and over the car. This diverts the flow away from the underbody channels. Therefore, the downforce produced at the rear axle will be reduced. By utilizing a concave upper surface, the effects of a front wing can be mimicked without diverting flow from underneath the car. This can lead to a much better distribution of downforce between the front and rear axle.

*Illustration of a clearly identifiable wing and a concave upper surface.

The placement of the rear wing presents another interesting problem to the designer. The main role of a single rear wing is to aid the flow exiting from beneath the car. Therefore, the wing is placed low enough to interact with the flow beneath the car. This decreases the amount of downforce that the wing itself creates since it is moving through the highly disturbed flow field of the car body. A second wing is mounted as high as possible to produce more downforce from the undisturbed freestream air. The height is generally dictated by the regulations governing the overall height of the race car. This second wing can be seen on prototype and formula one race cars, but Indy rules only allow for the use of one wing at the rear of the car.

*Illustrations of rear wings for various race cars mentioned.

The tires also create a significant amount of drag for open­wheel race cars. This is due to the separation of the flow behind the tires. Several tricks have been used to decrease this drag. Most of these involve simple plates designed to divert the air around the tire, thereby, limiting the amount of flow separation behind the tire.

The race car designers are not the only ones concerned with aerodynamics. The drivers themselves 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 will minimize drag and therefore consume less fuel. The drag on the lead car is also reduced as the flow separation at the rear of the car diminishes due to the presence of the second car. One major drawback to drafting is a reduction of stability for both cars. As the distance between the two cars decreases, the lift at the rear axle on both cars increases. This is amplified for the second car as the lift at the front of the car decreases.

*Picture of drafting in a NASCAR race.

For prototype and Indy race cars which incorporate underbody channels, drafting is not a desirable option. The flow from the lead car is highly disturbed due to the vortices exiting from the channels. This can reduce the effectiveness of the drafting car's aerodynamic devices. Therefore, in these types of racing, a lead driver might use his vehicles aerodynamics to slow down the competition by making them drive through `dirty' air.

The emphasis may change, be it downforce for Indy racing or drag for drag racing, but aerodynamics plays an important role in automobile racing. When regulations limited the engine power and tire size, the production of aerodynamic downforce kept speed rising. As speeds continue to increase, new regulations have developed putting limitations on aerodynamic devices. For example, the sliding skirt was banned along with the use of active aerodynamic devices such as fans and moveable wings. Still, a continuous game of cat and mouse is being played by designers and the regulating bodies of automobile racing.

As the aerodynamics of race cars change, so do the techniques of the race car driver. For a NASCAR driver, drafting will always play a major role since costs are kept down by only allowing limited changes to production vehicles. On the other hand, with ever changing rules and regulations, more dramatic changes in driving technique may occur in other types of racing.

The role of aerodynamics in automobile racing is extremely complex. All of the interactions which occur on a given vehicle make the flow field of every car very complex. A new dimension 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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