People usually put spoilers on cars without realising what it will do. They dont know how it works and use their butt dyno to see results. This time we will discuss how it actually works.
For cutting-edge aerodynamics on production-based cars, look no further than Germany's DTM and Japan's Super GT cars. Both series have rules allowing significant use of aero aids (such as multi-element wings, diffusers, super-low side skirts, air dams, and splitters) while keeping reasonably faithful to the cars' original lines. These components combine to produce prodigious amounts of downforce, helping to keep the car planted through corners at ballistic speeds. They also improve braking performance and acceleration thanks to the added traction.
There is a price for this amount of downforce. Drag. Redirecting the energy of the airflow to hold a car down creates more resistance for the car to push against. But that's OK. Although drag reduces top speed somewhat, the increase in cornering speeds makes for faster lap times, so some drag is acceptable. Many race cars have drag coefficients of over 1.1, while modern production cars hover around 0.35. The big difference is that race cars have enough horsepower to compensate.
The key is to produce just enough downforce to maximize the average speed around the track. Produce too much downforce and the increased drag will slow the car excessively, too little downforce will hurt cornering speeds. It usually takes some experimenting with wing settings and other components to find the sweet spot where performance is optimized.
Keep in mind that the basic shape of a production car generates a lifting force when moving through air. The lift characteristics increase exponentially (to the second power) as speed increases. As a result, traction at high speed is reduced and the car's handling can become vague and unresponsive, especially when cornering. By adding downforce, some or all of this unwanted lift can be reduced.
Several aero components are mounted nearer the front, others are usually located closer to the rear. For example, a wing is usually mounted on the trunk lid; an air dam is attached to the front bumper cover. The effect produced by the component will generally be concentrated near to where it's mounted. Consequently, the rear tires will 'feel' more of the wing's downforce than the front tires. Since a wing is usually mounted behind the rear wheels, there will usually be a decrease in the load acting on the front tires due to the fulcrum effect; downforce from the wing will actually lift the front of the car.
To improve high-speed handling, we would normally add downforce in proportion to the car's lengthwise weight distribution. In many front-wheel-drive cars, which have about a 60/40 split, adding downforce in the same percentages to the front and rear retains a balanced handling feel.
If a wing is added to an already 'balanced' car, then the tendency will be to increase understeer because of the slight front-end lift. Since most production-based cars are designed to understeer, the addition of a wing will make this tendency even worse at higher speeds. To correct understeer, the easy fix is to add downforce to the front. A properly designed air dam - with or without a splitter - will add some much-needed downforce.
When we want more downforce at both ends, a wing or spoiler can be used at the rear in conjunction with an air dam and/or splitter at the front. The size and design of the wing and the size and type of the air dam and splitter will determine how the high-speed handling will be affected. Usually the air dam is a fixed size and shape, whereas the wing can be adjusted for more or less downforce, depending on the wing's angle of attack relative to the oncoming airstream. Increasing the nose-down attitude will result in more downforce - up to a point. Altering the wing's angle of attack will fine-tune the high-speed handling balance.
By using some of the various aids below, you can properly tune the aerodynamic characteristics of your car not only for downforce, but also for cooling and stability. Some of these parts might be overkill in anything but competition, but all are readily available as bolt-on or home made parts for most imports.
We'll call automotive wings 'inverted wings' to avoid confusion with aircraft wings. Although aircraft wing sections are similar, the automotive version is inverted with the more curved surface on the bottom, hence the name. An inverted wing is a device that generates downforce by creating a pressure difference between the top and bottom wing surfaces. The oncoming air splits at the wing's leading edge, where some air goes over and the rest goes under the wing. Because of the wing's profile, the air going over the top is moving slower than the air on the bottom. In addition, Bernoulli's law states that slower-moving air possesses a higher static pressure. As a result, the higher-pressure air on top pushes down more than the lower pressure air on the bottom pushes up. This pressure difference, in addition to the plane-view surface area (angle of attack) of the wing, is what creates downforce. The presence of the wing modifies airflow over the car, resulting in slight pressure differences that need to be considered for the generation of overall downforce.
Most garden-variety wings have a constant cross section along the wingspan. Other more sophisticated wings change in both airfoil type and size in addition to a step in the wing's angle of attack (at approximately 20 to 25 per cent of the wingspan) towards the end of the span. These complex three dimensional '3D' wings can be more effective than the simple examples because the design takes into account the actual flow arriving at the wing. At both ends, the air coming off the rooftop-to-window juncture has a different angle of approach compared with the air going over the middle of the roof. By designing a wing to take into account this local airflow condition, more downforce and less drag can be achieved.
While some wings are single-element, others may have many elements. A single-element wing can generate a significant amount of downforce. However, in some forms of racing where multiple elements are allowed, downforce can be doubled or even tripled. For example, Formula One cars can have as many as four elements, contributing to the staggering amount of overall downforce produced. Multi-element designs are also used on wings with long chords (the length when you look at the cross section). Smooth airflow tends to detach from the surface of the wing when it has to travel a long distance. By using multiple elements, air from the bottom is allowed to cross over to the top and continue to flow across the topside of the next element. This minimizes flow separation over the wing just as a jet liner with flaps fully extended.
There are some devices that have been proven to augment the downforce of inverted wings. One used prominently on race car wings is called the Gurney flap, or wicker bill. The Gurney flap is usually a very small bent angle attached to the wing's trailing edge. The upward-protruding edge helps produce more downforce by increasing the vertical airflow deflection. Gurney flaps are efficient at improving downforce up to the size of approximately four per cent of the airfoil's chord length. More downforce can be generated with larger Gurney flaps, but drag increases quickly as the flap size increases. For a 10-inch wing chord, an effective Gurney flap of two-fifths of an inch could augment the wing's downforce by as much as 25 per cent.
The GT-R's underbody has two main diffusers to help drive the low-pressure flow beneath the car. The obvious one is the upswept duct at the rear and below the bumper. The not-so-obvious one is located directly behind the splitter leading into the front wheel wells. Aerodynamically, both of these diffusers achieve the same thing: minimizing pressure under the car. In addition to the rear diffuser, a number of vertical 'fences' are used so the airflow moves more efficiently underneath.
A rear diffuser helps drive the under-car flow by exposing it to the turbulent low-pressure wake region behind the car, using this low pressure to suck the flow out. In addition, the diffuser slows the air emerging from the underbody region by expanding it through a larger-area opening. They are effective in generating large amounts of downforce by increasing air speed underneath, thereby reducing pressure. Since this low-pressure region acts on a large surface area, plenty of downforce can be generated. Even if pressure below the diffuser is only half a psi lower than outside, over a 3x6-foot area, that equates to over 1000 pounds of downforce.
Vertical fences are installed within the diffuser channel to ensure that flow remains attached to the diffuser. Since the diffuser ceiling slopes upwards, airflow there is slowing down, resulting in increased pressure. Aerodynamicists call this type of region an adverse or unfavorable pressure gradient, since maintaining attached flow almost always requires that the flow speed increase throughout the region where it moves over a surface. Flow separation - and the resultant loss of flow velocity - would reduce downforce significantly if nothing were done to prevent it. The fences act as vortex generators to assist in energizing the flow through the diffuser, which help maintains attached flow and allows the air to fill in the wake.
Also known as dive planes or dive plates, since they resemble the winged appendages on submarines, canards help generate downforce in two different ways. First, the canard redirects the oncoming air's momentum upwards, which causes a downward force on the canard. This is only moderate, since the velocity near the skin is significantly slower than in the free stream. In addition, canards generate strong vortices that travel down the sides of the car and act as a barrier. If the canards are positioned correctly, these strong vortices act to keep high-pressure air around the car from entering the low-pressure underbody region, thus maintaining more downforce. If air was allowed to enter the underside, the pressure would inevitably rise, reducing downforce. Therefore, these strong vortices act like a virtual curtain or dam, restricting higher-pressure air around the car's sides from entering the underbody region. As a result, the low pressure under the car is maintained and downforce is maximized. Unfortunately, canards are not that efficient , since the strong vortices create a significant amount of drag. They are more useful for fine-tuning aerodynamic balance.
Side skirts are used to reduce the amount of air that goes under the car from the sides. If an air dam is used, air under the car is at a low pressure, which causes the higher-pressure air on the outside of the car to come rushing in. The effectiveness of the skirts depends primarily on how close to the ground the lower edge can be maintained. That edge should be less than a half-inch from the ground, otherwise the skirts' effectiveness diminishes rapidly as the gap increases.
Side ducts are primarily seen on race cars for two reasons, because brake and engine cooling is crucial, and because most serious race cars will use a front underbody diffuser that channels airflow toward the rear of the front wheel well. Conventional fender designs trap much of the turbulent air coming off the top and back of the tire generated by the counter rotation of the tires and wheels. Combined with hot air moving through the engine bay and brakes, this generates losses and drag. Side ducting not only provides a smooth outlet for these hot and turbulent gasses, but also turns the flow to exit smoothly along the side of the car instead of directly outward, which would interfere with the turbulent curtain generated by the canards. This reduction of air stagnation inside the bay also helps pull more fresh air through the cooling system.
The air dam's job is to restrict the amount of air going under the car. By using a vertical barrier made from either a composite material or aluminum sheet, the air dam effectively reduces the opening leading to the underside of the car. By restricting flow under the car, more air is forced around the sides and over the top of the bodywork at higher pressure. The limited air forced underneath has to pass through faster and thus at a lower pressure which causes a suction effect. Air dams are more common in production cars with higher ride heights and bumpers.
Splitters, the horizontal plate extending forward and underneath the air dam, use the same principle but operate differently. Since the front of the car is a blunt shape, the oncoming air is slowed substantially, resulting in a high-pressure zone known as a stagnation point. By placing a horizontally protruding splitter plate right in the thick of this high-pressure zone, a large amount of efficient downforce can be generated. The splitter, hence its name, splits the high-pressure zone from the low-pressure high-speed flow moving under the car. Pressure varies with the car's speed squared, so downforce increases quickly as the speed increases. Generally, the effects are felt at speeds over 75mph. Downforce can be increased or decreased, depending on the amount of exposed splitter area, and an adjustable splitter area can be used to fine-tune the aerodynamic balance. As is true with most race cars, the Nismo GT-R uses a splitter only, on account of its low ride height and large ducts that feed its engine bay.
Unlike the prominent ram-type intakes seen behind the driver's head in Formula One cars, NACA ducts are submerged into the bodywork. When they were developed for the National Advisory Committee for Aeronautics (NACA) in 1945, these ducts where called 'submerged-duct entrances'. NACA ducts are low-drag intake channels used for a variety of cooling requirements such as brakes, engine, and even the poor overheated driver. The NACA duct's distinctive geometry includes a widening mouth at the inlet, with the duct floor slightly opening up the flow area. Extensive wind tunnel testing of various designs has resulted in the best compromise of flow rate to drag. In the case of this GT car, the NACA duct on the hood feeds small airboxes that direct cool air into the front brakes. Sharp wall-edges effectively generate vortices that help keep the flow attached to the diffuser-like slope floor. These edges have to be sharp (unlike many aftermarket parts copies), otherwise the flow would separate, reducing the duct's efficiency.
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