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Aero Basics

The principals involved in modern hi-performance aerodynamics.

By Michael Knowling

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When travelling at high speeds, a vehicle's aerodynamics can be either your best friend or your worst enemy. Driving a car possessing no aerodynamic development at anything over about 80 km/h often reveals major wind noise, buffeting, drawn-out acceleration and a worrying lack of stability. However an aerodynamic car fully decked out with a rear spoiler, front dam and side-skirts might actually feel more stable as it gets faster.

Here we'll show you the principles of aerodynamics and how they can relate to your own vehicle.

The Overall Shape of the Car

This is the one thing that we're stuck with and, unfortunately, the overall shape of a car is ill-suited to producing high-speed stability. A car's relatively flat undercarriage and curved top section conspire to create an overall shape akin to that of an aircraft's wing - which, obviously, produces lift.

All road cars have a body shape that produces an overall lifting effect. It is generally the shape and size of the top curve of a vehicle that determines its aero characteristics, such as drag and lift - without taking into account wings etc.

Flow over the Body

The aim of a smooth body profile is to achieve attached laminar airflow (without any turbulence) over the entire length of the car. Turbulence will dramatically increase aerodynamic drag and it can also be detrimental to the use of a downforce-producing rear wing. Other side effects of this "confused" turbulent air can include frequently dirty windows due to a lack of an airflow stream, and reduced efficiency of window wipers caused by buffeting.

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However, on its own, laminar flow wrapped over the entire car does produce high levels of aerodynamic lift that makes a car feel unstable. Therefore a balancing factor, such as a spoiler, is usually incorporated in the design of these sleek-bodied vehicles. The Mazda RX7 of the Seventies was amongst the earliest production cars to incorporate a body that had excellent laminar flow, giving it a 0.36 Cd - which, today, is about the same as a poor sedan.

Drag Co-efficient

Think of a car's co-efficient of drag (Cd) as its aerodynamic resistance to forward motion. A car that has a low Cd will be able to travel at the same speed as a less aerodynamic car while consuming considerably less engine power. Conversely, it will have a potentially higher top speed and it will also accelerate quicker in the upper speed ranges.

But note that the car's Cd doesn't take into account its frontal area, only the contours along its length. So the total aerodynamic drag of a car is its Cd multiplied by the frontal area - meaning the narrower and lower a car is, the better. All modern design cars have a relatively low Cd figure, but it is interesting to learn that a set of add-on roof racks might increase this drag figure by near 40 per cent - say bye, bye to any potential gains!

Boat Tailing/Body Tapering

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Many late model cars have a rear body section that, when observed from a plan-view, gradually narrows as it nears the rear. This reduces the area of turbulence that gets left behind the car when it is in motion. The area of turbulence is called the "wake" - the same as the wake left behind by a boat. The general idea of body tapering (on any part of a car) is to create a smaller area of following turbulence. Another trick that reduces both aerodynamic lift and drag is to sharply cut away the car's rear trailing edge - thus preventing attached flow wrapping over the end of the boot-lid. Many aerodynamic cars have this feature, while those that don't usually have a curved trailing edge purely for styling reasons. The current-shape Audi sedan successfully utilises both the boat-tailing and sharp air separation concepts.

Parasitic Aero Devices

Note: Most add-on aerodynamic devises on their own are intended to provide "downforce". However, in most applications they will only reduce amount of the car's total aerodynamic lift - not give overall downforce. Very few cars have a body that creates a net downforce.

Rear Wings

A wing has a cross-section that develops lift (or downforce when it's upside down) when it has laminar airflow flow over both surfaces. High downforce wings also have huge drag, and so these types are used only on top-level high-performance cars. However, using a wing with a relatively thin cross-sectional shape (called a "section"), allows good downforce while trading off a minimal amount of drag.

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Quite often, end plates are used with a wing to prevent the air from spilling over the ends and not actually going over or under its section. These can be disguised as support pillars or mounts, but they all offer this same advantage. The "angle of attack" of the rear wing will also effect its downforce. This angle must be carefully defined as a too-steep angle will create turbulence and drag, and a too-gentle angle will reduce the generated downforce.

The location of a rear wing must always be in the path of undisturbed laminar airflow - thus enabling smooth flow over and under the wing surfaces. On a sedan, this means a wing should be positioned at the extreme rear edge of the car and usually quite high above the boot level. This locates the wing further behind the rear window so there's more chance of laminar flow approaching it, and it also gives a better leverage effect on the bodywork.

The vehicle's stability is also improved when the wing is placed as far back as possible, as it produces a more rearward "centre of pressure". The centre of pressure is the point where all aerodynamic forces appear to act on the vehicle's body. Elevated wing height helps to further ensure the provision of clean laminar airflow and prevents the close proximity of the boot-lid interfering with flow underneath the wing.

On hatchbacks or vehicles with a boxed-off rear section, wings must be positioned at the back of the roof just before the cut-off. This is to provide laminar flow for the operation of the wing - ie the same as a sedan. Note that wings on a hatchback vehicle will have no downforce effect when they are positioned halfway down the tailgate (in the car's wake!).

Rear Spoilers

A rear spoiler is designed to "spoil" or re-direct the attached flow of air over the car's trailing edge. This provides downforce by eliminating the wrap-around airflow that would otherwise travel over the rear trailing edge - thus further increasing the car's overall wing shape that, as we said, generates lift. Spoilers are generally inferior in efficiency to a wing, as they usually offer downforce at the expense of aerodynamic drag.

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The most famous factory rear spoiler in Australia is that fitted to the VL Walkinshaw Commodore. This sizable structure serves to raise the height of the boot-lid to allow continued attached laminar airflow along the roof, rear window and the boot-lid. It has a wind tunnel developed up-angle to produce downforce, and its top surface also extends beyond the rear cut-off of the car. This extension increases the downforce effect of the spoiler as it gives a greater effective surface area, plus it will also improve the body's directional stability. This happens because the car's centre of pressure will be located further behind its centre of gravity. The distance between both of these centres is the "static margin", and the larger this distance is, the greater aerodynamic stability the vehicle has. Note that this TWR-developed kit in its entirety reduces both the car's Cd and total aerodynamic lift.

Front Air Dam/Spoiler

To minimise front-end lift, it is advisable to have the front lower edge of the car (usually the bumper extension) reach down as far as possible. This prevents large amounts of air from getting under the car, creating high underside pressure and therefore aerodynamic lift. As a bonus to reducing lift, blocking off some of the under-car airflow will enhance the flow of air through the engine radiator. After passing through the core, this hot air will be able to exit the engine bay more efficiently via the low pressure area that is created under the vehicle.

Splitter

A forward protruding "splitter" catches on-coming air against the frontal area of the car and prevents it from spilling underneath. Instead, the air is forced to travel up and over the top of the car or down its sides. This air also has a downward acting force that pushes onto the surface of the splitter, as the high pressure in the front of the car tries to escape. Plus it puts more air through the rear wing (if there's one fitted).

A splitter works best on cars with a blunt front end that gives a large frontal high-pressure area to use. However it can also change the car's front "stagnation point", which is the point on the front of the car where air strikes and splits either upwards or downwards into different directions. By moving this stagnation point, the strategic location of intake ducts in a previously high-pressure area may be effected. It can also create a near drag-free source of downforce.

Side-skirts

The aim of side-skirts is to stop outside air from entering the low-pressure area that is often created underneath a car. This increases under-car "suction". Therefore side-skirts generally perform the best when they're used in conjunction with a front air dam or underbody venturi shaping, since the created low-pressure area can be maintained. And the closer to the ground a set of side-skirts reaches the more sealing effect they will have. This means a car with a full front spoiler/splitter and side-skirt combination will have a very low underside pressure until the very rear of the car.

Underside Airflow

A car with some underbody smoothness will benefit from a low underbody pressure, which reduces lift. However, the use of smooth or flat-bottomed underbellies is generally only applicable to racecars. A road car's diff(s), fuel tank, spare wheel well, driveshafts, exhaust all make it virtually impossible to encourage any undercar structural development. Some low-volume production cars are equipped with small front and/or rear undertrays that extend from near the axle line to the bottom edge of the bumper. However, we are yet to see results quantifying the use of these devices, as they are a relatively scarce fitment.

The most famous spate of undercar airflow development was in the awesome "ground effect era" of Formula One. In the late '70s, full underside trays were fitted to the cars combined with active side-skirts that barely even cleared the ground. Then virtually straight away, a rear mounted high-volume extraction fan began to be used to suck out the underlying air - thus literally creating an under-car vacuum! These factors enabled the cars to stick prodigiously to the track, even at significantly increased cornering speeds. But the real beauty of these "fan cars" was that they produced downforce independent of vehicle speed - meaning slower corners could be also taken with extra confidence and speed.

More recently, some racecars use "underbody tunnels" and "diffusers" to produce downforce. In both cases, air is carried from a flat-bottomed front section to the rear of the car where the floor angles upward. This gives an effect similar to a wing, but its forces acts upon the entire chassis of the car - so it is very effective. Put simply, the difference between a diffuser and a tunnel is that the latter has side plates, which can divide the area into individual tunnels.

But for most of us, the airdam/splitter and side-skirt route is the most practical way to achieving a low underside pressure.

Some Examples of Aerodynamic Concepts in Practice:

Ford Credit Falcon Racecar
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This racecar is dramatically lowered for less drag and to reduce lift. It uses a deep front spoiler for front downforce along with integrated ducting for a minimal amount of front drag. A forward-facing splitter is also used to prevent airflow under the body. Deep side-skirts and rear guard extensions maintain this area of low pressure under the vehicle to provide "suction onto the track". The rear wing is located on the extreme rear limit of the car and is elevated into the flow of laminar airflow to optimise its effectiveness, leverage and to move the car's centre of pressure rearward.

Holden Vectra
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The Opel-designed Holden Vectra has quite a low Cd figure. It achieves this through using smooth frontal contours to prevent turbulence, plus integrated mirrors and flush-fitting doors to maintain attached laminar flow. The sharp rear cut-off prevents excessive rear-end lift. The car's overall shape tapers to the rear very gradually, with attached flow continuing until the rear cut-off - thus giving excellent laminar flow.

Honda (Acura) NSX
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A NSX has achieved a Cd of 0.32 by using a sleek profile. At the front, the nose displaces a very small amount of air, picking it up and instantly achieving attached flow, which "holds on" until the sharply cut rear trailing edge. Cooling air for the rear-mounted engine enters through integrated side scoops that would create a minimal amount of flow disturbance, exiting underneath the car in its low-pressure area. By continuing the rear slope of the bodywork until the trailing edge, the wake created by the car is very small. Without the need to be mounted high, the rear wing is located in the path of laminar flow since there is no area of turbulence behind the rear window. The rear window also tapers in toward the centre of the vehicle to reduce (eliminate?) turbulence.

A very aero-sophisticated car!

Interested in do-it-yourself car aerodynamics? You’re sure then to be interested in the Amateur Car Aerodynamics Sourcebook, available now.

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