Aerodynamics

Drag and Lift

Drag

Aerodynamic efficiency of a car is determined by its Coefficient of Drag (Cd). Coefficient of drag is independent of area, it simply reflects the influence to aerodynamic drag by the shape of object. In theory, a circular flat plate has Cd 1.0, but after adding the turbulence effect around its edge, it becomes approximately 1.2. The most aerodynamic efficient shape is water drop, whose Cd is 0.05. However, we cannot make a car like this. A typical modern car is around 0.30.

Drag is proportional to the drag coefficient, frontal area and the square of vehicle speed. You can see a car travelling at 120 mph has to fight with 4 times the drag of a car travelling at 60 mph. You can also see the influence of drag to top speed. If we need to raise the top speed of Ferrari Testarossa from 180 mph to 200 mph like Lamborghini Diablo, without altering its shape, we need to raise its power from 390 hp to 535 hp. If we would rather spend time and money in wind tunnel research, decreasing its Cd from 0.36 to 0.29 can do the same thing.

Fastback

In the 60s, motor racing engineers started to take aerodynamics seriously. They discovered that if they reduce the slope of the back of a car to 20 degrees or less, the air flow will follows the roof line smoothly and dramatically reduce the drag. They termed this design as "Fastback". As a result, many racing cars, such as the Porche 935 / 78 "Moby Dick" shown here, added an exaggerately long tail and lower the back.

For a 3-box car, air flow leaves the car straightly at the end of roof line. The dramatic drop of rear screen creates a low pressure area around, this attracts some air flows back to complement, thus creates turbulence. Turbulence always deteriorates drag coefficient.

However, this is still better than something between a 3-box and a fastback. If the rear screen angle is around 30 to 35 degrees, the air flow will be very unstable. It could greatly deteriorate the high speed stability. In the past, car makers had little knowledge about this and created many cars like this.

Lift

Another important aerodynamic factor is Lift. Since air flow above the car travels longer distance than air flow underneath the car, the former is faster than the latter. According to Bernoullis Principle, the speed difference will generate a net negative pressure acted on the upper surface, which we call "Lift".

Like drag, lift is proportional to area (but surface area instead of frontal area), the square of vehicle speed and Lift Coefficient (Cl), which is determined by the shape. At high speed, lift may be increased to such an extent that the car becomes very unstable. Lift is particularly serious at the rear, you can easily understand, since a low pressure area exists around the rear screen. If the rear lift is not adequately counter, rear wheels will become easy to slip, and that is very dangerous for a car travelling at something like 160 mph.

Fastback is particularly bad in this aspect, because it has a very big surface area in contact with air flow. It seems that good drag and good lift are mutually exclusive, you can't have both of them. However, as we did more research on aerodynamics, we found there are some solution to achieve both of them ....

Aerodynamic Aid

Wing (rear spoiler)

In the early 60s, Ferrari's engineers discovered that by adding an air foil (we simply call "Wing") to the rear end, lift can be dramatically reduced or even generates net downforce. At the same time, drag is only slightly increased.

The wing has the effect of directing the majority of air flow to leave the roof straightly without going to the back, this reduce lift. (If we increase the wing angle, a hundred kilograms of downforce may even be available.) There is still a little bit air flow follows the back and leave the tail under the wing. This avoid turbulence that appears in non-fastback car, thus remain drag-efficient. Since there is too little air follows this route, its contribution to the lift can be easily cancelled by the wing.

Wing must be installed high in order to be benefited from the majority air flow. Escort RS Cosworth is right .... Cougar, well, seems to use wing as decoration only.

The first wing car was Ferrari 246SP endurance racer in 1962. Just one year later, 250GTO road car incorporated a small duck tail rear wing, a first for road car of course. However, wing did not get popular until Porsche launched its 911 RS 2.7 in 1972, whose big duck tail reduced lift by 75% at high speed. Just one year later, 911 RS 3.0 used a "Whale tail" wing which completely eliminated lift. It became a trademark for the later 911s.

Porsche's new 996 Carrera offers us some useful data :

	Front lift (at 157mph)	Rear lift (at 157mph)
Wing down	64 kg	136 kg
Wing up	5 kg	14 kg

Spoiler

Spoiler is the aerodynamic kit that alter the air flows underneath the car. We call those installed at the bottom edge of front bumper as "Chin Spoiler" or "Air Dam", and those installed at the bottom edge of the car's sides as "Skirt". To understand its principle, we must first talk about underside air flow.

Air flows underneath the car is always undesirable. There are many components, such as engine, gearbox, driving shaft, differential etc, exposed in the bottom of the car. They will obstruct the air flow, not only cause turbulence which increase drag, but also slow down the air flow thus increase lift. (Remember Bernoullis's Principle ?).

Spoiler is used to reduce underside air flow by encouraging air to pass either side of the car. As a result, drag and lift caused by underside air flow could be reduced. Generally speaking, the lower the spoiler locates, the better result obtain. Therefore you can see endurance racing cars having spoilers nearly touching the ground. Of course road cars cannot do so.

Smooth Undertray

We can also reduce the influence of underside airflow by covering the car's bottom by a smooth undertray, as shown in this Ferrari F355. This avoid turbulence and lift.

Ground Effect

To motor racing engineers, wing might be a good solution to lift, but still far away from what they really want. A typical formula one racing car corners at around 4g lateral acceleration, that requires substantial downforce to keep the tyres firmly on track. Install a huge wing with high angle can satisfy this requirement, but also deteriorates the drag coefficient.

In the 70s, Collin Chapman (again) invented a completely new concept to provide downforce without altering drag - Ground Effect. He incorporated an air channel into the bottom of his Lotus 72 racer. The channel is relatively narrow in front and expand towards the tail. Since the bottom is nearly touching the ground, the combination of channel and ground forms virtually a closed tunnel. When the car is running, air enters the tunnel in the nose and then expands linearly towards the tail. Apparently, air pressure is reducing towards the tail so that downforce will be generated.

Ground Effect is so superior than wing that it was soon banned in Formual One. In 1978, Brabham's Gordon Murray tried again with different means - instead of expansion channel, he used a powerful fan to create low pressure near the tail. Of course FIA banned it again.

Ground effect is not too suitable for road cars. It requires the bottom to be very close to the ground to form a closed tunnel. For racing car, this is no problem. But road cars should have much higher ground clearance to suit different rough roads, up hill and down hill etc. This greatly reduce the effectiveness of Ground Effect. McLaren F1 road car followed Brabham's trick by using 2 electric fans to create ground effect, but honestly speaking, no tester had ever praised its down force. Dauer 962, a so-called "road car" but it is actually a road-legal Porsche 962 endurance racing car, use conventional air-channel ground effect as the race car. Adjustable ride height allow it to run in rough road (slowly) and make good use of Ground Effect in Germany's Autobahn. Nevertheless, it can barely generate 40% downforce of the racing car.

Cd World Record

Cd	Year	Model	Remark
0.137	1986	Ford Probe V	Concept car
0.19	1996	GM EV1	Electric car
0.25	1999	Honda Insight	Hybrid car
0.25	2000	Lexus LS430	--
0.25	2000	Audi A2 "3-litre"	--
0.26	1989	Opel Calibra	2.0i base model
0.26	2000	Mercedes C180	--
0.27	1996	Mercedes E230	--
0.27	1997	VW Passat	--
0.27	1997	Lexus LS400	--
0.27	1998	BMW 318i	--
0.27	2000	Mercedes C-class	C200 up to C320