Aerodynamic efficiency of a car is determined by its Drag Coefficient ( Cd ). Drag Coefficient is independent of area, it simply reflects the influence to aerodynamic drag by object's shape. In theory, a circular flat plate has Cd 1.0, but after adding the turbulence effect around its edge, it would 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.
The drag is proportional to the drag coefficient, frontal area and the square of vehicle speed. You can see a car travelling at 120mph has to fight with 4 times the drag of a car travelling at 60mph. You can also see the influence of drag to the top speed. If we need to raise the top speed of Ferrari Testarossa from 180mph to 200mph like Lamborghini Diablo, without altering its shape, we need to raise its power from 390hp to 535hp. If we rather spend time and money in wind tunnel research, decreasing its Cd from 0.36 to 0.29 can do the same thing.
In the 60's, 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", added an exaggerately long tail and lower the back.
For a 3-box car, air flows will leave the car straightly at the end of roof line. The dramatical 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 has little knowledge about this and created many cars like this.
Another important aerodynamic factor is Lift. Since air flow in above the car travel 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 be easy to slide, and that is very dangerous for a car travelling at something like 160mph.
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.....
|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.|
Porsche's new 996 Carrera offers us some useful data :
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 reduced 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.
|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 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 race car.