F1 Insight - Ground Effect.

A racing designer's aim is for increased downforce, allowing greater cornering speeds. This started in the mid-60's, when wings were first plonked on F1 cars. However these wings were not enough for some designers and went on to create more extravagant methods of reaching this goal.

It was towards the late 60's when some designers realized that substantial further downforce is available by understanding the ground to be part of the aerodynamic system.

The basic idea was to create an area of low pressure underneath the car, so that the higher pressure above the car will apply a downward force. To maximize the force the designers want the maximum area at the minimal pressure. Racing car designers have achieved low pressure in two ways:
Firstly, by using a fan to pull air out of the cavity;
Secondly, to design the underside of the car so that incoming air is accelerated through a narrow slot between the car and the ground, lowering pressure by Bernoulli's principle.

Many Formula One designs came close to the ground effect solution which would eventually be implemented by Lotus and Colin Chapman. In the late 1960's Tony Rudd and Peter Wright at (BRM) experimented on track and in the wind tunnel with long aerodynamic section side panniers to clean up the turbulent airflow between the front and rear wheels. However both left the team shortly after and the idea was not taken further. Robin Herd, of March, used a similar concept on the 1970 March Formula One car after Peter Wright had told him about the experiments he had done at BRM. In both cars the sidepods were too far away from the ground for significant ground effect to be generated, and the idea of sealing the space under the wing section to the ground had not yet been developed.

In the 1970's Gordan Murray went one better. Brabham's BT46B used a large fan to reduce underbody air pressure.

In 1977 Rudd and Wright, now at Lotus, developed the Lotus 78 'wing car', based on a concept from Lotus owner and designer Colin Chapman. Its sidepods, bulky constructions between front and rear wheels, were shaped as inverted aerofoils and sealed with flexible "skirts" to the ground. The team won 5 races that year, and 2 in 1978 while they developed the much improved Lotus 79. The most notable contender in 1978 was the Brabham BT46B Fancar, designed by Gordon Murray. It's fan, spinning on a horizontal, longitudinal axis at the back of the car, took its power from the main gearbox. The car avoided the sporting ban by claims that the fan's main purpose was for engine cooling as less than 50% of the airflow was used to create a depression under the car. It raced just once, with Niki Lauda winning at the Swedish Grand Prix. An example of the supreme advantage this provided was shown by the way that as Lauda was approaching another car to pass, there was an oil slick on the track. While the other car had to slow down, Lauda simply accelerated over it as the fan was powered by the gearbox, thus the higher the engine speed the higher the grip.

The car was short lived. Brabham's owner, Bernie Ecclestone, who had recently become president of the Formula One Constructors' Association, reached an agreement with other teams to withdraw the car after three races. However, motorsport's world governing body, the Fédération Internationale de l'Automobile decided to ban 'fan cars' with almost immediate effect. The Lotus 79, on the other hand, went on to win 6 races and the world championship for Mario Andretti and gave team-mate Ronnie Peterson a posthumous second place, demonstrating just how much of an advantage the cars had. In following years other teams copied and improved on the Lotus until cornering speeds became dangerously high, resulting in several severe accidents in 1982 (most notably the death of Gilles Villeneuve), flat undersides became mandatory for 1983.

Why was it dangerous?

Part of the danger of relying on ground effects to corner at high speeds is the possibility of the sudden removal of this force; if the belly of the car contacts the ground, the flow is constricted too much, resulting in almost total loss of any ground effects. If this occurs in a corner where the driver is relying on this force to stay on the track, its sudden removal can cause the car to abruptly lose most of its traction and skid off the track.

Nevertheless, Formula One cars still generate a proportion of their overall downforce by this effect, vortices generated at the front of the car are used to seal the gap between the sidepods and the track and a small diffuser is permitted behind the rear wheel centerline to re-accelerate the high speed underbody airflow to free flow conditions. High nose designs, starting with the Tyrrell 019 of 1990, optimize the airflow conditions at the front of the car.

Source: wikipedia.org