Ground effect is term applied to a series of aerodynamic effects used in car design, which has been exploited to create downforce, particularly in racing cars. This has been the successor to the earlier dominant aerodynamic theory of streamlining. IndyCars employ ground effect to some extent, but Formula One and most other racing series worldwide currently use design constraints to heavily limit its effectiveness.
In racing cars, a designer's aim is for increased downforce, increasing grip and allowing for greater cornering speeds. (Starting in the mid 1960s 'wings', or inverted airfoils, were routinely used in the design of racing cars to increase downforce, but this is not ground effect.) This kind of ground effect is easily illustrated by taking a tarpaulin out on a windy day and holding it close to the ground, it can be observed that when close enough to the ground the tarp will suddenly be sucked towards the ground.
However, substantial further downforce is available by understanding the ground to be part of the aerodynamic system in question. The basic idea is to create an area of low pressure underneath the car, so that the higher pressure above the car will apply a downward force. Naturally, to maximize the force one wants the maximum area at the minimal pressure. Racing car designers have achieved low pressure in two ways: first, by using a fan to pull air out of the cavity; second, 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.
Jim Hall built Chaparral cars to both these principles. His 1961 car attempted to use the shaped underside method but there were too many other aerodynamic problems with the car for it to work properly. His 1966 cars used a dramatic high wing for their downforce. His Chaparral 2J "sucker car" of 1970 was revolutionary. It had two fans at the rear of the car driven by a dedicated two-stroke engine; it also had "skirts", which left only a minimal gap between car and ground, to seal the cavity from the atmosphere. Although it did not win a race, some competition had lobbied for its ban, which came into place at the end of that year. Movable aerodynamic devices were banned from most branches of the sport.<ref name="Ground Effects">Template:Harvnb</ref>
Formula One was the next setting for ground effect in racing cars. Several Formula One designs came close to the ground effect solution which would eventually be implemented by Lotus. In 1968 and 1969, Tony Rudd and Peter Wright at British Racing Motors (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. Both left the team shortly after and the idea was not taken further. Robin Herd at March Engineering, on a suggestion from Wright, used a similar concept on the 1970 March Formula One car. 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.<ref name="Ground Effects" />
On a different tack, Brabham designer Gordon Murray used air dams at the front of his Brabham BT44s in 1974 to exclude air from flowing under the vehicle. Upon discovering that these tended to wear away with the pitching movement of the car, he placed them further back and discovered that a small area of negative pressure was formed under the car, generating a useful amount of downforce - around 150 lbs. McLaren produced similar underbody details for their McLaren M23 design.<ref name="Ground Effects" />
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 design of the radiators, embedded into the sidepods, was partly based on that of the De Havilland Mosquito.<ref>Template:Harvnb</ref> 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. Its 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. The car's supreme advantage was proven after the track became oily. While other cars had to slow, Lauda was able to accelerate over the oil due to the tremendous downforce, which rose with engine speed.<ref>Template:Harvnb</ref> The car was also observed to visibly squat when the engine was revved at a standstill.<ref>8W - Why? - Brabham BT46B</ref> 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 the Fédération Internationale de l'Automobile (FIA), governing body of Formula One and many other motor sports, decided to ban 'fan cars' with almost immediate effect.<ref>Template:Harvnb</ref> The Lotus 79, on the other hand, went on to win six 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.<ref>Template:Harvnb</ref> 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.
The effect was used in its most effective form in IndyCar designs. Racing series based in Europe and Australia have mainly followed the lead of Formula One and mandated flat undersides for their cars. This heavily constrains the degree to which ground effect can be generated. Nonetheless, as of 2007, 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.
Note that while such downforce-producing aerodynamic techniques are often referred to with the catch-all term "ground effect", they are not strictly speaking a result of the same aerodynamic phenomenon as the ground effect which is apparent in aircraft at very low altitudes.
Porpoising is a term that was commonly used to describe a particular fault encountered in ground effect racing cars.
Racing cars had only been using their bodywork to generate downforce for just over a decade when Colin Chapman's Lotus 78 and 79 cars demonstrated that ground effect was the way to go in Formula One, so naturally at this point under-car aerodynamics were still very poorly understood. To compound this problem the teams that were keenest to pursue ground effects tended to be the more poorly-funded British "garagiste" teams, who had little money to spare for wind tunnel testing and tended simply to mimic the front-running Lotuses.
This led to a generation of cars that were designed as much by hunch as by any great knowledge of the finer details, making them extremely pitch sensitive. As the centre of pressure on the sidepod aerofoils moved about depending on the car's speed, attitude and ground clearance, these forces interacted with the car's suspension systems and cars began to resonate, particularly at slow speeds, rocking back and forth - sometimes quite violently. Some drivers were even known to complain of sea-sickness... This back-and-forth rocking motion, like a porpoise diving into and out of the sea as it swims along at speed, is what gives the phenomenon its name.
Ground effects were largely banned from Formula One in the early 1980s, but Group C sportscars and other racing cars continued to suffer from porpoising until gradually better knowledge of ground effects allowed designers to minimise the problem.