Ground effect (cars)

In car design, ground effect is a series of aerodynamic effects which have been exploited to create downforce, particularly in racing cars. This has been the successor to the earlier dominant aerodynamic theory of streamlining. American racing IndyCars employ ground effects in their engineering and designs. Similarly they are also employed in other racing series to some extent; however Formula One and many other racing series, primarily across Europe, employ regulations (or complete bans) to limit its effectiveness on safety grounds. In car design, ground effect is a series of aerodynamic effects which have been exploited to create downforce, particularly in racing cars. This has been the successor to the earlier dominant aerodynamic theory of streamlining. American racing IndyCars employ ground effects in their engineering and designs. Similarly they are also employed in other racing series to some extent; however Formula One and many other racing series, primarily across Europe, employ regulations (or complete bans) to limit its effectiveness on safety grounds. In racing cars, a designer's aim is for increased downforce and grip to achieve higher cornering speeds. A substantial amount of downforce is available by understanding the ground to be part of the aerodynamic system in question, hence the name 'ground effect'. Starting in the mid-1960s, 'wings' were routinely used in the design of race cars to increase downforce (this is not a type of ground effect). Designers shifted their efforts at understanding air flow around the perimeter, body skirts, and undersides of the vehicle to increase downforce with less drag than compared to using a wing. 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 be drawn towards the ground. This is due to Bernoulli's principle; as the tarp gets closer to the ground, the cross sectional area available for the air passing between it and the ground shrinks. This causes the air to accelerate and as a result pressure under the tarp drops while the pressure on top is unaffected, and together this results in a net downward force. The same principles apply to cars. The Bernoulli principle is not the only mechanic in generating ground effect downforce. A large part of ground effect performance comes from taking advantage of viscosity. In the tarp example above neither the tarp nor the ground is moving. The boundary layer between the two surfaces works to slow down the air between them which lessens the Bernoulli effect. When a car moves over the ground the boundary layer on the ground becomes helpful. In the reference frame of the car, the ground is moving backwards at some speed. As the ground moves, it pulls on the air above it and causes it to move faster. This enhances the Bernoulli effect and increases downforce. It is an example of Couette flow. 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. American Jim Hall built his developed Chaparral cars to both these principles, pioneering them. 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. 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. At about the same time, Shawn Buckley began his work in 1969 at the Univ. of California - Berkeley on undercar aerodynamics sponsored by Colin Chapman, founder of Formula One Lotus. Buckley had previously designed the first high wing used in an IndyCar, Jerry Eisert's 'Bat Car' of the 1966 Indianapolis 500. By proper shaping of the car's underside, the air speed there could be increased, lowering the pressure and pulling the car down onto the track. His test vehicles had a Venturi-like channel beneath the cars sealed by flexible side skirts that separated the channel from above-car aerodynamics. He investigated how flow separation on the undersurface channel could be influenced by boundary layer suction and divergence parameters of the underbody surface. Later, as a mechanical engineering professor at MIT, Buckley worked with Lotus developing the Lotus 78. 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 70 kg (150 lb). McLaren produced similar underbody details for their McLaren M23 design.