ground effect (cars)

8/11/2019 Ground Effect (Cars)

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Ground effect (cars) 1

Ground effect (cars)

Ground effect is a 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.

Theory

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', were routinely used in the design of racing cars to increase downforce, but this is

not ground effect.) Substantial further downforce is available by understanding the ground to be part of the

aerodynamic system in question. 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 asa 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 or 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.

History

Jim Hall built Chaparral cars to both theseprinciples. His 1961 car attempted to usethe 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.[1]

Formula One was the next setting for ground effect in racing cars. Several Formula One designs came close to theground effect solution which would eventually beimplemented by Lotus. In 1968and 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.[1]

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.
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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.[2][3][4]

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 pitchingmovement 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.[1]

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 design of the radiators, embedded into the

sidepods, was partly based on that of the de Havilland Mosquito

aircraft.[5] 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.[6]

The car was also observed

to visibly squat when the engine was revved at a standstill.[7]

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 Fdration Internationale de l'Automobile (FIA), governing body of Formula

One and many other motor sports, decided to ban 'fan cars' with almost immediate effect.[8]

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; flat undersides became mandatory for 1983.[9]

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 slow down

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.

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.
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Porpoising

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 future 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

known to complain of sea-sickness. This rocking motion, like a porpoise diving into and out of the sea as it swims at

speed, gives the phenomenon its name. These characteristics, combined with a rock-hard suspension resulted in the

cars giving an extremely unpleasant ride, so much so that Formula One world champion drivers Mario Andretti and

Alan Jones left Formula One because of it.

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 better knowledge of ground effects allowed designers to minimise the

problem.

References

[1][1] Nye 1985, p. 94

[2][2] S. Buckley, "Vehicle Surface Interaction" Ph.D. Dissertation, University of California - Berkeley, Sept., 1972

[3][3] B. Shawn Buckley, "Road Test Aerodynamic Instrumentation", SAE paper 741030, 1974-02-01

[4][4] B. Shawn Buckley, Edmund V. Laitone, "Air Flow Beneath an Automobile", SAE paper 741028, 1974-02-01

[5][5] Nye 1985, p. 96

[6][6] Nye 1985, p. 130

[7] 8W - Why? - Brabham BT46B (http://www.forix. com/8w/fancar.html)

[8] Henry 1985, pp. 186187

[9][9] Nye 1985, p. 33

Henry, Alan (1985),Brabham, the Grand Prix Cars, Osprey, ISBN 0-905138-36-8

Nye, Doug (1985),Autocourse History of the Grand Prix car 1966 - 1985, Hazleton publishing, ISBN

0-905-138-37-6

External links

Photoessayist.com: The Chaparral 2J (http://www.photoessayist.com/canam/chaparral/index.html)

VintageRPM: Chaparral history (http://www.vintagerpm.com/can-am-history-of-chaparral/)

8W: Brabham-Alfa BT46B "fan car" (http://8w.forix.com/fancar.html)

Dennis David: Lotus 79 (http://www.ddavid.com/formula1/lotus79.htm)
http://www.ddavid.com/formula1/lotus79.htmhttp://8w.forix.com/fancar.htmlhttp://www.vintagerpm.com/can-am-history-of-chaparral/http://www.photoessayist.com/canam/chaparral/index.htmlhttp://www.forix.com/8w/fancar.htmlhttp://en.wikipedia.org/w/index.php?title=Porpoisehttp://en.wikipedia.org/w/index.php?title=Lotus_79http://en.wikipedia.org/w/index.php?title=Lotus_78http://en.wikipedia.org/w/index.php?title=Colin_Chapman


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