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Ground Effect

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Let’s demystify the myth around Ground Effect.

The 1970s and early 1980s witnessed development in racing technology at a furious pace. Racecar engineers and designers knew that working with airflow around the car can generate downforce.

Initially, the focus was to make the car streamlined and to reduce drag. However, as the vehicles became faster, they started getting more unstable. At high speed, streamlined cars were producing aerodynamic lift. They were trying to get airborne resulting in loss of grip.

Engineer’s first tried to solve this by disturbing the airflow by using spoilers. Next, they tried increasing the airflow on the upper part of the car.

The desire to further increase the grip led to a significant revolution in motorsport design, the introduction of inverted wings which produce negative lift or downforce.

Jim Hall, the first person to race a car with aerofoils, appeared in Chaparral 2E at Bridgehampton in 1966. There are two ways to generate downforce. One is with all manner of wings and spoilers on the surface of the vehicle. The other is with ground effects. The fundamental law governing ground effect is Bernoulli’s Principle, i.e. as the speed of air (or other “fluid”) increases, pressure decreases provided the elevation from the datum is same.

The proximity of the sidepod’s entry to the ground augmented the effect by accelerating the air as it gets engulfed through the small gap between the track surface and the bottom of the radiator. The airspeed increases very rapidly as the gap closes to almost nothing (hence ‘ground effect’). With the skirts then preventing the air from escaping out from the sides, causing the acceleration of the air into the channel – and therefore its pressure reduction – was spectacular. It resulted in building a low-pressure region applied across the full width of the floor. The difference between the under body’s air pressure and that of the free stream airflow’s pressure above meant suction of the car to the ground. Furthermore, that downforce created hardly any drag, unlike that generated by the upper body wings pressing down upon the car.

In 1960s Can-Am sports cars series, fiddled with the idea of using the underside of the car to generate low-pressure region which made the car stick to the track. Getting the principle to work on a skinny-bodied, open-wheel single seater initially seemed unfeasible. The breakthrough car was the Lotus 78, which ushered F1 into the era of ground effect. The Lotus 78 which stormed the race track in 1977 used inverted aerofoils placed within the sidepods, creating two large venturi tunnels, but it was the use of sliding skirts that were the real coup and radically increased downforce.

The evolution of Ground Effect on the Lotus 78 came about by accident. Peter Wright was at Imperial College, London, testing out the idea of putting water radiators in the leading edge of the side-pods. In a bid to try and improve on the unsuccessful Lotus 77, Wright was using a wind tunnel with a rolling road, a novelty at the time. However, the instrumentation began to show some unrepeatable results. Closer examination showed that the side-pods on the model were sagging, and as they got closer to the floor of the wind tunnel, the downforce increased. That indicated two things: (1) that the side pods had started to generate downforce, and (2) that it had something to do with the gap between their edges and the ground.

Mario Andretti won four races with Lotus 78 but had many reliability problems throughout the year. The “problem” with the Lotus 78 was the position of the centre of pressure was too far forward in the car. The result was a lack of straight-line speed and oversteer. Lotus 79 solved these problems, and Mario Andretti became champion in 1978.

While the “more is better” ground effect theory seemed good on paper – and indeed in the wind tunnel – in reality, these cars proved highly unpredictable, never able to guarantee a constant level of downforce. The low-pressure area, in particular, proved hard to control, often moving around, leading to an alarming new phenomenon known as “porpoising” where the car would dip and heave, or in some cases disappearing altogether when a skirt became dislodged or broken.

The flow volume between the vehicle and the ground is strongly dependent on the car’s attitude relative to the ground. Small ground clearance results in positive lift since there is almost no airflow between the underbody and the track. With increasing ground clearance, the airflow produces low-pressure causing the overall lift to be lowered to negative values and then to rise again as ground clearance increases. An increase in ground clearance causes the flow velocity under the car to decrease. More downforce can be generated using a diffuser between the rear wheels. The air enters the diffuser in a low-pressure, high-velocity state after accelerating under the car. By gradually increasing the cross-sectional area of the diffuser, the air progressively slows down and returns to its original free-stream speed and pressure. The diffuser’s purpose is to decelerate the air without it separating from the tunnel walls, which would cause a stall, reducing the downforce and inducing a large drag force.

It is possible to create a low-pressure area, by installing an inverted wing close to the diffuser exit. The diffuser-wing combination permits a higher air mass flow rate through the diffuser, thus resulting in higher downforce. The Brabham BT46B, also known as the Fan Car, was a cunning Gordon Murray racer, designed to one-up the dominant Lotus 79 in the 1978 Formula One season.

Murray took the inspiration from the great pioneer of aerodynamics, Jim Hall’s   Chaparral 2J Can-Am car of 1970: which used fans to suck the air from beneath. Murray installed a single giant fan connected by clutches to the car’s engine. After sealing the engine bay to prevent atmospheric air from entering the low-pressure zone created by the fan, the car was good to go. Niki Lauda won 1978 Swedish Grand Prix, driving Fan Car by a margin of over half a minute, defeating Andretti in Lotus 79.

The only time, BT46B revved and throttled across the finish line of a grand prix.

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. 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. 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.

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