Use the Force – Ground-Effect Aerodynamics

2016 / 2017 Drive-My

Ground-effect aerodynamics burst onto the car scene in the late “70s. But had a pioneer aviator already figured it out half a century earlier? Time for Drive-My to prove how a pre-war design might have worked. Words: Charles Armstrong-Wilson. Images: 3D Engineers/TotalSim.

Look under the tail of a high-performance road car and you should see the floor sweeping up towards you and possibly divided by vertical vanes. This is a diffuser, part of a package of features that allows the car to generate aerodynamic downforce from the air flowing underneath. It’s an effect that took to the racetracks in the late 1970s – but if one engineer had had his way, grand prix cars would have used aerodynamics to stick them to the ground 50 years earlier.

Downforce works on the Bernoulli principle, familiar to anyone who stuck out physics to the bitter end at school. Daniel Bernoulli (1700-1782) devised equations that describe the properties of airflow and noted that air flowing through a restriction (or venturi) speeds up, but its pressure drops. So when applied to a car rushing along the road, the air forced underneath speeds up while the pressure drops. This low pressure under the car puts more load onto the tyres, generating more grip, allowing it to corner faster. Handy for a road car, essential for a racing car – and it was no accident that the idea first took hold in grand prix racing.

People first began putting wings on racing cars in large numbers back in the late 1960s. Initially it was to counteract their tendency to take off. But teams quickly realised that the more the wings pressed down on the cars, the better the tyres worked. On the principle that if some is good, more must be better, a young engineering graduate at BRM called Peter Wright proposed making wing-shaped side pods. His concept ran, but died in the flames of the team’s political turmoil before any benefits could be established.

Later he tried the same principle while working for a company called Specialised Mouldings, when it was contracted to make auxiliary tanks for the March 701 F1 car. ‘I suggested inverted aerofoil-shaped side tanks,’ recalls Wright, ‘and they appeared in several races fitted to the cars.’ With the benefit of hindsight, he adds: ‘I doubt they generated much downforce, but they housed the fuel with no drag penalty.’

Come 1974, Wright was recruited by Team Lotus and put to work looking at aerodynamics to try and rethink the grand prix car from first principles. Working with chief designer Ralf Bellamy, they spent long periods in the Imperial College wind tunnel with a quarter- scale test model. This was built around a slim chassis, with the radiators and fuel tanks housed in Wright’s aerofoil side pods as a way of accommodating them without a drag penalty. Crucially, they added vertical plates down the sides of the side pods.

‘By the end of a week of tunnel testing, the strong, wooden model would have been so modified with card, modelling clay and sticky tape that it had usually lost most of its structural integrity,’ says Wright.

Then, after one busy week, the tunnel stopped producing consistent readings. ‘Something was wrong,’ he recalls, ‘and, looking carefully at the model, it became clear that the side pods were sagging under load.’ Bellamy and Wright quickly figured out that the side pods were generating downforce. Also, the more they sagged, the more the gap between the floor and the vertical sideplates closed, increasing downforce.

This almost accidental discovery quickly led to the design that defined ground-effect racing cars: the 1978 World Championship- winning Lotus 79. Its wing-shaped side pods evolved into smoothly blended venturis, as they had become known, housed in the pods either side of the slim chassis. The Lotus also had sliding skirts bridging the gap between the side plates and the road, preventing air leaking into that all-important low-pressure area. And it developed much more downforce than any car using just wings had before. To quote the old chestnut, it could, in theory, drive along the ceiling held up only by air pressure.

Soon everyone had to copy Lotus, but the phenomenon was not without its problems. Cornering speeds rocketed, as did the loads on the chassis, causing failures the engineers had never seen before. These also resulted in huge accidents and the sport’s governing body was forced to step in with new rules. First skirts were banned, then flat bottoms were ruled out for 1983 in an effort to exclude this new technology once and for all.

But what has been learnt is not easily forgotten and the teams still found ways of generating substantial amounts of downforce from the flat bottoms using ground effect. Today, ground effect is used throughout motor sport and, on a modern grand prix car, contributes around 40% of the total downforce. Road-going supercars also use the principle to counteract lift and keep them planted on the road at high speeds.

So, there you have it, a history of ground effect as applied to racing cars. But is it? There is an intriguing aside to this story that for years has left some compelling, unanswered questions. Half a century before ground effect burst onto the racing scene in 1978, a letter appeared in the September 1928 edition of the august British journal The Automobile Engineer. It was from a Monsieur R Prevost who signed himself as the President of the Technical Committee of the Air Club of Algeria. It outlined a proposal for a car that used the airflow under its bodywork to pull it down onto the road, overcoming its tendency to become ‘unstuck’ or take off. He described his objective as being to ‘find a form of keel (bodyshape) capable of offering the minimum resistance to advance in the air, while at the same time giving the machine the maximum stability and power of keeping to the track’.

Above Aviation enthusiast Rene Prevost suggested the idea of ground effect in a letter to a magazine in 1928. Drive-My put his theories to the test using virtual modelling software.

It all sounds rather familiar, as does his description of the underbody as ‘nothing more than a venturi tube’.

Monsieur Prevost even appreciated the importance of closing the sides of the gap under the car to stop air pouring into the low-pressure area, although he conceded that there has to be a gap to stop the car scraping along the road. The idea of sliding skirts had not occurred to him.

It seems that he had conceived the idea of using air pressure to push the car onto the road decades before wings came into common use. Furthermore, he had pretty much worked out the concept of the ground effect venturi some 50 years before it hit the tracks in earnest. He was surely some kind of creative genius, but just who was he?

Rene Prevost was born in Asnieres, north of Paris, in 1886. During the First World War he was a balloonist and, in 1919, having qualified as an engineer, he became a director of the Bureau Veritas in Algeria. However, he was not a well man and often had to walk with sticks, probably due to what we now call osteoarthritis. Despite this impediment, Prevost became a leading light in the Air Club of Algeria from 1920 to 1956. Elected vicepresident of the club in 1930, he also became vice-president and secretary general of the Federation of Aero Clubs of North Africa in 1933. That year he was also awarded the Legion of Honour. He finally became president of the Aero Club of Algeria in 1945.

Remarkably, as an amateur working outside motor sport, he seems to have figured out the fundamental elements of how to use ground effect on a car. The big question, though, is would it have worked? Drive-My thought it would be interesting to find out. Using the latest software, it is possible to simulate the airflow over Prevost’s concept and produce real-world figures for the forces it would have generated.

All Drive-My had to go on was a line drawing of the basic shape and a very small photo of the wind tunnel model. So we turned to Stuart Brown of 3D Engineers, the man responsible for the digital Bugatti project featured in the January issue, for help. Could he make a reasonable stab at recreating the car?

Working with the computer aided design (CAD) software SolidWorks, Brown began by incorporating everything we knew. ‘I started by using the diagram to define the shape,’ he says, ‘but it was very basic and didn’t give me all the information I needed. Fortunately the picture of the wind tunnel model gave me the shape of the sides and the way the inner and outer surfaces blended together. Also, on the diagram, the steering wheel position was clearly wrong, so I took it from the photo. The best moment was when I finished; I lined up the CAD model at the same angle as the model in the photo and it looked identical.’

Confident that we had captured the shape accurately, we took the model to TotalSim, experts in the use of computational fluid dynamics (CFD). This is software that can be used to model the flow of a fluid and is widely used in motor sport for developing the aerodynamics of cars. With clients across all areas of motor sport, including Formula 1, TotalSim knows how to generate reliable figures using this ground-breaking technology. What was proprietor Rob Lewis’s first reaction on seeing the design? ‘I’m just amazed that someone did it and wind tunnel-tested it all those years ago,’ he said.

To test the concept, the model was run at three different speeds to see how much downforce it generated. Running at 80mph represents the maximum speed at which a 1920s racing car might have been expected to take a corner on period tyres, and would tell us whether it would be likely to make a carfrom that period faster around a racetrack. Then it was run at 120mph, the cornering speed of a modern racing car with downforce, so we could compare its efficiency. Finally, it was run at 200mph – the goal Land Speed Record cars were aiming for at the time – to see how it could have helped.

Every effort was made to ensure the simulation was realistic. TotalSim’s Graham Martin positioned a driver in the cockpit, created a moving road under the car at the test speed, and even rotated the tyres to simulate the effect of the tread revolving in the airflow. ‘It was a pretty simple CFD model compared with modern race cars,’ he says.

So, what happened? At 80mph the model generated downforce as hoped, but a modest 76kg, about the same as having a passenger in the cockpit. Once up to 120mph the downforce increased to 172kg. Again significant, but a fraction of the 550-600kg or so a modern Formula 1 car would be generating. Finally, at 200mph the downforce increased to 495kg or about half a tonne.

More interesting, though, is where this downforce acts on the car. Oddly, the test shows that at all speeds it seems to be consistently acting just ahead of the rear axle. This would mean the car would have excellent traction but would understeer badly if it tried to corner at speed. A curious result; especially when you look at the CpZ plot. This is a map of where the vertical forces act on the underside of the car. It shows a nice big blue area representing a downward force extending across most of the body.

Where, then, is it all going wrong? Martin points out the culprit, a red area just under the nose that is a stagnation point. This is producing significant lift and counteracting much of the downforce being generated in the middle of the car.

So was the design a failure? Lewis says not, pointing out that lowering the nose of the car by just 20% would kill the stagnation point and, with it, the lift it was generating. And if you subtract the lift at the front from the total downforce, that increases by around 44% and moves forward to a third of the wheelbase ahead of the rear axle.

The figures are nowhere near those produced by the early ground-effect racing cars such as the Lotus, and Peter Wright points out that skirts were the crucial element in generating those. However, Lewis observes that it would have made a noticeable contribution to traction; vital for Land Speed Record breaking where getting up to speed as quickly as possible is all-important. That was also where its inventor saw the principal benefits. ‘He put the car in the right valley,’ says Lewis. ‘He just needed to walk down a few steps and he would have been there.’ But why didn’t anyone take up the idea?

Lewis has a theory on that too. ‘The thing with motor sport that really annoys me is the lack of willingness to look at new ideas. This poor guy probably went to some teams and said “Look what I’ve got” and they went “We’re not interested”. If motor sport then was anything like it is today, they would have dismissed it.’

And what happened to Monsieur Prevost? He left Algeria at the age of 70 and retired to Marseilles. He was last heard of living there in 1974, coincidentally the year that Wright was recruited to Lotus and started work on the wind tunnel project that led to ground effect revolutionising racing. Unfortunately we don’t know whether Rene Prevost lived to see the principles he envisaged being applied to racing cars for real. By then he would have been 88 and he certainly never met Wright. But it took 50 years for engineers to catch up with his ideas and for that he surely deserves a plaque in the near-misses hall of fame.

Thanks to Stuart Brown at 3D Engineers,; Rob and Graham at TotalSim,; and Pierre Jarrige,

Above and left Lift simulations were based on Provost’s drawings and revealed significant downforce (shown in blue); ’78 Championship-winning Lotus 79 defined ground-effect racing cars.

Above and top Editor’s response to Prevost’s letter in The Automobile Engineer questioned how driveshafts would upset the ground effect; 1970 March 701 won three of its first four races.

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