Sports cars new 'shark-fin'

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In the first of a new series, we look at the FIA’s efforts to prevent LMP cars from taking flight

Next year’s LMP sports cars will look very different: they will all sport a large dorsal fin. It’s not pretty, but it should obviate the frightening sideways barrel roll or ‘blowover’, spectacularly demonstrated by Stéphane Ortelli at Monza in 2008 when his Courage entered Italian airspace before re-entry. Ortelli’s was one of a spate of rolls which caused the FIA to set up a working group, composed of the ACO, sports car manufacturers and engineers, charged with finding answers.

This wasn’t the first urgent investigation into flying sports cars. There can’t be anyone who didn’t marvel at Peter Dumbreck’s escape from the triple backflip and pike his Mercedes performed at Le Mans in 1999. In a straight line his CLR headed skywards, tumbling upwards to disappear into a wood. When they found it, it was just Kevlar confetti. Dumbreck, astonishingly, was unhurt. Then as now an FIA working group devised new rules, cutting the risk of repeats.

Let’s discuss the backflips first. With the relatively flat undersides of early-1990s sports cars it was possible to generate downforce by tipping the car slightly nose-down and creating a venturi – the principle which says if you squeeze air through a narrow space its speed rises but its pressure drops. The elements are always the same – a funnel inlet, a narrow venturi or throat where the pressure is low, and a widening exit – and as Lotus showed with the 78 in Formula 1, if a car’s underside is that shape it will be sucked towards the track.

This nose-down stance causes drag, so on higher-speed tracks the temptation was to reduce or reverse the rake, raising top speeds. But this made the centre of pressure or CoP (the aerodynamic equivalent of centre of gravity) unstable, leaving the car very sensitive in pitch angle – over a brow or running in messy air behind another car the CoP could shift backwards. If the nose lifted beyond a critical amount pressure underneath would surge, the nose lifted and the car would take off, rotating backwards as it flew. It was not a new problem – 1970s Can-Am cars often flew over the brow at St Jovite, and it also happened to Porsche, BMW and others in the ’90s, but those multiple Merc flips catapulted it into public awareness.

Regulations phased in from 2004 stipulated a 20mm plank down the centre of the underside, that even running nose-up there was still some front downforce. In addition rear wings now had to be smaller and further forward. Running in more turbulent air they created less downforce, reducing the pressure needed on the front axle to balance the car. Overall downforce fell by perhaps 20 per cent, cutting speeds too.

These changes removed the backflip danger, but in 2008 a series of aerial rolls highlighted the blowover problem. These accidents centred on cars spinning and flipping while travelling sideways. And it wasn’t because the wheels dug in – they were rolling the other way. Whether the slide began simply through losing tyre grip or as a result of a puncture, video shows the cars lifting at the side moving forwards, with a lazy barrel roll before a drastic touchdown.

Such rolls were not new – for a textbook example, type ‘Masahiro Hasemi, Nissan, Fuji 1991’ into YouTube. Just why there were half a dozen similar events in 2008 isn’t clear, but it’s assumed to be simply because designers had by then regained the speed lost to the new rules.

When designers studied these rollovers the scenario became clear. With the car moving sideways, the flat side now meeting the airstream produced low pressure along the top edge, lifting it upwards, with high pressure underneath the bottom edge and low pressure at the far side reinforcing the effect. With the tyres off the ground, the driver would need a joystick to regain control. The problem had been postponed by another requirement of the post-2004 rules, the chamfer to the lower body sides; in these sideways slides this moved the lowest undercar pressure towards the centre, reducing the overturning force, but ever higher speeds were now overcoming this. An interim measure for 2009 was to radius the plank, shifting the lowest pressure point even more central, but it was clear substantially more action was required.

So what we’ll see for 2011 is the sort of fin F1 cars are now sporting. And it’s not there merely to keep the car straight. It works by offering an aero-obstacle once the car has yawed, or rotated, sideways beyond recovery, for whatever reason. Air rushing over the car side meets the fin and piles up against it, boosting pressure above the car side and cancelling the deadly lifting force. Because it’s towards the rear it also has an arrow effect to help straighten the vehicle. The driver is still having a major moment, but at least his tyres should stay on the Tarmac.

All sports cars in LMP1 and 2, open and closed, will have to comply, adding a fin of up to 0.3sq m within a defined space. It may look like a simple addition, but thousands of hours have gone into ensuring that it works. Next year the blowover should be over.

With thanks to the Lola Design Office for their consultation. For more on Lola click on to www.lola-group.com

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