THE FUTURE OF AERODYNAMICS
Cars are starting to look the same and its because of physics
There is a basic tension between the aerodynamics engineers and the car designers. Each side will say that their missions are in harmony, that good design should not preclude good aerodynamics, and that aerodynamics should not limit design. But that’s not entirely true. The wind doesn’t care whether your grille is distinctive. The wind wants your car to look like a raindrop. Anything else is a compromise. Yes, every sliver of kilometre per litre or range matters, but visual distinction sells cars. ‘I always ask: “Have we perfected the bottom of the car?”’ says Andrew Smith, global head of design for Cadillac. ‘Let’s work on the part that nobody can see.’ The aerodynamicist, then, must please both the designers and the laws of physics, creating shapes that are visually distinctive, but are also aerodynamically anonymous, helping the car slip through the air without any drama. Maybe you’ve seen it: three lanes of midsize crossovers that could trade badges and nobody would notice. All those shapes are dictated by interior space, power-train packaging, government regulations, and production feasibility. But most of all, they’re dictated by aerodynamic efficiency. When we started asking auto manufacturers how that works, we realised that it’s not incredible that so many cars look so similar: It’s incredible that cars look different at all.
And this can lead to some novel solutions. Consider the grille found on the electric Jaguar I-pace. Why does an electric car even have a grille, a relic of the internal-combustion engine? Standing next to a red I-pace in the parking lot of Jaguar Land Rover’s new North American headquarters in Mahwah, New Jersey, I ask. ‘ We’re still building our brand,’ says Wayne Burgess, the production studio director at Jaguar Land Rover. ‘All the other models have a grille, so we didn’t want to abandon that with the I-pace. But we made it functional even though there’s no radiator behind it.’ He’s referring to how the lower half of the grille feeds air to heat exchangers for the battery’s cooling system, and the top half opens out to the bonnet, creating a path through which air can accelerate over the windshield and roof and down over the rear window to the upright tail. ‘ You want to keep the air attached to the car, and then suddenly detach it, to minimise turbulence,’ he says. ‘ That’s why, in a plan view, looking down, the sides of the car are flat, but in profile you see curves at the tops of the fenders.’ It makes sense. When it’s moving, this shape is efficient and distinct, a shark fin moving through water. But Burgess does seem just a little bit wistful when he compares the I-pace’s flat flanks to the Jaguar F-type’s more curvaceous fenders. ‘ The F-type has that beautiful Coke-bottle shape,’ he says, ‘ but it creates turbulence. The air gets detached.’ You can’t have sexy, flared fenders and low drag.
AERO R&D A few weeks later, I’m at GM’S Technical Center up in Warren, Michigan, standing inside GM’S new wind tunnel. The tunnel’s supervisor, Nina Tortosa, fires up the 820 kw fan and spins the rolling road under a 40 per cent-scale Chevy Silverado. These treadmill systems are now standard in wind tunnels because, in the real world, your car moves, but the road and air don’t. The wind tunnel must reverse both parameters, or else the results will all be skewed.
I always thought that wind tunnels blew smoke over cars so everyone could see how the air behaved. It turns out no one really does that. The process is messy, so it’s mainly a stunt for photos or visitors.
Naturally, I want to see it. Tortosa points a wand over the top of the overgrown toy truck and the smoke glides smoothly across, tracing the shape of the bonnet and roof before going all chaotic over the bed and tailgate. She moves the wand under the truck and reveals that, yes, the exhaust system, suspension, and axles make for turbulence. It’s an empirical demonstration of why manufacturers use front air dams – and even heightadjustable suspension – to minimise air going under the truck. You want as much air as possible going over the smooth parts up top rather than across the lumpy underbody.
In addition, GM uses supercomputers to run elaborate fluid-dynamics simulations, the results of which guide engineers toward problem areas. ‘It showed a
circulation bubble on the quarterpanel of the Volt,’ Tortosa says. ‘ We couldn’t see that in the tunnel.’
But a wind tunnel is still essential. ‘Once I build a computer model, if we change something, it takes a day to run the new calculations,’ says Ken Karbon, global head of simulation for aerodynamics. ‘ Whereas, how many different iterations could you test in the tunnel?’ He directs the question to Tortosa. ‘ Thirty,’ she says. ‘ We could try 30 different things in a day.’ It’s a lot of work that results in small but vital margins. ‘ We can affect aero by perhaps 10 per cent one way or the other – so if the coefficient of drag is .30, maybe we can get it down to .27,’ Karbon explains. ‘And that might represent a tenth of a kilometre per litre in fuel economy, depending on the vehicle.’
We go next door to the centre’s original wind tunnel, which is simply unfathomably huge. The fan looks like a propeller from the Queen Mary, fitted with six beautiful laminated spruce blades – wood is better than even carbon fibre, because if a piece of car flies off and damages a blade, technicians can easily repair it. The space is shaped roughly like an oval track, so the air circulates through the test section with the vehicle and back to the fan, requiring less power. You could host Wimbledon inside the chamber behind the fan. Seeing this stadium-size facility in person now makes it pretty obvious why not everyone has one. It is a very expensive tool, and a fantastic and complicated machine, so much so that no two tunnels produce the exact same results.
‘ The three US companies all have wind tunnels, and we all test each other’s cars,’ says Tortosa. ‘ We share the results, because each wind tunnel is different. So we like to know what our truck got in Ford’s tunnel, and vice versa.’ This professional aero courtesy does not extend to foreign companies.
TECHNOLOGY, NOT COMPROMISE
Aerodynamic principles are fixed, non-negotiable. So how will GM, Jaguar, Cadillac, and every other manufacturer continue to make cars more efficient while avoiding homogeneity? How do we get performance cars that have high top speed (low drag) and high downforce (better handling)? One answer kept coming up: active aero, cars that don’t just passively zip through the air, but manipulate it to their benefit. This is the way you resolve all the compromises in performance and styling. You build a Transformer. MercedesAMG’S upcoming 750 kw supercar, the Project One, is a case in point. Before going to New Jersey and Michigan, I’m at the Miami International Boat Show with the chief design officer for Mercedes, Gorden Wagener. Looking at a fullsized clay reproduction of the Project One, I remark on the cleanliness of its silhouette. ‘ Well, there is a lot going on under the car that you cannot see,’ Wagener says. ‘ But also, there’s active aero. There’s a big wing there, but you don’t see it when it’s parked.’
Active aero satisfies both designers and aerodynamicists because it’s at once effective and mostly invisible. Lamborghini’s ALA (Aerodinamica Lamborghini Attiva) system, as gets deployed on the Huracán Performante, doesn’t look like much more than a wing on the rear deck. But that wing’s hollow, with airflow through each of its stanchions controlled by an electric motor – and there’s a similar system in the front spoiler to balance front – rear lift. On a straight stretch, the motors can channel air under the wing for low drag. Close the air intakes and the wing makes high downforce, for extra grip in corners. And the car can vary the system from side to side for ‘aero vectoring,’ which gives you even more control when steering into tight corners. Now, I’ve driven the Huracán’s predecessor, the Gallardo, at 320 km/ h
on a supersonic runway in Florida, and it was spooky, wandering and floating across the tarmac. I didn’t go that fast in the Performante, though as fast as I did go, it was glued to the road, with none of the Gallardo’s aerodynamic imbalance. Lamborghini has always prioritised top speed over handling, but active aero allows for both.
The Performante set a lap record at the Nürburgring, the tried-and-tested measure of high-speed stability. But never forget it will also easily gallop to 325 km/ h.
Suzy Cody, GM’S head of vehicle performance for aerodynamics, says this technology is the bridge between design and engineering. ‘Look,’ she says, ‘ it doesn’t matter how great your aerodynamics are if only ten people buy the car. Design matters. And active aero helps enable design.’ But what if, I posit, there’s a massive propulsion breakthrough? Right now, aerodynamics are tied to fuel consumption and electric range. What if we had batteries that were good for 965 km of range and could fully charge in ten minutes? Could we stop worrying about every crease in the bodywork? Could we just give those designers the flared fenders and not sweat the small stuff? In other words, would the aero cease to be such a big deal? Cody, unsurprisingly, appeared aghast that I would even suggest such a thing. ‘Even if you had a battery like that, good aerodynamics gives you other options. You could have a small battery or make the car cheaper, give it more space, make it quieter. Aero will always be important.’
GM uses supercomputers to run elaborate fluid-dynamics simulations – but the wind tunnel will produce quicker results. It’s in use 24 hours a day, seven days a week.
A TEACHABLE MOMENT LE MANS, 1999: Cresting a hill + low downforce = flight. Somehow, no one was hurt.
SEMI-INTERESTING AERODYNAMICS FACTOID NO. 1 The shape known in physics as the Sears-haack body produces the lowest drag per volume at a supersonic speed. Imagine a chubby toothpick.