Aero-car designs go with the flow

Aerodynamics is the science of performance in one of its purest forms. CHARLIE WATSON talks to Saab specialist Hakan Danielson about the effect of the carmaker's aircraft-engineering heritage

As the dark shadow of post-war austerity shaded everyday life in northern Europe, the first Saab car was unveiled in the Swedish city of Linkoping.

It was June, 1947, and the Saab 92's sleek, ultra-modern lines were a revelation to the assembled motoring writers. Here was a car for the future, a bold statement of optimism and possibility.

But the Saab 92's qualities went far beyond its looks.

Taking the pre-war German DKW (early Audi) car as a benchmark, the design team used its aeronautical skill to create a car with an aircraft-wing profile that minimised power demand and fuel consumption and delivered a 50 per cent improvement in acceleration.

"That car was incredible," says Hakan Danielson, aerodynamics manager for Saab Technical Development. "In fact, it took us until the launch of the Saab 9-5 in 1997 to come up with a car that was better aerodynamically than the Saab 92."

A reorganisation of the Australasian operation means Saab's 9-5 series is being marketed hard in New Zealand with price cuts aimed at grabbing a bigger share of the high-end performance market.

Today, Saab's commitment to aerodynamics - with safety, one of the marque's brand pillars - is as strong as ever. From analysing the air-channelling properties of a front spoiler to mapping airflow through the engine compartment, the focus on aerodynamics can be encapsulated in one word: performance.

The best-known measure of aerodynamic performance is the drag coefficient (Cd) value. A low Cd value - below 0.32, say - means that the car offers low resistance to oncoming air and is more efficient, requiring less engine power and consuming less fuel.

The Saab 9-5, for example, has a Cd value of 0.29 and the estate a value of 0.31. The latest Mercedes-Benz, the C-Class, has a Cd value of 0.26.

"Mention no names," says Danielson, "but some manufacturers' cars have, until recently, had Cd values as high as 0.4, which means that driving them is like going permanently uphill.

"So there's no doubt that the Cd value is the quality measurement of a car, but it's also essential to point out that aerodynamics is about much more than Cd values.

"Stability, noise, in-car air quality, engine compartment contamination and factors such as dirt deposits and water that affects visibility ... these aspects of performance are all affected by a car's aerodynamic properties, and every Saab produced requires a slightly different emphasis.

"The Saab 9-5 Aero, for instance, is a car we know is going to be driven at high speed so, although its Cd value is very good, when we tested it we concentrated hard on stability."

The development process - like that of all carmakers - begins with sketches, but quickly moves to clay models.

Eight scale models are built - each one a half-model mounted on a mirror - and are evaluated visually for their aerodynamics properties.

The shortlist is then whittled down first to four and then to two models, a main and a back-up, and testing is switched to the advanced wind tunnel at Stuttgart University in Germany.

There the university's aerodynamics team works closely with Saab engineers, adjusting the one-fifth scale models between tests and running overnight computer fluid dynamics calculations to predict the impact of further possible changes and to be sure that scaling effects - the consequences of working with scale models - are within tolerable limits.

Then development of the models is frozen while a full-size model of the first-choice design is built and taken to the General Motors' wind tunnel in Detroit, the biggest in the world. GM owns Saab.

"The first full-size tests are for wind noise and for aerodynamic values," says Danielson, "and when we have confirmed the results of our one-fifth scale work we come back for testing at the Motor Industry Research Association wind tunnel in Britain."

During testing, any problems are identified and the design adjusted. The Saab 9-5, for instance, had a slot introduced into its rear spoiler to direct air over the rear window and reduce dirt deposits.

"With many wagons," said Danielson, "the last 100 mm of the roof is slightly inclined by a few degrees. But this can cause unpredictable airflow and even juddering at high speeds. To counter that we put a small lip at the rear of the roof.

"It's a small design change but an effective and important one for the driver."

The importance of aerodynamics is set to grow.

Mercedes-Benz has certainly recognised its importance with the new C-Class sedan. The goal now for carmakers is to be able to achieve a Cd value of 0.25.

"There isn't a government in the world that doesn't want cars to be more environmentally friendly, so carmakers are producing lighter cars with better fuel economy," Danielson says.

"But the lighter the car, the better the aerodynamics you need to keep it on the road.

"Since I started at Saab things have changed a lot, especially in computing where we now have the power to perform sophisticated Cd analysis.

"But in the end it doesn't matter to me whether we get data from a computer, from a wind tunnel, experiment, or from wherever - the real problem is what to do about it, how to eliminate problems and optimise performance. For me and for Saab, that's why aerodynamics is so important."