Rounded forms not necessarily the most aero

[Frank Lee]

Originally published Friday, January 2, 2009 at 12:00 AM

Cutting-edge aerodynamics: Auto designers discover surprising new things about saving fuel

Usually, aerodynamic shapes are rounded forms that slip through the air. But the wind tunnel is proving that counterintuitive, edgy automotive shapes such as the Ford Flex or Toyota FT-HS can reduce the drag coefficient and save fuel or battery power.

By PHIL PATTON

The New York Times

Wayne Koester was pleasantly surprised. Koester, who was a Ford aerodynamicist at the time, had been assigned to turn the popular boxy Fairlane design study that was introduced at the 2005 Detroit Auto Show, inspired by the woody station wagons of the 1940s, into the production Flex crossover. But he had to produce a shape slippery enough to provide acceptable fuel mileage, and he feared the boxy show car would have to be radically revised.

To his surprise, in hundreds of wind-tunnel tests the original edges produced less drag than curved ones. In the bumper, headlights and hood, in fact, aerodynamics were improved by carefully designed edges.

Usually, aerodynamic shapes are rounded forms that slip through the air. But the wind tunnel is proving that counterintuitive, edgy shapes can reduce the drag coefficient and save fuel or battery power.

Many people associate aerodynamics with getting rid of edges and corners. But redoubled efforts to reduce drag have led engineers and designers to some surprising shapes. The discoveries offer both practical and aesthetic implications.

"Round is not always aero," said J C. Mays, group vice president for design and chief creative officer of Ford. Some cars, including Porsches, have famously proved more aerodynamic when turned tail-first in the wind tunnel.

Koester's box turned out to be a success. His team at Ford found that the box cut 0.02 off the Flex's coefficient of drag, beating the competitors and providing a respectable 24 mile-a-gallon highway rating.

The future of Toyota

Toyota has begun using the term aerocorner for angular but aero shapes. The aerocorners include edges and bulges on the front of Toyota's small city car, the iQ, its FT-HS concept and the next generation Prius hybrid. The idea, said Kevin Hunter, president of Toyota's Calty Design Research studio in Newport Beach, Calif., is to smooth the flow of air down the sides of the car. The aerocorner pushes airflow out to reduce turbulence around the front-wheel housing, he explained.

The aerocorner is one of several aero elements Toyota has listed as keynotes of all future design strategy, as decreed by Toyota's design chief, Wahei Hirai.

The other elements include forward-projecting front bumpers that send air around the car rather than under it, avoiding the drag of the messy underbody, still the aerodynamically crudest place on a car. Covering the underbody completely would be expensive and impractical, but some cars, including the Nissan GT-R and Ferrari California, cover parts of the underbody.

The difference between the concept version of the Chevrolet Volt, the plug-in hybrid that General Motors first showed in 2007, and the production version shown in September, had to do with aerodynamics, said chief designer Bob Boniface.

"Aero is more about the rear of the car than the front," said Freeman Thomas, director of strategic design at Ford, as he walked through displays at the recent Los Angeles Auto Show. Problem-causing drag behind the car stems not so much from frontal resistance as from swirling eddies of turbulent air at the rear. Designers now speak of managing airflow and having the air cleanly released at the back.


Efficiency is the key driver in aerodynamic design. "Remember, at highway speed about 60 percent of energy is used overcoming air resistance," Hunter said. But aerodynamicists also have to worry about interior noise, stability and potentially dangerous lift at higher speeds.

The new shapes of aero can take small forms: a tiny tablike shape sits in front of the rear wheel of the Chevrolet Traverse crossover, and its roof rack has been lowered flush with the roof. The Chevrolet Volt lacks the wheel covers of earlier electrics and hybrids. "The wheels fill the wheel wells and we don't need those," Boniface said.

The Volt also offers a surprisingly sharp upright edge on its rear flanks, which makes for more controlled detachment of airflow from the side of the vehicle. A small, almost invisible edge on the bulging taillights of the new Mazda 3 is there to help snap the airflow neatly off the back of the car.

Slits, not grilles

The new aero is visible in lower, wider grilles on many models. "We are trying to redefine the grille," said Hunter of Toyota's future designs. "It will be minimized quite a bit and given a different proportion. It is possible to have a thinner slit versus a big opening."

Lower grilles take advantage of higher air pressure close to the ground. There is new emphasis across the auto industry on reducing cooling drag, from air pulled into the engine and radiator, Hunter said.

Some grilles are larger for aesthetic reasons rather than functional ones. "The grille is about symbolism," Thomas said. Some small air slits are disguised as larger "mouths," with closed-off portions.

Reduced drag does not necessarily mean increased beauty, however. The tall, humpbacked silhouette of the Prius is not exactly elegant, but Hirai, the Toyota design chief, has said that it will continue in the next-generation car. The grilles of the Ford Fiesta and Ford Ka are larger at the bottom for better airflow.

Most cars have two-part grilles. The top area of the grille has been the symbolic mouth — lower vents were visually disguised or de-emphasized. Now there is a change, according to Hunter. "There is a shift to emphasis on the lower grille," he said.

"As designers, we are using that as an aesthetic advantage that we can leverage into other models," Hunter said. "It is also a positive symbolic advantage for us, since fashion is still very important. It gives a good blend of fashion and function."

Aero: the early years

The idealized low-drag shape, the earliest aerodynamics researchers decided, was the teardrop form: a rounded front and a long tapering tail, like a drop of water falling from a faucet. But by the 1930s, after research by Dr. Wunibald Kamm in Germany, it was widely understood that slicing off the back of the vehicle produced an aerodynamic result that was nearly as satisfactory and eminently more practical. (Experimental designs like the Oldsmobile Aerotech of 1987 demonstrated the aerodynamic long tail.)

When aerodynamics became a concern, with the arrival of cars like the Chrysler Airflow and Lincoln Zephyr in the 1930s, some designers feared it would make all cars look alike. Harley Earl, the design chief of GM at the time, said he thought 1940s streamlining was producing an inverted-bathtub look, as seen in the Chrysler Thunderbolt concept car or in postwar Hudsons.

But discoveries about the science of aero and new ideas about its art created fresh shapes. Earl developed big fenders and the tail fin to defeat the bathtub. In the 1990s, ovals were decorated with large traditional grilles and aerodynamically harmless creases.

Still, the wind tunnel continues to bring new discoveries. "Aero," said Michael Simcoe, a veteran exterior designer at GM, "remains a black art."

Motoring | Cutting-edge aerodynamics: Auto designers discover surprising new things about saving fuel | Seattle Times Newspaper

[Frank Lee]

More about the Ford:

Ford Flex Aerodynamics

[Ryland]

Am I understanding it correctly that they are not saying that the tear drop shape is bad but that boxes are not as bad as some might think.

[ChazInMT]

Great read Frank. Thanks!

[KamperBob]

It's interesting how articles like this emphasize wake importance yet it seems 9 out of 10 wind tunnel pix show approach not departure flow. That surely helps keep the eyes off the prize...

[fusion210]

From the Ford Flex article.

At 55 mph, the Flex needs only 8.90 horsepower

Does that sound wrong to anyone else?

[BHarvey]

Originally Posted by KamperBob

It's interesting how articles like this emphasize wake importance yet it seems 9 out of 10 wind tunnel pix show approach not departure flow. That surely helps keep the eyes off the prize...

Exactly!

[mort]

Originally Posted by fusion210

From the Ford Flex article.

Does that sound wrong to anyone else?

They must mean 8.9 HP just to overcome wind drag. And another 8 or 9 for rolling.

-mort

[Joenavy85]

Originally Posted by mort

They must mean 8.9 HP just to overcome wind drag. And another 8 or 9 for rolling.

-mort

sounds like a full diaper load there. if that was the case they could gear the thing so that the engine was at idle going that speed. imagine the mileage you could get then, my jeep's v6 sucks down about .4 GPH at idle, that would make 137.5 MPG!!!

[Frank Lee]

Originally Posted by mort

They must mean 8.9 HP just to overcome wind drag. And another 8 or 9 for rolling.

-mort

I couldn't find CdA for it but plugged Cd .355 into the aero calculator and jockeyed the frontal area around til it spit out approx 8.9 HP for aero load at 55 mph. Frontal area value seems a bit small... but anyway, the rolling resistance HP requirement should be about 3.5. I don't think the 8.9 is the total HP requirement at 55 mph; besides, the fe is too poor for that to be the case.