Test The Limits

Drag Queens: Blowin' in the Wind

By Steve Remondi

June 24 2014

  You know that old saying, "Close but no cigar?" That's how we felt after reading Car & Driver's first-ever aerodynamic "comparo." (You just gotta love the title of the article: "Drag Queens." :-) ) At Exa, we are all about raising awareness of the impact of aerodynamics on both design and engineering. Simply put, we believe that to make increasingly tough CAFE and other regulatory requirements while building beautiful cars, mastering the aerodynamics early in the program has increasingly become the leading indicator of the overall program's success. And we were glad to see that C&D largely agrees with us:

Like a thief in the night, wind resistance is a stealthy intruder that saps your speed and murders your mileage without leaving fingerprints. The gentle murmur of air streaming over, under, and through your car belies the wind’s heinous ways. Even if there’s no alternative to driving through Earth’s atmosphere, we can at least fight wind resistance with science. Aerodynamics—the study of air in motion—can lift our top speeds, curb our fuel consumption, and, if we’re smart about it, keep our tires stuck to the pavement.

So far, so good. But like the poor dude getting blown away in a sandstorm in the photo nearby, C&D missed the most important part of aerodynamic testing: wind tunnels don't actually measure the real aerodynamic performance of a car. In fact, they can -- and often do -- mislead the design and engineering team. Forget how expensive wind tunnels are and how hard it is to get time in them. They have bigger problems than cost and availability. Wind tunnels don't have a road. The car being tested doesn't actually move through the atmosphere.  Wind tunnels may have the value of being comfortable for the design team because they are familiar (and until recently, they were all we had). But they simply are not good tests of the aerodynamics of a car.  And C&D admits as much:

All wind tunnels strive to accurately quantify the aerodynamics a car will experience in the real world. The vehicle and the tunnel constitute a system with complex interactions. As a result, drag and lift measurements on a particular vehicle can vary from one tunnel to another... A group of vehicles may rank differently in different tunnels...This is why most manufacturers have so little faith in aero numbers measured outside their own facilities[emphasis mine]

That's why there's no cigar for this C&D's story: at the end of the day, despite the fact that the cars were tested in the same wind tunnel, all that C&D achieved was measurements of those cars at that moment in that wind tunnel. Nobody will ever drive those cars in that wind tunnel. As a result, their results do not predict the real world performance of those cars on a real road, in real temperatures and with real variations in atmospheric pressure. But (not surprisingly), at Exa we can tell a program team what those results will be, with our PowerFLOW aerodynamic simulation software. So, which car magazine wants to partner with us to design and execute a real 21st century "aero comparo?" One that is simulation-based -- but which accurately predicts real-world performance? Drop me a line at editors@thevortex.guru; let's talk.