Streamlining Cars: Then and Now
Wind tunnels hold a special fascination for designers not only of aircraft, but of any vehicle for which speed and fuel economy are considerations. Although the dinosaurs of the 1950s will remain sentimental favorites of aficionados for years to come, the fuel crunch of the 1970s led to a revolution in automobile design toward smaller and more streamlined designs. And the wind tunnel is the primary tool used by engineers to test those designs.
The wind resistance arising from a car's aerodynamic design is a function of many independent factors--mass, speed, and smoothness, to name a few. Engineers deal with these factors and more to arrive at a single number which says it all--the drag coefficient, abbreviated Cd. Simply stated, the lower the Cd, the less resistance a car offers to the wind.
To give a few examples, the worst possible streamlining would be expected from a parachute, which is designed to maximize wind resistance. The Cd of a parachute is about 1.35. The lowest possible resistance is desirable in the airplane wing, which has a Cd of about 0.05. Automobile Cd figures lie between these two extremes. In the past 60 years, automakers have managed to cut Cd figures for production models nearly in half, from about 0.70 to about 0.40. In a practical sense, gas mileage is increased by 5 percent for every 10 percent improvement in aerodynamics.
Wind tunnel tests have led to automobile designs which, although they may never be a common sight on the streets, probably can not be significantly improved upon aerodynamically. Ford's Probe IV is a state-of-the-art experimental vehicle which boasts a Cd of only 0.15--about the same as an F-15 jet airplane. Chevrolet's experimental Citation IV has a Cd of 0.18.
Among commercially available 1984 model street cars, the most aerodynamically efficient was the Audi 5000S, with a Cd of 0.33. The Chevrolet Corvette was close behind with a Cd of 0.34. Most other street models have approximate values of 0.40 (my wife's Chevrolet Chevette, I have discovered, is at the high end of the spectrum with 0.45).
Many of the factors that contribute to air resistance can be deduced by common sense. These include the amount of frontal area, external door pillars, projections such as ornamentation and trim, and high-profile windshields.
There are other factors which aren't so obvious. Wheel wells provide an enormous source of drag, but what is there to do about them? A smooth underbody is just as important as the top, but unseen streamlining beneath seldom receives much emphasis.
Sometimes the wind tunnel provides the designers with results that all their experience and intuition would have cried out against. That is why it is such a valuable tool.
As an example, in 1921 Germany's Dr. Edmund Rumpler used experience that he had gained from the aircraft industry during World War I to design a car. The Rumpler Wagon, as it was called, was a peculiar-looking affair, teardrop-shaped when viewed from above, and looking somewhat like a boat mounted on wheels from the side. Recently, curious engineers trundled one of the fragile relics into Lockheed's wind tunnel in Georgia and turned on the fan, probably just for a laugh.
The engineers were stunned when the puttery little car, which Rumpler had designed without ever having heard of a computer or a wind tunnel, turned in a Cd value of barely 0.27--better than any car on the market in 1984.
