A while back, gave me a good adventure in its Untertürkheim wind tunnel, where I also got to meet Dr. Teddy Woll, senior manager, aerodynamics and wind tunnels, a man who clearly shares my enthusiasm for automotive history as well as technology.
I asked about vintage cars tested in wind tunnels and sure if he didn't have fascinating numbers. I dug out other tunnel tidbits from various sources, including this very magazine as far back as August 1982.
Cd, a Measure of Sleekness
For starters, a car's (or, for that matter, any shape's) coefficient of drag is a measure of its aerodynamic sleekness. Multiply this CD by the car's frontal area, A, and the airflow's dynamic pressure, q, and you get its total drag: D = CD x A x q.
Or, working the other side of the tunnel, when you know q from tunnel conditions and A from the car, and then measure D with the car on the tunnel's six-component balance, you can compute CD = D/(A x q).
What about q? In fact, q = 1/2 d x V2, where d is the density of the air (depending on temperature, humidity, etc.) and V is the velocity of its flow. For comparing cars in the same tunnel under the same conditions, CD x A is a valid means of comparison.
Ford's Model T and Other Extremes
I'd have thought the Model T was a regular barn door, but indeed its coefficient of drag is 0.79—whereas a barn door is more like 1.17. In theory, a flat surface would have a CD of 1.00, but in reality turbulence around its edges bump this up a bit.
Also, as another prototypical CD, imagine a hemisphere followed by a 20-degree cone. No less than Alexander Gustave Eiffel—yes, that Eiffel—figured out that such a shape had drag characteristics equivalent to a CD of 0.09. A high-speed aircraft wing is even better, around 0.04.
Returning to the Model T, Dr. Woll tells me its frontal area, big flat windshield and all, is 28.0 sq. ft. and hence its CD x A value is 22.1. Let's keep this handy for other automotive comparisons.
Historical automotive shapes
Other values shared by Dr. Woll are shown here.
Note the Beetle and Gullwing share the same frontal area, but the latter shows the benefit of sleekness. The 300SLR, the Moss and Jenks 1955 Mille Miglia car, isn't as sleek as the Gullwing, but it presents a smaller target to the wind.
By a long shot, each beats the Model T—or that barn door.
An Unlisted Reynolds Number
Though factored out of the foregoing comparison, it's fun to talk about V, the velocity of the flow, and its effect on verisimilitude of wind tunnel models. It's essential that the modeled object and flow of air (or any other fluid) cooperate in transforming smooth (or "laminar") behavior into turbulence in a manner similar to full-size conditions. For instance, it's intuitively clear that a 1/24-scale model disrupts the air—that is, causes less turbulence—in a given airflow than its full-size counterpart would.
This transition from laminar to turbulent behavior is characterized by the Reynolds Number of the situation. Reynolds Number is a dimensionless factor that's proportional to the fluid's density, its velocity and the length of the object, and inversely proportional to the fluid's viscosity. To wit, Re = (d x V x L)/u.
This is quite a mouthful, but here's a takeaway: If you're working with a 1/4-scale model, you need four times the air speed to preserve the Reynolds Number describing this laminar/turbulent transition.
Actually, it's a little easier than this. As long as the Reynolds Number is sufficiently large, model-to-full-scale correlation is acceptably close. As a guideline, though, the smaller the model, the higher velocity of the airstream to preserve verisimilitude.
Fooling with Re = (d x V x L)/u
Or you can work other variables in the Reynolds Number characterization. If the fluid is more dense than air—water, for instance—then the flow velocity can be proportionately lower. This is why Dan Gurney's All American Racers and others have employed water tunnels.
Another way to raise density is to boost the pressure of the fluid. And, indeed, it's been hinted that some Formula 1 teams have gone to the extreme of pressurized wind tunnels.
Others have suggested that the game isn't worth the candle. But then again, back in 1982, there were experts who felt the same about moving ground planes. Today, many tunnels—especially those devoted to open-wheel race cars—have moving floors.
F-Cell in Monaco—and Soon Everywhere!
As a complete change of pace—and climate—in the south of France I drove the Mercedes-Benz F-Cell, its latest hydrogen fuel cell car, as well as the Smart in EV form.
The F-Cell is built on the company's B-Class platform (think size), another engineering tour de force in that from the onset the B-Class was designed for multiple propulsion concepts. The fuel cell variant has no compromise of passenger or cargo space, a non-recessed load floor being the only giveaway of this flexible packaging.
A colleague and I drove the F-Cell from Nice Airport to Monaco along the Bas Corniche, the lower road skirting Riviera harbors. Our return trip came on the Grande Corniche and autoroute, where I ever so briefly probed the car's claimed 170-km/h (105-mph) top speed. Alas, our trip wasn't long enough to assess a reported range of 400 km (248 miles) based on the F-Cell's relatively modest 3.7 kg of onboard H2 compressed to 10,000 psi.
The big news is production readiness of the F-Cell. It'll be in U.S. fleets by midyear, reportedly at a lease price comparable to that of a fully equipped C-Class (perhaps $500/month?). Availability to ordinary folks is scheduled for 2015.
Where do we sign up?
Hydrogen-Exhaling Bugs in Your Fuel Tank?
Researchers at Virginia Tech, Oak Ridge National Lab and the University of Georgia have concocted a novel mix of enzymes, wood scraps and water that yields hydrogen pure enough to power a fuel cell. The process occurs at ordinary atmospheric pressure and modest temperature, around 86 degrees Fahrenheit, with a significantly high hydrogen generation rate.
It uses polysaccharides, complex carbohydrates of the form C6H10O5 found in plant cellulose. Its catalyst is a blend of enzymes and coenzymes never found together in nature. Add these to the mixture of polysaccharides and water, and the result is high-quality hydrogen and CO2.
Details are given in "Spontaneous High-Yield Production of Hydrogen from Cellulosic Materials and Water Catalyzed by Enzyme Cocktails," published in the Wiley journal ChemSusChem, Chemistry and Sustainability, Vol. 2, Issue 2.