Common Misconceptions in Aerodynamics: Part 1

Frequently, online articles or forum posts about aerodynamics are succinct, with simple explanations and guidelines for modifying your car. The problem with a lot of these is that they are wrong. Not just fuzzy on a few details here and there but outright, completely, 100% incorrect. They are not borne out by the vast body of literature on car aerodynamics or by direct, hands-on experimentation. The counter to these, unfortunately, cannot be as simple because, as we will see, the science of air flows is by its very nature complex and hard to understand. This series will argue against some of the most insidious myths, debunked with my own test results and citations from experts as supporting evidence (which I encourage you to read for yourself).

Aerodynamic "Templates" and "Ideal" Shapes Aren’t Applicable to Real Cars
 
The claim: Everything we need to know about car aerodynamics was discovered in the 1920s. Using a single ideal profile, we can create a universally applicable "template" to guide the shape of modifications and assess the aerodynamic performance of production cars.
 

This is a screenshot of an online tool for overlaying the "template" on your car.

The "template" in action. It's so easy! Hmm....

 
The reality: We’ll start with the big one. Visit any web forum, comments section or website even tangentially related to aerodynamics and you will undoubtedly see an author or poster (if not multiple posters) waxing on about the "ideal" teardrop shape, often claiming that applying a "template" of this teardrop to any car—regardless of body style, shape, or details—will lead to the optimum solution to reducing drag.
 
This supposition has its roots in some very old yet very robust research. In the early 1920s two engineers working at the Zeppelin company, Paul Jaray and his employee Wolfgang Klemperer, conducted research on a streamlined body ("teardrop" shaped) gradually brought closer to the floor in a wind tunnel. They found that the drag increased sharply as it approached the floor, but this drag increase could be mitigated if the body was cut in half lengthwise, with the flat side sitting just above the ground. Then, they put rudimentary wheels on it and found their resulting "car" had much lower drag than cars at the time—or even cars today. Jaray went on to experiment with real cars, licensing his designs to several manufacturers after leaving Zeppelin in 1923 to start his own autobody company, and patented what he called the combination form where several "ideal" teardrop shapes are stacked on each other in various ways to produce an actual car in which people can sit, luggage can be stored, etc.
 
Modern internet know-it-alls have taken this and run with it. They like to claim that the "ideal" car produced by Klemperer and Jaray in the Zeppelin wind tunnel is the be-all, end-all of aerodynamics, and if we would just make our cars look like that as much as possible our problems will all be solved ("the only pathway to low drag," one commenter on a well-known web forum is fond of writing). However, there are several issues with this:
 
1) Klemperer’s and Jaray’s model was just that: a model. It had no cooling airflow, no doors or windows, no interior, not even wheel openings (the wheels were partial, stuck onto its bottom side). It was in no way representative of a real, usable car.
 
2) Jaray’s attempts to design real car bodies emulating the "ideal" form were failures, as far as matching the drag figure of the model is concerned. While they were more efficient than typical car bodies of the time, they didn’t even come close to the low drag of the wind tunnel model; the closest he got was a drag coefficient about twice as high for the real thing compared to the "ideal."
 
3) There isn’t one "ideal" low-drag shape. In the 1960s, AJ Scibor-Rylski experimented with developing a low-drag body in the City University of London wind tunnel; his "whale car" model looks very different from Klemperer’s/Jaray’s and actually develops less drag. Similarly, modern solar cars and some concept cars achieve lower—sometimes much lower—drag figures than the 1920s "ideal" and don’t resemble its shape at all. The idea that there is one "ideal" teardrop—in effect, a "one size fits all" approach—is not supported in any way by the subsequent decades of research, design, and experimentation, or by anything in the literature.
 
4) Applying an "ideal" teardrop shape to a portion of an existing car, which can have any sort of body geometry preceding, surrounding, or following that teardrop application, is completely nonsensical. Fluid behavior does not follow our intuited progression of events: changing the flow pattern at the back of a car affects what happens at the front as much as changing the front affects the back, and the flow over cars is characterized by interactions (meaning if you change something like the size of the cooling air inlet you might affect flow speed over the roof, which in turn will alter the lift, which might affect the wake, etc.). Because of this, you can’t just stick an "ideal" shape somewhere on a car and expect that this will automatically be the best solution to a flow problem.
 
Wolf-Heinrich Hucho, the scion of modern vehicle aerodynamics, discussed the differences between aeronautics (the field in which early researchers like Klemperer and Jaray worked) and aerodynamics in the first chapter of Aerodynamics of Road Vehicles (1998). He pointed out, "With regard to their geometry, vehicles differ from airplanes in that their bodies cannot be separated into discrete fluid dynamic components which are clearly discernible…. The shape has to be considered as a whole" (emphasis added).
 

2021 Chevrolet Suburban

 
You can see the reality of this simply by looking at any modern car, such as the 2021 Chevrolet Suburban above. Ahead of the front wheels you can usually find a small plate, positioned with its face to the oncoming airflow. By itself, this plate has very high drag; however, underneath the front bumper cover or engine undertray and ahead of the spinning wheel and tire, it reduces the drag of the car as a whole. The shape of a single part of the car cannot be considered independently because of the interactions between every part of the flow over a car, often very complex interactions.
 
Proponents of template theory almost always simplify their arguments into two-dimensional principles that, on their face, seem to make sense. If only real cars were that simple—but they aren’t. The airflow over cars is very complicated, more so than over the elements of an airplane such as wing sections. RH Barnard, in Road Vehicle Aerodynamic Design (2009), cautioned, "Once again, it is necessary to remember that road vehicles and their air flow patterns are highly three-dimensional. It is important to avoid simply looking at the centre-line shape as if it extended to infinity on both sides." "Template" proponents will claim they don’t do this; most of them base their argument on the "half-body of revolution" ie one that appears hemispherical if looked at head-on (again, based on Klemperer’s and Jaray’s model). However, there are exactly zero cars today that have hemispherical body geometry, and applying a hemispherical, teardrop tail to a car may or may not reduce its drag more than another shape—or at all!
 
Here's an example of a typical "template" interaction. Person A comes on a forum asking for advice on aerodynamic modification and Person B, fully indoctrinated, posts something like this:
 

*knock knock* Hi, do you have a moment for us to share a message about a wonderful new church?

 
Neither Person A nor Person B has any idea what the characteristics of the airflow are over that car. If there is attached flow, building up to the "template" profile might increase drag by enlarging the size of the wake; if the flow is not attached, raising the profile may or may not keep it attached, or it may be unnecessarily high—perhaps the trunk lid would only need to come up a couple of inches to keep attached flow? If it is attached already, perhaps adding a box cavity or some tapered length or a spoiler extension at the back of the trunk would decrease the wake size and drag? Then there’s the question of the three-dimensional shaping; a profile view overly simplifies the situation. How does the degree of rounding of the C-pillars affect things? What if they were sharper? Would that reduce drag or increase it? What about stability? How on earth would anyone know without testing these parameters?
 
The very concept of a "template"—any template at all—violates the basic process of aerodynamic development as it is given in every single book on the subject: given a car of a particular shape, you first need to assess it for its existing flow characteristics, then test various changes to see what helps and what doesn’t. Deciding that some "ideal" shape can preclude all that is nothing more than living in a fantasy land.
 

It can be amusing to watch "template" creators or promoters confronted with reality. They almost invariably resort to personal attacks such as this one:
 

And sometimes, people see right through the BS:

 
Well said, Random Internet Person.