Common Misconceptions in Aerodynamics: Part 7

Continuing our dissection of the untruths you'll encounter online, today we'll visit the idea that simple "rules of thumb" can be applied to any car for maximum drag reduction.

"Rules of Thumb" are Not Reliable

The claim: Modifying the aerodynamics of your car is as easy as following some simple rules for boat tail angles, diffuser angles, air dam height, and more. Lowest drag is guaranteed if you adhere to these rules.

One particularly disingenuous feature of these "rules of thumb" is their common attribution to authoritative sources. Hucho did indeed include figures of various streamlined shapes in his book, but nowhere does he give a rule about maximum taper angle.

The reality: Rules of thumb abound in online aerodynamics discussions and message boards. One well-known web forum has accepted, for example, that boat tail extensions should not deviate more than 22 degrees from horizontal and that air dams should not extend lower than the lowest component underneath a car body (these rules are repeated ad infinitum in forum threads and included in stickies, along with many others).
 
The problem with these rules of thumb is that they are only sometimes right. There may indeed be a car which, when a tapered tail is added to it, suffers from increased drag if the angle exceeds 22 degrees. But not every car; it depends on its geometry and the properties of the flow over it. Yet this is how rules of thumb are transmitted and presented: as absolute and exact.
 
Aerodynamics textbooks don’t generally give rules of thumb because such simplifications are not appropriate and often misleading when applied to the complex flow over a car. And when they do, they are neither absolute nor exact.
 
A rare example can be found in Scibor-Rylski’s Road Vehicle Aerodynamics (1984). In the chapter on aerodynamics and styling, the author writes, “In general we can assume that the local declination of the surface to the airstream should not exceed 3 to 5 degrees at the rear part of the car.” Notice the difference between this advice and typical rules of thumb: this is general rather than exact; it gives a range rather than a specific, absolute number; and it qualifies the advice as an assumption, something that may be proven wrong in the future and is therefore subject to emendation.
 
Hucho touched on the subject of rules of thumb, writing in Aerodynamics of Road Vehicles (1998), “It is true that the mass of published experimental data is now so great that, with the aid of this information, it is possible to estimate drag from the geometrical particularities of any specific configuration…. However, as in the past, experiment remains the only means of obtaining reliable quantitative data for drag” (emphasis original).
 
We should also distinguish between rules of thumb that are drawn from actual research and those that appear to have sprung up ex nihilo, such as the air dam height proscription I mentioned at the outset of this post. A rule such as “removing external mirrors reduces drag by 4%” has its basis, at least, in reality: almost all cars see a drag reduction from getting rid of protuberances such as mirrors, even if the exact percentage reduction varies. There is a grain of truth in this “rule,” and a home modifier can be reasonably certain that removing the mirrors on their car will reduce its drag.
 
Similarly, the oft-repeated 22-degree boat tail angle “rule” has its basis in research conducted by W.A. Mair in the 1960s and 1970s. Mair used axi-symmetric bodies (shapes that are symmetric across their centerline side-to-side and top-to-bottom) with various lengths of tapered tail on them; basically, these looked like missiles with tail cones. He found that, for a shape like this, a drag minimum was achieved when the taper angle approached 22° in zero-yaw flow; as yaw angle increased, the 22° tail made more drag than steeper angles. Internet sages have decided that this means a 22-degree taper is the optimal angle for an extension on any car (this is the origin of the idea that you can use an aerodynamic “template” overlaid on any car to determine its optimal shape). This isn’t true, but it at least has its roots in established research.
 
On the other hand, “air dams should only extend as low as the lowest component hanging under the car” does not appear to have its origin in any published research; I have several aerodynamics textbooks and fluid mechanics textbooks in my library, and this appears in none of them. In fact, Aerodynamics of Road Vehicles (which is widely seen as the definitive reference for car aerodynamics) mathematically relates air dam height and underbody roughness to arrive at an equation for predicting optimum height that has nothing to do with the minimum height of underbody components—and is, again, an estimate only and must be quantified by testing as the author wrote previously (he goes on to give examples of air dam optimization with measured results on several production cars).
 
“Rules” in aerodynamics textbooks most often take the form of equations like this, relating two or more variables such as, in the preceding example, air dam height and surface roughness. These are almost always empirical rather than theoretical equations, drawn from experimental results, subject to uncertainty and possibly nowhere close to true—very different than the “rules of thumb” online commenters are more than happy to repeat. Instead of adhering to some rule that may or may not apply to your car specifically, you will need to test in order to optimize modifications to your car.

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