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A Practical Guide to Aerodynamic Modification

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Updated August 15, 2023 Tuft testing shows the streamlines on a car as the yarn aligns itself with airflow while you drive. Gas prices have recently reached their highest level in nearly a decade. You may find yourself looking at your car, wondering if it’s possible to use less fuel on your long commute and keep some money in your pocket. You may have heard of people who modify their cars to get better fuel economy. You might have even seen cars like the Aerocivic, a weird-looking contraption that was reported on in mainstream media articles during the gas price spike of 2008-09. Would doing something like that work on your car? Can you modify the aerodynamics of your car at home? The good news is, you can! The better news is, you don’t have to (and shouldn’t) make your car look like the Aerocivic. Air drag has an influence on the fuel economy of cars, and that influence is greater the faster you typically drive. You can also do a lot more with airflow than just reduce drag. Many peo
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No, Cars Are Not Wings

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It’s a common refrain: “Cars are shaped like wings, so they make lift.” Shockingly, one of these was published by a major automotive magazine. The only problem is, it isn’t true. Cars are entirely different from wings (or 2D sections of wings used for analysis and design called airfoils ). Here’s why. Airfoils Plot of a NACA 0012 airfoil. The numbering system gives us information about the airfoil: it has no camber (curve), there is no position of maximum camber, and its thickness is 12% of its chord. Moving at 60 m/s, and with an angle of attack of just 1.7 °, this symmetric airfoil with a chord (length) of 2.0 m will generate a massive 820 N lift per meter of wing width! The behavior of airfoils is described by a branch of aerodynamics called Thin Airfoil Theory (TAT) that is well-developed and characterized by straightforward math that can, to a surprisingly high degree of accuracy, predict the performance of wings. Theodore von Kármán wrote in 1954:   “Mathematical theories from th
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What is Entrainment?

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Lots of technical terms in aerodynamics are thrown around by amateurs without a real, working knowledge of what they mean  and look like in practice, and consequently, without clarity of thought (I was certainly guilty of this for a long time). I’ve started to try and address this with a glossary, but here I’ll focus on one term in particular: entrainment . What is entrainment, and what is it good for?   Entrainment   To “entrain” is to cause something to be drawn along or to follow, and this works in fluids due to pressure differences. If I  “push” a fluid somewhere—say, out of a tube or nozzle—so that it moves at some speed and has a lower pressure than the fluid surrounding it, more of that surrounding fluid will want to come along for the ride, moved by the pressure differential. This is entrainment , and it’s a way of getting flow for “free” since you get more flow out than you put in.   Jet Pump   Now, the fun part. You can build a simple device at home to see entrainment in a
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Aerodynamic Modification Doesn't Have to be Expensive or Complicated

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Reading through a Youtube post recently on the perceived roadblocks to aerodynamic modification, I was disheartened to see many commenters espouse the idea that effective mods require huge expenditures of money and time, lots of trial and error, or won’t be effective at (legal) road speeds and therefore aren’t worth doing. These attitudes couldn’t be more wrong: aerodynamic modification of road cars can be done very simply and cheaply, and the results can be dramatic.   Myth: To effectively modify your car’s aerodynamics requires a powerful computer and CFD or thousands of dollars’ worth of wind tunnel time.   Reality: CFD and wind tunnels are the two test environments most amateurs are somewhat familiar with and put a lot of faith in. However, there are huge problems with both of these, as I’ve intimated before in several posts .   CFD, or computational fluid dynamics, uses mathematics to model airflows. This can be a very useful tool in the initial development phase of a new ca
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Optimizing a Tail for Low Drag: Part 4

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Curvature Now that I know from my first round of testing which panel orientations have attached flow, how the pressure behaves with changes in angle, and a rough prediction of pressure drag reductions from all that information, I’ll move on to a larger test buck that will start to approximate a full tail. This buck is almost as long as my maximum length requirement and, rather than a flat panel like my first board approximations , has some curvature in it. Specifically, the extension here bends from an angle of about 20° from horizontal at its front to 23° at its trailing edge, in between the shallowest and middle angles I tuft- and pressure-tested: “Conventional wisdom” says to bend the tail in a convex curve like this for lowest drag. But is that “wisdom” correct? Only testing will answer that. I’ll test this buck in this configuration as well as with an added spoiler that, when affixed to the tail, is horizontal: More fun with the miter saw. All in, I’m still sitting at $0 inve
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